Studies of isotope effects on chemical shifts in intramolecular hydrogen bonded compounds with low barriers to hydrogen transfer; NMR structural studies of proteins, nucleic acids and biomolecules in general, structure-function relationships. NMR studies in environmental chemistry and biology. Studies of H- and metal tautomerism. Isolation, characterization and synthesis of new natural products. Compound classes of particular interest, proteins, nucleic acids, Schiff bases, b-thioxoketones and b-diketones.
Abstract: kappa-Carrageenan was hydrolyzed with mild hydrochloric acid and separated into a series of oligosaccharides, the sequences and structures of which were investigated by double-quantum filtered correlation spectroscopy (DQF-COSY), total correlation spectroscopy (TOCSY), heteronuclear multiple-quantum coherence (HMQC), and heteronuclear multiple-bond correlation (HMBC) techniques, respectively. The chemical structures and conformations of the individual sugar residues were identified, as well as the sequential connectivity of the oligosaccharides. The interresidue nuclear Overhauser effects (NOEs)/rotating frame Overhauser effects (ROEs) revealed an ordered helical structure of the carrageenan oligosaccharide chains. Therefore, a general two-dimensional (2-D) NMR methodology for the unambiguous sequence and structure analysis of kappa-carrageenan-derived oligosaccharides was established in this study.
Abstract: Secondary deuterium isotope effects on C-13 and N-15 nuclear shieldings in a series of cyclic enamino-diesters and enamino-esters and acyclic enaminones and enamino-esters have been examined and analysed using NMR and DFT (B3LYP/6-31G(d,p)) methods. One-dimensional and two-dimensional NMR spectra of enaminocarbonyl and their deuterated analogues were recorded in CDCl3 and CD2Cl2 at variable temperatures and assigned. (1)J(NH), coupling constants for the derivatives of Meldrum's and tetronic acids reveal that they exist at the NH-form. It was demonstrated that deuterium isotope effects, for the hydrogen bonded compounds, due to the deuterium substitution at the nitrogen nucleus lead to large one-bond isotope effects at nitrogen, (1)Delta N-15(D), and two-bond isotope effects on carbon nuclei, (2)Delta C(ND), respectively. A linear correlations exist between (2)Delta C(ND) and (1)Delta N-15(D) whereas the correlation with delta NH is divided into two. A good agreement between the experimentally observed (2)Delta C(ND) and calculated d sigma C-13/dR(NH) was obtained. A very good correlation between calculated NH bond lengths and observed NH chemical shifts is found. The observed isotope effects are shown to depend strongly on Resonance Assisted Hydrogen bonding. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: Trichloroacetic acid (TCAA) is a pollutant with several sources and is also formed naturally in soil. We show that almost all investigated environmental compartments (soil, soil water, groundwater, spruce needles and throughfall, but not rain) contain compounds, which make false positives in the thermal decarboxylation method often used for determination of TCAA in environmental samples. The compounds are dominating quantitatively over TCAA in soil, soil water and groundwater, while TCAA is dominating in needle and throughfall samples. The compounds behave differently from TCAA with regard to the velocity and the pH-dependence of the chloroform release. We did not manage to reveal the whole chemical structure of the compounds, but a trichloroacetyl group seems to be the only plausible structure giving rise to CHCl3 both upon heating and under alkaline conditions. Besides the trichloroacetyl group, the compounds did in general contain a carboxylic acid group, although in needle and throughfall samples, trichloroacetyl compounds with a neutral charge at pH 7.5 seemed to co-exist with the carboxylic acids. Trichloroacetyl groups in humic substances and possibly other macromolecular structures contribute to the major portion of the total trichloroacetyl- CHCl3 in topsoil, but smaller molecules with less UV-VIS absorption seem to constitute the major part of trichloroacetyl-CHCl3 in soil water and groundwater. The trichloroacetyl containing compounds are most likely naturally occurring compounds formed in the natural chlorination processes in soil, but additional studies are needed to substantiate this hypothesis.
Abstract: Pollution with organochlorines has received major attention due to various environmental effects but it is now increasingly recognized that they also take part in biogeochemical cycles and that natural background concentrations exist for several chlorinated compounds We here report the natural occurrence and cycling of organic compounds with a trichloromethyl moiety in common The study areas are temperate coniferous forests Trichloromethyl compounds can be found in all compartments of the forests (groundwater soil vegetation and throughfall) but not all compounds in all compartments The atmospheric input of trichloromethyl compounds is found to be minor with significant contributions for trichloroacetic acid (TCAA) only In top soil where the formation of the compounds is expected to occur there is a clear positive relationship between chloroform and trichloroacetyl containing compounds Other positive relations occur which in combination with chlorination experiments performed in the laboratory point to the fact that all the trichloromethyl compounds may be formed concurrently in the soil and their subsequent fates then differ due to different physical chemical and biological properties TCAA cannot be detected in soil and groundwater but sorption and mineralization experiments performed in the laboratory in combination with analyses of vegetation show that TCAA is probably formed in the top soil and then partly taken up by the vegetation and partly mineralized in the soil Based on this and previous studies a conceptual model for the natural cycling of trichloromethyl compounds in forests is proposed (C) 2010 Elsevier B V All rights reserved
Abstract: Deuterium equilibrium isotope effects (EIEs) for a cage diol and 2,6-dihydroxyacylaromatics and complexes thereof containing intra- or intermolecular hydrogen bonds have been calculated using harmonic and anharmonic vibrational frequencies using GAussian '03 and the HF, B3LYP, and MP2 levels of theory. The predicted isotope effects have been compared with experimental NMR data, and the origins of the isotope effects have been characterized in terms of zero-point vibrational energy differences and enthalpic and entropic contributions to the free energy difference between labeled species. Reliable free energy predictions based upon harmonic frequencies were found for systems whose isotope effects, re governed by bond stretching effects and for systems whose isotope effects are determined by low-frequency Vibrational modes. In contrast, thermochemical predictions based upon anharmonic frequencies were found to be far less consistent. Vibrational entropy is predicted to play an important role in modulating, and, in Some cases, governing isotopic site preferences in hydroxyl-derived intra- and intermolecular hydrogen bonds.
Abstract: Soil organic matter (SOM) is generally believed not to influence the sorption of glyphosate in soil. To get a closer look on the dynamics between glyphosate and SOM, we used three approaches: I. Sorption studies with seven purified soil humic fractions showed that these could sorb glyphosate and that the aromatic content, possibly phenolic groups, seems to aid the sorption. II. Sorption studies with six whole soils and with SOM removed showed that several soil parameters including SOM are responsible for the strong sorption of glyphosate in soils. III. After an 80 day fate experiment, similar to 40% of the added glyphosate was associated with the humic and fulvic acid fractions in the sandy soils, while this was the case for only similar to 10% of the added glyphosate in the clayey soils. Glyphosate sorbed to humic substances in the natural soils seemed to be easier desorbed than glyphosate sorbed to amorphous Fe/Al-oxides. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: Alkylated hydroxyflavothiones, namely flavothione, 5-hydroxyflavothione, 5,7-dihydroxyflavothione (chrysinthione), 7-dodecyloxy-5-hydroxyflavothione, 7-butyloxy-5-hydroxyflavothione, 2',3,4',7-tetramethoxy-5-hydroxyflavothione, 3,3',4',7-tetra methoxy-5-hydroxyflavothione, 7-butyloxy-4',5-dihydroxyflavothione and 7-butyloxy-4',5-hydroxyflavanonethione have been synthesized from the corresponding hydroxyflavones in two steps, alkylation of the non-hydrogen-bonded hydroxyl groups by bromoalkanes or dimethyl sulfate followed by conversion of the carbonyl group to a thione using Lawesson's Reagent under microwave irradiation and solvent-free conditions. Part of the alkylated flavanone, 7-butyloxy-4',5-dihydroxyflavanone, was oxidized during the treatment with Lawesson's reagent to yield a second product 7-butyloxy-4',5-dihydroxyflavothione in addition to the target product butyloxy-4',5-hydroxyflavanonethione. Deuterium isotope effects on 13C chemical shifts have been measured in hydroxyflavones, isoflavones, flavanones and the thio analogs. Formal four-bond deuterium isotope effects on 13C chemical shifts, n Delta C=S(OD) are very sensitive to variations in structures and substitution patterns. Density functional theory (DFT) calculations are carried out to obtain geometries. Correlations relating distances around the hydrogen bond system to the deuterium isotope effects on 13C chemical shifts are discussed. 13C chemical shifts are calculated by DFT methods. Effects of thiocarbonyl anisotropies are suggested. Copyright (C) 2009 John Wiley & Sons, Ltd.
Abstract: Quantum mechanical calculations are presented that predict that one-bond deuterium isotope effects on the N-15 chemical shift of backbone amides of proteins, (1)Delta N-15(D), are sensitive to backbone conformation and hydrogen bonding. A quantitative empirical model for (1)Delta N-15(D) including the backbone dihedral angles, I broken vertical bar and I, and the hydrogen bonding geometry is presented for glycine and amino acid residues with aliphatic side chains. The effect of hydrogen bonding is rationalized in part as an electric-field effect on the first derivative of the nuclear shielding with respect to N-H bond length. Another contributing factor is the effect of increased anharmonicity of the N-H stretching vibrational state upon hydrogen bonding, which results in an altered N-H/N-D equilibrium bond length ratio. The N-H stretching anharmonicity contribution falls off with the cosine of the N-H center dot center dot center dot O bond angle. For residues with uncharged side chains a very good prediction of isotope effects can be made. Thus, for proteins with known secondary structures, (1)Delta N-15(D) can provide insights into hydrogen bonding geometries.
Abstract: NMR investigations have been carried out on the B1 domain of protein G. This protein has six lysine residues, of which three are consistently found to form surface-exposed salt bridges in crystal structures, while the other three are not. The N zeta and H zeta chemical shifts of all six lysines are similar and are not affected significantly by pH titration of the carboxylate groups in the protein, except for a relatively small titration of K39 N zeta. Deuterium isotope effects on nitrogen and proton are of the size expected for a simple hydrated amine (a result supported by density functional theory calculations), and also do not titrate with the carboxylates. The line shapes of the J-coupled N-15 signals suggest rapid internal reorientation of all NH3+ groups. pK(a) values have been measured for all charged side chains except Glu50 and do not show the perturbations expected for salt bridge formation, except that E35 has a Hill coefficient of 0.84. The main differential effect seen is that the lysines that are involved in salt bridges in the crystal display faster exchange of the amine protons with the solvent, an effect attributed to general base catalysis by the carboxylates. This explanation is supported by varying buffer composition, which demonstrates reduced electrostatic shielding at low concentration. In conclusion, the study demonstrates that the six surface-exposed lysines in protein G are not involved in significant salt bridge interactions, even though such interactions are found consistently in crystal structures. However, the intrahelical E35-K39 (i,i+4) interaction is partially present.
Abstract: The review covers the use of chemical shifts, isotope effects on chemical shifts and coupling constants in the description of Schiff bases. o-Hydroxy Schiff bases are often tautomeric and special attention is devoted to this feature. Nuclei in question are H-1, C-13, N-15, F-19 and O-17. As well primary as secondary isotope effects are treated. Deuterium isotope effects on C-13 and N-15 chemical shifts are treated in detail and the theoretical background is dealt with. Finally, various applications of Schiff bases in biology and stereochemistry are discussed.
Abstract: Two new geranylated xanthones, 6-O-methylcowanin (4) and oliverixanthone (5), along with five known compounds, cowanin, rubraxanthone, cowaxanthone, cowanol, and beta-mangostin, have been isolated from the bark of Garcinia oliveri. For comparison of their biological activities, one mono- and seven di-O-alkylated alpha-mangostin derivatives were synthesized from alpha-mangostin. The structures of all compounds were assigned by spectroscopic methods (1D and 2D NMR and MS). Cytotoxicity of selected xanthones against MCF-7 and DLD-1 cell lines was examined. Evaluation of the structure-activity relationship showed that alpha-mangostin had the strongest activity, and all the O-alkylated alpha-mangostin derivatives showed reduced activity compared to the naturally occurring alpha-mangostin.
Abstract: The OH chemical shift of the enol form of nitromalonamide is found at 18.9 ppm both in DMSO-d(6) and in DMF-d(7) indicating a very strong hydrogen bond. The OH chemical shift is insensitive to temperature changes. Contrary to the large OH chemical shift, a small two-bond deuterium isotope effect of 0.135 ppm due to deuteration at the OH position is found at the enolic carbon. This is confirmed by density functional theory calculations. The observed effects are interpreted as due to an equilibrium between identical enolic forms. These show a strong OH center dot center dot center dot O hydrogen bond as well as a NH center dot center dot center dot O-N=O hydrogen bond. Copyright (C) 2008 John Wiley & Sons, Ltd.
Abstract: Humic substances (HS) often are dominant constituents of soil organic matter. In this study 28 fractions of HS were purified and chemically characterized from horizons of three Danish soils differing in texture. A commercial humic acid (HA) was included for comparison. The HS fractions were chemically characterized by CHNS analysis, pH-titration and liquid state C-13-NMR experiments. Amino acids and carbohydrates could clearly be differentiated from other O- and N-aliphatic compounds. The data were analysed by principal component analysis. The analysis revealed greater difference between HS fractions than between locations. The characteristics of the HS fractions were compared with several models in the literature and an average structural model for one HA and one FA fraction is proposed.
Abstract: A series of 5-acyl-3-methylrhodanines have been synthesized. The structures of those are described using H-1 and C-13 NMR, deuterium isotope effects on C-13 chemical shifts and DFT, calculations. The compounds exist as exocyclic enols with intramolecular hydrogen bonds to the C = ONCH3 oxygen. No signs of enhanced mesoionic character were found.
Abstract: Structures of several beta-amino-functionalized secondary thioamides capable to form intramolecular hydrogen bond N-H center dot center dot center dot N have been determined by X-ray diffraction and ab initio calculations. Both methods show comparable results for the bond lengths and angles, although the N center dot center dot center dot N distances is slightly longer in the calculated structures. Particular attention has been directed to the deformations of the thioamide group caused by steric constraints. Out-of-plane deformations of the thioamide group are described in terms of bending (chi(C) and chi(N)) and twisting (tau) parameters. The X-ray and ab initio results show that the thioamide group displays high resistance to steric stress which is in this case caused by intramolecular hydrogen bond and deviations of the thioamide group from planarity are small if any. Some sp(2) bond angles in the thioamide group of the molecules under exploration as well as some tetrahedral bond angles in the saturated parts of the molecules are changed from ideal values in order to retain the thioamide group planarity. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Structures of several beta-amino-functionalized secondary thioamides capable to form intramolecular hydrogen bond N-H center dot center dot center dot N have been determined by X-ray diffraction and ab initio calculations. Both methods show comparable results for the bond lengths and angles, although the N center dot center dot center dot N distances is slightly longer in the calculated structures. Particular attention has been directed to the deformations of the thioamide group caused by steric constraints. Out-of-plane deformations of the thioamide group are described in terms of bending (chi(C) and chi(N)) and twisting (tau) parameters. The X-ray and ab initio results show that the thioamide group displays high resistance to steric stress which is in this case caused by intramolecular hydrogen bond and deviations of the thioamide group from planarity are small if any. Some sp(2) bond angles in the thioamide group of the molecules under exploration as well as some tetrahedral bond angles in the saturated parts of the molecules are changed from ideal values in order to retain the thioamide group planarity. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Five crude extracts were made from leaves and stems of Jasminum subtriplinerve Blume (Oleaceae) and investigated for antimicrobial, antioxidant and cytotoxic activities. The extractions were done with petroleum ether, ethyl acetate, ethanol, methanol or water. All extracts exhibited anti-bacterial activity except the water fraction. On the other hand, all extracts exhibit antioxidant activity except the petroleum ether fraction using the DPPH radical scavenging assay. Only the petroleum ether fraction showed a cytotoxicity activity against tested cell-lines, Hep-G2 and RD with IC50 values of 19.2 and 20 mu g mL(-1), respectively. From the petroleum ether and ethyl acetate extracts, two triterpenes namely 3 beta-acetyl-oleanolic acid and lup-20-en-3 beta-ol and a sterol, stigmast-5-en-3 beta-ol were isolated. The structure of those compounds were elucidated by spectrometric methods IR, MS, 1D-NMR, 2D-NMR and simulated ACD/NMR spectra. The data presented here indicate that J. subtriplinerve do contain compounds with interesting biological activity.
Abstract: Humic substances (HS) are the dominant constituents of soil organic matter (SOM). The interactions between the phenylurea herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) and several HS fractions, purified from various soil horizons, were studied. One commercial humic acid (HA) was included for comparison. Diuron was shown to adsorb significantly, but reversibly, to purified HA while sorption to fulvic acid (FA) was less pronounced. The sorption abilities of the purified HS fractions were correlated with their total aromatic content In natural soils, SOM was the main adsorbent of diuron, but the organic matter partition coefficient was larger in sandy compared to clayey soils. Degradation of diuron in natural soils was slow and incomplete. Inoculation of a sandy C-horizon with a diuron-degrading bacterial strain led to substantial diuron degradation, but the addition of purified FA and HA to these inoculated soils decreased this degradation. The main metabolite produced during diuron degradation, 3,4-dichloroaniline, was bound irreversibly to HS within days after formation.
Abstract: The vibrational structure of the title compound (1,3-diphenyl-3-thioxopropane-1-one, TDBM) was studied by a variety of experimental and theoretical methods. The stable ground state configuration of TDBM was investigated by infrared (IR) absorption measurements in different media, by linear dichroism (LD) polarization spectroscopy of samples partially aligned in a stretched polymer matrix, and by Raman spectroscopy. The investigation of the metastable photoproduct of TDBM was based on the previously published spectrum of the product trapped in argon matrix [Y. Posokhov, A. Gorski, J. Spanget-Larsen, F. Duns, P.E. Hansen, J. Waluk, Chem. Phys. Lett. 350 (2001) 502]. The observed vibrational spectra were compared with theoretical transitions obtained with B3LYP/cc-pVTZ density functional theory (DFT). The results leave no doubt that the stable ground state configuration of TDBM corresponds to the intramolecularly hydrogen bonded-enol form (e-CCC), and that the photoproduct corresponds to an "open", non-chelated enethiol form (t-TCC), thereby supporting the previous conclusions by Posokhov et at. No obvious indications of the contribution of other forms to the observed spectra could be found. (C) 2006 Elsevier B.V. All rights reserved.
Abstract: Tautomerism in aromatic systems with oxygen substitutents is rare. This is investigated in 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene (1) and in 2,7-diacetyl-1,8-dihydroxy-3,6-dimethylnaphthalene (2). The tautomeric nature of 2-acetyl-1,8-dihydroxy-3.6-dimethylnaphthalene is supported by long-range hydrogen-hydrogen coupling between the OH-1 and the OH-8 and by the isotope effects on C-13 caused by deuteration at the CH3C=O methyl group. Compound 2 participates in a degenerate equilibrium between two equivalent nonsymmetrical rotamers (2A and 2B), each having two intramolecular O center dot center dot center dot HO hydrogen bonds: one involving an acetyl oxygen and the neighboring hydroxyl group, and the other between the oxygen centers at positions 1 and 8. In addition, each rotamer is involved in a tautomeric equilibrium, with a structure having an OH-substituted exocyclic double bond (2AT or 2BT). DFT calculations for a large set of compounds highlight the factors controlling the unusual rotational and tautomeric behaviors. A very important factor seems to be the repulsive interaction between the O-1 and O-8 centers, which is modulated by formation of an OH-1 center dot center dot center dot O-8 or OH-8 center dot center dot center dot O1 hydrogen bond. Steric interactions, mesomeric release of electrons from the oxygen at position 8, and a strong OH center dot center dot center dot O=C hydrogen bond are other factors. Solid-state C-13 NMR spectra of 2,7-diacetyl-1,8-dihydroxy-3,6-dimethylnaphthalene at different temperatures demonstrated no averaging in the solid, whereas partially deuterated 2-acetyl-1,8-dihydroxy-3,6-dimethylnaphthalene showed an isotope effect at C-1 of 1.5 ppm, indicating tautomerism in the solid state. Copyright (c) 2006 John Wiley & Sons, Ltd.
Abstract: ortho-Hydroxythioacetophenone and four structurally related ortho-hydroxyaryl methyl thioketones have been prepared from the corresponding ketones by reaction with gaseous H2S and HCl in ethanol under strictly controlled reaction conditions. The remarkable stability of the new monomeric thioketones seems to be due to a strong intramolecular O-H center dot center dot center dot S=C hydrogen bonding. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: 3-Acyltetronic acids bearing different 3- and 5-substituents have been examined focussing on tautomerism and inter- and intramolecular hydrogen-bonding properties of these beta,beta'-tricarbonyl compounds in solution as well as in the solid state. Spectroscopic methods like NMR, IR, Raman-spectroscopy as well as X-ray diffractometry and MAS-NMR for the solid state have been applied. In a solution of CDCl3, the acids exist as cis/trans pair both involving the 3-acyl group in a ratio 60/40. The pair also involving the carbonyl group at C-4 is tautomeric and the most abundant, whereas the other isomer only shows one form with an exo-cyclic double bond. NMR and IR measurements are in agreement. In the solid state, only one of the four possible tautomers is found. DFT-calculations on the B3LYP/6-31G** level helped to verify the assignment of the IR- and NMR-spectra and yielded an estimation of the relative thermodynamic stabilities of the tautomers of several 3-acyltetronic acids. Low temperature NMR experiments gave an insight into the equilibria. Deuterium isotope effects on the C-13 NMR chemical shifts have been observed for 5,5-dimethyl 3-pivaloyltetronic acid at low temperature in order to examine the fast internal equilibria. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: The syntheses of the square-planar platinum(II) complexes [Pt(L)(tba)] (L = 5,5'-dichloro-2,2'-dihydroxyazobenzenate (dhab), (5-chloro-2-hydroxyphenylazo)-3-oxo-N-phenylbutanamidate (hpab) or (5-chloro-2-hydroxyphenylazo)-2-naphtholate (hpan) and (tba) = tributylamine) is reported, together with those of the complete set of analogous complexes [M(L)(py)] (M = nickel(II), palladium(II), platinum(II) and (py) = pyridine). The coordinating nitrogen has been assigned to the azo-nitrogen attached to the 5-chloro-2-hydroxyphenyl substituent in both [Ni(h-pab)(py)] and [Pt(hpan)(tba)] by single crystal X-ray diffraction methods. It has been established that complexes with different group 10 metals are iso-structural by isomorphology of the crystals as determined by X-ray powder diffraction studies on the complexes containing pyridine in the fourth coordination site. Furthermore. we present a method to determine the coordination geometry by comparison of calculated C-13 chemical shifts for possible coordination modes optimized by ab initio methods with experimentally measured 13C chemical shifts. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Photochromism of p-methyl(thiobenzoyl) acetone (1) has been studied in argon and xenon cryogenic matrices. Application of linearly polarized fight to induce the phototransformation resulted in partial alignment of both the initial structure and the photochromic product. Different orientations were achieved by using irradiation wavelengths corresponding to differently polarized electronic transitions. This was followed by measurements of linear dichroism (LD) in the IR region. The analysis of the IR spectra, combined with the results of DFT B3LYP/cc-pVDZ calculations enabled determining the structures of the most stable ground state species and of the photoproduct. similarly to the recently reported cases of thioacetylacetone and monothiodibenzoylmethane, the initial structure of 1 corresponds to an intramolecularly hydrogen-bonded enol form, and the photochromic species to an "open", nonchelated enethiolic form. Vibrational assignments have been made for both species, greatly helped by the analysis of the LD spectra. It is concluded that DFT calculations for I quite reliably predict not only vibrational frequencies and intensities, but also transition moment directions, in both IR and electronic spectra. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: A series of thioanilides and corresponding anilides, some of which contain fluorinated phenyl rings, have been synthesized as model compounds. They all contain rather strong intramolecular hydrogen bonds. the strength of which varies. Deuterium isotope effects on F-19 and C-13 chemical shifts due to deuteriation at the NH proton show interesting new long-range isotope effects on chemical shifts that may be related to the existence of an intramolecular hydrogen bond and to transmission of the isotope effect due to all electric field effect. Deuterium isotope effects on chemical shifts report oil variations in hydrogen bonding. for example, as a function of changes in substituents or temperature. Deuteriation leads to a strengthening of the hydrogen bond. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: Schiff bases of gossypol with benzylamine, methylamine, 4-aminoacetophenone and 4-fluoroaniline have been synthesized and characterized by NMR spectroscopy. All the Schiff bases of gossypol are in the enamine form according to (3)J(HC,NH) and (1)J(,H) coupling constants. The spectra are basically unchanged by change of solvent (CD2Cl2, THF-d(8) and CD3OD) and by variation of temperature. For the derivative of benzylamine, deuterium isotope effects on C-13 chemical shifts are determined. They support strongly the enamine form and serve as a reference for other tautomeric Schiff bases. Structures and NMR nuclear shieldings of model compounds (the second monomer is replaced by a 2-hydroxybenzene ring) have been calculated by density functional theory (DFT) methods. A good correlation is observed between calculated C-13 nuclear shieldings of the enamine form and observed C-13 chemical shifts. Copyright (c) 2005 John Wiley S Sons, Ltd.
Abstract: Dihydrocyclocitral, a useful reagent in organic synthesis, has been synthesized in high yield and with high stereo selectivity from citronellal under microwave irradiation in two steps, involving acetic anhydride under base catalysis, then p-toluenesulfonic acid adsorbed on silica gel under solventless conditions (80% yield, reaction time 22 min). (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: A series of sterically hindered o-hydroxy Schiff bases derived from o-hydroxyaceto- and benzophenones with very short intramolecular hydrogen bonds were described qualitative and quantitatively by deuterium isotope effects on C-13 chemical shift, (n)Delta C(XD), (n)Delta F(XD), (1)J(N,H) coupling constants, delta NCH3 chemical shifts and UV spectra. All the investigated compounds are found to be tautomeric. The tautomeric character is described by the signs of the deuterium isotope effects on the C-13 chemical shifts. For the 3-nitro-5-chloro derivatives at low temperature, the equilibrium is shifted almost fully toward the proton transferred form in CD2Cl2. Intrinsic deuterium isotope effects on chemical shifts of these compounds as well as (1)J(N,H) coupling constants suggest that a zwitterionic resonance form is dominant for the proton transferred form. Structures, H-1, F-19, and C-13 chemical shifts, and deuterium isotope effects on C-13 chemical shifts are calculated by ab initio methods. The potential energy functions and the total deuterium isotope effects are calculated, and they are shown to correspond well with the experimental findings.
Abstract: The enol forms of the beta-diketones, benzoylacetones, have been studied using long-range carbon-hydrogen couplings involving the chelate OH proton, (OH)-H-1 chemical shifts, C-13 chemical shifts and deuterium isotope effects on C-13 chemical shifts. Studies were done in the temperature range from 268 to 181 K. The compounds are shown to be tautomeric, in opposition to a more symmetrical, delocalised, close to one-potential well structure as found in the solid at very low temperature. The same is true for dibenzoylmethane. The isotope effects on chemical shifts are very sensitive gauges of structure in these almost symmetrical systems. Equilibrium constants are determined and related to other similar compounds. Copyright (c) 2005 John Wiley & Sons, Ltd.
Abstract: We have analyzed deuterium isotope effects on C-13 chemical shifts in a series of o-hydroxy Schiff bases by applying factor analysis. Two orthogonal factors were obtained that explain about 80 and 10% of the variance of the data. The numerical values of these factors can be related to H-1 NMR chemical shifts of the proton involved in the intramolecular bonds delta(XH) (X=O or N). Such a relation allows one to identify clusters of compounds with different tautomeric forms of hydrogen bonding. Application of a similar approach to solution C-13 NMR chemical shifts produces three important factors. which have a different structure to factors describing isotope effects. This illustrates well the different nature of chemical shifts and isotope effects. the three factors explain about 54, 15, and 13% of variance. They can be rationalized and are strongly related to the electronic properties and location of substituents.
Abstract: A series of o-hydroxy acyl aromatics of the type 1,3-diacetyl-2,4,6-trihydroxybenzene (1), 6-methoxy-1,3-diacetyl-2,4-dihydroxy- (2) 2,4,6-trihydroxy- 1,3,5-triacetylbenzene (3) and I -acetyl-2-naphthol (4) have been investigated by means of single crystal X-ray diffraction, solution and solid state NMR spectroscopy and theoretical calculations. The structures of 1 and 2 exhibit interesting hydrogen bonds, planar structures and have, as a consequence, unexpected geometrical parameters (interatomic distances, bond lengths and valence angles) and depletion of electron density of the aromatic rings. Also significant packing effects are present. For 4, a twist of the carbonyl group is observed together with an out of plane bending of the C-C = O bond leading to the formation of a C = (OH)-H-...-O-C hydrogen bond which is almost coplanar with the naphthalene rings. Solid state NMR spectra show lack of C-3 symmetry for 3. Solution NMR spectra show-in the case of 1 and 2-quite different behaviour. A complex averaging-observed for I in solution-is unravelled at low temperature. Compounds 1-3 show large two-bond deuterium isotope effects, (2)Delta C-2(OD), on C-13 chemical shifts. This indicates strong hydrogen bonds. These can be understood in terms of an electronic effect caused by bond localisation of the benzene ring and a steric effect caused by either neighbouring CH3CO, OH or OCH3 groups leading to shorter (OHO)-O-... and (OO)-O-... distances and, consequently, stronger hydrogen bonds. A general scheme for distinguishing between steric twist (as seen in 4) and steric compression as seen in 1-3 is suggested. An experimental method based on isotope effects at the chelate proton of compounds deuteriated at the CD3CO groups is demonstrated. A Bader atom in molecules is done to investigate hydrogen bonding. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: The intercalating nucleic acid (INA) presented in this paper is a novel 1-O-(1-pyrenylmethyl)glycerol DNA intercalator that induces high thermal affinity for complementary DNA. The duplex examined contained two INA intercalators, denoted X, inserted directly opposite each other: d(C(1)T(2)C(3)A(4)A(5)C(6)X(7)C(8)A(9)A(10)G(11)C(12)T(13)):d(A(14)G(15) C(16)T(17)-T(18)G(19)X(20)G(21)T(22)T(23)G(24)A(25)G(26)). Unlike most other nucleotide analogues, DNA with INA inserted has a lower affinity for hybridizing to complementary DNA with an INA inserted directly opposite than to complementary unmodified DNA. In this study we used two-dimensional H-1 NMR spectroscopy to determine a high-resolution solution structure of the weak INA-INA duplex. A modified ISPA approach was used to obtain interproton distance bounds from NOESY cross-peak intensities. These distance bounds were used as restraints in molecular dynamics (rMD) calculations. Twenty final structures were generated for the duplex from a B-type DNA starting structure. The root-mean-square deviation (RMSD) of the coordinates for the 20 structures of the complex was 1.95 Angstrom. This rather large value, together with broad lines in the area of insertion, reflect the high degree of internal motion in the complex. The determination of the structure revealed that both intercalators were situated in the center of the helix, stacking with each other and the neighboring nucleobases. The intercalation of the INAs caused an unwinding of the helix in the insertion area, creating a ladderlike structure. The structural changes observed upon intercalation were mainly of local character; however, a broadening of the minor groove was found throughout the helix.
Abstract: Thioacetylacetone and its variously deuterated isotopomers have been investigated using electronic and vibrational spectroscopy combined with quantum chemical calculations. Thioacetylacetone is known for its photochromic properties, but the structures of the initial and final forms have been the subject of a long debate. Analysis of the IR spectra recorded in low-temperature argon and xenon matrices, room-temperature solutions, and in the gas phase has allowed us to establish the nature of the photochromic species and of its precursor. Similar to the case of another beta-thioxoketone, monothiodibenzoylmethane, the photo-product has been assigned to the nonchelated SH exo-rotamer of the (Z)-enethiol tautomeric form, whereas the dominant ground state species corresponds to the chelated (Z)-enol tautomeric form. Detailed vibrational assignments have been proposed for both forms based on quantum chemical calculations and polarization experiments. In the case of the chelated (Z)-enol species prevailing in the ground state, a second-order perturbative anharmonic effects associated with the intramolecular hydrogen bond, leading to a shift more than 600 cm(-1) of the wavenumber of the OH-stretching vibration. A small fraction of the SH endo-rotameric chelated (Z)-enethiol form was also detected under unperturbed conditions. The (Z)-enethiol form can be converted into the (Z)-enol form by irradiation at 290 nm.
Abstract: This contribution describes the study of hydrogen bonding in secondary oxamides, monothiooxamides, and dithiooxamides by ab initio calculations, X-ray diffractions, NMR spectra, thermal analysis, and variable-temperature infrared and Raman spectroscopy. The results can all be interpreted as a function of the change in the strength and the nature of the hydrogen bonding by substituting oxygen for sulfur in the series CH3-HNCOCONHCH3, CH3HNCSCONHCH3, CH3HNCSCSNHCH3 and by changing the steric influence of the alkyl group in a series of oxamides (RHNCOCNHR; R = CH3, C2H5, iC(3)H(7), tC(4)H(9)).
Abstract: Polyphosphate (polyp) is presumably central to phosphate (P) metabolism of arbuscular mycorrhizal (AM) fungi, but its synthesis, location and chain lengths are poorly characterized. Here, we applied noninvasive and nondestructive nuclear magnetic resonance (NMR) spectroscopy to obtain novel information on AM fungal polyp. In Vivo P-31 NMR spectroscopy was used to characterize polyp and other P pools in external hyphae and in mycorrhizal roots of associations between Glomus intraradices and cucumber (Cucumis sativus). A time-course study of P-starved external hyphae supplied with additional P showed that polyp appeared more rapidly than vacuolar inorganic P. These P metabolites also appeared in the roots, but later. Polyp considerably exceeded amounts of vacuolar inorganic P, where it was located in acidic, presumably vacuolar compartments, and had a short average chain length. The rapid synthesis of polyp might be important for the maintenance of effective hyphal P uptake. Our data support the hypothesis that polyp is the major P species translocated in the tubular vacuolar network, the presence of which was previously demonstrated in AM fungi.
Abstract: A series of o-hydroxy Schiff bases showing tautomerism to a varying degree are investigated together with compounds totally at the proton transfer form (enamines). Deuterium isotope effects on C-13 chemical shifts are measured at different temperatures. Structural features determining the equilibrium constants are discussed. DFT calculations are done in order to obtain structures. Schiff bases derived from dehydracetic acid (3-acetyl-6-methyl-pyran-2,4-dione) as well as from 1,3,5-triacetyl-2,4,6-trihydroxybenzene are found to be fully at the PT-form. These forms are characterised and the use of these compounds as reference for the PT-form in general is discussed. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: A comparison of hydrogen bond strengths in various enamines was made by monitoring the differential shifts Deltadelta(X) as the difference of NMR chemical shifts between E and Z forms of the nuclei directly involved in hydrogen bonding, i.e., X = N-15, H-1, O-17 atoms. The interpretation of these differential values was aided by ab initio calculations and X-ray derived geometric parameters for selected compounds. It is shown that Deltadelta((NH)-H-1) and Deltadelta(N-15) give conclusive results and their changes are rationally interpreted by invoking established contributing effects that influence the values of chemical shifts. The Deltadelta(O-17) parameter is sensitive to intramolecular geometry, mainly bond angles around oxygen and co-planarity of the atoms forming the hydrogen bonds. The latter factor is important in determining the strength of hydrogen bonds. Even a weak acceptor such as the lactone function gives a relatively strong hydrogen bond as compared with sp(3)-hybridized sulfonyl or sulfinyl acceptors. The hybridization of the penultimate atom of an acceptor plays a crucial role in determining the strength of hydrogen bonding.
Abstract: The binding constants of camptothecin, topotecan and its lactone ring-opened carboxylate derivative to DNA octamers were measured by UV and NMR spectroscopy. The self-association of topotecan (TPT) was also measured. The carboxylate form of TPT binds in the same way as the lactone, but more weakly. Titration of TPT into d(GCGATCGC)(2) shows a preferred location stacked onto the terminal G1 base. However, the intermolecular NOEs cannot be reconciled with a single conformation of the complex, and suggest a model of a limited number of conformations in fast exchange. MD calculations on four pairs of starting structures with TPT stacked onto the G1-C8 base pair in different orientations were therefore performed. The use of selected experimental "docking" restraints yielded ten MD trajectories covering a wide conformational space. From a combination of calculated free energies, NOEs and chemical shifts, some of the structures produced could be eliminated, and it is concluded that the data are consistent with two major families of conformations in fast exchange. One of these is the conformation found in a crystal of a TPT/DNA/topoisomerase I ternary complex.
Abstract: NH chemical shift temperature coefficients have been measured in a large series of N-substituted-3-piperidinethiopropionamides in which the (NN)-N-... distances are short but of varied length, as well as in a couple of the corresponding amides and in some simpler amides and thioamides. Geometries are calculated by means of ab initio DFT methods. The N-substituted-3-piperidinethiopropionamides show in most cases strong intramolecular N-(HN)-N-... hydrogen bonds according to IR spectra and ab initio calculations. For compounds with rather short (NN)-N-... distances the S=C-N-H moiety is non-planar. Dihedral angles as small as 160degrees are found. The NH chemical shift coefficients measured in non-polar solvents in all the N-substituted-3-piperidinethiopropionamides are more negative (-8 to -17 ppb/K) than in non-hydrogen bonded thioamides. For the latter in non-polar solvents like CDCl3 and toluene the temperature coefficients are as small as -1 to -4 ppb/K. The large negative effects can be related not only to the non-planarity of the thioamide group in a way that the more pronounced the non-planarity the more negative the temperature coefficients, but also to strong hydrogen bonding and the fact that the acceptor is a nitrogen. For similar amides with non-planar amide groups and nitrogen acceptor large negative temperature coefficients are likewise seen. In polar solvents like DMF the effects in simple thioamides are uniform and close to -6 ppb/K, whereas in the more complex compound like 4p(t) the temperature coefficient is close to 0. An essential feature of measuring temperature coefficients of compounds without strong intramolecular hydrogen bonds in non-polar solvents and at low temperatures is to keep the concentration low enough to avoid dimerisation. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: The crystal structures of two ortho-hydroxy ketones (5-chloro-2-hydroxyacetophenone (I K) and 3,5-dichloro-2-hydroxyacetophenone (II K)) have been determined with X-ray diffraction at 100 K. A comparison of steric effects on properties of pseudoaromatic hydrogen bonds in ortho-hydroxy acetophenone and ortho-hydroxy ketimines have been carried out with the application of crystallographic data. Nuclear quadrupole resonance (NQR) spectra have been measured and interpreted. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Parent genistein and its new amine complexes with morpholine and piperazine were studied comparatively in the solid and liquid states by X-ray crystallography and C-13 and N-15 NMR spectroscopy. Biochanine A and its complexes were used as reference. Secondary deuterium isotope effects on C-13 chemical shifts in solution were studied in parent isoflavones and their morpholine and piperazine complexes to aid in evaluation of the electronic distribution in both systems. In addition, to quantify the extent of proton transfer as well as to establish strong hydrogen bonding of the 7-OH group in a morpholine complex, proton transfer from the 7-OH group to the piperazine nitrogen atom was also confirmed by C-13 NMR in the solid state and by X-ray studies. The effect of 7-OH deprotonation yields a high frequency shift of 7-8 ppm on the C-7 carbon atom of the piperazine complex whereas it is as large as 12 ppm in the morpholine complex in the solid. The former trend is confirmed from solution state concentration studies which also show that the isoflavones have a strong tendency to form complexes with bases. Depending on the pK(a) difference between the isoflavones and the base this leads either to proton transfer and ion-pair formation or, in the case of a larger pK(a) difference, to a hydrogen bonded ion pair. The concentration studies show formation of a 1 : 1 genistein-piperazine complex in DMSO. Addition of water leads to formation of solvent separated ions. The C-5 OH group is involved in strong intramolecular hydrogen bonding leading to a pseudo aromatic ring extending the aromatic part of the drug pharmacophore. The analysis also suggests the way that both the C-7 and C-4' hydroxyl group of genistein may participate in stabilising the ternary inhibitor complexes of tyro sine-specific kinases or DNA topoisomerase II.
Abstract: C-H coupling constants of acetyl groups of a series of substituted 2-hydroxyacetophenones have been measured. These (1)J(C,H) couplings are then intercompared and compared to acetophenone in order to elucidate the cause of the variation. Structures are calculated using ab initio DFT methods. A comparison of 2-hydroxyacetophenone and acetophenone shows an increase of similar to0.5 Hz. An OR group in the 6-position of 2-hydroxyacetophenone leads to an increase of (1)J(C,H) of similar to1 Hz and substitution of an acetyl group at 3- or 5-positons likewise leads to an increase, whereas OR groups in 3-, 4- or 5-positions lead to small negative changes compared to the corresponding 2-hydroxyacetophenones. The variations in the (1)J(C,H) coupling constants of 6-OR substituted 2-hydroxyacetophenones are discussed as a function of C-H hydrogen bonding and electric field effects. The former is shown not to be at play, whereas the latter is clearly active, but in some instances in an indirect fashion.
Abstract: Parent genistein and its new amine complexes with morpholine and piperazine were studied comparatively in the solid and liquid states by X-ray crystallography and C-13 and N-15 NMR spectroscopy. Biochanine A and its complexes were used as reference. Secondary deuterium isotope effects on C-13 chemical shifts in solution were studied in parent isoflavones and their morpholine and piperazine complexes to aid in evaluation of the electronic distribution in both systems. In addition, to quantify the extent of proton transfer as well as to establish strong hydrogen bonding of the 7-OH group in a morpholine complex, proton transfer from the 7-OH group to the piperazine nitrogen atom was also confirmed by C-13 NMR in the solid state and by X-ray studies. The effect of 7-OH deprotonation yields a high frequency shift of 7-8 ppm on the C-7 carbon atom of the piperazine complex whereas it is as large as 12 ppm in the morpholine complex in the solid. The former trend is confirmed from solution state concentration studies which also show that the isoflavones have a strong tendency to form complexes with bases. Depending on the pK(a) difference between the isoflavones and the base this leads either to proton transfer and ion-pair formation or, in the case of a larger pK(a) difference, to a hydrogen bonded ion pair. The concentration studies show formation of a 1 : 1 genistein-piperazine complex in DMSO. Addition of water leads to formation of solvent separated ions. The C-5 OH group is involved in strong intramolecular hydrogen bonding leading to a pseudo aromatic ring extending the aromatic part of the drug pharmacophore. The analysis also suggests the way that both the C-7 and C-4' hydroxyl group of genistein may participate in stabilising the ternary inhibitor complexes of tyro sine-specific kinases or DNA topoisomerase II.
Abstract: An improvement of a method for determination of the distribution coefficient for polycyclic aromatic compounds (PAC) between organic carbon and water (K-proportional to) by high-performance liquid chromatography (HPLC) is presented in this paper. By use of silica-based HPLC columns with chemically immobilized humic acid (Aldrich, Zigma-Aldrich, Taufkirchen, Germany), the retention of PAC to humic acid can be measured, and the results can, when they are compared to those from a blank column, be used to a direct determination of K-proportional to. These values agree quite well with literature K-proportional to values for two- to four-ringed PAC. Our work has shown that it is possible to use this method to investigate how environmental parameters like electrolyte concentration. pH, and temperature affect the sorption of PAC to humic acid. Thermodynamic parameters can be estimated from variable temperature experiments. The advantage of the method is its cost and manpower effectiveness.
Abstract: This paper describes the purification and characterization of microviridin J, a newly discovered metabolite of Microcystis that causes a lethal molting disruption in Daphnia spp., upon ingestion of living cyanobacterial cells. Microviridin J consists of an acetylated chain of 13 amino acids arranged in three rings and two side chains. Unlike other known isoforms of microviridin, microviridin J contains arginine that imparts a unique solution conformation characterized by proximal hydrophobic interactions between Arg and other regions of the molecule. This eventually results in the formation and stabilization of an additional ring system. Microviridin J potently inhibits porcine trypsin, bovine chymotrypsin, and daphnid trypsin-like proteases. The activity against trypsin is most likely due to Arg and its distinctive conformational interactions. Overall, the data presented for microviridin J emphasize once again the ability of cyanobacteria to produce numerous and potent environmental toxins.
Abstract: Nitrogen (N) fixation and assimilation in pea (Pisum sativum) root nodules were studied by in vivo N-15 nuclear magnetic resonance (NMR) by exposing detached nodules to N-15, via a perfusion medium, while recording a time course of spectra. In vivo P-31 NMR spectroscopy was used to monitor the physiological state of the metabolically active nodules. The nodules were extracted after the NMR studies and analyzed for total soluble amino acid pools and N-15 labeling of individual amino acids by liquid chromatography-mass spectrometry. A substantial pool of free ammonium was observed by N-15 NMR to be present in metabolically active, intact nodules. The ammonium ions were located in an intracellular environment that caused a remarkable change in the in vivo N-15 chemical shift. Alkalinity of the ammonium-containing compartment may explain the unusual chemical shift; thus, the observations could indicate that ammonium is located in the bacteroids. The observed N-15-labeled amino acids, glutamine/glutamate and asparagine (Asn), apparently reside in a different compartment, presumably the plant cytoplasm, because no changes in the expected in vivo N-15 chemical shifts were observed. Extensive N-15 labeling of Asn was observed by liquid chromatography-mass spectrometry, which is consistent with the generally accepted role of Asn as the end product of primary N assimilation in pea nodules. However, the Asn N-15 amino signal was absent in in vivo N-15 NMR spectra, which could be because of an unfavorable nuclear Overhauser effect. gamma-Aminobutyric acid accumulated in the nodules during incubation, but newly synthesized N-15 gamma-aminobutyric acid seemed to be immobilized in metabolically active pea nodules, which made it NMR invisible.
Abstract: Differences in magnetic susceptibility between various compartments in heterogeneous samples can introduce unanticipated complications to NMR spectra. On the other hand, an understanding of these effects at the level of the underlying physical principles has led to the development of several experimental techniques that provide data on cellular function that are unique to NMR spectroscopy. To illustrate some key features of susceptibility effects we present, among a more general overview, results obtained with red blood cells and a recently described model system involving diethyl phthalate in water. This substance forms a relatively stable emulsion in water and yet it has a significant solubility of similar to5 mmol L-1 at room temperature; thus, the NMR spectrum has twice as many resonances as would be expected for a simple solution. What determines the relative intensities of the two families of peaks and can their frequencies be manipulated experimentally in a predictable way? The theory used to interpret the NMR spectra from the model system and cells was first developed in the context of electrostatics nearly a century ago, and yet some of its underlying assumptions now warrant closer scrutiny. While this insight is used in a practical way in this article, the accompanying article deals with the mathematics and physics behind this new analysis. (C) 2003 Wiley Periodicals, Inc.
Abstract: Sea anemones produce a family of 18-20 kDa proteins, the actinoporins, that lyse cells by forming pores in cell membranes. Sphingomyelin plays an important role in their lytic activity, with membranes lacking this lipid being largely refractory to these toxins. The structure of the actinoporin equinatoxin II in aqueous solution, determined from NMR data, consists of two short helices packed against opposite faces of a beta-sandwich structure formed by two five-stranded beta-sheets. The protein core has extensive hydrophobic interfaces formed by residues projecting from the internal faces of the two beta-sheets. N-15 relaxation data show uniform backbone dynamics, implying that equinatoxin II in solution is relatively rigid, except at the N terminus; its inferred rotational correlation time is consistent with values for monomeric proteins of similar mass. Backbone amide exchange rate data also support the view of a stable structure, even though equinatoxin II lacks disulfide bonds. As monitored by NMR, it unfolds at around 70 degreesC at pH 5.5. At 25 degreesC the structure is stable over the pH range 2.5-7.3 but below pH 2.5 it undergoes a slow transition to an incompletely unfolded structure resembling a molten globule. Equinatoxin II has two significant patches of positive electrostatic potential formed by surface-exposed Lys and Arg residues, which may assist its interaction with charged regions of the lipid head groups. Tyr and Trp residues on the surface may also contribute by interacting with the carbonyl groups of the acyl chains of target membranes. Data from mutational studies and truncated analogues identify two regions of the protein involved in membrane interactions, the N-terminal helix and the Trp-rich region. Once the protein is anchored, the N-terminal helix may penetrate the membrane, with up to four helices lining the pore, although other mechanisms of pore formation cannot be ruled out. (C) 2002 Elsevier Science Ltd.
Abstract: The inherent chemical properties of eight different dissolved organic matters (DOMs) originating from soil, surface and groundwater have been analysed. The samples consist of isolated fulvic acids (FA), humic acids (HA), and humic substances (HS), i.e. natural mixtures containing a humic and a fulvic fraction. The humic substances have been characterised by elemental analysis, size exclusion chromatography, E-2/E-3 and E-4/E-6 UV absorption ratios, and liquid-state C-13-NMR spectroscopy. The information contents of the different analytical methods have been investigated by pattern recognition, i.e. cluster analysis and principal component analysis (PCA). A comparative study of the information contents of DOM descriptors derived from different analytical methods is presented. Through extraction of information content of the individual analytical methods the inherent properties of DOM are quantified. Pattern recognition revealed significant quantitative differences in the inherent properties of DOM of different origin and type. PCA based on the NMR descriptors showed highest explained variance. However, all models showed low robustness due to the limited number of samples. The supervised pattern recognition, i.e. PCA, indicates a classification of DOMs into groups of similar properties by an increase in the number of samples. Furthermore that the number of groups may be higher and more continuously distributed than the conventional classification into fulvic acids, humic acids or humic substances. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: Deuterium isotope effects on N-15, C-13 and H-1 chemical shifts have been measured in the protonated forms of DMAN, 4-bromo, 4-picryl, 4-nitro and 2- and 2,7-chloro derivatives. Structures have been geometry optimised using BPW91/6-31(d)G density functional theory (DFT) methods and show good correspondence to experimental X-ray data. N-15, C-13 and H-1 chemical shifts are calculated using GIAO/DFT methods. Chemical shifts for the two tautomers are calculated. Equilibrium constants are determined from (1)J(N,H) coupling constants. A comparison of (1)J(N,H) and (1)J(N,D) couplings for the non-symmetrical derivatives show that upon deuteriation the equilibrium is shifted towards the dominant form. The experimental deuterium isotope effect on chemical shifts of DMANH(+) shows only small values despite the apparently strong intramolecular hydrogen bonds as judged from primary isotope effects. The counter ion has little effect on the one-bond isotope effect. Secondary isotope effects on N-15 and C-13 chemical shifts for the non-symmetrical derivatives are divided into intrinsic and equilibrium contributions. The one-bond deuterium intrinsic isotope effect on N-15 chemical shifts is close to 0.3 ppm and is related to charge delocalisation and the close proximity of the positive charge. The equilibrium contributions to the isotope effects at the nitrogens and at carbons are seen to increase throughout the series in parallel with the increase of the equilibrium constant. For H-1 resonances only equilibrium contributions are found at the aromatic hydrogens. An important feature in the analysis of the protonated DMANs is the multi-isotope effect approach. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: The sorption of the pyrethroid, esfenvalerate, to the dissolved and/or dispersed fraction of eight different natural humic compounds has been investigated. The dissolved organic matters (DOMs) included in this study originate from ground water, soil pore water, and surface waters. Sorption was modelled at DOM concentration levels where equilibrium partitioning of esfenvalerate between DOM and the aqueous bulk phase prevails. The inherent characteristics of the eight different humic materials, quantified in the preceding paper by Thomsen et al. (2002, this issue (PII: S0045-6535(02)00335-1)), have been used as explanatory variables for modelling this equilibrium partitioning. Using a reverse QSAR approach based on by projection-into-latent-structure regression (PLS-R) inherent sorbent properties determining for the sorption affinity of esfenvalerate to DOM were analysed. For all humic substances a decrease in the DOM-normalised equilibrium-partitioning coefficient, K-DOM, with increasing concentration of DOM was observed. Significant variations in K-DOM values, as function of the inherent characteristics of the individual humic substances, were found at DOM concentrations of 75 and 100 ppm, respectively. The latter is a strong indication of variations in sorption mechanisms of esfenvalerate to DOM of varying inherent properties. Groupings in the principal property space quantifying DOMs may indicate that separate models are needed for quantifying the equilibrium partitioning to different classes of DOM. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: Compounds of three groups of parent secondary aliphatic beta-sulfonyl enamines, unsubstituted or with an alkyl substituent in the alpha or beta positions, have been studied. The beta-sulfonyl enamines are found in equilibria between the E and Z enamine and imine forms in varying amounts in CDCl3 solution. H-1, C-13 and N-15 data were acquired and used for the assignment of the three species in solution. Considerable variations in the amounts of enamine E and Z and imine forms are found, depending on the substitution pattern. The conformational space of the equilibrated tautomers have been investigated using steady-state NOE experiments, dynamic NMR and (n)J(H, H) and (n)J(C, H) long-range spin-spin coupling constants, as well as ab initio HF and DFT calculations for structure determination and relative energy evaluations. Based on NOE, saturation transfer experiments and conformational analysis, an equilibrium scheme is proposed showing the mechanism and the involvement of water in the transformation of beta-sulfonyl imines into the corresponding enamines.
Abstract: This mini review describes intrinsic deuterium isotope effects on C-13 chemical shifts of rigid hydrogen bonded compounds primarily in solution. The steric effects of intramolecularly hydrogen bonded compounds are dissected into different interactions leading either to steric compression or to steric twist. One-bond isotope effects involving CH(D) bonds are analyzed in terms of substituent effects and the question is raised whether isotope effects can be useful in the study of CH-hydrogen bonds.
Abstract: Inositolphosphates from barley low-phytate grain mutants and their parent variety were analysed by metal-dye detection HPLC and NMR. Compound assignment was carried out by comparison of retention times using a chemical hydrolysate of phytate [Ins(1,2,3,4,5,6)P-6] as a reference; Co-inciding retention times indicated the presence of phytate, D/L-Ins(1,2,3,4,5)P-5, Ins(1,2,3,4,6)P-6, D/L-(1,2,4,5,6)P-5, D/L-(1,2,3,4)P-4, D/L-Ins(1,2,5,6)P-4 and D/L-Ins(1,4,5,6)P-4 in PLP1B mutants as well as the parent variety. In grain extracts from mutant lines PLP1A, PLP2A and PLP3A unusual accumulations of D/L-Ins(1,3,4,5)P-4 were observed whereas phytate and the above-mentioned inositol phosphates were present in relatively small amounts. Assignment of D/L-Ins(1,3,4,5)P-4 was corroborated by precise co-chromatography with a commercial Ins(1,3,4,5)P-4 standard and by NMR spectroscopy. Analysis of inositol phosphates during grain development revealed accumulation of phytate and D/L-Ins(1,3,4,5)P-4, which suggested the tetrakisphosphate compound to be an intermediate of phytate synthesis. This assumption was strengthened further by phytate degradation assays showing that D/L-Ins(1,3,4,5)P-4 did not belong to the spectrum of degradation products generated by endogenous phytase activity. Metabolic scenarios leading to accumulation of D/L-Ins(1,3,4,5)P-4 in barley low-phytate mutants are discussed.
Abstract: Heterocyclic, aliphatic amines add to acyclic and cyclic alpha,beta -unsaturated thioamides yielding beta -amino-functionalized derivatives. In the case of cyclic acceptors, the formation of both kinetic and thermodynamically controlled products is observed. Tailoring of cis or trans products is thus possible. A mechanism for the addition to cyclic acceptors is proposed and evidence presented to support it. Ease of addition is studied in relation to the structure of the acceptors. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: The proton transfer equilibrium in series of Schiff bases derived from 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-acetonaphthone were measured by means of H-1, C-13 and N-15 NMR spectra and deuterium isotope effects on C-13 and N-15 chemical shifts at variable temperature and in different solvents. The latter made it possible to investigate a broad range of equilibrium positions. All compounds exist mainly as the NH tautomer over the full range of temperatures in CDCl3 solution. The position of the equilibrium was estimated using (1)J(N-15,H-1) and (3)J((NH)-N-15,H-1) coupling constants. The correlation between deuterium isotope effects (n)Delta C-13-2(XD) or (n)Delta N-15(XD), where X = O or N, and the mole fraction of the NH form show the characteristic, non-monotonic function as for other intramolecular hydrogen bonded systems. The position of the minimum depends on substituents and the maximal negative values of (n)Delta C-13-2(XD) varies with substituents and solvents. Copyright (C) 2001 John Wiley & Sons, Ltd.
Abstract: When lipophilic compounds like diethyl phthalate (DEP) were added to water, two sets of resonances appeared in the H-1 NMR spectrum, whereas when added in concentrations above similar to3.5 mM to erythrocytes in a high haematocrit suspension, only one set of resonances was observed at the low-frequency position. The appearance of one set of resonances at lower frequency was found to be common to a series of lipophilic compounds in erythrocytes. The appearance of the NMR spectra is ascribed to the existence of an emulsion, meaning two different phases of a compound: a "droplet" (resonances to lower frequency) and aqueous dissolved phase (resonances to higher frequency). The absence of the resonances from the dissolved phase in erythrocyte solution is ascribed to exchange broadening. The absolute chemical shift of the compound in its "droplet'' phase was also measured using a cylindrical/spherical microcell. This arrangement mimicked the geometry of the dissolved versus the phase-separated species and thus obviated the effect of a difference in magnetic susceptibility between the "droplet" solute and its aqueous solution. Factors influencing the formation of emulsion phases such as erythrocytes, haemoglobin and smaller proteins were investigated; they are found to be effective in the order given.
Abstract: Two different humic acids (HA) and a fulvic acid (FA) were chemically immobilized to a high performance liquid chromatography (HPLC) silica column material. The immobilization was performed by binding amino groups in HA/FA to the free aldehyde group in hlutardialdehyde attached to the silica gel. The HPLC column materials were compared with a blank column material made by applying the same procedure but without immobilizing HA or FA. Also, a column was made by binding carbonyl groups in HA to amino groups attached to the silica gel. The humic substances were selected to secure appropriate variation of their structural features. The retention factors of 45 polycyclic aromatic compounds (PAC) to the four columns were determined by HPLC. The advantage of the technique is a large number of compounds can easily be studied. The binding procedure does not appear to cause a drastic selection between the HA molecules. The k ' values obtained for the two Aldrich HA columns agree in general reasonably. The retention or sorption of the compounds increased with the size of the PAC and the number of lipophilic substituents, but decreased when polar substituents were present. The PAC retention was much stronger to the two HA columns than to the FA and blank column, both for hydrophobic polycyclic aromatic hydrocarbons (PAH) and the polar PAC. Other factors impacting the PAC binding may be specific interactions with HA and the ionic strength of the aqueous phase. The technique has been applied to do direct determinations of K-oc.
Abstract: The title N-allylthioamides (1a-f) were synthesized in the reaction of allyl isothiocyanate with enamines (1a-c) and 1,3-diketones (1d-f), respectively carried out in an acetonitrile solution in the presence of DBU. When treated with the bromine-dioxane complex or with iodine, they underwent cyclization to the corresponding derivatives of 4,5-dihydrothiazole (2a-g). NMR spetroscopy made it possible to elucidate the tautomeric structures of the thioamides and thiazolines.
Abstract: A dumbbell double-stranded DNA decamer tethered with a hexaethylene glycol linker moiety (DDSDPEG), with a nick in the centre of one strand, has been synthesised. The standard NMR methods, E.COSY, TOCSY, NOESY and HMQC, were used to measure H-1, P-31 and T-1 spectral parameters, Molecular modelling using rMD-simuiated annealing was used to compute the structure. Scalar couplings and dipolar contacts show that the molecule adopts a right-handed B-DNA helix. in 38 mM phosphate buffer at pH 7. Its high melting temperature confirms the good base stacking and stability of the duplex, This is partly attributed to the presence of the PEG(6) linker at both ends' of the duplex that restricts the dynamics of the stem:pentamers and thus stabilises the oligonucleotide. The inspection of the global parameters shows that the linker does not distort the B-DNA geometry. The computed structure suggests that the presence of the nick is not disturbing the overall tertiary structure, base pair geometry or duplex base pairing to a substantial extent. The nick has, however, a noticeable impact on the local geometry at the nick site, indicated clearly by NMR analysis and reflected in the conformational parameters of the computed structure. The H-1 spectra also show much sharper resonances in the presence of K+ indicating that conformational heterogeneity of DDSDPEG is reduced in the presence of potassium as compared to sodium or caesium ions, At the same time the H-1 resonances have longer T-1 times, This parameter is suggested as a sensitive gauge of stabilisation.
Abstract: Regioisomeric monoacyl- and monobenzoyl-substituted pyrenes are diprotonated in FSO3H . SbF5 (4:1)/SO2ClF to give persistent carboxonium-pyrenium dications, whereas diacetyl- and dibenzoyl-pyrenes are diprotonated to give dicarboxonium dications. The resulting dications were studied by low-temperature NMR at 500 MHz. Conformational aspects of the carboxonium group in various regioisomers are addressed by a combination of NOED spectra and 2D-NMR and AM1 calculations. Charge delocalization pathways are gauged and compared on the basis of the magnitude of Delta delta C-13 values.
Abstract: Deuterium isotope effects on C-13 chemical shifts are studied in a series of o-hydroxythioamides, (2)Delta-thiazolines and enolic 5-acyl-2-thiobarbituric acids. Novel 2-hydroxy-1-thiocarboxamide naphthalenes show steric isotope effects of opposite sign to those observed in 2-hydroxy-1-acetylnaphthalenes and pyrenes. The o-hydroxyaromatic (2)Delta-thiazolines show tautomeric behaviour and accordingly large isotope effects. Tautomerism of these and the thioamides are discussed in relation to structure. Intramolecular hydrogen bonding of non-RAHB type show only weak effects. The enolic mono N-alkyl 5-acyl 2-thiobarbituric acids show two isomers, both of which are involved in enol-enol tautomerism.
Abstract: Isotope effects on chemical shifts, (n)Delta(13)C(D), (n)Delta(1)H(D), (1)Delta(15)N(D) and (1)Delta(13)C(O-18), and solvent isotope effects in proteins are reviewed and references are provided to related cases. The isotope effects included are motivated by the effects occurring in perdeuterated proteins and effects of C-13 and N-15 labelling combined with effects of an H2O-D2O mixture. The focus is on structural information. The following isotope effects depend on structure: (1)Delta(15)N(D), (1)Delta(13)C(D), (2)Delta(13)C-alpha(ND), (3)Delta(13)C-alpha(ND), and (3)Delta(13)C-beta(ND). Equilibrium isotope effects on chemical shifts are briefly discussed by the elegant use of (1)Delta(13)C(O-18) and (1)Delta(13)C(N-15) in the determination of pK(a) values. Ab initio calculations of isotope effects show promise both in traditional cases and for describing the new type of isotope effects observed in paramagnetic proteins. Copyright (C) 2000 John Wiley & Sons, Ltd.
Abstract: The primary deuterium and tritium isotope effects, delta(XH) - delta(XD/T), were measured for 55 compounds having one or more intramolecular hydrogen bonds. The primary isotope effects were measured at various temperatures. For compounds displaying tautomerism the primary isotope effects are found to have contributions from both intrinsic and equilibrium isotope effects. The primary tritium isotope effect, (P)Delta(H-1, H-3), and the primary deuterium isotope effect, (P)Delta(H-1, H-2), shown to be related by (P)Delta(H-1, H-3) = 1.4(P)Delta(H-1, H-2) This finding is valid for both tautomeric compounds and compounds with localized hydrogen bonds. Large negative primary tritium and deuterium isotope effects were observed for compounds displaying tautomerism and having sulfur as donor or acceptor. These isotope effects show a strong temperature dependence, which is related to the change in equilibrium due to isotope substitution. For the compounds with localized hydrogen bonds, the primary deuterium and tritium isotope effects correlated with the two bond deuterium isotope effect on C-13 chemical shifts. The primary deuterium and tritium isotope effects are therefore a measure of the hydrogen bond strength for compounds with localized hydrogen bonds. Copyright (C) 2000 John Wiley & Sons, Ltd.
Abstract: Isotope effects on chemical shifts of intramolecularly hydrogen bonded systems are reviewed. The effects are conveniently divided into localized (intrinsic) and equilibrium isotope effects. The review covers both primary and secondary isotope effects on chemical shifts. For the localized one ii is very important to distinguish between RAHB and non-RAHB types. For the RAHB systems the OH group is shown to form a stronger hydrogen bond than the OD group, whereas the opposite is true for non-RAHB. Theoretical calculations at the ab initio level (DFT) can be used to provide reliable structures, chemical shifts and isotope effects. Large intrinsic secondary isotope effects can to a good degree be related to the change in the OH(D) bond length upon deuteriation. (2)Delta C(XD) isotope effects are shown to be good measures of hydrogen bond strength. So far no evidence for heavy atom movement has been convincingly advanced for RAHB systems. A number of isotope effect types have now been studied in depth, (n)Delta C(XD), X=O, S or N, (1)Delta N(D), (1)Delta O(D), (5)Delta O(D), (n)Delta H(OD), (n)Delta F(D). It is concluded that the possible over determination of isotope effects in hydrogen bonded systems provide a very powerful tool in studies of structure of hydrogen bonded systems. Isotope effects are studied in detail in sterically hindered systems and parameters are available to distinguish between twist of e.g. RCO groups and steric compression. Furthermore, twist of phenyl rings may also be monitored. Proton transfer reactions such as tautomerism have been studied extensively. Equilibrium isotope effects on chemical shifts have been reported in a large number of cases. The magnitude of the equilibrium isotope effects depends on the equilibrium constant. A series of parameters have been suggested as a good way to establish tautomerism in a number of difficult cases. Both deuterium and tritium primary isotope effects have now been reported in a large number of systems. Large intrinsic primary isotope effects is a good proof of double potential wells. Primary isotope effects are also studied in tautomeric systems and at different temperatures and can under such circumstances be both positive and negative.
Abstract: The applicability of HECADE approach to the measurement of C-13, 1 long-range coupling constants, which was difficult to achieve by other methods, is demonstrated. The coupling magnitudes obtained allow the qualitative characterization of rotational degrees of freedom in all isomers of the beta-sulfonylenamines studied in terms of the prevailing conformation contribution to conformational averaging. Copyright (C) 2000 John Wiley & Sons, Ltd.
Abstract: A chemical shift vs. temperature analysis of beta-thioxoketones has been performed for the four beta-thioxoketones, thioacetylacetone (1), benzoylthioacetone (2), thiobenzoylacetone (3) and monothiodibenzoylmethane (4), to test this method as a general way of obtaining the individual chemical shifts of tautomers involved in tautomeric equilibria. Both C-13 and chelate H-1 resonances far 1 and 2 showed a coalescence point subsequently followed by observation of two sets of resonances when lowering the temperature. Analysis of chemical shifts and isotope effects on these reveals that a three component system is involved in the tautomeric equilibria for 1 and 2. The three components are the intramolecularly hydrogen-bonded (Z)-enol form (A), the intramolecularly hydrogen-bonded (Z)-enethiol form (B) (which are interconverting rapidly by intramolecular proton transfer/electron redistribution) and the non-proton chelated (Z)-enethiol form (C). This third species is observable at low temperature in CD2Cl2 as well as in mixtures of freons. The hydrogen-bonded (Z)-enol and (Z)-enethiol forms A and B appear to be in equilibrium at all obtainable temperatures. The analysis of the data for 3 and 4 leads to Delta H degrees and Delta S degrees values as well as chemical shifts for the individual tautomers. It is demonstrated how deuteriation of the chelate proton may lead to analysis of a complex three species equilibrium system of which only one component can be observed directly. The large negative isotope effects observed are due to large equilibrium isotope effect contributions. A very large shift in the equilibrium is observed upon deuteriation. The negative primary isotope effects found for the chelate protons are resolved into intrinsic and equilibrium parts. The large positive intrinsic effects clearly point to a two-potential well in agreement with results from UV and IR measurements and indicate strong hydrogen bonds. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: The applicability of HECADE approach to the measurement of C-13, 1 long-range coupling constants, which was difficult to achieve by other methods, is demonstrated. The coupling magnitudes obtained allow the qualitative characterization of rotational degrees of freedom in all isomers of the beta-sulfonylenamines studied in terms of the prevailing conformation contribution to conformational averaging. Copyright (C) 2000 John Wiley & Sons, Ltd.
Abstract: P-31 nuclear magnetic resonance (NMR) spectroscopy was used to study phosphate (P) metabolism in mycorrhizal and nonmycorrhizal roots of cucumber (Cucumis sativus L) and in external mycelium of the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith. The in vivo NMR method allows biological systems to be studied non-invasively and non-destructively. (3)1P NMR experiments provide information about cytoplasmic and vacuolar pH, based on the pH-dependent chemical shifts of the signals arising from the inorganic P (P-i) located in the two compartments. Similarly, the resonances arising from alpha, beta and gamma phosphates of nucleoside triphosphates (NTP) and nucleoside diphosphates (NDP) supply knowledge about the metabolic activity and the energetic status of the tissue. In addition, the kinetic behaviour of P uptake and storage can be determined with this method. The (3)1P NMR spectra of excised AM fungi and mycorrhizal roots contained signals from polyphosphate (PolyP), which were absent in the spectra of nonmycorrhizal roots. This demonstrated that the P-i taken up by the fungus was transformed into PolyP with a short chain length. The spectra of excised AM fungi revealed only a small signal from the cytoplasmic P-i, suggesting a low cytoplasmic volume in this AM fungus.
Abstract: A chemical shift vs. temperature analysis of beta-thioxoketones has been performed for the four beta-thioxoketones, thioacetylacetone (1), benzoylthioacetone (2), thiobenzoylacetone (3) and monothiodibenzoylmethane (4), to test this method as a general way of obtaining the individual chemical shifts of tautomers involved in tautomeric equilibria. Both C-13 and chelate H-1 resonances far 1 and 2 showed a coalescence point subsequently followed by observation of two sets of resonances when lowering the temperature. Analysis of chemical shifts and isotope effects on these reveals that a three component system is involved in the tautomeric equilibria for 1 and 2. The three components are the intramolecularly hydrogen-bonded (Z)-enol form (A), the intramolecularly hydrogen-bonded (Z)-enethiol form (B) (which are interconverting rapidly by intramolecular proton transfer/electron redistribution) and the non-proton chelated (Z)-enethiol form (C). This third species is observable at low temperature in CD2Cl2 as well as in mixtures of freons. The hydrogen-bonded (Z)-enol and (Z)-enethiol forms A and B appear to be in equilibrium at all obtainable temperatures. The analysis of the data for 3 and 4 leads to Delta H degrees and Delta S degrees values as well as chemical shifts for the individual tautomers. It is demonstrated how deuteriation of the chelate proton may lead to analysis of a complex three species equilibrium system of which only one component can be observed directly. The large negative isotope effects observed are due to large equilibrium isotope effect contributions. A very large shift in the equilibrium is observed upon deuteriation. The negative primary isotope effects found for the chelate protons are resolved into intrinsic and equilibrium parts. The large positive intrinsic effects clearly point to a two-potential well in agreement with results from UV and IR measurements and indicate strong hydrogen bonds. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: The primary deuterium and tritium isotope effects, delta(XH) - delta(XD/T), were measured for 55 compounds having one or more intramolecular hydrogen bonds. The primary isotope effects were measured at various temperatures. For compounds displaying tautomerism the primary isotope effects are found to have contributions from both intrinsic and equilibrium isotope effects. The primary tritium isotope effect, (P)Delta(H-1, H-3), and the primary deuterium isotope effect, (P)Delta(H-1, H-2), shown to be related by (P)Delta(H-1, H-3) = 1.4(P)Delta(H-1, H-2) This finding is valid for both tautomeric compounds and compounds with localized hydrogen bonds. Large negative primary tritium and deuterium isotope effects were observed for compounds displaying tautomerism and having sulfur as donor or acceptor. These isotope effects show a strong temperature dependence, which is related to the change in equilibrium due to isotope substitution. For the compounds with localized hydrogen bonds, the primary deuterium and tritium isotope effects correlated with the two bond deuterium isotope effect on C-13 chemical shifts. The primary deuterium and tritium isotope effects are therefore a measure of the hydrogen bond strength for compounds with localized hydrogen bonds. Copyright (C) 2000 John Wiley & Sons, Ltd.
Abstract: Regioisomeric monoacyl- and monobenzoyl-substituted pyrenes are diprotonated in FSO3H . SbF5 (4:1)/SO2ClF to give persistent carboxonium-pyrenium dications, whereas diacetyl- and dibenzoyl-pyrenes are diprotonated to give dicarboxonium dications. The resulting dications were studied by low-temperature NMR at 500 MHz. Conformational aspects of the carboxonium group in various regioisomers are addressed by a combination of NOED spectra and 2D-NMR and AM1 calculations. Charge delocalization pathways are gauged and compared on the basis of the magnitude of Delta delta C-13 values.
Abstract: The proton transfer equilibrium in a series of Schiff bases derived from substituted salicylic aldehydes and aliphatic amines has been investigated by means of variable temperature multinuclear magnetic resonance and the deuterium isotope effect on C-13 nuclear shielding. Most of the compounds exist in two tautomeric forms in CDCl3. The populations of the tautomeric forms have been estimated using (3)J(NH,H) and (1)J(N-15,H) coupling constants. One goal is to describe the characteristics of both tautomers. Deuterium isotope effects on C-13 chemical shifts ((n)Delta C(XD)) are shown to depend on the position of the tautomeric equilibrium. (n)Delta C-2(XD) and (n)Delta C-1'(XD) are found to be a sensitive non-monotonic function (S-shape) of the mole fraction. The pattern of the dependence seems also to be general for other tautomeric compounds with intramolecular hydrogen bonds, but with much lower barriers to interconversion like beta-diketones and piroxicam as well as organic acid-pyridine complexes with intermolecular hydrogen bonds. This new feature is a valuable tool in the characterisation of tautomeric and other equilibrium systems.
Abstract: Deuterium isotope effects on O-17 chemical shifts are studied in a series of intramolecularly hydrogen bonded o-hydroxy acyl aromatics and beta-diketones. Geometries and chemical shifts are calculated using DFT ab initio methods, Relationships between hydrogen bond strength represented as either R-O ... O, RO-H or R-O ... H, and the experimental parameters, (5)Delta(17)O(OD) and delta(17)O are demonstrated for non-tautomeric compounds. These plots do also provide a method for establishing tautomerism in new compounds. (5)Delta(17)O(OD) as well as delta(17)O are found to be dominated by hydrogen bonding in terms of RO-H or R-O ... H distances and not influenced very much by the R-O ... O distance. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: Truncated BPTI missing residues 1 and 2 is investigated together with variants thereof (Lys-15, Arg-17, and Arg-42 are replaced by other residues in various combinations). A comparison of the X-ray structure of BPTI with that of 3-58BPTI(K15R,R17A,R42S) shows only minor variations for the backbone, but the lack of salt bridge between the terminals and the lack of two N-terminal residues provide a structure open at one end. Comparisons of amide exchange rates show a dramatic increase for the most slowly exchanging NH protons of 3-58BPTI and the analogues thereof, as compared to those of the wildtype despite only small differences in the structures, The amide exchange rates for truncated analogues increase with decreasing TTEP (temperature top endothermic peak) values. On the basis of the known structural changes comparisons to C-13 chemical shifts are made. C-13 chemical shifts are assigned using the D-isotope and HMBC techniques. Excellent resolution is obtained in these 1D natural abundance spectra. C-13 NMR chemical shifts are shown to be able to gauge structural changes. A comparison of C-13 chemical shifts of WT BPTI (aprotinin) and 3-58BPTI reveals effects caused by (i) the removal of the salt bridge of the terminii, (ii) the charge of the N-terminus, and (iii) the increased mobility of the side chain of Tyr-23, Small effects are also seen due to a conformational change of the aromatic ring of Phe-4. Ring current shifts at C-13 chemical shifts are calculated. The difference in the calculated ring current effects are small comparing the wild-type with 3-58BPTI(K15R,R17A,R42S) provided the structures are relaxed. Protein unfolding as a function of pH and temperature is studied by DSC. Unfolding occurs at lower temperature with N-terminally truncated analogues, and the maximum is shifted toward higher pH.
Abstract: Highly stable crowded carbenium ions such as (1-pyrenyl)diphenylmethylcarbenium ion (2) and 1,6-(3) and 1,8-bis(diphenylmethylenium)pyrene [dication] (4) and their dibrominated analogues (3Br and 4Br) have been studied at low and at ambient temperatures. 2 shows a conventional two-ring flip (ph,ph), whereas the disubstituted pyrene derivatives show one-ring flips (py) and two-ring flips (ph,ph) with a higher rotational barrier in agreement with AM1 calculations. A series of calculations show that the AM1 method gives the transition-state energies in best agreement with experiment. The propeller-shaped geometry of these molecules is reflected in characteristic low-frequency resonances of the phenyl rings. At low temperature, 3 and 4 exist as rotational isomers with C-2 and C-s (or C-i) symmetry. In 4, steric interaction makes the two rotamers slightly different in energy (1 kcal). For 4 or 4Br, the C-13 chemical shift differences between the C-2 and the C-s species of 4 or 4Br correlate roughly with the calculated charge differences between the C-2 and the C-s species. The charge at the C+ carbon is most extensively delocalized in 2, whereas in 3 and 4 with two C+ groups the pyrene moieties are less effective in charge delocalization.
Abstract: Primary and secondary deuterium isotope effects on H-1 and C-13 chemical shifts are measured in citrinin, a tautomeric compound with an unusual doubly intramolecularly hydrogen bonded structure. The isotope effects are to a large extent dominated by equilibrium contributions and deuteration leads to more of the deuterated enol forms rather than the deuterated acid form. H-1, C-13 and O-17 nuclear shieldings are calculated using density functional ab initio methods. A very good correlation between calculated nuclear shieldings and experimental H-1 and C-13 chemical shifts is obtained. The tautomeric equilibrium can be analyzed based on the isotope effects on C-6 and C-8 carbons and shows an increase in the o-quinone form on lowering the temperature. Furthermore, upon deuteration the largest equilibrium shift is found for deuteration at OH-8 and the shift in the tautomeric equilibrium upon deuteration is increasing at lower temperature.
Abstract: O-17 chemical shifts were measured in 40 enamines activated in the beta-position by C=O, COO, NO2, SO and SO2 groups. Data for the oxygen-containing series of o-hydroxyacyl aromatics are also included for comparison. Intramolecular hydrogen bonding in the enamines is discussed in terms of the acceptor and donor groups and the separating link. O-17 chemical shifts, the two-bond deuterium isotope shifts on C-alpha C-13 shifts and H-1 NH or OH chemical shifts are correlated to show the interrelations of these parameters in elucidating intramolecular hydrogen bonds and their strength in a wide variety of compounds. O-17 chemical shifts in open-chain compounds are shown to reflect intramolecular hydrogen bonding by a change to lower frequency whereas for five-membered rings steric effects cause higher frequency chemical shifts. (C) 1998 John Wiley & Sons, Ltd.
Abstract: Low temperature protonation of 1-nitropyrene and its N-15--labeled isotopomer with FSO3H . SbF5 (1:1) ('magic acid))-SO2ClF (or SO2) or with FSO3H . SbF5 (4:1)SO2ClF generates either the dihydroxyiminiumpyrenium dication (NO, diprotonation) or the hydroxyiminiumpyrenium dication as the principle NMR observable persistent species (depending on the sample concentration, reaction time and the superacid). The latter is independently generated by diprotonation of authentic l-nitrosopyrene. Variable formation of dihydroxyiminiumpyrenium and hydroxyiminiumpyrenium dications is also observed in the protonation of sterically crowded 1-nitro-2,7-di-tert-butylpyrene, which gives the corresponding dihydroxyiminiumpyrenium dication in FSO3H . SbF5 (1:1)-SO2ClF (or SO2) and the hydroxyiminiumpyrenium dication by low temperature reaction with FSO3H . SO2ClF or CF3SO3H . SO2. Protonation of the buttressed 1-nitro-2,4,6,8,10-pentaisopropylpyrene and its N-15-labelled isotopomer produces the dihydroxyiminiumpyrenium dication (no NMR evidence for the formation of the hydroxyiminiumpyrenium dication) which, as shown before (J; Chem. Sec., Perkin Trans. 2, 1995, 537), undergoes a facile cyclization to the oxazoline-fused pyrenium cation for which N-15 NMR data are now presented. Diprotonation and subsequent cyclization of the singly N-15-labelled 1,3-dinitro-2,4,6,8,10-pentaisopropylpyrene are also studied. Whereas our work reaffirms the generality of NO2 diprotonation in nitropyrenes, it focuses attention on an additional pathway leading to =NH(OH) dication formation. PM3 calculations are used as a complementary tool to examine the geometries and energies of the resulting dications.
Abstract: A series of N-15 enriched Schiff bases have been investigated by means of N-15 NMR at variable temperature (300-200 K). Most realiable results are obtained below 250 K, as the exchange of the chelate proton is sufficiently slow on the NMR time scale at this temperature. The N-15 chemical shifts are found to vary linearly with the one bond N-15-H coupling constants. Deuterium isotope effects on N-15 chemical shifts, (1) Delta(15)N(D), show a characteristic S-shape, when plotted vs. the one-bond N-15-H coupling constants. From this graph the intrinsic isotope effects of the two forms can be estimated as (5) Delta(15)N(D)(int)similar to-1.5 ppm and (1) Delta(15)N(D)(int)similar to 3 ppm. The intrinsic isotope effects for the OH-form is shown to vary only little for the various substituted compounds investigated. The difference in N-15 chemical shifts of the OH-form minus the NH-form is estimated to 100-140 ppm. The dependence of (1) Delta(15)N(D) on the mole fraction is similar to that found for (2) Delta C(OD) for other intramolecularly hydrogen bonded cases, beta-diketones and Mannich bases and looks very similar to that determined for (1) Delta(15)N(D) of the organic acid-pyridine complex.
Abstract: (n)J(C-13,(OH)-H-1) carbon-hydrogen couplings were measured for a broad series of intramolecularly hydrogen-bonded compounds, some of which display tautomerism. A plot of J(obs)(C-3,OH) + J(obs)(C-1,OH) cs. delta OH showed reasonable correlation both for compounds displaying tautomerism and for those with localized hydrogen bonds. Ketones and aldehydes fall on one line and esters on another line corresponding to a lower sum. The (4)J(obs)(C-1,OH) coupling depends on orbital overlap between the hydrogen-bonded hydrogen and the carbonyl oxygen. This coupling can therefore also be useful for monitoring twisting of the carbonyl group out of the plane of the hydrogen bond. An interesting finding is that for aromatic compounds (3)J(obs)(C-2,OH)(cis) is larger than that for an olefin both having the same H-1 OH chemical shift. A plot of J(obs)(C-3,OH) cs. J(obs)(C-1,OH) is very useful for monitoring tautomerism, and J(obs)(C-1',OH) and J(obs)(C-1 ",OH) are both well suited for estimating the mole fractions of tautomeric systems. (C) 1998 John Wiley & Sons, Ltd.
Abstract: Deuterium isotope effects on chemical shifts, (n)Delta C(OD),have been measured in a series,of o-hydroxy acyl aromatics of the type 2-hydroxyacetophenone (1) and 1,3,5-triacetyl-2,4,6-trihydroxybenzene (3). (2)Delta C(OD) increase as the number of neighboring hydrogen-bonded moieties increase. The calculated molecular ab initio geometries with Density Functional Theory(BPW91/6-31 G(d(p)) (5D) with p functions on the chelate protons only) show a large increase in R-OH in going from 1 to 3 and a large corresponding decrease in the C=O ... H-O distance. R-O ... O, A(OH ... O), R-OH ... O, as well as ROH and R-C=O correlate linearly as do (2)Delta C(OD) and R-O ... O. The nuclear shielding(1) and the first derivative of the C-13 nuclear shielding with respect to O-H bond stretching, (d sigma/dR(OH)), has been calculated with the 6-31G(d) (6D) basis set using the GIAO/B(PW91) method (exchange term only). (Chemical shift and nuclear shielding are used intermittently. It should be remembered that they lead to different signs.) The change in the ROH distance upon deuteriation (Delta R-OH(D),) was obtained from a potential scan of OH bond stretching and analyzing the data with a fitted Morse function. Isotope effects are calculated as the product of d sigma/dR(OH) and Delta R-OH(D)(D) The variations in the calculated (2)Delta C(OD) are dominated by Delta R-OH(D.) The calculated (n)Delta C(OD) correlate well with experimental isotope effects. Three parameters, (2)Delta C(OD), Delta R-OH(D), and R-O ... O all show promise as gauges of hydrogen bond strength. Calculated OH and H-1 chemical shifts in general show good agreement with experimental values (RMSD = 0.40 ppm) as do the C-13 chemical shifts (RMSD = 1.9 ppm). The large experimental (2)Delta C(OD) values can be understood in terms of a steric effect caused by the neighboring CH3CO group leading to shorter OH ... O and O ... O distances and consequently stronger hydrogen bonds.
Abstract: Deuterium isotope effects on C-13 nuclear shielding, (n) Delta C(OD), were investigated for a series of enolic triacetylmethane, 2-acyl-1,3-cycloalkanediones, 5-acyl Meldrum's acids and 5-acyl-1,3-dimethylbarbituric acids at different temperatures. The enoloc 2-acyl-1,3-cycloalkanediones, 5-acyl Meldrum's acids and 5-acyl-1,3-dimethylbarbituric acids all exhibit intramolecular enol-enol tautomerism. For the first two the equilibrium constants were estimated from the deuterium isotope effects on the enolic and carbonylic carbons. The equilibrium constants were estimated to be 1.5 for the enolic 2-acyl-1,3-cyclohexanediones and 2-acetyl-1,3-cyclopentanedione, favouring the form having an endocyclic enolic double bond, and 0.8 for 5-acyl-1,3-dimethylbarbituric acids, favouring the form having an exocyclic enolic double bond. Apparently, the equilibrium position is unaffected by increasing the size of the acyl group, and therefore no distinct effects caused by steric hindrance were observed. The non-hydrogen-bonded alpha-carbonyl group of enolic triacetylmethane, the 2-acyl-1,3-cycloalkanediones, 5-acyl Meldrum's acids and 5-acyl-1,3-dimethylbarbituric acids cause a high frequency shift of the OH H-1 chemical shifts. A plot of the latter against the sum of (2) Delta C(OD) (4) Delta C(OD) shows a larger sum for the compounds apparently exhibiting intramolecular enol-enol tautomerism than for compounds apparently not exhibiting such tautomerism. (C) 1998 John Wiley & Sons, Ltd.
Abstract: Two-electron oxidations (SbF5/SO2ClF) of 6-fluoro-benzo[a]pyrene, 3, 6-chloro-benzo[a]pyrene 4, and their corresponding 7-methyl derivatives 5 and 6 as well as 1-fluoropyrene,;7, 1-chloropyrene, 8, and 1-fluoro-2,7-di-tert-butylpyrene, 9, allowed the charge delocalization mode, substituent effects, one-bond and long-range C/F coupling constants, and the tropicity of the resulting persistent PAH dications to be examined by NMR for the first time. For comparison, detailed NMR studies of the dications of 7-methylbenzo[a]pyrene, 2, and the parent benzo[a]pyrene, 1, are also reported. Total deshieldings Sigma Delta delta C-13 in the resulting dications are ca. 195-211 ppm/e for the benzo[a]pyrenium and ca. 200-209 ppm/e for the pyrenium dications, respectively. Their AM1-calculated Delta Delta H(f)degrees values are between 421 and 427 kcal/mol and 444 and 449 kcal/mol, respectively, well within the previously predicted dication formation limit of 463 +/- 4 kcal/mol.
Abstract: Prediction of 1-octanol water partition coefficients for a range of polar N-PAC from HPLC capacity coefficients has been investigated. Two commercially available columns, an ODS column and a Diol column were tested with water-methanol eluents. The best prediction of log K-ow for N-PAC was achieved using a Diol column with an eluent of 35 % MeOH and 65 % water. The results indicate that the Diol column, in reversed phase mode, is able to form hydrogen bonds with a solute. Different LFERs between retention and log K-ow was found for polar and nonpolar compounds. In general log K-ow increased with size and log K-ow for N-PAC was 1.1-1.3 lower than log K-ow for the equivalent PAH. Shielding of the nitrogen atom in the N-PAC compounds caused an increase in log K-ow. (C) 1997 Elsevier Science Ltd.
Abstract: Aldrich humic acid was chemically immobilized to the silanol surface of a column material to be used for highperformance liquid chromatography (HPLC). The retention factors to the humic acid column material of 45 polycyclic aromatics compounds (PAC) were determined by HPLC. The PAC include PAH, N-, S-, O-PAC and substituted PAC (9-substituted anthracenes, bromopyrenes, and quinoline derivatives). The sorption coefficient of quinoline to humic acid was directly determined at different pH. The good correlation achieved between the HPLC retention factors and literature K-oc values, including the presented one of quinoline, was applied to determine K-oc of 39 other PAC. The determined K-oc values were parametrized with regard to size, ring heteroatoms, and steric and substituent effects and were compared with literature values of water solubility and recently determined octanol-water partition coefficients. It is shown that the sorption of PAC to humic a cid is not only affected by hydrophobic interactions but also by hydrogen and especially ionic bonds. The investigation shows that the application of humic acid stationary HPLC phases is a valuable supplement to other techniques for determination of K-oc.
Abstract: The interesting deuterium isotope effects of gossypols have been reinvestigated and the very large two-bond isotope effect, (2) Delta C-6(OD), is ascribed to electric field effects. Common to the investigated compounds is the presence of intramolecular hydrogen bonds. A feature strongly related to the strength of the intramolecular hydrogen bond is intermolecular OH exchange. Electron-attracting substituents at the 3- and 5-positions of 2-hydroxyacl aromatics increase the acidity of the 2-OH proton and therefore the intermolecular exchange, but not the hydrogen bond strength, whereas alkyl groups ortho to the intramolecularly hydrogen bonded OH prevent the OH group from swinging oat and therefore prevent intermolecular exchange. Conformational equilibria were studied in 2-hydroxy-3-nitro-6-methoxyacetophenone. Surprisingly, the form with the weaker intramolecular hydrogen bond to the nitro group is dominant at ambient temperature, whereas it is the opposite at 160 K. For 2-hydroxy-5-methyl-3-nitroacetophenone a similar pattern is seen, but with much less of the form having hydrogen bonding to the nitro group at ambient temperature. 2-Acetyl-1, 8-dihydroxy-3,6-dimethylnaphthalene is involved in tautomerism of the enolic beta-diketone type and large deuterium isotope effects on the C-13 and OH chemical shifts are observed. (C) 1997 by John Wiley & Sons, Ltd.
Abstract: Superacid protonation studies are reported on 2-methyl-(1), 3-methyl-(2), 3,6-dimethyl-phenanthrene (3) and 4H-cyclopenta[def]phenanthrene (4). In FSO3H-SO2CIF persistent monocations 1H(+)-4H(+) are generated, 3 gives a symmetrical dication by protonation with FSO3H . SbF5(4:1)-SO2CIF or with 'magic acid'-SO2ClF. 1 and 2 are partially diprotonated in 'magic acid'-SO2ClF. The observed mono-and di-cations are in most cases those predicted by AM1 to be the most stable, Correlation between the magnitude of Delta delta(13C), and AM1 calculated changes in carbon changes [Delta q = q(c)(ion) - q(c)(neutral)] is also examined. The charge alternation paths in the mono-and di-cations are compared and discussed, A parallel is drawn between stable ion and AM1 studies of methylphenanthrenes and the solvolytic studies of K-region and non-K-region phenanthrene oxides.
Abstract: Secondary isotope effects on C-13 chemical shifts have been measured in a series quinolinols, quinaldinic acid N-oxides and quinoline-2-carboxyamide N-oxide. For 8-quinolinol N-oxides a good correlation was found between delta OH and (n) Delta C(OD) isotope effects, The OH and C-13 chemical shifts and (n) Delta C(OD) show very small temperature dependences. The primary isotope effects are small, positive and temperature insensitive, Furthermore, they increase with increasing (n) Delta C(OD). All features point towards a localised hydrogen bend in an asymmetric double well potential. The quinaldinic acid N-oxides show long-range isotope effects on C-13 chemical shifts of both signs with (2) Delta C=O(OD) rather small. The primary isotope effects of the quinaldinic acid N-oxide is of order of 0.5 ppm, whereas for its 4-ethoxy-derivative is smaller, similar to 0.3 ppm. The OH chemical shifts resonate at the low field similar to 18-20 ppm and the OH resonance is fairly broad at room temperature, especially for the 4-ethoxy-derivative The temperature effects on the chemical shifts, primary and secondary isotope effects are small. For quinaldinic acid N-oxides the asymmetric broad quasi-single potential is suggested. For quinoline-2-carboxyamide N-oxide the isotope effects are small, indicating rather weak hydrogen bond. (C) 1997 Elsevier Science B.V.
Abstract: Chrysene (1), 6-fluorochrysene (2), 6-chlorochrysene (3), and 6-bromochrysene (4) are cleanly monoprotonated in FSO3H . SbF5 (ca. 10:1)/SO2ClF at the C-12 position. 6-Acetylchrysene (5) is CO-protonated in FSO3H/SO2ClF with significant charge delocalization into the chrysene and provides a model for a C-6-protonated chrysenium cation. 4H-Cyclopenta[def]chrysene (6) is protonated at C-5 (site of bromination and acetylation). The observed chrysenium (methanochrysenium) cations are those predicted by AM1 to have the lowest energies. The NMR characteristics of the resulting arenium ions are discussed and the he Delta delta Cs-13 are compared with AM1 calculated changes in charges [Delta q(c) = q(c)(ion) - q(c)(neutral)]. Possible relationships between the charge delocalization path in chrysenium ions and metabolic activation of chrysenes by electrophilic pathways via the bay-region epoxide ring opening (--> PAH-DNA adduct) are evaluated.
Abstract: Deuterium isotope effects on C-13 chemical shifts have been studied in a series of substituted N-alkyl and N-phenyl keto-enamines. The intramolecularly hydrogen bonded Z-forms show the largest two-bond isotope effects, (2) Delta C-1(ND). Methyl-substitution al C-l leads to a larger two-bond isotope effect in the N-phenyl-substituted derivatives. This effect is ascribed to steric compression. Space-filling substituents at the ortho-position of the N-phenyl ring lead to a decrease of the two-bond isotope effect. A correlation is found between (2) Delta C-1(ND) and (3) Delta C-2'(ND). The latter becomes negative in the sterically hindered cases. (3) Delta C-2'(ND) may therefore be used as a gauge of the twist of the phenyl ring. o-Hydroxy substitution of the CO-phenyl rings enables intramolecular hydrogen bonding to the carbonyl group. This kind of hydrogen bond with two donors to one acceptor leads to smaller (2) Delta C-2(ND) and (2) Delta C-2 ''(OD) isotope effects equivalent to weaker hydrogen bonds for the Z-isomer. This is ascribed to competition for the acceptor. For the E-isomer (2) Delta C(OD) is enhanced. The same feature is seen for N,N-dimethylamino enamines. This increase is ascribed to delocalization of the nitrogen lone-pair onto the carbonyl oxygen, thereby strengthening the hydrogen bond and thus leading to larger two-bond, (2) Delta C(OD), isotope effects.
Abstract: The relative stability, magnitude, and mode of charge delocalization into the pyrene moiety (Py) were evaluated for a series of tertiary and secondary 1-pyrenylmethylcarbenium ions PyC+R1R2 and PyC+R3H (with R-1 = R-2 = Me and Ph; R-3 = Me, Ph, CH2Ph, and (CH2)(10)Me), 4-pyrenylmethylcarbenium ions (with R-1 = R-2 = Me; R-3 = Me and Ph), and 2-pyrenylmethylcarbenium ions (with R-1 = R-2 = Me). The carbocations were generated from the corresponding carbinols by protonation with FSO3H/SO2ClF. The primary 1-pyrenyl- and 2-pyrenylmethyl cations could not be generated by ionization of their primary alcohols. Within the tertiary and the secondary carbocations, pi-charge delocalization into the pyrene moiety and the pyrenium ion character of the resulting carbocations increase in the order: 1-pyrenyl (alpha) > 4-pyrenyl (alpha beta) > 2-pyrenyl (beta). The NMR characteristics of the resulting ions are discussed and compared. The connection between charge delocalization/ stability in the regioisomeric pyrenylmethyl carbocations and the magnitude of carcinogenic activity in the benzannelated pyrenes, for which the bay-region diol-epoxides are the ultimate carcinogens, are discussed.
Abstract: Deuterium isotope effects on F-19 chemical shifts caused by deuteriation at OH or NH groups have been determined for intramolecularly hydrogen bonded compounds including fluorinated o-hydroxyacyl aromatics, enaminones, o-hydroxyazo and hydrate compounds. The o-hydroxyazo and hydrate compounds represent tautomeric and non-tautomeric cases. Deuterium isotope effects on fluorine chemical shifts for o-hydroxyacyl aromatics are found to parallel deuterium isotope effects at the carbon ipso to fluorine. For the azo and hydrate compounds very long-range effects are seen formally over ten bonds. Through-space effects are observed in the case of spatially close nuclei like 2-fluorobenzamide-N-alpha. The isotope effects on F-19 chemical shifts can, in p-fluorophenyl substituted cases, be used to monitor the change in equilibrium upon deuteriation and therefore to estimate the importance of hydrogen bonds.
Abstract: Deuterium isotope effects of C-13 chemical shifts are studied in a series of enol and keto forms of beta-ketoamides and the corresponding thioamides. In addition, the 2,6-cyclohexanediketo-1-amides and thioamides are studied. The effects of ring size (five- and six-membered rings) on the isotope effects and the tautomeric nature of the systems are also looked into. Rather unusual isotope effects are found for the amides, indicating a tautomeric system of the -C=ONHRCOHNHR type. This is supported by the O-17 chemical shift studies. The isotope effects of the simple amides are compared with those of the tetracyclines and piroxicams. The study of N-phenyl-3-phenyl-3-oxo-propiothioamide at low temperature reveals that this thioamide exists as a mixture of s-cis and s-trans species. The isotope effects and the influence of intramolecular hydrogen bonding in the two species can thus be studied. Thioamides of indan-1,3-diones show tautomeric behaviour, as revealed by very large deuterium isotope effects of both signs. Deuteriation shifts the equilibrium in the direction of the thioamide. Finally, the tendency of a series of beta-hydroxy esters, thioesters, anhydrides, amides, thioamides, aldehydes and ketones to become tautomeric is discussed in terms of hydrogen bonding, isotope effects, (2) Delta C(OD), and the nature of the acceptor.
Abstract: The complete assignment of the H-1 and C-13 NMR spectra of two carbothioamide- substituted meroterpenes is presented, Resonance assignments were achieved by the use of one- and two-dimensional MMR, NOED, selective decoupling measurements and the deuterium isotope effect on the C-13 chemical shifts. Six-membered ring conformations were determined by analysis of proton spin coupling constants with the aid of spectral simulations in conjunction with molecular mechanics calculations and equations correlating coupling constants and dihedral angles. The anisotropy effect of benzene rings was used for structural assignments.
Abstract: Parent dihydropyrene 1 and 2,7-di-tert-butyldihydropyrene 3 are monoprotonated with FSO3H/SO2ClF to give their persistent monoarenium ions 1H(+) and 3H(+) by the attack of proton at C-3 (peri to the ethano-bridge not at C-l as previously suggested). Dihydropyrene 3 is diprotonated in FSO3H.SbF5 1:1 ''Magic Acid''(R)/SO2ClF to give the symmetrical dication H-3(2)+2, similar diprotonation of 1 with ''Magic Acid''/SO2ClF or with FSO3H.SbF5 (4:1)/SO2ClF gave the diprotonated species H-1(2)+2 in a mixture. NMR characteristics of the mono- and dications are discussed. On raising temperature or on prolonged cold storage, 1H(+) and 3H(+) are converted to,their corresponding pyrenium cations (2H(+) and 4H(+)). Formation of 2H(+) from 1 is more rapid than conversion of 3 to 4H(+). Parent pyrenium cation was independently generated by protonation with FSO3H/SO2ClF. When a mixture of 2 and 1 is reacted with FSO3H/SO2ClF (dry ice/acetone temperature) only 2H(+) is seen in the NMR (concomitant presence of the radical cation 1(+.) is inferred from EPR). Similar protonation of a mixture of 6-chlorochrysene 5 and 1 with FSO3H/SO2ClF leads to NMR observation of 1H(+) (with concomitant presence of 5(+.)); on raising temperature 1H(+) is converted to 2H(+). The nature of the paramagnetic radical cation (RC) present in the arenium ion samples influences the position, and resolution of the NMR spectra. This approach may prove useful in NMR studies of large polycyclic aromatic hydrocarbons PAHs where concomitant RC formation greatly diminishes the quality of the NMR spectra.
Abstract: Fluorescence excitation and emission behavior are reported for methylcoronene, 1,2-dimethylcoronene, 2-isopropylpyrene, 4-isopropylpyrene, 2,4,7,1O-tetraisopropylpyrene, 1,3,6,8-tetraisopropylpyrene, 1,3,5,7,9-pentaisopropylpyrene, 2,7-di-tert-butylpyrene, 2,7-di-(dimethylpropyl)pyrene, 1,3,6,8-tetracyclopentylpyrene and 1,3,6,8-tetracyclohexylpyrene dissolved in various organic solvents of varying solvent polarity. Experimental results are used to test a potential screening method for determining whether a given alkyl-substituted polycyclic aromatic hydrocarbon (PAH) solute is likely to exhibit probe character, as evidenced by a systematic variation in the ratios of the fluorescence intensities of select emission bands with solvent polarity. Of the eleven solutes studied, only 4-isopropylpyrene, methylcoronene and 1,2-dimethylcoronene showed useful solvent polarity probe character. 1,3,6,8-Tetraisopropylpyrene, 1,3,5,7,9-pentaisopropylpyrene, 1,3,6,8-tetracyclohexylpyrene and 1,3,6,8-tetracyclopentylpyrene exhibited slight probe behavior; however, the dynamic ranges covered in emission intensity ratios were believed to be too small for many practical applications when compared against the experimental uncertainty associated with the measured values.
Abstract: Deuterium isotope effects on C-13 nuclear shielding, ''Delta C(OD), were investigated for a series of enolic beta-diketones at different temperatures, The investigated enolic beta-diketones cover a broad range of tautomeric equilibrium constants (K). The equilibrium constants were estimated from O-17 and C-13 chemical shifts, C-13 chemical shifts and the deuterium isotope effects show changes with temperature, which are due to a change in the tautomeric equilibrium, It is shown that the variation of K with deuterium substitution depends on K, This has the important consequence that the equilibrium isotope effects for a series with different K may go through a maximum, If the sum of the deuterium isotope effects on C-13 chemical shifts for the carbonyl and enolic carbons is above 0.8 ppm for five-membered and 1.2 ppm for six-membered ring compounds, the system is tautomeric. This statement holds for sterically non-hindered compounds, The intrinsic two-bond deuterium isotope effects for an intramolecular hydrogen bonded system with a chelate six-membered ring with optimal geometry and a localized double bond are estimated to be 1.2 ppm, Knowing the intrinsic contribution, deuterium isotope effects can be used to estimate the position of tautomeric equilibria for beta-diketones. H-1 chemical shifts of OH groups display a linear relation with the molar fraction X.
Abstract: The low temperature protonations of sterically crowded molecules 2,4,6,8,10-pentaisopropyl-1-nitropyrene 1, 2,4,6,8,10-pentaisopropyl-1,3-dinitropyrene 12, 3,6,8,10-tetracyclohexyl-1-nitropyrene 16, 2,7-di-tert-butyl-1-nitropyrene 19, 2,7-di-tert-butyl-1,8-dinitropyrene 21, 1,3,6,8-tetraisopropyl-4-nitropyrene 23 and parent 1-nitropyrene 28, all possessing buttressed nitro groups, were studied in various superacid media under persistent ion conditions. Nitro group diprotonation was observed in all cases to give N,N-dihydroxyiminium-pyrenium (or oxoiminium-pyrenium) dications. The resulting dications derived from 1, 12, 16 and 23 undergo a facile intramolecular nitro group yclisation (which is usually complete within minutes at -75 degrees C --> room temp.) to give oxazoline-(or oxazine-)pyrenium cations 6 (and 11), 14 (and 15), 18 and 25. The charge distribution patterns (probed by C-13 and H-1 chemical shift analysis) for the iminium-pyrenium dications and their cyclised derivatives illustrate extensive charge delocalisation away from the iminium group at the alternating carbons of the periphery, similar to alkylpyrenium and fluoro(alkyl) pyrenium ions. The remote a positions carry substantial positive charge, which increases with the presence of inductively stabilising alkyl (cycloalkyl) substituents. The assignment of the iminium carbon was proved by independent synthesis and protonation of the 15-N labelled 1. Quenching of 6, 18 and 14 (containing 15) does not furnish their N-hydroxyoxazoline derivatives (like 5) nor the alkylnitropyrene precursors. Instead, the corresponding nitroso-alkylpyrenium and its derived radical cation salts (like 9a and 9RC) are obtained (EPR), which when re-dissolved in (CFSOH)-S-3-H-3 or (FSOH)-H-3 reform the cyclised pyrenium ions. Formation of persistent radical cations was also detected in some cases upon storage of the nitroalkylpyrene samples in (CFSOH)-S-3-H-3 ('TfOH') at room temperature. Ring opening of 25 and reduction upon quenching produces a different type of persistent nitrosoalkylpyrene radical (27 <-> 27a) with smaller a(N). The mechanistic aspects of nitro group cyclisation/ring opening are discussed.
Abstract: A series of intramolecularly hydrogen-bonded enamines, enols and enethiols with ester carbonylic, ketonic carbonylic, thioester carbonylic, nitro and sulphoxide accepters were investigated to obtain C-13 chemical shifts and deuterium isotope effects. Results from 33 new compounds and six remeasurements are compared with already existing data. An important aim was to show that isotope effects on chemical shifts are useful descriptors of hydrogen-bonded systems and not only a parameter proportional to the C-13 chemical shifts. Substituent effects were studied and the donors and accepters ranked according to their abilities to support hydrogen bonding. Steric effects strengthen the hydrogen bonding in cyclic five-membered beta-diketones. Plots of two-bond [(2) Delta C(OD)] vs. four-bond isotope effects [(4) Delta C(OD)] show that (4) Delta C(OD) increases with increasing hydrogen bond strength and that large deviations from this relationship can be an indicator of tautomerism.
Abstract: Benzo[a]pyrene 1 and dibenzo[a,e]pyrene 2 are monoprotonated with FSO3H-SO2ClF to give persistent arenium ions of protonation at C-6 (1H(+)) and C-8 (2H(+)), respectively. The low temperature protonation of benzo[e]pyrene 3 under a variety of conditions produced a mixture of arenium ions of attack at C-1 (3aH(+)) and C-3 (3bH(+)). The charge distribution pattern in the resulting arenium ions (as probed by low temperature C-13 and 1H NMR spectroscopy) remains very much analogous to those of alkyl(cycloalkyl)pyrenes, showing significant phenalenium ion character (predominant charge alternation at the periphery of the pyrene moiety). The arenium ion energies and charges were examined by semiempirical theory (AM1). For 1, in agreement with experiment, protonation at C-6 is most favoured and the changes in charge distribution Delta Q[q(c)(ion) - q(c)(neutral)] indicated a very distinct phenalenium unit. Attack at other sites is less favoured, with that at C-2 being least likely. The extent of positive charge delocalisation becomes more limited as the arenium ion energies increase. In accord with experiment, AM1 calculations indicate that for 2, the arenium ion of protonation at C-8 is the most stable followed by those of C-1 and C-3 which are 4.5 and 5.1 kcal mol(-1) less stable. Charge delocalisation away from the protonation site in these cations is predicted to be more extensive and involve a phenalenium moiety. For 3, in line with experiment, the arenium ion energies for attack at C-l and C-3 are almost identical and most favoured. The results indicate that despite the pronounced effect that benzannellation has on carcinogenicity, the preferred site(s) of electrophilic attack are those which lead to the most highly delocalised carbocations. Among these, formation of a robust phenalenium ion moiety is always favoured. Possible implications of these findings in relation to carcinogenesis models mediated by arenium ions are discussed. The oxidation dications of 1-2 were also probed by the AM1 method.
Abstract: A routine method for searching the conformational space of small flexible chain molecules in solution is presented. This involves the quantitative use of NOEs aided by molecular modeling. The l diastereomer of model compound (+/-)-N-(1-phenylethyl)-2-(ethylsulfinyl)-1-propenylamine (1) was studied in CDCl3 and pyridine-d(5) solutions using H-1 NMR. It was shown to undergo self-association in CDCl3. Comparison of calculated and observed steady-state NOE values and the kinetics of NOEs shows that the compound has the E geometry around the double bond. The conformational space accessible to the sulfinyl enamine was searched using the program PCMODEL with the help of ancillary programs and programs to calculate theoretical NOEs. These conformations could be limited substantially by using coupling constants and by comparing observed NOEs to those calculated by assuming conformational averaging about a small number of energy minima.
Abstract: The structures of acylpyran-diones, -triones and acylpyridinediones have been studied primarily by deuterium isotope effects on C-13 chemical shifts. The 3,5-diacetyltetrahydropyran-2,4,6-trione forms a double tautomeric system involving one of the carbonyl carbons of the anhydride moiety. This compound also exists as a minor symmetrical isomer with two intramolecular hydrogen bonds to the same acceptor. This isomer shows isotopic perturbation of the OH proton resonance upon deuteriation. A similar situation is found for 1,5-diphenylpentane-1,3,5-trione. The 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione is found to be tautomeric and mainly in the 4-hydroxy form. The corresponding 5-acetyl derivative forms a very weak hydrogen bond as is also found in the 5-ethoxycarbonyl-6-methylpyridine-2,4(3H)-dione. The same pattern is found for 3- and 5-acetyl-6-methylpyridine-2,4(3H)-dione. This difference in the two-bond deuterium isotope effect is related to the bond orders of the bonds linking the hydrogen bond donors and accepters and reflects the strength of the intramolecular hydrogen bonds. The 3-acetyl-4-hydroxy-2(1H)-quinolones are tautomeric in a similar fashion. The formal hydroxypyridines are shown by isotope effects to be of the 2-pyridone form. The formal imines of most of the above compounds have also been studied and are shown to exist in their keto-enamine forms. In the case of 3-(1-amino)ethylidenequinoline-2,4(1H,3H)-diones and 2-(1-amino)ethylidene-6,7-dihydro-5H-benzo[ij]quinolizine-1,3(2H)-diones two different forms with hydrogen bonds to either the carbonyl at C-4 or the amide carbonyl group at C-2 are observed. Deuterium isotope effects on chemical shifts again turned out to be crucial in the structure elucidation.
Abstract: In FSO3H-SO2ClF at low temperature, the symmetrical all alpha-substituted tetraisopropylpyrene 1, tetracyclohexylpyrene 2, and tetracyclopentylpyrene 3 monoprotonate at the ipso position to give 1H(+), 2H(+) and 3H(+), respectively. Whereas 1H(+) is directly observable by low-temperature NMR spectroscopy before it rearranges to 1aH(+), the sterically more congested pyrenium ions 2H(+) and 3H(+) are not persistent, rearranging rapidly to the NMR-observable 2aH(+) and 3aH(+) These rearranged pyrenium cations are the first examples of:pyrenium ions of ap attack. Ambient disproportionation of 2 in triflic acid 'TfOH' gives 1,3,6,8,7-pentacyclohexylpyrene. The 1,3,6-triisopropylpyrene 4 is protonated at the available alpha position to give 4H(+). The 2,7-di-tert-pentylpyrene 5 and 2,7-di-tert-butylpyrene 6 are alpha monoprotonated, and 2-tert-pentylpyrene 7 is exclusively monoprotonated at the alpha position ortho to the tert-pentyl group (-->7H(+)). The 4-isopropylpyrene 8 is monoprotonated at the 'remote' alpha positions to give 8H(+) and 8aH(+) in near equal amounts. The charge distribution in the resulting monopyrenium ions was probed with C-13, H-1 and two-dimensional NMR(HECTOR, COLOC and COSY) experiments. The positive charge is extensively delocalized away from the site of-attack and exists at the alternating carbons of the periphery. Thus the phenalenium ion character of the pyrenium ion of a attack and phenanthrenium ion character of the pyrenium ion of alpha beta attack are shown. Low-temperature reaction bf 2 with FSO3H.SbF5(4:1)-SO2ClF gives a varying amount of sulfonylation sigma-complex 2SO(2)H(+), the oxidation dication 2(2+) and a 1,3-diprotonation species, 2D(2+) (D = dynamic). Likewise. protonation of 6 and 7 with FSO3H.SbF5(1:1) magic acid(R')-SO2ClF leads to diprotonation forming, 6D(2+) and 7D(2+) in addition to dealkylation to give R(+). The 1,3-diprotonation species consist of singly charged phenalenium ions and tertiary carbocations in which the alkyl group at the 2-position is dynamic and alkyl cation-like.
Abstract: A precondition for the chemotherapeutic treatment of a variety of virally-induced human diseases and malignant conditions is a highly selective interaction of the drug molecule to be used with it's biological target. To ensure the development of novel, effective drugs, it is essential that the biological target is well characterised with regard to it's structure and activity. Such characterisation relies upon adequate amounts of pure target being available. One of the most important enzymatic importers for antimetabolities is the enzyme thymidine kinase. In this article an in vitro protein expression system is described which facilitates the production of milligram amounts of pure and biologically active thymidine kinase, from a number of important biological sources. Results have shown that the in vitro produced enzyme has the exact biochemical properties of the in vivo enzyme. Thus the in vitro protein expression system is an ideal vehicle to facilitate an in depth investigation of the enzyme's biological properties.
Abstract: The H-1 and C-13 NMR spectra of the soluble square-planar o,o'-dihydroxyazoarene-platinum(II) complexes [Pt-(L) (tba)] (L = 5,5'-dichloro-2,2'-dihydroxyazobenzenate (dhab), (5-chloro-2-hydroxyphenylazo)-3-oxo-N-phenylbutanamidate (hpab), or (5-chloro-2-hydroxyphenylazo)-2-naphtholate (hpan) and tba = tributylamine) and the protonated ligands have been assigned by use of homo- and heteronuclear 2D correlated NMR spectroscopy. The assignment of spectra of the analogous complexes [M(L)(py)] (M = nickel(II), palladium(TI), and platinum(II) and py = pyridine) has been achieved by comparison of the chemical shifts. C-13 chemical shifts of corresponding carbon signals of the 5-chloro-2-hydroxyphenyl group, common to all the eight complexes with hpab and hpan, are very similar to each other, and in turn these shifts are very similar to the one set of signals from the 5-chloro-2-hydroxyphenyl groups in the four complexes of the symmetrically substituted ligand dhab. These chemical shifts exhibit the same periodic change when the ligand is coordinated to different group 10 metal ions, and the signals are assumed to originate from 5-chloro-2-hydroxyphenyl groups with equal coordination environment in all the reported complexes; thus C-13 NMR spectroscopy is believed to constitute an excellent way of establishing isostructurality. On the basis of the crystal structures of complexes with the unsymmetrical ligands hpab and hpan, all the reported complexes with these ligands are N-alpha-isomers coordinated to the metal at the azo nitrogen attached to the 5-chloro-2-hydroxyphenyl substituent. The changes in C-13 chemical shift with coordination can be used to distinguish N-alpha- from N-beta-isomers. Deuterium isotope effects on C-13 chemical shifts in the hpab complexes with one exchangeable amine proton and in the free unsymmetrically substituted ligands (hpab)H-2 and (hpan)H-2 themselves reveal intramolecular hydrogen bonds and predominant tautomeric forms, these being possible indicators for predicting the identity of the ligating nitrogen. (n)J(195Pt- C)(13) couplings in the [Pt(L)(tba)] complexes have been measured at low field, and the magnitudes of these couplings are analyzed in terms of coupling paths in the two annulated five- and six-membered rings formed by the ligand and the central platinum(II) ion.
Abstract: NOE experiments at low temperature were used to establish the structure of the E- and Z-isomers of the enamine (BUSOCH)-S-t=CHNHBUt. The NOE experiments revealed large intermolecular NOE effects between E- and Z- isomers indicating the formation of dimers of specific orientation. NH chemical shifts and their temperature dependence showed that the E- and Z- isomers behave differently. The NH chemical shifts of the former go to higher frequencies on lowering the temperature, whereas the opposite holds for the (Z)-NH resonance. Similar trends are found on increasing the concentration. The orientation around the C-1-N bond can be probed by (3)J(H-1,NH) or by (1)J(N,H). The N-15 chemical shifts were also investigated in a series of beta-sulphinylenamines. They are mainly dominated by alpha- and beta-effects and show only a slight dependence on the E/Z isomerism, which seems to indicate a weak hydrogen bond in the Z-isomer at high concentration. Pertinent NMR parameters are now available for a general structure determination of geometrical isomers of beta-sulphinylenamines.
Abstract: A series of sterically hindered o-hydroxy aromatic ketones were synthesized, including benzene, naphthalene, phenanthrene and pyrene derivatives. Deuterium isotope effects on the C-13 chemical shifts of 2-hydroxy-1-acenaphthone and other sterically hindered, intramolecularly hydrogen-bonded aromatic ketones (OH exchanged) are shown to be unusual. The two-bond isotope effects are very large. Likewise ape the istope effects on C=0, C-1, C-3 and C-4 carbon resonances and some show unusual signs. These unusual effects are explained by a higher degree of twist in the deuterio than the protio compound Steric isotope effects are also observed on OH chemical shifts of sterically hindered o-hydroxy acetyl aromatic compounds deuteriated at the methyl group. These isotope effects show non-additivity. For one-bond isotope effects, (1) Delta(13)C(O-18), hydrogen bonding leads to a decrease, whereas twisting of the carbonyl group leads to an increase. Two hydrogen bonds to the same acceptor has a reduced cumulative effect, Data for sterically hindered, hydrogen-bonded compounds are found tea fall outside the correlation between delta(O-17) and (1) Delta(13)C(O-18).
Abstract: C-13=O, (CH3)-C-13, N-15 and O-17 isotropic NMR shieldings of model amides, acetamide and N-methylacetamide, in the presence of point charges and of hydrogen fluoride simulating dipolar perturbations, have been calculated by the ab initio LORG method. Positive and negative charges produce similar but opposite changes. The calculated effects are exaggerated compared to experiment, but the trends are reproduced. The results are analyzed in terms of electric field effects and contributions along the bond directions, assuming a cos theta angular dependence between the charge and the bond that is polarized, and a 1/r2 distance dependence. The parametrized expressions can form a basis for a protocol for analyzing changes in C-13, N-15 and possibly O-17 amide and peptide shieldings caused by charged perturbers. Conversely, the location of an atom or group changing charge can be inferred from the chemical shift response of nuclei in its vicinity.
Abstract: Deuterium isotope effects on C-13 chemical shifts are investigated in intramolecular hydrogen-bonded systems. As models, o-hydroxydibenzoylmethane and tetracycline (TC) are chosen. The OH proton is exchanged and the deuterium isotope effects at carbons, (n)DELTAC(OD), are studied in both localised and tautomeric situations. Factors influencing the isotope effects are the nature of donors, acceptors, the intervening double bonds and local substituents. The isotope effects of the D ring of TC are shown to be similar to isotope effects in o-hydroxyacetophenone. The central beta-diketone system in both compounds is shown to be tautomeric and strongly shifted towards a form with a keto group forming a hydrogen bond to the phenolic hydroxy group of ring A. This double hydrogen bond to one acceptor weakens both hydrogen bonds. Interesting long-range effects are emerging from this situation. The A ring of TC is shown to contain a C=C(OH)NH2 moiety.
Abstract: Low-temperature protonations of 2-fluoropyrene (9), 2-fluoro-7-tert-butylpyrene (10), their corresponding tetrahydro analogs 11 and 12, 1-fluoropyrene (4), 4-fluoropyrene (15), and its derivative 14 are studied in various superacid media. The isomeric fluoro(alkyl)pyrenes are fully monoprotonated in FSO3H/SO2ClF and their tetrahydro and hexahydro analogs 11, 12, and 14 are monoprotonated with FSO3H.SbF5 (4:1)/SO2ClF. The resulting persistent fluoro(alkyl)pyrenium ions are probed by multinuclear NMR (H-1, C-13, F-19). Whereas a vicinal fluorine in the beta position increases the stability of the pyrenium ion of alpha protonation leading to a single alpha pyrenium ion, fluorine in the alphabeta or alpha position is not sufficiently stabilizing to alter the normal site of protonation; a attack is still observed. C-13, H-1, and F-19 NMR data illustrate that the positive charge is extensively delocalized away from the site of attack and exists predominantly at the alternating carbons of the periphery, indicative of significant phenalenium ion character in the fluoro(alkyl)pyrenium ions of alpha attack. The fluoronium ion character of the resulting delocalized arenium ions is deduced based on the increase in 1J(C-F), deshielding of the fluorine, and reduced deshielding of the fluorine-bearing carbons as compared to position of these carbons in the analogous non-fluorinated pyrenium ions. With 10 the presence of fluorine and the tBu group in 2, 7 positions (beta) allows their relative stabilization effects on an adjacent carbenium ion formed by vicinal protonation (alpha) to be compared. Whereas kinetic protonation produces both 10-H+ and 10a-H+ in near equal amounts, subsequent thermodynamic equilibration gives almost pure 10-H+. A rearrangement of the tetrahydropyrene derivative (12-H+ --> 12a-H+) was also observed.
Abstract: One-bond deuterium isotope effects on nitrogen nuclear shielding, 1DELTAN(D) and two-bond isotope effects at H-1 nuclear shielding, 2DELTAH(D), have been investigated in a series of inclusion complexes. The hosts comprise, SC-24, [2.2.2], [2.2.1], [2.2.1]D, [2.2] DD, K5, 18-crown-6 and 18-crown-6 tetracarboxylic acid. The structure of the host [2.2.1 ] is discussed based on isotope effects and H-1 chemical shifts, and an interesting exchange reaction is observed between CDCl3 and ammonium ions in the [2.2.1] complex. The counterion dependence is shown to be zero for ammonium ions fully included in cages such as SC-24 and [2.2.2] and for 18-C-6(COOH)4. For cryptands and podands such as [2.2.1] and K5 a weak counterion dependence of 2DELTAH(D) opposite to that found for ammonium ions in water is observed, while 1DELTAN(D) and N-15 chemical shifts depend very strongly on the counterions. as also found for ammonium ions in water solution. The anilinium ion shows effects similar to those observed for the ammonium ion. 1DELTAN(D) isotope effects correlate well with deltaN, and 2DELTAH(D) correlate with deltaNH. The correlation between 1DELTAN(D) and 2DELTAH(D) is different for ammonium ions in water and ions included in crowns, cryptands or podands. This shows that different mechanisms are operating. The 2DELTAH(D) isotope effects and deltaNH chemical shifts depend on the distance to the nearest acceptor (oxygen or nitrogen), and they also depend on the type of acceptor. 2DELTAH(D) isotope effects are found to be a good gauge of N ... N or N ... O distances of the inclusion complexes. The 2DELTAH(D) isotope effects vary with temperature, but the dramatic changes in the H-1 NMR spectra of the host are not reflected in the isotope effects. The one-bond couplings, 1J(N, H), correlate with 2DELTAH(D). The variations are much larger for the cryptands than found in water solutions. 1DELTAN(D) and 2DELTAH(D) are calculated in NH3D ... NH3 complexes using the ab initio/LORG method. 1DELTAN(D) increases with an increase in the distance R(N...N), and 2DELTAH(D) changes from negative to positive values for longer R(N...N), values. Both isotope effects depend on R as 1/r2, where r is the distance from the nitrogen of ammonia to the midpoint of the N-D bond.
Abstract: Sterically crowded alkylpyrenes 2-7 undergo two-electron oxidation with SbF5/SO2ClF (or SbF5.FSO3H (9:1)/SO2ClF) to give persistent alkylpyrene dications. In agreement with the charge distribution pattern observed in the parent pyrene dication and with semiempirical calculations (AM1) on isomeric monoisopropylpyrenes dications; the deshielding order C(alpha) > C(alphabeta) > C(beta) was found in their carbon-13 NMR spectra regardless of the number and position of the alkyl substituents. In most cases, the observed DELTAdelta13C for the aromatic ipso-carbon signals of the alkylpyrenium dications relative to those of 1(+2) were slightly larger than expected for a normal substituent effect in the alkylpyrene precursors. Only minor changes (DELTAdelta13C) in the unsubstituted positions were observed, except for the ring carbons in highly crowded locations for which a noticeable upfield shift was detected. The H-1 NMR spectra of alkylpyrenium dications showed a much greater substituent dependency. Low temperature reactions of the less-reactive pyrene and 2,7-di-tert-butylpyrene with SbF5/SO2ClF led primaryl to sulfonylation sigma-complexes rather than oxidation dications. Persistent alkylpyrene radical cations were found to coexist with the alkylpyrenium ions of protonation in FSO3H/SO2 (ESR detection) without any noticeable line broadening in the NMR spectra of the Wheland intermediates.
Abstract: A series of 2,6-dihydroxy acylaromatic compounds were investigated to characterize the rotational and hydrogen bonding properties of the carbonyl group. Deuterium isotope effects on H-1 and C-13 chemical shifts due to deuteriation of OH groups were determined at both ambient and low temperature. In the latter case isotope effects on chemical shifts of the individual rotamers can be determined. Deuteriation of one of the OH groups may lead to isotopic perturbation of the tautomeric equilibrium of the carbonyl group and the two hydroxyl groups. The perturbation was found to be larger in ketones than in esters. Complete band shape analysis of the OH resonances of the esters and ketones in a temperature interval above and below the coalescence temperature led to DELTAG(not-equal), AH(not-equal) and DELTAS(not-equal) values for various concentrations of added THF-d8. DELTAS(not-equal) was found to be strongly negative. Temperature coefficients for the shift of the OH resonances showed large variations for esters and ketones owing to the different hydrogen bond patterns. The esters have two intramolecular hydrogen bonds, one strong and an additional weaker one between the OH and OR groups. The second OH group Of the ketones was shown to point primarily towards C-5. Increasing amounts of THF-d8 increased the amount of this rotamer. The anisotropy of the XC=O group at C-2, C-6 was shown to lead to a low-field shift of C-2, very different from that found for C=O groups without hydrogen bonds. The anisotropy caused by OH groups can also be estimated. On the basis of the thermodynamic parameters, a model for the rotation of the ester group is suggested. The rate determining step involves both intramolecular hydrogen bonds, which are twisted out of the ring plane to form hydrogen bonds to the solvent or other hydrogen bond acceptors.
Abstract: Deuterium isotope effects on C-13 nuclear shielding, (n)DELTAC(OD), were investigated for a large number of substituted ortho-substituted aromatic hydroxyacyl compounds of the type salicylaldehyde, 2-hydroxyacetophenone, etc. The isotope effects were analysed as a function of substitution patterns. 2DELTAC-2(OD) shows a linear dependence on deltaOH, with one slope for 4- and 6-substituted derivatives and another for 5-substituted derivatives. A similar pattern is also found for the long range isotope effects. In addition, isotope effects also correlate with C-13 chemical shifts. Isotope effects in o-hydroxyacetophenones can be predicted from the deltaOH and (n)DELTAC(OD) values of salicylaldehyde, and vice versa. The isotope effects were analysed in terms of a vibrational term related to the intramolecular hydrogen bond and an electronic term related to the substituent influence on the transmission of the effect along the pathway between the deuterium-substituted oxygen and the carbon in question. 2DELTAC(OD) isotope effects are found to depend on the one-bond bond order of the C=C bond of the unsubstituted hydrocarbons. Long-range isotope effects are observed to the olefinic carbons of 2-hydroxychalcone, whereas no effects beyond C-1' are seen in o-hydroxyketones or o-hydroxybenzophenones. The tautomeric equilibrium of usnic acid is perturbed by the removal of the weak hydrogen bond between 9-OH and the carbonyl group at C-1.
Abstract: Long-range deuterium isotope effects on C-13 chemical shifts, (n)DELTAC(OD), were studied in the intramolecularly hydrogen-bonded purpurogallins (benzotropolones). A very large long-range isotope effect from the hydrogen-bonded 4-OH(D) is observed over six bonds at C-7. Further, long-range isotope effects are transmitted over as many as eight bonds. The transmission pathway including the carbonyl carbon is favoured, and some of the long-range effects are transmitted via the carbonyl group. Large long-range isotope effects are thus observed in hydrogen-bonded systems with effective conjugation to the carbonyl group via alternating double bonds. This study indirectly supports recent theoretical predictions. The C-13 and H-1 NMR spectra of purpurogallin and its 2,3-dimethyl, 2,3,6-trimethyl and 2,3,4,6-tetramethyl derivatives are assigned, and the hydrogen bond patterns are established based on the OH chemical shifts and 2DELTAC(OD) isotope effects.
Abstract: H-1 and N-14 nuclear shieldings of the ammonium ion and of its mono- and tetrahydrated species in directional as well as non-directional coordinations are calculated ab initio using the localized orbital/local origin (LORG) method; counterpoise calculations are included to test for basis set superposition effects. The variation of the H-1 and N-14 shieldings with N-H distance is essentially linear in the bonding region, while the large-scale variations illustrate the transition from solvated ammonium to ammonated hydroxonium complexes. The deuterium isotope effects for these systems are evaluated in a static approximation, and for the directionally coordinated NH4+ (H2O)4 ion the computed two-bond deuterium isotope effect on H-1 correctly reproduces the unusual (i.e. negatively signed) experimental result, while the bare ion and non-directionally coordinated NH4+(H2O)4 yield positively signed effects. The decisive factor for the sign inversion is found to be the directional solvation of the deuterium atom, with an ammonium-water distance not exceeding ca. 3 angstrom, and the effect can be traced to a difference in the distance variation of the shielding perpendicular and parallel to the N-H bond. For the N-14 nuclear shielding again the results obtained for directionally coordinated NH4+(H2O)4 are in better agreement with experiments than the results obtained for the bare ion or for non-directional solvation.
Abstract: Deuterium isotope effects on C-13 nuclear shielding were investigated for a series of o-amino and acetamido azo compounds and for amino- and acetamido-9,10-anthra- and leucoanthraquinones. A variety of these compounds form intramolecular hydrogen bonds and are possibly tautomeric in nature. Deuterium isotope effects on C-13 chemical shifts were used to probe both the tautomerism and the nature of the hydrogen bond. Deuterium isotope effects over two bonds reflect the strength of the hydrogen bond. In general, isotope effects do not depend on the solvent and, in this study, are independent of temperature. In the acetamido compounds isotope effects on the internal aromatic carbons, 2-DELTA-C(ar)(ND), vary much more than the 2-DELTA-CO(ND) values of the external carbonyl carbon. Plots of 2-DELTA vs. delta(H) accordingly show different slopes. Investigations of these types of compounds allow a correlation between the hydrogen-bond enthalpy and 2-DELTA-CO(ND) of amides. 2-DELTA-CO(ND) values therefore form a basis for extending the use of deuterium isotope effects to, for example, biomolecules. Diamino-9,10-leucoanthraquinone represents, to our knowledge, the first case of a compound in which isotope effects from both a hydrogen-bonded and a free ND deuterium can be observed. Bis(phenylamino)leucoanthraquinone exhibits interesting equilibrium isotope effects over many bonds. Many of these isotope effects are non-additive. Hydrogen bonding is discussed in general, based on these observations.
Abstract: Protic mono- and dinitration of 1,3,5,7,9-pentaisopropylpyrene (1) occurred at the available alpha positions to give 2 and 3. Despite steric crowding, 2 and 3 did not exhibit a torsional barrier to i-Pr rotation at ambient temperatures; however, buttressing of the peri i-Pr groups was evident (H-1 NMR, UV, and force field energy minimizations). Persistent (dihydroxyiminium)pyrenium dications 2a+2 and 3a+2 were formed by low temperature protonation of 2 and 3 with CF3SO3H (TfOH)/SO2 or with FSO3H/SO2. Intramolecular cyclization of the nitro group of 2a+2 gave the rearranged pyrenium ion 2c+. 1 reacted with NO2+BF4- in acetonitrile solvent to give two pyrenium ions stable at rt, viz. the Wheland intermediate of alpha-nitration 2b+2 and the (dihydroxyiminium)pyrenium dication 2a+2; the latter was also the predominant pyrenium ion formed in the reaction of 1 with NO+BF4- in acetonitrile. Reaction of 1 with NO2+BF4- in chloroform solvent gave alpha-nitration products and a persistent radical cation RC. The simultaneous presence of alpha-nitration products and a persistent pyrenium RC was also observed in the reaction of 1 with NO+BF4- in chloroform, where broader NMR line widths and a stronger ESR signal suggested more extensive oxidation. Protic and aprotic nitrations of 1,3,6,8-tetraisopropylpyrene (10) occurred at the alpha-beta positions; a minor addition product (26) was also found. A mixture of isomeric dinitropyrenes was obtained in NO2+ nitration of 1-isopropylpyrene (13). In line with low temperature protonation studies, aprotic nitrations of 2-isopropyl- and 4-isopropylpyrenes occurred predominantly at the alpha positions. The crowded pyrenium ion of 3 and 2 transfer nitrate to aromatics (toluene, mesitylene, benzene) under mild conditions in competition with a more facile transalkylation.
Abstract: Four N-terminal extended species of the wild-type bovine pancreatic trypsin inhibitor (WT-BPTI), Arg-BPTI (1-BPTI), Met-Glu-Ala-Glu-BPTI (4-BPTI), Ser-Ile-Glu-Gly-Arg-BPTI (5-BPTI) and Gly-Ser-Ile-Glu-Gly-Arg-BPTI (6-BPTI) have been studied by H-1 n.m.r. The overall structure of the protein is largely unaffected by the addition of extension peptides. pH titration effects on the C-terminal Ala 58 H(beta) chemical shift indicate that the structure of 1-BPTI at neutral pH is very, similar to that of the WT protein, with a salt bridge between the main chain terminal charges. A salt bridge interaction is prevented by addition of the longer extension peptides. Temperature stabilities are measured by high temperature hydrogen isotope exchange and by microcalorimetry. The stability of 1-BPTI is equal to that of WT-BPTI. A slight decrease in stability is observed for longer extensions, following the order WT-BPTI = 1-BPTI < 5-BPTI = 6-BPTI < 4-BPTI. Small changes in chemical shift are observed for 30 invariant resonances in 4-, 5- and 6-BPTI and for a subset of this group in 1-BPTI. These protons are distributed over about half of the BPTI molecule. The size of the chemical shift changes for many resonances follow the same ranking as the temperature stability. The chemical shift effects are attributed to charge and dielectric effects from extension peptides that probably share a common orientation on the surface of BPTI.
Abstract: Deuterium isotope effects on carbonyl C-13 magnetic shielding were measured for the backbone carbonyl groups in BPTI (basic pancreatic trypsin inhibitor), and interpreted as a measure of hydrogen bond energies. The effects originate from peptide amide proton deuterium substitution and were observed on carbonyl carbons separated by two or three covalent bonds from the amide H/D. Two-bond isotope effects depend on the energy of the hydrogen bond donated by NH/D. Calibration of the effect with model compound data leads to hydrogen bond enthalpies < 4.7 kcal/mol. Isotope effects over three bonds from the amide H/D to the carbonyl carbon of the same amino acid residue are observed for seven carbonyl resonances in BPTI. The three-bond isotope effects are highly related to the various backbone conformations. The largest effects are observed for residues with an approximate syn- periplanar conformation of the H-N-C(a)-C = O atoms, as realized for many residues in the BPTI antiparallel beta-sheet. The residues that show measurable three-bond effects have unusually short distances between H and O. The size of this effect decreases rapidly with increased O..H distance in the open five-membered ring. This observation suggests appreciable interactions in these rings.
Abstract: The concentration dependence of the H-1 and C-13 NMR chemical shifts in D2O and in CDCl3 solutions were determined for (+/-)-alpha-phenylethylamine (I). Aqueous solutions of (-)-I and (+)-I, 50% enantiomerically enriched in (-)-and (+/-)-2,2,2,-trifluoro-1-phenylethanol and the (+)-and (-)-N-formyl derivatives of I, were also studied. H-1 nuclear Overhauser enhancements were used to check the conformations of the solutes at various concentrations and H-1 T1 values were used to monitor the changes in molecular tumbling in solutions. The results are interpreted in terms of a spontaneous aggregation of solute molecules in water, with the possible determination of the critical micelle concentration. The time-dependent splittings in the H-1 NMR spectra suggest further, more detailed, studies of the structures of the aggregates and the possibility of chiral recognition in water.
Abstract: Proton-detected heteronuclear multiple-bond H-1-C-13 correlations (HMBC) previously have been used for assignment purposes in a variety of isotopically enriched proteins. In the present study it is demonstrated that the technique yields an almost complete assignment of the natural abundance C-13 Spectrum of the protein basic pancreatic trypsin inhibitor (BPTI). In addition, the technique permits additional H-1 assignments to be made for this well-studied protein. The intensities of observed correlations permit rough estimates to be made of 2J(C,H) and 3J(C,H) coupling constants. These couplings can be used for conformational studies of both the side chains and the backbone. Intra- and interresidue coupling between C-alpha-H and the carbonyl carbon provides information about the backbone angles-psi and phi. Side-chain conformations can be determined from both two- and three-bond carbon-hydrogen coupling constants. The present study of BPTI together with its known high-precision solution structure yields an experimental correlation between resonance intensities and secondary structure. The spectra show the potential of the method in analyzing C-13 NMR spectra of nonenriched proteins. The method yields C-13 NMR chemical shifts, which are versatile parameters to be used to monitor structural changes, titrations, etc.
Abstract: Equilibrium displacements within the enol-enthiolic tautomeric systems of beta-thioxoketones caused by long-range isotope effects were monitored by H-1 and C-13 NMR spectroscopy. Thioacetylacetone and 1,3-diphenyl-3-thioxopropanone deuteriated at various positions were investigated. Both compounds are in fast (Z)-enol-(Z)-enethiol equilibrium on the NMR time scale. The investigations showed that CD3 and C6D5 groups changes the equilibrium so that the tautomer having a C = X bond next to the CD3 or C6D5 groups decreases. Deuteriation at the methine position pushes the equilibrium towards the (Z)-enol form. The study further showed that deuteriation at the phenyl ring next to the C = O group causes the largest effect if the deuterium is at the ortho position and the smallest if it is at the para position. Long-range effects on equilibrium are discussed in general in order to establish a common pattern. Deuterium scrambling is observed both during the coupling reaction of ethyl thionoacetate with hexadeuterioacetone and during preparation of specifically labelled acetophenone. The latter compounds are synthesized by acetylation of specifically labelled benzenes. The scrambling reaction is suggested to be a transdeuteriation of the deuteriated benzenes rather than involving the acetophenones.
Abstract: Deuterium isotope effects on carbonyl C-13 magnetic shielding were measured for the backbone carbonyl groups in BPTI (basic pancreatic trypsin inhibitor), and interpreted as a measure of hydrogen bond energies. The effects originate from peptide amide proton deuterium substitution and were observed on carbonyl carbons separated by two or three covalent bonds from the amide H/D. Two-bond isotope effects depend on the energy of the hydrogen bond donated by NH/D. Calibration of the effect with model compound data leads to hydrogen bond enthalpies < 4.7 kcal/mol. Isotope effects over three bonds from the amide H/D to the carbonyl carbon of the same amino acid residue are observed for seven carbonyl resonances in BPTI. The three-bond isotope effects are highly related to the various backbone conformations. The largest effects are observed for residues with an approximate syn- periplanar conformation of the H-N-C(a)-C = O atoms, as realized for many residues in the BPTI antiparallel beta-sheet. The residues that show measurable three-bond effects have unusually short distances between H and O. The size of this effect decreases rapidly with increased O..H distance in the open five-membered ring. This observation suggests appreciable interactions in these rings.
Abstract: The concentration dependence of the H-1 and C-13 NMR chemical shifts in D2O and in CDCl3 solutions were determined for (+/-)-alpha-phenylethylamine (I). Aqueous solutions of (-)-I and (+)-I, 50% enantiomerically enriched in (-)-and (+/-)-2,2,2,-trifluoro-1-phenylethanol and the (+)-and (-)-N-formyl derivatives of I, were also studied. H-1 nuclear Overhauser enhancements were used to check the conformations of the solutes at various concentrations and H-1 T1 values were used to monitor the changes in molecular tumbling in solutions. The results are interpreted in terms of a spontaneous aggregation of solute molecules in water, with the possible determination of the critical micelle concentration. The time-dependent splittings in the H-1 NMR spectra suggest further, more detailed, studies of the structures of the aggregates and the possibility of chiral recognition in water.
Abstract: 1-Isopropyl- (1), 2-isopropyl- (2), 4-isopropyl- (3), 1,3,6,8-tetraisopropyl- (4), and 1,3,5,7,9-pentaisopropylpyrene (5), 2,7-di-tert-butylpyrene (6), and 1,3,5,8-tetracyclohexyl- (7) and 2,4,7,9-tetracyclopentylpyrene (8) in FSO3H or CF3SO3H (TfOH) in SO2 or SO2ClF solvent gave stable monopyrenium ions. In agreement with theory, exclusive alpha protonation occurred at low temperature (-75 --> -65-degrees-C) irrespective of the position of the substituents. The position of alpha-protonation is controlled by inductive stabilization of the alkyl (cycloalkyl) groups. Unlike hexahydropyrene which is diprotonated in FSO3H.SbF5 (1:1) Magic acid, with isopropylpyrenes stable dications could not be generated; in SO2 solvent the Wheland intermediates of sulfinylation were observed, whereas in SO2ClF solvent oxidation and monoprotonation were competitive. Charge distribution patterns in the sulfinylation sigma-complexes are similar to those of protonated pyrenium ions. Stable pyrenium cations deprotonate or desulfinylate on quenching without dealkylation or disproportionation. At higher temperatures (ca. -40-degrees-C), ipso-protonated 4 undergoes isomerization in FSO3H/SO2 solvent; other alkyl (cycloalkyl)pyrenium cations show no isomerization/disproportionation. Upon standing in Magic Acid, hexahydropyrene is oxidized to pyrene.
Abstract: It is shown that by considering the C-13-C-C-H coupling constant between the alpha-CO2- and the beta-protons in amino-acids it is possible to obtain side-chain conformational information concerning the rotamer populations which is not available from studies of H-H coupling constants alone.
