Abstract: Exploration of intrachain hydrodynamics of dsDNA within channels has been limited to indirect analysis of global coil dynamics. In this Letter, we isolate hydrodynamic interactions within single molecules of dsDNA confined to slitlike channels by making use of density covariance measurements. We show that the strength of hydrodynamic interactions in DNA is dependent on the intrachain correlation length and that screening by symmetry in slitlike confinement results in a screening length that is proportional channel height. Moreover, we directly show the partial draining nature of the blobs formed by dsDNA in slits and predict under what conditions a dsDNA blob should obey nondraining Zimm behavior. DOI: 10.1103/PhysRevLett.110.068101
Abstract: Topoisomerase II (TOP2) regulates the topology of DNA by catalysis of a double strand passage reaction. Inhibition of this reaction prevents cell replication, and, thus, is a pathway targeted by anticancer drugs. Some details regarding the cell-killing mechanism are unknown and assays to screen for anticancer drugs are not well established. Here, we study the gelation of linear and circular DNA using microrheology assays. Gelation of the DNA-enzyme mixture was examined by tracking of multiple colloidal probe particles. The mean square displacements of the probe particles were analyzed by the time-cure superposition procedure as well as the classical derivation of the dynamic moduli. First, the passage reaction was inhibited by AMP-PNP, a non-hydrolyzable analog of ATP. The results showed gelation due to the formation of a self-catenated network of circular DNA molecules. Next, when TOP2 was inhibited by the anti-cancer drug ICRF-193, we observed a similar change in rheology. Based on these findings, we propose a cell-killing mechanism by gelation of the genome through TOP2-mediated interlocking of looped DNA segments of the replicated, intertwined chromosomes.
Abstract: We report an approach to study the in situ conformational response of single biomolecules such as DNA to a change in environmental solution conditions. These conditions are, for example, the composition of the buffer or the presence of protein. For this purpose, we designed and fabricated a nanofluidic device featuring two arrays of parallel nanochannels in a perpendicular configuration. The cross-sections of the channels are rectangular with a diameter down to 175 nm. These lab-on-a-chip devices were made of polydimethylsiloxane (PDMS) cast on a high quality master stamp, obtained by proton beam writing and UV lithography. Biomolecules can be inserted into the device through the array of channels in one direction, whereas the buffer can be exchanged through the intersecting array of channels in the other direction. A buffer exchange time inside the grid of nanochannels of less than one second was measured by monitoring the conductivity of salt solutions. The exchange time of a protein was typically a few seconds, as determined by imaging the influx of fluorescence labelled protamine. We demonstrate the functionality of the device by investigating the compaction of DNA by protamine and the unpacking of pre-compacted DNA through an increase in the concentration of salt.
Abstract: HU is a protein that plays a role in various bacterial processes including compaction, transcription and replication of the genome. Here, we use atomic force microscopy to study the effect of HU on the stiffness and supercoiling of double-stranded DNA. First, we measured the persistence length, height profile, contour length and bending angle distribution of the DNA-HU complex after different incubation times of HU with linear DNA. We found that the persistence and contour length depend on the incubation time. At high concentrations of HU, DNA molecules first become stiff with a larger value of the persistence length. The persistence length then decreases over time and the molecules regain the flexibility of bare DNA after ∼2 h. Concurrently, the contour length shows a slight increase. Second, we measured the change in topology of closed circular relaxed DNA following binding of HU. Here, we observed that HU induces supercoiling over a similar time span as the measured change in persistence length. Our observations can be rationalized in terms of the formation of a nucleoprotein filament followed by a structural rearrangement of the bound HU on DNA. The rearrangement results in a change in topology, an increase in bending flexibility and an increase in contour length through a decrease in helical pitch of the duplex.
Abstract: Blob theory has been widely applied to describe polymer conformations and dynamics in nanoconfinement. In slit confinement, blob theory predicts a scaling exponent of 2/3 for polymer diffusivity as a function of slit height, yet a large body of experimental studies using DNA produce a scaling exponent significantly less than 2/3. In this work, we develop a theory that predicts that this discrepancy occurs because the segment correlation function for a semiflexible chain such as DNA does not follow the Flory exponent for length scales smaller than the persistence length. We show that these short length scale effects contribute significantly to the scaling for the DNA diffusivity, but do not appreciably affect the scalings for static properties. Our theory is fully supported by Monte Carlo simulations, quantitative agreement with DNA experiments, and the results reconcile this outstanding problem for confined polymers.
Abstract: Artificial nanowalkers are inspired by biomolecular counterparts from living cells, but remain far from comparable to the latter in design principles. The walkers reported to date mostly rely on chemical mechanisms to gain a direction; they all produce chemical wastes. Here we report a light-powered DNA bipedal walker based on a design principle derived from cellular walkers. The walker has two identical feet and the track has equal binding sites; yet the walker gains a direction by pure physical mechanisms that autonomously amplify an intrasite asymmetry into a ratchet effect. The nanowalker is free of any chemical waste. It has a distinct thermodynamic feature that it possesses the same equilibrium before and after operation, but generates a truly nonequilibrium distribution during operation. The demonstrated design principle exploits mechanical effects and is adaptable for use in other nanomachines.
Abstract: An easy method is introduced allowing fast polydimethylsiloxane (PDMS) replication of nanofluidic lab-on-chip devices using accurately fabricated molds featuring cross-sections down to 60 nm. A high quality master is obtained through proton beam writing and UV lithography. This master can be used more than 200 times to replicate nanofluidic devices capable of handling single DNA molecules. This method allows to fabricate nanofluidic devices through simple PDMS casting. The extensions of YOYO-1 stained bacteriophage T4 and lambda-DNA inside these nanochannels have been investigated using fluorescence microscopy and follow the scaling prediction of a large, locally coiled polymer chain confined in nanochannels. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4740231]
Abstract: Monte Carlo simulations are used to study the knotting probability of circular DNA confined in a slit. We systematically vary the slit height, the width, and the contour length of the DNA molecule. We find that the trend in knotting probability with respect to slit height can be monotonic or nonmonotonic, depending on the width and contour length. The nonmonotonic trend is caused by two competing factors: the increase of the effective persistence length and the increase of segment density by slit confinement. These factors are antagonistic, in the sense that the increase in effective persistence length disfavors knot formation, whereas the increase in segment density favors the knotting probability. Our simulation results bring to light the importance of both chain length and width for slit-confined circular DNA and can be used to guide future experiments which aim to produce populations of knotted DNA through cyclization or catalyzed double-strand passage reactions in confinement.
Abstract: The effects of the like-charged proteins bovine serum albumin and hemoglobin on the conformation and compaction of single DNA molecules confined in rectangular nanochannels were investigated with fluorescence microscopy. The channels have lengths of 50 mu m and cross-sectional diameters in the range of 80-300 nm. In the wider channels, the DNA molecules are compressed and eventually condense into a compact form with increasing concentration of protein. In the narrow channels, no condensation was observed. The threshold concentration for condensation depends on the channel cross-sectional diameter as well as the ionic strength of the supporting medium. The critical values for full compaction are typically less than one-tenth of a millimolar. In the bulk phase and in the same environmental conditions, no condensation was observed. Anisotropic nanoconfinement hence facilitates compaction of DNA by negatively charged protein. We tentatively interpret this behavior in terms of enhanced depletion interaction between segments of the DNA molecule due to orientation order imposed by the channel walls.
Abstract: DNA conformation in slitlike confinement is studied using Monte Carlo simulation and scaling theory. We focus on analysing the in-plane DNA extension as a function of slit height and DNA properties, such as contour length, persistence length and width. Similar to tube confinement, we identify an extended de Gennes regime which is prevalent in many experimental studies. However, unlike previous studies in tubes, we find two highly confined regimes (so called Odijk regimes) that depend on chain crossing. Our results support the majority of experiments which display a gradual transition into the Odijk regime and give new insight into the importance of chain width for DNA in nanoslits.
Abstract: We present a novel device for the separation of microparticles in a single channel, which is made of inversely asymmetric Brownian ratchets. It enables separation into two different fractions with an adjustable threshold and can be modeled with good agreement. This device serves as proof of concept for an extremely compact class of microsieves.
Abstract: Currently, no theory is available to describe,the conformation of DNA confined in a channel when the nanochannel diameter is around the persistence length. Back-folded hairpins in the undulating wormlike chain conformation result in the formation of loops, which reduces the stretch of the molecule in the longitudinal direction of the channel. A Cooperativity Model is used to quantify the frequency and size of the loop domains. The predictions agree with results from the Monte Carlo simulation.
Abstract: Increasing evidence supports the contribution of local inflammation to the development of Alzheimer’s disease (AD) pathology, although the precise mechanisms are not clear. In this study, we demonstrate that the pro-inflammatory protein S100A9 interacts with the A beta 1-40 peptide and promotes the formation of fibrillar beta-amyloid structures. This interaction also results in reduced S100A9 cytotoxicity by the binding of S100A9 toxic species to A beta 1-40 amyloid structures. These results suggest that secretion of S100A9 during inflammation promotes the formation of amyloid plaques. By acting as a sink for toxic species, plaque formation may be the result of a protective response within the brain of AD patients, in part mediated by S100A9.
Abstract: We experimentally and numerically study the effects of macromolecular crowding agents on DNA structure when confined to a nanochannel. Curiously, DNA response to crowding is significantly different between bulk phase, nanoslit confinement, and nanotube confinement. Coarse grained Brownian dynamics simulations reproduce trends seen in the experiments and allow us to develop a deeper understanding of the key physics at play in these systems. It is proposed that the occupancy of free volume next to the channel wall by crowders causes an effective reduction in confining dimensions of the channel that initially swells DNA in nanoconfinement.
Abstract: With a view to determining the distance between the two opposing duplexes in supercoiled DNA, we have measured small angle neutron scattering from pHSG298 plasmid (2675 base pairs) dispersed in saline solutions. Experiments were carried out under full and zero average DNA neutron scattering contrast using hydrogenated plasmid and a 1:1 mixture of hydrogenated and perdeuterated plasmid, respectively. In the condition of zero average contrast, the scattering intensity is directly proportional to the single DNA molecule scattering function (form factor), irrespective of the DNA concentration and without complications from intermolecular interference. The form factors are interpreted with Monte Carlo computer simulation. For this purpose, the many body problem of a dense DNA solution was reduced to the one of a single DNA molecule in a congested state by confinement in a cylindrical potential. It was observed that the interduplex distance decreases with increasing concentration of salt as well as plasmid. Therefore, besides ionic strength, DNA crowding is shown to be important in controlling the interwound structure and site juxtaposition of distal segments of supercoiled DNA. This first study exploiting zero average DNA contrast has been made possible by the availability of perdeuterated plasmid.
Abstract: The flow properties of DNA are important for understanding cell division and, indirectly, cancer therapy. DNA topology controlling enzymes such as topoisomerase II are thought to play an essential role. We report experiments showing how double-strand passage facilitated by topoisomerase II controls DNA rheology. For this purpose, we have measured the elastic storage and viscous loss moduli of a model system comprising bacteriophage lambda-DNA and human topoisomerase I la using video tracking of the Brownian motion of colloidal probe particles. We found that the rheology is critically dependent on the formation of temporal entanglements among the DNA molecules with a relaxation time of similar to 1 s. We observed that topoisomerase II effectively removes these entanglements and transforms the solution from an elastic physical gel to a viscous fluid depending on the consumption of ATP. A second aspect of this study is the effect of the generic topoisomerase II inhibitor adenylyl-imidodiphosphate (AMP-PNP). In mixtures of AMP-PNP and ATP, the double-strand passage reaction gets blocked and progressively fewer entanglements are relaxed. A total replacement of ATP by AMP-PNP results in a temporal increase in elasticity at higher frequencies, but no transition to an elastic gel with fixed cross-links.
Abstract: We introduce an extension of the simple on-off ratchet by including a second asymmetric sawtooth potential with half the periodicity and inverse asymmetry in the ratchet cycle. As a result of this additional potential, the Brownian particles exhibit reversal of the direction of their mean displacement when relevant parameters such as the on time of the potentials are varied. This direction reversal offers new opportunities for microfluidic particle separation in sieve devices. Based on the results of our extended ratchet model, we propose two designs. Compared to the classical microfluidic sieve, the proposed devices can be made of significantly smaller sizes without sacrificing the resolution of the separation process. In fact, one of our devices can be reduced to a single channel. We study our extended ratchet model by Brownian dynamics simulations and derive analytical and approximative expressions for the mean displacement using an extension of the method of discrete steps. We show that these expressions are valid in relevant regions of the parameter space and that they can be used to predict the occurrence of direction reversal.
Abstract: The effect of dextran nanoparticles on the conformation and compaction of single DNA molecules confined in a nanochannel was investigated with fluorescence microscopy. It was observed that the DNA molecules elongate and eventually condense into a compact form with increasing volume fraction of the crowding agent. Under crowded conditions, the channel diameter is effectively reduced, which is interpreted in terms of depletion in DNA segment density in the interfacial region next to the channel wall. Confinement in a nanochannel also facilitates compaction with a neutral crowding agent at low ionic strength. The threshold volume fraction for condensation is proportional to the size of the nanoparticle, due to depletion induced attraction between DNA segments. We found that the effect of crowding is not only related to the colligative properties of the agent and that confinement is also important. It is the interplay between anisotropic confinement and osmotic pressure which gives the elongated conformation and the possibility for condensation at low ionic strength.
Abstract: The viscoelastic moduli of lambda-phage DNA through the entanglement transition were obtained with particle tracking microrheology. With increasing frequency, the viscous loss modulus first increases, then levels off, and eventually increases again. Concurrently, the elastic storage modulus monotonously increases and eventually levels off to a constant high frequency plateau value. Once the DNA molecules become entangled at about ten times the overlap concentration, the elastic storage modulus becomes larger than the viscous loss modulus in an intermediate frequency range. The number of entanglements per chain is obtained from the plateau value of the elasticity modulus. The longest, global relaxation time pertaining to the motion of the DNA molecules is obtained from the low shear viscosity as well as from the lowest crossover frequency of the viscous loss and elastic storage moduli. The concentration dependencies of the low shear viscosity, the number of entanglements per chain, and the relaxation time agree with the relevant scaling laws for reptation dynamics of entangled polyelectrolytes in an excess of simple, low molecular weight salt with screened electrostatic interactions.
Abstract: The adsorption of lambda-phage DNA onto mica was investigated with atomic force microscopy. We found that the morphologies depended on the solvent conditions in the sample preparation procedure. Flat-lying networks of hybridized single-stranded DNA were obtained if ultrapure water was used. If buffered conditions are maintained during the whole of the preparation procedure, single double-stranded DNA molecules are adsorbed. The adsorbed double-stranded DNA molecules subsequently can be condensed in situ on the surface by a brief rinse with anhydrous ethanol in the presence of divalent magnesium cations. The majority of these surface-directed and ethanol-induced condensed structures are toroids, but a small fraction of rods also has been observed. Analysis of the height and lateral dimensions shows that the toroids are single-molecular and disk-like with a height of one to two DNA diameters. The thin toroid morphology appears to be a general phenomenon of surface-directed condensation, irrespective of the nature of the condensing ligands and the specific surface interaction.
Abstract: Monte Carlo simulations were done to study the conformation of supercoiled DNA confined in a nanochannel. The molecule has a superhelical density of around -0.05 and is bathed in a monovalent salt solution with an ionic strength of 2, 10, or 150 mM. The cross-sectional diameter of the circular shaped nanochannel was varied in the range of 10 to 80 nm. The conformational properties were characterized by the writhing number and the distribution in the distance between the two opposing strands of the superhelix. With increasing confinement, as set by a smaller tube diameter and/or decreased screening of the Coulomb interaction, the supercoil becomes more tightly interwound and long-range structural features such as branching and the formation of hairpins are progressively suppressed. Analysis of the energetics shows a concurrent increase in electrostatic energy and energy of interaction of the supercoil with the wall, but the elastic twisting energy decreases. Confinement in a nanochannel or otherwise hence results in a decrease in the absolute value of the twist exerted on the duplex. The bending energy remains approximately constant, which means that there are no significant deflections from the wall. The simulation results are interpreted with theory based on the wormlike chain model, including the effects of the wall, charge, elasticity, and configurational entropy. It was found that the theory is reasonably successful in predicting the structural response to the confinement at the local level of the diameter and pitch of the supercoil. (C) 2008 American Institute of Physics. [DOI:10.1063/1.2992076]
Abstract: All atom molecular dynamics simulations with explicit water were done to study the interaction between two parallel double-stranded DNA molecules in the presence of the multivalent counterions putrescine (2+), spermidine (3+), spermine (4+) and cobalt hexamine (3+). The inter-DNA interaction potential is obtained with the umbrella sampling technique. The attractive force is rationalized in terms of the formation of ion bridges, i.e., multivalent ions which are simultaneously bound to the two opposing DNA molecules. The lifetime of the ion bridges is short on the order of a few nanoseconds.
Abstract: Single T4-DNA molecules were confined in rectangular-shaped channels with a depth of 300 nm and a width in the range of 150-300 nm casted in a poly(dimethylsiloxane) nanofluidic chip. The extensions of the DNA molecules were measured with fluorescence microscopy as a function of the ionic strength and composition of the buffer as well as the DNA intercalation level by the YOYO-1 dye. The data were interpreted with the scaling theory for a wormlike polymer in good solvent, including the effects of confinement, charge, and self-avoidance. It was found that the elongation of the DNA molecules with decreasing ionic strength can be interpreted in terms of an increase of the persistence length. Self-avoidance effects on the extension are moderate, due to the small correlation length imposed by the channel cross-sectional diameter. Intercalation of the dye results in an increase of the DNA contour length and a partial neutralization of the DNA charge, but besides effects of electrostatic origin it has no significant effect on the bare bending rigidity. In the presence of divalent cations, the DNA molecules were observed to contract, but they do not collapse into a condensed structure. It is proposed that this contraction results from a divalent counterion mediated attractive force between the segments of the DNA molecule.
Abstract: A hexagonal liquid crystal of DNA fragments (double-stranded, 150 basepairs) with tetramethylammonium (TMA) counterions was investigated with small angle neutron scattering (SANS). We obtained the structure factors pertaining to the DNA and counterion density correlations with contrast matching in the water. Molecular dynamics (MD) computer simulation of a hexagonal assembly of nine DNA molecules showed that the inter-DNA distance fluctuates with a correlation time around 2 ns and a standard deviation of 8.5% of the interaxial spacing. The MD simulation also showed a minimal effect of the fluctuations in inter-DNA distance on the radial counterion density profile and significant penetration of the grooves by TMA. The radial density profile of the counterions was also obtained from a Monte Carlo (MC) computer simulation of a hexagonal array of charged rods with fixed interaxial spacing. Strong ordering of the counterions between the DNA molecules and the absence of charge fluctuations at longer wavelengths was shown by the SANS number and charge structure factors. The DNA-counterion and counterion structure factors are interpreted with the correlation functions derived from the Poisson-Boltzmann equation, MD, and MC simulation. Best agreement is observed between the experimental structure factors and the prediction based on the Poisson-Boltzmann equation and/or MC simulation. The SANS results show that TMA is too large to penetrate the grooves to a significant extent, in contrast to what is shown by MD simulation.
Abstract: We report the design and structural characterization of cationic diblock copolymer vesicles loaded with plasmid DNA based on a single emulsion technique. For this purpose, a DNA solution was emulsified in an organic solvent and stabilized by an amphiphilic diblock copolymer. The neutral block forms an interfacial brush, whereas the cationic attachment complexes with DNA. A subsequent change of the quality of the organic solvent results in the collapse of the brush and the formation of a capsule. The capsules are subsequently dispersed in aqueous medium to form vesicles and stabilized with an osmotic agent in the external phase. Inside the vesicles, the plasmid is compacted in a liquid-crystalline fashion as shown by the appearance of birefringent textures under crossed polarizers and the increase in fluorescence intensity of labeled DNA. The compaction efficiency and the size distribution of the vesicles were determined by light and electron microscopy, and the integrity of the DNA after encapsulation and subsequent release was confirmed by gel electrophoresis. We demonstrate reverse transfection of in vitro cultured HeLa cancer cells growing on plasmid-copolymer vesicles deposited on a glass substrate.
Abstract: Encapsulation of dsDNA fragments (contour length 54 nm) by the cationic diblock copolymer poly(butadiene-b-N-methyl-4-vinyl pyridinium) [PBd-b-P4VPQ] has been studied with phase contrast, polarized light, and fluorescence microscopies, as well as scanning electron microscopy. Encapsulation was achieved with a single emulsion technique. For this purpose, an aqueous DNA solution is emulsified in an organic solvent (toluene) and stabilized by the amphiphilic diblock copolymer. The PBd block forms an interfacial brush, whereas the cationic P4VPQ block complexes with DNA. A subsequent change of the quality of the organic solvent results in a collapse of the PBd brush and the formation of a capsule. Inside the capsules, the DNA is compacted as shown by the appearance of birefringent textures under crossed polarizers and the increase in fluorescence intensity of labeled DNA. The capsules can also be dispersed in an aqueous medium to form vesicles, provided they are stabilized with an osmotic agent [poly(ethylene glycol)] in the external phase. It is shown that the DNA is released from the vesicles once the osmotic pressure drops below 10(5) N/m(2) or if the ionic strength of the supporting medium exceeds 0.1 M. The method has also proven to be efficient to encapsulate pUC18 plasmid in submicrometer-sized vesicles, and the general applicability of the method has been demonstrated by the preparation of the charge inverse system: cationic poly(ethylene imine) encapsulated by the anionic diblock poly(styrene-b-acrylic acid).
Abstract: The structure of spherical micelles of the diblock poly(styrene-block-acrylic acid) [PS-b-PA] copolymer in water was investigated up to concentrations where the polyelectrolyte coronal layers have to shrink and/or interpenetrate in order to accommodate the micelles in the increasingly crowded volume. We obtained the partial structure factors pertaining to the core and corona density correlations with small angle neutron scattering and contrast matching in the water. The counterion structure factor was obtained with small angle x-ray scattering (SAXS) with a synchrotron radiation source. Furthermore, we have measured the flow curves and dynamic visco-elastic moduli. The functionality of the micelles is fixed with a 9 nm diameter PS core and a corona formed by around 100 PA arms. As shown by the SAXS intensities, the counterions are distributed in the coronal layer with the same density profile as the corona forming segments. Irrespective ionic strength and micelle charge, the corona shrinks with increasing packing fraction. At high charge and minimal screening conditions, the polyelectrolyte chains remain almost fully stretched and they interdigitate once the volume fraction exceeds the critical value 0.53+/-0.02. Interpenetration of the polyelectrolyte brushes also controls the fluid rheology: The viscosity increases by three orders of magnitude and the parallel frequency scaling behavior of the dynamic moduli suggests the formation of a physical gel. In excess salt, the coronal layers are less extended and they do not interpenetrate in the present concentration range.
Abstract: Polymer gels are well known in the oil industry, but their potential for use as barriers to contaminant transport has not previously received significant study. As a first step, this paper examines the potential for a polyelectrolyte gel to serve as a barrier to the migration of sodium chloride. Two series of tests are reported. These involve the use of hydrogen pulsed field gradient - nuclear magnetic resonance (HPFG-NMR) to measure the self-diffusion on a microscopic scale and the use of magnetic resonance imaging (MRI) to monitor Na+ and H+ migration in the polymer gel with time. It is shown that the gel, which has a hydraulic conductivity of 2 x 10(-12) m/s, has a diffusion coefficient similar to that of compacted clay and greater sorption of Na+ than is typical for compacted clay.
Abstract: The structure of spherical micelles of the diblock copolymer poly(styrene-block-acrylic acid) in water was investigated with small angle neutron scattering and contrast matching. We have monitored intermicelle correlation and the extension of the polyelectrolyte chains in the coronal layer through the overlap concentration. Irrespective of ionic strength, the corona shrinks with increasing packing fraction. Furthermore, at high charge and minimal screening conditions, the corona layers interpenetrate once the volume fraction exceeds the critical value 0.53+/-0.02.
Abstract: The diffusion of ions in polyelectrolyte gels has been investigated both theoretically and experimentally. A simple phenomenological model was first developed, building upon the geometric obstruction theory developed originally for the diffusion of neutral and charged species in neutral gels. The electrostatic obstruction is accounted for by (a) adopting an effective chain diameter obtained from the Dobrynin scaling theory for polyelectrolytes and (b) adding an exclusion layer of thickness delta(c(s)) around the charged chains to describe the electrostatic interaction between the ions and the chains. Then the diffusion of water and organic ions in an organically cross-linked polyacrylamide gel was studied experimentally using nuclear magnetic resonance (NMR). The relative diffusion coefficients D-r = D-g/D-o (D-g = diffusion in gel, D-o = diffusion in solvent) of the different species were measured for various polymer, cross-linker, and salt concentrations. The calculated predictions and the measured data were found to be in good agreement. For water molecules, we obtain D-r = 0.92 +/- 0.01 independently of salt content, which is consistent with a pure geometric obstruction. For monovalent anions and cations (butyrate, Bu-, and tetramethylamonium, TMA(+)) D-r increases from about 0.5 to 0.75 when the ion concentration increases from 0 to 150 mol/m(3). For the divalent cation (putrescine, Pu2+) the D-r are generally higher and increase from 0.6 to about 0.92, indicating an almost complete elimination of the electrostatic obstruction, consistent with a more efficient screening of the charges in the chains.
Abstract: The interaction of the divalent metal ions Mg(2+), Cd(2+), and Ni(2+) with liquid crystalline NaDNA solutions (molar ratios Me(2+)/DNA-phosphate </=0.050) was investigated by polarized light microscopy and multinuclear (31)P, (2)H, and (23)Na NMR. Our findings show that the state of the cholesteric NaDNA phase at equal MgCl(2), CdCl(2), or NiCl(2) concentration is affected in a different way and to a different extent by the nature of the divalent cation. Indeed, we found that the occurrence of an isotropic phase is markedly favored by Mg(2+), and to a lower extent by Cd(2+), and that the pitch of the cholesteric phase decreases in the presence of Cd(2+) or Ni(2+). (23)Na NMR spectroscopy also shows differences between the binding behavior of Mg(2+) and the transition metal ions in the counterion atmosphere around DNA. The results are discussed in terms of different binding modes of the metal ions.
Abstract: We have investigated the phase behaviour of pUC18 plasmid solutions with phase separation experiments and polarized light microscopy. Furthermore, the configuration of the superhelix is monitored with small-angle neutron scattering. The phase diagram is interpreted with liquid crystal theory including the effects of charge, orientation entropy, excluded volume, as well as the elastic, entropic and electrostatic contributions to the molecular free energy.
Abstract: The relaxation dynamics of the spin 3/2 density operator in the presence of fluctuating and static quadrupolar interactions is reviewed. The nuclear magnetic resonance (NMR) line shapes are analyzed for any value of the static quadrupolar interaction, ranging from isotropic systems to systems exhibiting large splitting far exceeding the line widths. Pulse sequences optimized for the elimination of line broadening due to an inhomogeneous static quadrupolar interaction and for the detection of nuclei involved in slow molecular motion and/or in anisotropic, liquid crystalline environment are discussed. in Part II, the dynamics of spin 3/2 in the presence of a (pulsed) radio frequency (RF) field is reviewed. (C) 2003 Wiley Periodicals, Inc.
Abstract: The relaxation dynamics of the spin-3/2 density operator under strong, on-resonance radio-frequency (RF) irradiation and in the presence of fluctuating and static quadrupolar interactions is reviewed. The evolution under the lock is analyzed for any value of the static quadrupolar interaction, ranging from isotropic systems to systems exhibiting large splitting far exceeding the line widths. For isotropic systems, relaxation under off-resonance spin-locking conditions and in the multiple-pulse quadrupolar echo (QE) experiment is reviewed also. Spin-lock pulse sequences optimized for the selective detection of nuclei involved in slow molecular motion and/or in anisotropic, liquid crystalline environment are discussed. in Part I, the classical relaxation dynamics of spin 3/2 was reviewed. (C) 2003 Wiley Periodicals, Inc.
Abstract: To describe the structure and dynamics in a lyotropic xanthan liquid crystal, we have done Na-23 and N-14 NMR experiments. The spectra are consistent with a powder average over randomly oriented liquid-crystalline domains. From N-14 quadrupolar echo experiments it was concluded that the exchange among the several micron sized domains is slow with residence times in the millisecond range. Conventional spin-lock experiments N-14 and Na-23) as well as spin-lock experiments filtered through the quadrupolar spin polarization order (Na-23 only) revealed the existence of an extremely low frequency, submegahertz dispersion in the spectral densities pertaining to both sodium and ammonium counterions. The corresponding correlation times are on the order of tens of microseconds and agree with a structural correlation length on the order of 500 nm. The latter correlation times reflect the rearrangement of counterions among polymer segments over the same spatial extent as the polymer persistence length.
Abstract: The critical concentrations pertaining to the liquid crystal formation of pUC18 plasmid in saline solutions were obtained from (31)P nuclear magnetic resonance, polarized light microscopy, and phase equilibrium experiments. The transition is strongly first order with a broad gap between the isotropic and anisotropic phase. The critical boundaries are strongly and reversibly dependent on temperature and weakly dependent on ionic strength. With polarized light microscopy on magnetically oriented samples, the liquid crystalline phase is assigned cholesteric with a pitch on the order of 4 microm. Preliminary results show that at higher concentrations a true crystal is formed. The isotropic-cholesteric transition is interpreted with lyotropic liquid crystal theory including the effects of charge, orientation entropy, and excluded volume effects. It was found that the molecular free energy associated with the topology of the superhelix is of paramount importance in controlling the width of the phase gap. The theoretical results compare favorably with the critical boundary pertaining to the disappearance of the isotropic phase, but they fail to predict the low concentration at which the anisotropic phase first appears.
Abstract: A new method for selectively detecting sodium ions in anisotropic environments is presented. A spin-lock (SL) sequence, followed by a coherence transfer pulse, generates rank-two zero-quantum coherences, and converts them into observable transverse magnetization. The quadrupolar polarization is only generated when there are residual quadrupolar couplings in the sample, and provided the SL field strength is comparable to these couplings. This filter has proved to be more efficient than a double-quantum magic-angle (DQ-MA) filter in generating observable signal from ions in anisotropic media in both a nasal bovine cartilage sample and a liquid crystalline DNA sample. Finally, the SL filtering technique does not rely on a flip angle effect for the selection of the desired signal component, as does a DO-MA filter, and may therefore prove desirable in an imaging experiment, due to its better tolerance to phase and flip angle imperfections. Magn Reson Med 47:68-74, 2002. (C) 2002 Wiley-Liss, Inc.
Abstract: With a view to determine the configuration and regularity of plectonemically supercoiled DNA, we have measured the small angle neutron scattering from pUC18 plasmid in saline solutions. Furthermore, we have derived the mathematical expression for the single chain scattering function (form factor) of a superhelical structure, including the longitudinal and transverse interference over the plectonemic pitch and radius, respectively. It was found that an interwound configuration describes the data well, provided interactions among supercoils are accounted for in the second virial approximation. The opening angle was observed to be relatively constant and close to 58 degrees, but it was necessary to include a significant distribution in radius and pitch. For diluted supercoils with vanishing mutual interaction, the derived structural results agree with independent measurements, including the distribution in linking number deficit as determined by gel electrophoresis. With increasing plasmid concentration, prior and covering the transition to the liquid-crystalline phase, the radius and pitch are seen to decrease significantly. The latter observation shows that compaction of negatively supercoiled DNA by confinement results in a decrease in writhing number at the cost of a positive twist exerted on the DNA duplex. It is our conjecture that the free energy associated with this excess twist is of paramount importance in controlling the critical boundaries pertaining to the transition to the anisotropic, liquid-crystalline phase.
Abstract: We have derived approximate analytic solutions to the master equation describing the evolution of the spin I = 3/2 density operator in the presence of a radio-frequency (RF) field and both static and fluctuating quadrupolar interactions. Spectra resulting from Fourier transformation of the evolutions of the on-resonance spin-locked magnetization into the various coherences display two satellite pairs and, in some cases, a central line. The central line is generally trimodal, consisting of a narrow component related to a slowly relaxing mode and two broad components pertaining to two faster relaxing modes. The rates of the fast modes are sensitive to slow molecular motion. Neither the amplitude nor the width of the narrow component is affected by the magnitude of the static coupling, whereas the corresponding features of the broad components depend in a rather complicated manner on the spin-lock field strength and static quadrupolar interaction. Under certain experimental conditions, the dependencies of the amplitudes on the dynamics are seen to vanish and the relaxation rates reduce to relatively simple expressions. One of the promising emerging features is the fact that the evolutions into the selectively detected quadrupolar spin polarization order and the rank-two double-quantum coherence do not exhibit a slowly relaxing mode and are particularly sensitive to slow molecular motion. Furthermore, these coherences can only be excited in the presence of a static coupling and this makes it possible to discern nuclei in anisotropic from those in isotropic environment. The feasibility of the spin-lock pulse sequences with limited RF power and a nonvanishing average electric field gradient has been demonstrated through experiments on sodium in a dense lyotropic DNA liquid crystal. (C) 2001 Academic Press.
Abstract: A detailed line shape analysis of Na-23 nuclear magnetic resonance spectra within dense suspensions (12% vol/vol) of Laponite, clay exhibits a macroscopic ordering of these charged anisotropic colloids within a nematic phase. The angular variation of the order parameter limits to 20% of the maximum amount of disorder in these dense suspensions. By contrast, dilute Laponite suspensions (1-4% vol/vol) remain isotropic while the variation of Na-23 relaxation rates over a broad range of frequencies indicates a local ordering of the clay platelets within microdomains of the same spatial extent than the particle diameter (300 Angstrom).
Abstract: The dimension of spherical micelles of the diblock copolymer poly(styrene-block-acrylic acid) [PS(20)-b-PA(85)] was investigated as a function of the degree of ionization of the polyelectrolyte in the coronal layer. To describe the structural arrangement of the blocks, the partial structure factors pertaining to PS-PS and PA-PA density correlations as well as the composition structure factor were obtained with small-angle neutron scattering and contrast matching in water. The PS blocks form a densely packed spherical core with a radius 4.5 nm and an aggregation number similar to 100; the core structure does not depend on the corona charge to a significant degree. The extension of the PA chains in the coronal layer, and, hence, the micelle radius, are found to be highly sensitive to the degree of ionization. At full ionization, the PA chains are almost fully stretched with a density scaling proportional to the inverse second power of the radius away from the core. For lower charge fraction, the results are interpreted in terms of scaling theory for star-branched polyelectrolytes, including the effects of charge annealing. The dimension of the micelles was found to be controlled by the balance of the elastic, conformational, stretching forces and the osmotic pressure exerted by the counterions trapped within the corona.
Abstract: To describe the effect of salt on the structural arrangement of the blocks in aqueous poly(styrene-bloch-acrylic acid) [PS(20)-b-PA(85)] solutions, the partial structure factors pertaining to PS-PS and PA-PA density correlations, as well as the composition structure factor were obtained with small-angle neutron scattering and contrast matching in the water. The copolymers self-assemble with an aggregation number similar to 100 into spherical micelles made of a PS-block core, surrounded by a coronal layer formed by the PA blocks. The addition of salt has no effect on the size of the core and the aggregation number. At full corona charge and minimal screening conditions, the PA chains are almost fully stretched in the radial direction away from the core. With increasing salt concentration, the micelle contracts and the corona chain statistics can be described with a two-region density-scaling model. In the inner coronal region the statistics is unaffected by the salt, whereas for larger radial distances the scaling is similar to neutral polymer stars. Both the micelle radii and the crossover distance between the two different density-scaling regimes comply with theory for osmotic star-branched polyelectrolytes in the salt-dominated regime. For low fractional corona charge, the density scaling is determined by charge annealing effects. Here, the addition of salt does not affect the density scaling, but the micelle nevertheless contracts and eventually precipitates at high ionic strength. Despite the high salt concentrations required to compete with the salinity in the coronal layer generated by the counterions coming from the polyelectrolyte blocks, the range in micelle dimension is similar to the one that can be covered by variation in pH.
Abstract: We have derived the differential equations that describe the dynamics of spin-3/2 nuclei in the presence of radiofrequency (RF) fields and both static and fluctuating quadrupolar interactions. The formalism presented was used to predict the sodium triple-quantum-filtered (TQ-filtered) signal loss in a whole-body scanner, where the widths of the hard 90 degrees RF pulses are on the same order of magnitude as the transverse relaxation times. A small piece of bovine nasal cartilage, known for exhibiting residual quadrupolar splittings, was used to test the theory. The sample was modeled as consisting of small domains, each characterized by a static quadrupolar interaction constant, with an overall Gaussian distribution across the sample. An increase of about 15% in the TQ-filtered signal strength, as the 90 degrees RF pulse width was decreased from 500 to 100 mus, was predicted and demonstrated experimentally for this particular sample. (C) 2000 Academic Press.
Abstract: With a view to describe the structural arrangement of the tetramethylammonium (TMA(+)) counterions in simple salt-free aqueous poly(styrene-block-acrylic acid) [PS(20)-b-PA(85)] solutions, the partial structure factors pertaining to PS-PS, PS-TMA, and TMA-TMA density correlations were obtained with small-angle neutron scattering and isotopic labeling of the counterion. The contributions to the scattering related to the PA-blocks are blanked by contrast matching in water. The copolymers self-assemble with an aggregation number similar to 100 into spherical micelles made of a PS-block core, surrounded by a coronal layer formed by the PA-blocks. At the present level of ionization (50%), the PA chains are almost fully stretched in the radial direction away from the core. A comparison of the counterion involved partial structure factors with the previously reported relevant structure factors pertaining to the PA-blocks shows that the counterion distribution along the radius is very close to the one for the segments of the corona-forming blocks. Within a 10% error margin, all counterions are trapped in the coronal layer, and there is no indication for charge annealing effects and/or counterion migration toward the outer corona region.
Abstract: In solutions of tetramethylammonium (TMA(+)) DNA (double stranded) without added low-molecular-weight salt, the counterion radial density is calculated using the cylindrical Poisson-Boltzmann equation with a distance-dependent quasimacroscopic dielectric permittivity. Comparisons with small-angle neutron scattering data indicate that any inhomogeneity in dielectric permittivity is confined to one or two solvent layers from the DNA surface. At feast for TMA(+), which may be too large to penetrate the grooves of DNA to any significant extent, dielectric inhomogeneity modeled in this way has no detectable effect on the radial distribution.
Abstract: The partial structure factors pertaining to DNA-DNA, DNA-polyamine, and polyamine-polyamine density correlations in DNA fragment (contour length 54 nm) solutions have been measured with small angle neutron scattering and contrast matching in water. The effect of the polyamines putrescine and spermidine on the DNA molecular structure is gauged from the limiting behavior of the DNA-DNA partial structure factor at high values of momentum transfer. The double layer structure and the extent to which the polyamines can approach the DNA are derived from the DNA-polyamine and polyamine-polyamine partial structure factors. For this purpose, the structure factors are interpreted with the correlation functions derived from the classical Poisson-Boltzmann and the modified Poisson-Boltzmann equations and/or Monte Carlo simulation. For simple salt free DNA with tetramethylammonium or putrescine counterions, spatial fluctuations in the charge density are discussed in terms of the charge structure factor. The structural arrangement of putrescine and spermidine can be fully rationalized in terms of their valence. In the case of spermidine, it is necessary to include ionic correlation effects, but this could be accomplished by modeling the ligands as hard spheres. The polyamines have no detectable effect on the DNA molecular structure and are too large to penetrate the grooves to any significant extent. These results imply that DNA condensation in the presence of polyamines is largely governed by electrostatic interactions, rather than by the binding of the multivalent cation per se. (C) 1999 American Institute of Physics. [S0021-9606(99)50847-7].
Abstract: The persistence length DNA hexagonal-cholesteric phase transition upon dilution and/or increase in solvent ionic strength is investigated with polarized light microscopy. The ionic strength dependence of the transition follows Lindemann criterion C-L = 0.098 +/- 0.003, i.e., the hexagonal lattice melts when the root-mean-square fluctuations in transverse order exceed 10% of the interaxial spacing. The spacings are derived from density and the fluctuations are estimated with a theory of undulation enhanced electrostatic interactions. Additional support for this theory is given by the DNA equation of state and anisotropic neutron radiation scattering from magnetically aligned cholesteric samples just below the phase transition.
Abstract: The structure function of DNA fragments (contour length 54-104 nm) immersed in aqueous salt solutions is determined with small angle neutron scattering. From the limiting behavior at high values of momentum transfer an effective diameter or second moment of the DNA cross section of 1.6 nm is derived. For low values of momentum transfer and/or high ionic strength, the effects of supporting electrolyte, DNA;concentration, and DNA contour length can be rationalized with virial theory including screened electrostatics. With excess monovalent salt concentration exceeding, e.g., 1 M, neutral polymer behavior is recovered.
Abstract: The critical volume fractions pertaining to the formation of DNA liquid crystals were obtained from polarization microscopy, P-31-nmr, and phase separation experiments. The DNA length (approximately one to two times the persistence length 50 nm), ionic strength, and counterion variety dependencies are reported. The cholesteric-isotropic transition is interpreted in terms of the coexistence equations, which are derived from the solution free energy including orientational entropy and excluded volume effects. With the wormlike chain as reference system, the electrostatic contribution to the free energy is evaluated as a thermodynamic perturbation in the second virial approximation with a Debye-Huckel potential of mean force. The hard core contribution has been evaluated with scaled particle theory and/or a simple generalization of the Carnahan-Starling equation of state for hard spheres. For sufficiently high ionic strengths, the agreement is almost quantitative. At lower amounts of added salt deviations are observed, which are tentatively attributed to counterion screening effects. The contour length dependence agrees with a DNA persistence length 50 nm. (C) 1998 John Wiley & Sons, Inc.
Abstract: DNA and synthetic poly(styrenesulfonate) (PSS) solutions without excess simple salt were investigated with small-angle neutron scattering. For both polyelectrolytes, the transition from the rod to the coil regime was covered by an appropriate choice of molecular weights. The polymer, polymer-counterion, and counterion partial structure functions were obtained using contrast variation. For PSS, the single-chain scattering (form function) was observed from samples with zero-average polyion scattering length density contrast. The PSS polymer structure can be described by a locally rodlike configuration, but the projected monomer repeat distance 0.17 nm is smaller than the value expected for a fully stretched (trans) conformation. The PSS persistence length is of order 10 nm and does not agree with any theoretical analysis based on either the bending rigidity of a wormlike chain or modern variational results. The interpolymer structure was derived and compared with results based on the random-phase approximation. Poor agreement was observed, due to the high linear polyion charge density and, hence, strong electrostatic coupling. For highly charged linear polyelectrolytes, it was shown that from the full set of partial structure functions information on the radial counterion profile can be obtained without resorting to a model describing chain correlations. For PSS and DNA, the data agree with the counterion distribution obtained from the classical Poisson-Boltzmann theory and the cylindrical cell model, if the momentum transfer is far greater than the inverse persistence length.
Abstract: The partial structure functions of aqueous solutions of rod-like DNA fragments and polystyrenesulfonic acid are calculated in the modified Poisson-Boltzmann theory. The cylindrical cell model appropriate for linear polyelectrolyte solutions with monovalent counterions and without any added salt is utilized. The predicted results are compared with the corresponding results from the classical Poisson-Boltzmann theory, and experimental small angle neutron scattering data obtained in the monomer concentration range 0.05-0.2 mol/dm(3). It is seen that both the modified Poisson-Boltzmann and the Poisson-Boltzmann results lead to avery good fit to the experimental structure functions.
Abstract: Poly(styrenesulfonate) ion exchange resins were investigated with small angle neutron scattering. The polymer, polymer counterion, and counterion partial structure functions and charge structure function were obtained with contrast matching in water. In the reciprocal space interval q < 1 nm(-1), all partial structure functions display an upturn. This is attributed to different swelling ofregions with differences in cross-link concentration, resulting in similar long range static heterogeneity in polymer and small ion density. The charge structure function was found to satisfy the sum rules and demonstrates the strong suppression oflarge scale temporal fluctuations. At larger wave vectors, the structure functions reflect the distribution of counterions with respect to the polymer charges. An effective charge screening length is obtained, and its value agrees with a strong association of the ionic species.
Abstract: Cholesteric liquid crystals of persistence length DNA fragments (contour length 55 nm) were studied with small angle neutron scattering. The cholesteric axis was oriented either perpendicular or parallel to the incoming neutron beam with the help of a magnetic field in the range 0.8-1.65 T. A single diffraction peak is observed, which shows in the perpendicular configuration an angular anisotropy in intensity. A moderate magnetic field strength dependence of the anisotropy has been observed, but at the higher field strength the liquid crystal is nearly completely aligned. A decrease in peak intensity as well as anisotropy is observed with increasing ionic strength, but fixed DNA concentration. This indicates a decrease in position and orientation order with increased screening of electrostatic interactions. From a comparison of the anisotropic data to the scattering contribution of a single fragment, the standard deviation of the presumed Gaussian orientation distribution could be derived. The results compare favorably with the second virial theory of lyotropic liquid crystals, provided that electrostatic interactions in terms of an ionic strength dependent effective diameter and DNA flexibility are taken into account.
Abstract: The Na-23 relaxation in isotropic and anisotropic aqueous, salt-free DNA fragment (500 Angstrom) solutions is investigated. The temperature dependence of the coupling constants and correlation times are reported. Premelting effects occur in the isotropic phase, whereas this is not observed in the anisotropic solution. Slow dynamic processes in the frequency range 200-9000 Hz are absent in both phases.
Abstract: Small angle neutron scattering experiments have been performed on liquid crystalline 163 basepair NaDNA fragments in aqueous solution in the concentration range 190-285 mg ml-1. To induce a macroscopic alignment, a magnetic field was applied either parallel (B(parallel-to) configuration) or perpendicular (B(perpendicular-to) configuration) to the incoming neutron beam. The isotropic scattering pattern in the B(parallel-to) configuration and the anisotropic scattering in the B(perpendicular-to) configuration agree with the cholesteric structure of the liquid crystalline solutions. From the anisotropic scattering in the B(perpendicular-to) configuration, information on the orientation ordering can be derived. For this purpose, the experimental data are compared to the form function of a uniform rod including a gaussian orientation distribution. The standard deviation of this distribution is approximately 20-degrees for all concentrations investigated here.
Abstract: A neutron diffraction isotopic substitution experiment was carried out on the chloride ion in an aqueous solution of the polybase linear poly(ethylenimine) and DCl at high polymer charge density. The composite radial distribution function of all atomic species about Cl- (G(Cl)(r)) is compared with that of Cl- in a low molecular weight salt solution, and with that in a solution of the nonionic polymer poly(ethylene oxide). For the first time, it is observed that the presence of a linear polyion seriously affects G(Cl)(r) in an r-space region commonly referred to as the first hydration shell. This observation is suggestive of close contact between polyelectrolyte and counterion. The nuclear magnetic relaxation rate of Cl- is strongly enhanced in the presence of the polycation, and it is concluded that an increase in the coupling constant may at least be partly responsible for the observed NMR effect.
Abstract: The spectral density functions of the H-2-methylene nuclei in poly(ethylene oxide) (PEO) have been determined by magnetic relaxation measurements in the 0-90 MHz range at six applied field strengths. The relaxation rates were determined in a concentration interval from 0.1 to 40 monomolal. No molecular weight dependence was observed in the 3000 less-than-or-equal-to M(w) less-than-or-equal-to 50 000 range. In the dilute ana semidilute regimes, the spectral density function is interpreted in terms of anisotropic reorientation of a hydrodynamic unit. The corresponding dimensions of the dynamic unit are similar to those of a Kuhn segment. At a concentration where the dynamic unit can no longer reorient freely, the nature of the reorientational dynamics of the polymer segments is severely altered. This point is indicative of the start of the concentrated regime. Both the spectral density curve and the effective activation energies become strongly dependent on the polymer concentration. The H-1- and H-2-NMR relaxation rates show qualitatively different molecular weight and concentration dependencies at a polymer concentration greater than the crossover concentration c**. From the determination of H-1-NMR relaxation rates in mixtures of perdeuterated and hydrogenous polymers at c(p) > c**, it becomes clear that the different behavior of H-1 and H-2 is singularly due to an additional intermolecular dipolar coupling in the case of H-1. The latter interaction is modulated on relatively long time scales (tau(c) > 10(-8) s).
Abstract: The partial and charge structure functions of 163 base-pairs DNA fragment solutions are experimentally determined by small-angle neutron scattering at a relatively low concentration of 0.05 M nucleotides/L. In the momentum transfer range q > 0.05 Angstrom(-1), the data agree with theoretical model calculations according to the cell model together with a distribution for the counterions around the rodlike DNA fragment obtained from the Poisson-Boltzmann equation. For q > 0.075 Angstrom(-1), the scattered intensities agree with previous results on similar, but higher concentrated (i.e., 0.1 M nucleotides/L) solutions [van der Maarel; et al. J. Phys. II (France) 1992, 2, 109]. Unlike the previous data, the present results can be compared with the theoretical curves up to sufficiently low q values to check the theoretically expected low q upward curvature of the monomer-counterion and the counterion-counterion structure functions. A shell-like step radial counterion distribution as well as a model in which the condensed counterions are uniformly distributed within a cylindrical volume including the DNA molecule are checked and found to be in disagreement with the experimental data.
Abstract: The dynamics of spin I=1 nuclei in a radio-frequency (rf) field and with both a static and fluctuating quadrupolar interaction are discussed. Approximate expressions of the relaxation rates are derived under the assumption that the linewidths are much smaller than the characteristic line-splittings. The conventional spin-lock relaxation rate increases with increasing ratio of the spin-lock field strength and static quadrupolar coupling omega1/omega(Q) and is sensitive to the spectral density at frequency square-root omega(Q)2 + 4omega1(2). The rate shows a maximum in the presence of low frequency molecular motion. Some experiments are performed on a deuterium spin probe in a low molecular weight liquid crystal. The creation of multipolar states by continuous rf irradiation is demonstrated.
Abstract: The partial structure functions and the charge structure function are obtained for aqueous solutions of poly(styrenesulfonate) without excess low molecular weight salt. In the momentum-transfer range q > 0.1 angstrom-1, the structure functions are interpreted in terms of the correlation functions derived from the exact solution of the Poisson-Boltzmann equation in the cell model. The local polymer structure can well be approximated by a rod-like configuration with a rather high linear charge density. The derived charge density is, however, in agreement with the relatively low value of the osmotic coefficient for vinylic polyelectrolytes.
Abstract: The proton relaxation rates of normal as well as O-17-enriched water and the deuterium relaxation rate in H-2-enriched water were measured in 10 monomolal poly(ethyleneoxide) solutions. The O-17-H-1 intramolecular dipolar contribution to the proton relaxation rate was determined. From these experimental parameters the intramolecular OH-bond distance in the water molecule was calculated. It was found that this distance is increased by a small (1%) but significant amount compared to the pure water value. The results indicate that the solvent shows increased hydrogen bonding at the hydrophilic as well as the hydrophobic interface.
Abstract: The charge structure function of a rodlike polyelectrolyte within an electroneutral coaxial volume can be decomposed into two parts. A first contribution is given by fluctuations. The other is related to the existence of concentration profiles, formed by counterions about the polyion. Using this simple cell model, it is possible to identify these contributions separately in different wave vector ranges, as illustrated by recent small-angle neutron scattering. We discuss the implications of this decomposition, particularly with regard to the Poisson-Boltzmann approximation.
Abstract: Observations of the partial structure functions and the charge structure function are reported for an aqueous solution of monodisperse rodlike DNA fragments, without added simple salt. In the reciprocal space interval q greater-than-or-equal-to 0.075 angstrom-1, the neutron scattering data can be fitted by the correlation functions derived from the exact solution of the Poisson-Boltzmann equation in the cell model. The fit is equally good for all partial structure functions as well as the charge structure function. The cell model seems to be appropriate for this kind of solution.
Abstract: The relation between charge and dielectric structure functions is examined in the case of electrolytes and polyelectrolytes. When the coupling constant is larger than unity, characteristic features should be observed on the dielectric structure function, in addition to the classical screening effect. We point our the existence of experimental conditions which provide direct information of the dielectric structure at shorter wavelength (lambda less-than-or-equal-to 1 nm). They correspond to zero average contrast in scattering experiments.
Abstract: The time evolution of the spin S = 3/2 density operator due to relaxation in the presence of a sequence of (pi/2) pulses has been calculated using perturbation theory in an interaction representation in which no external time-dependent rf fields occur. It is shown that the relaxation under the effect of the pulse train is similar to T1-rho relaxation. If the magnetization is sampled between the pulses, outside extreme narrowing conditions the observed signal is characterized by a biexponential decay. The amplitude ratio of the fast and slowly relaxing component equals 0.8:0.2. The rate of the fast relaxing component is sensitive to the pulse cycle time in the presence of slowly fluctuating electric-field gradients. A full expression of the relevant spectral density function valid for (pi/2) pulses with arbitrary pulse width has been derived. In this derivation, the processes involved in determining the loss of correlation of the quadrupolar interaction are assumed to be independent and characterized by exponential correlation functions. Due to relaxation under the effect of the (pi/2) pulse train triple-quantum coherences may be excited. The latter coherences can be monitored by applying an additional coherence transfer pulse after the pulse train. The relaxation of sodium in an ion exchange resin in the presence of the (pi/2) pulse sequence has experimentally been studied and agrees with the theoretical analysis.
Abstract: The field dependence of the longitudinal and transverse nuclear magnetic relaxation rates of Na-23+ in aqueous crosslinked Na-poly(styrene sulfonate) (PSS) systems (ion exchange resins) has been obtained as a function of the degree of crosslinking. The relaxation is considerably enhanced relative to solutions of non-crosslinked NaPSS at equal ionizable group concentration. This is due to the dynamic constraints of the polymer chains, which render the averaging of the counter-ion chain interaction less efficient. The field dependence of the relaxation rates in the crosslinked NaPSS systems reveals two processes that are out of the extreme narrowing limit. This is in contrast to the relaxation behavior found in non-crosslinked NaPSS systems. To characterize these processes their correlation times were combined with constants of selfdiffusion to estimate the distances diffused by an ion in order to average the electric field gradient at its nucleus. These two distances are interpreted as characteristic length scales in the network. At all degrees of crosslinking it was found that the smallest of these length scales is roughly equal to the distance between two neighbouring crosslinks. The largest characteristic distance extends over several crosslinks and reflects inhomogeneities in the crosslink concentration. These conclusions were also reached from similar experiments on Li-7+ in LiPSS systems.
Abstract: The fractal distribution of pore sizes in a poly(4-vinylpyridine) resin cross-linked by diamagnetic (Cd2+) or paramagnetic (VO2+, Cu2+) divalent metallic ions has been characterized by three complementary techniques. Small-angle x-ray scattering has provided a direct determination of the surface fractal dimension D(f) approximately 2.4-2.6 for the pore system between 4 and 44.5 nm as well as the mass fractal of dimension 2.65-2.75 for the polymer network below 4 nm. Proton-pulsed-field-gradient experiments on adsorbed methanol have been performed at variable delays between the gradient pulses, covering the free and bounded diffusion regimes. They have evidenced the same fractal distribution of pore radii in the 1-50-mu-m range, providing an estimate of the solvent diffusion coefficient of the order of 5 X 10(-10) m2/s. The pores of radii between 3 nm and 1-mu-m have been sampled by the methanol H-2 and C-13 relaxations in resins cross-linked by paramagnetic ions. In this range, the paramagnetically enhanced relaxation is strongly dependent on the pore surface-to-volume ratio and exhibits a multiexponential behavior at short times. The analysis of the data confirms the D(f) value given by the two other methods and shows that the pores are surrounded by a solvent adsorption shell of thickness about ten molecular diameters. This is significant of the roughness of the interface between the bulk solvent and the polymer network suggested also by the reduction of the methanol diffusion coefficient, as compared with the neat liquid, as well as by the mass fractal dimension of this network. Thus, three quite different methods concur to show that this organic disordered material has a fractal distribution of pore sizes over 4 orders of magnitude. This result is of particular importance for studies on diffusion processes in cross-linked resins used in catalysis or chemical separation.
Abstract: We have investigated the diffusion of ions through a polyelectrolyte gel deemed appropriate to form composite liners and barriers to prevent leachate now from waste disposal sites. We developed a model for the diffusion of ions in the gel taking into account electrostatic obstruction. The non-linear macroscopic diffusion equation was solved using finite element methods, Self-diffusion coefficients were determined by nuclear magnetic resonance (NMR). Macroscopic diffusion data were obtained by NMR imaging. The data and the models suggest consistently that the gel could be rather efficient in retarding the flux of anions and cations.
