Sodium ion and solvent nuclear relaxation results in aqueous solutions of DNA

Unprecedented hour-long residence time of a cation in a left-handed G-quadruplex

Cations are critical for the folding and assembly of nucleic acids. In G-quadruplex structures, cations can bind between stacked G-tetrads and coordinate with negatively charged guanine carbonyl oxygens. They usually exchange between binding sites and with the bulk in solution with time constants ranging from sub-millisecond to seconds. Here we report the first observation of extremely long-lived K+ and NH4 + ions, with an exchange time constant on the order of an hour, when coordinated at the center of a left-handed G-quadruplex DNA. A single-base mutation, that switched one half of the structure from left- to right-handed conformation resulting in a right-left hybrid G-quadruplex, was shown to remove this long-lived behaviour of the central cation.

23Na NMR relaxation studies of the Na-DNA/drug interaction

This Minireview covers the methodological approaches of (23)Na NMR spectroscopy to the study of the structure and dynamics of the DNA molecule, in particular the application of the (23)Na NMR quadrupolar relaxation to investigate the perturbations on the polyion surface due to exogenous agents. A brief description of the (23)Na NMR quadrupolar relaxation and of the models used to describe the distribution of counterions around DNA, and the results of the application of the (23)Na NMR relaxation to the study of the cation-DNA interaction are also shown. Following sections present results of the investigation on ligand-DNA interaction and on ordered DNA systems.

Counter-ion dynamics in crosslinked poly(styrene sulfonate) systems studied by NMR

The field dependence of the longitudinal and transverse nuclear magnetic relaxation rates of 23Na+ 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 7Li+ in LiPSS systems.

The influence of water content on the 23Na and 31P NMR spectral parameters in solid Na-DNA

23Na and 31P MAS NMR spectra and spin-lattice relaxation times in a solid sample of Na-DNA were measured under very carefully controlled conditions of relative ambient humidity. The observed substantial changes of the NMR parameters are related to the water-induced transitions between the different molecular configurations of DNA, with the transition between A- and B-DNA occurring in the 30-60% range of relative humidity. Our work demonstrates that the previously measured NMR parameters are in error because the relative humidity of the system had not been controlled, rendering the results irreproducible.

A 23Na NMR study of the effect of D(+) and L(-) arabitol on NaDNA in aqueous solution

The 23Na NMR quadrupolar relaxation in NaDNA aqueous solutions has been investigated in the presence of D(+) and L(-) arabitol. Quite different results were produced by the enantiomers, i.e. the addition of D(+) arabitol produced a small increase of the 23Na NMR relaxation rates, while in the presence of L(-) arabitol a significant decrease was observed. These findings were analysed and discussed in terms of an effective interaction of L(-) arabitol with DNA.

Counterion condensation as saturation effect under the influence of ion hydration

Polyelectrolyte solutions are often described by structural theories. These theories in some cases yield values for the counterion concentration at the charged monomer surface that exceed the saturation concentration. This means a change of the ion properties due to ion immobilization or ion condensation in close vicinity to the polymer chain. The extent of this counterion condensation (CIC) and the respective surface potential are calculated from the saturation concentrations of the electrolyte involved including the influence of ion hydration on the effective dielectric number. In this paper, we shall consider all these influences by a fundamental differential equation and a set of explicit formulae yielding quantitative expressions without linearization. All calculations are based on the abstraction of an idealized elementary cell.

Study by (23)Na-NMR, (1)H-NMR, and ultraviolet spectroscopy of the thermal stability of an 11-basepair oligonucleotide

23Na-NMR, (1)H-NMR, and ultraviolet (UV) spectroscopy have been used to study the thermal stability of the double helix structure of an 11-basepair oligonucleotide. The denaturation curves obtained by (23)Na-NMR and UV are analyzed using a two-state model. The melting temperature and DeltaH(0) obtained are identical within experimental error, suggesting that modifications in the ionic atmosphere, probed by (23)Na-NMR, and the modifications in the basepair stacking, probed by UV, occur at the same temperature. Additional dynamical information on the denaturation process has been obtained by (1)H-NMR: slow exchange is observed between the thymine methyl resonances, and the disappearance of imino protons shows that a single basepair opening does not contribute significantly to proton exchange.

Sequence-specific binding of counterions to B-DNA

Recent studies by x-ray crystallography, NMR, and molecular simulations have suggested that monovalent counterions can penetrate deeply into the minor groove of B form DNA. Such groove-bound ions potentially could play an important role in AT-tract bending and groove narrowing, thereby modulating DNA function in vivo. To address this issue, we report here (23)Na magnetic relaxation dispersion measurements on oligonucleotides, including difference experiments with the groove-binding drug netropsin. The exquisite sensitivity of this method to ions in long-lived and intimate association with DNA allows us to detect sequence-specific sodium ion binding in the minor groove AT tract of three B-DNA dodecamers. The sodium ion occupancy is only a few percent, however, and therefore is not likely to contribute importantly to the ensemble of B-DNA structures. We also report results of ion competition experiments, indicating that potassium, rubidium, and cesium ions bind to the minor groove with similarly weak affinity as sodium ions, whereas ammonium ion binding is somewhat stronger. The present findings are discussed in the light of previous NMR and diffraction studies of sequence-specific counterion binding to DNA.

Sodium and chlorine ions as part of the DNA solvation shell

The distribution of sodium and chlorine ions around DNA is presented from two molecular dynamics simulations of the DNA fragment d(C(5)T(5)). (A(5)G(5)) in explicit solvent with 0.8 M additional NaCl salt. One simulation was carried out for 10 ns with the CHARMM force field that keeps the DNA structure close to A-DNA, the other for 12 ns with the AMBER force field that preferentially stabilizes B-DNA conformations (, Biophys. J. 75:134-149). From radial distributions of sodium and chlorine ions a primary ion shell is defined. The ion counts and residence times of ions within this shell are compared between conformations and with experiment. Ordered sodium ion sites were found in minor and major grooves around both A and B-DNA conformations. Changes in the surrounding hydration structure are analyzed and implications for the stabilization of A-DNA and B-DNA conformations are discussed.

23Na NMR studies of Na-DNA in the solid state

23Na MAS, CP/MAS and quadrupole nutation NMR spectra and the 23Na spin-lattice relaxation times in Na-DNA with and without competing species (Mg2+, ethidium bromide and [Ru(phen)3]2+) reveal the presence of two sodium sites with different NMR parameters. While in the presence of Mg2+, sodium resides far from the surface of the DNA molecule, ethidium bromide and [Ru(phen)3]2+ displace sodium closer towards the surface.

The interaction of DNA with intercalating agents probed by sodium-23 NMR relaxation rates

We have investigated the changes of Na-23 NMR spin-lattice and spin-spin relaxation rates of Na-DNA in dilute aqueous solutions, induced by the intercalating drugs ethidium, propidium, 6,7-dihydro-pyrido[2',1':3,4]pyrazino[1,2-f]-phenanthridinediium dihydrate (dq2pyp) and by the electrostatic binder Mg2+. It has been found that the Na-23 spin-lattice relaxation is monoexponential, while the spin-spin relaxation follows a biexponential law. From the linear trends of the relaxation rates observed in the titration experiments of Na-DNA with the drugs we inferred the validity of a two-site model in the fast exchange limit. The relaxation rates of sodium in the bound state have been estimated by using the fraction of bound sodium ions per phosphate charge, predicted by the counterion condensation theory, and the number of sodium ions released per bound drug, calculated from salt dependent equilibrium binding studies. The results show that the addition of the competitors slow down the broad component of the spin-spin relaxation rate of bound sodium in the order dq2pyp approximately propidium < ethidium < Mg2+. This reduction is shown to be due mainly to the decrease of the quadrupolar coupling constant for the slow motions, thus indicating a decrease of the average electric field gradient at the sodium ions close to the DNA surface. We also show that the broad component of the spin-spin relaxation rate linearly correlates with the relative non-polyelectrolyte free energy. This result is discussed in terms of the non-polyelectrolyte interactions affecting R2bB, which can be partitioned into stacking interaction and interactions involving the molecular moieties of the intercalators exposed to the solvent in the minor groove.

23Na NMR study of the effect of organic osmolytes on DNA counterion atmosphere

The effect of different organic osmolytes on the DNA counterion condensation layer has been investigated by 23Na NMR relaxation measurements. The zwitterionic compounds glycine, beta-alanine, 4-aminobutyric acid, and 6-aminocaproic acid have shown an increasing capacity to decrease the amount of sodium ions in the vicinity of the macromolecule. The experimental data have been correlated with the dielectric constant increase in their corresponding solutions and have been compared with the prediction of counterion condensation theory. Polyols (sorbitol and mannitol) did not display the same effect. These compounds largely increase the relaxation rate of sodium ions in the proximity of DNA, unlike the zwitterionic compounds. This probably results from a perturbation of the water dynamic around the macromolecule, of the primary or secondary hydration shell of the sodium nuclei involved, or both.

Importance of coulombic end effects on cation accumulation near oligoelectrolyte B-DNA: a demonstration using 23Na NMR

The local cation concentration at the surface of oligomeric or polymeric B-DNA is expected, on the basis of MC simulations (Olmsted, M. C., C. F. Anderson, and M. T. Record, Jr. 1989. Proc. Natl. Acad. Sci. USA. 86:7766-7770), to decrease sharply as either end of the molecule is approached. In this paper we report 23Na NMR measurements indicating the importance of this "coulombic" end effect on the average extent of association of Na+ with oligomeric duplex DNA. In solutions containing either 20-bp synthetic DNA or 160-bp mononucleosomal calf thymus DNA at phosphate monomer concentrations [P] of 4-10 mM, measurements were made over the range of ratios 1 < or = [Na]/[LP] < or = 20, corresponding to Na+ concentrations of 4-200 nM. The longitudinal 23Na NMR relaxation rates measured in these NaDNA solutions, Robs, are interpreted as population-weighted averages of contributions from "bound" (RB) and "free" (RF) 23Na relaxation rates. The observed enhancements of Robs indicate that RB significantly exceeds RF, which is approximately equal to the 23Na relaxation rate in an aqueous solution containing only NaCl. Under salt-fre-tconditions ([Na]/[P] = 1), where the enhancement in Robs is maximal, we find that Robs--RF in the solution containing 160-bp DNA is approximately 1.8 times that observed for the 20-bp DNA. For the 160-bp oligomer (which theoretical calculations predict to be effectively polyion-like), we find that a plot of Robs v. [P]/[Na] is linear, as observed previously for sonicated (approximately 700 bp) DNA samples. For the 20-bp oligonucleotide this plot exhibits a marked departure from linearity that can be fitted to a quadratic function of [P]/[Na]. Monte Carlo simulations based on a simplified model are capable of reproducing the qualitative trends in the 23Na NMR measurements analyzed here. In particular, the dependences of Robs--RF on DNA charge magnitude of Z(320 vs. 38 phosphates) and (for the 20-bp oligomer) on [Na]/[P] are well correlated with the calculated average surface concentration of Na+. Thus, effects of sodium concentration on RB appear to be of secondary importance. We conclude that 23Na NMR relaxation measurements are a sensitive probe of the effects of oligomer charge on the extent of ion accumulation near B-DNA oligonucleotides, as a function of [Na] and [P].

23Na nuclear magnetic resonance study of intracellular sodium in vascular endothelial cells

23Na(+)-NMR was used to determine the intracellular mobility and cell membrane permeability of Na+ in porcine vascular endothelial cells. The cells were grown as monolayers onto microcarrier beads and perfused with a medium containing Dy(P3O10)2(7-) to shift the extracellular from the intracellular Na+ resonance. Using triple quantum coherence filtered NMR experiments and spin echoes, it was shown that not all intracellular Na+ ions are in the extreme narrowing limit. The triple quantum coherence filtered experiments resulted in an observed R2f = 2022 +/- 302 s-1 and R2s = 200 +/- 28 s-1. From spin-echo experiments we obtained R2f = 2200 +/- 355 s-1 and a R2s = 145 +/- 15 s-1. These values are similar to those found in other cell systems and indicate water-Na(+)-protein interactions. Using single quantum NMR, the Na+ permeability of the endothelial membrane was determined. To obtain the Na+ transcellular permeability coefficient the cells were treated with 50 microM ouabain in the perfusion medium. Ouabain inhibits the Na-K pump and caused the intracellular Na+ concentration to increase in time. The permeability coefficient was obtained from the time dependence of the intracellular Na+ concentration. Assuming a monolayer of rectangularly shaped cells, we obtained a value of P = 0.02.10-5 cm s-1.

Thermal denaturation of Na- and Li-DNA in salt-free solutions

Thermal denaturation of Na- and Li-DNA from chicken erythrocytes was studied by means of scanning microcalorimetry in salt-free solutions at DNA concentrations (Cp) from 4.5 x 10(-2) to 1 x 10(-3) moles of nucleotides/liter (M). Linear dependencies of DNA melting temperature (Tm) vs lgCp were obtained: [formula: see text] for Na- and Li-DNA, respectively. Microcalorimetry data were compared with the results of spectrophotometric studies at 260 nm of DNA thermal denaturation in Me-DNA + MeCl solutions at Cp congruent to (6-8) x 10(-5) M and Cs = 0-40 mM (Me is Na or Li, Cs is salt concentration). It was found that Eqs. (1) and (2) are valid in DNA salt-free solutions over the Cp range 6 x 10(-5)-4.5 x 10(-2) M. Protonation of DNA bases due to the absorption of CO2 from air in Na-DNA + NaCl solutions affects DNA melting parameters at Cs < 4 mM. Linear dependence of Tm on lga+ is found in Na-DNA + NaCl at Cs > 0.4 mM in the absence of contact of solutions with CO2 from air (a+ is cation activity). A dependence of [dTm/dlga+] on Li+ activity was observed in Li-DNA + LiCl solutions at Cs < 10 mM: [dTm/dlga+] increases from 17 degrees-18 degrees at Cs > 10 mM to 28 degrees-30 degrees at Cs congruent to 0.2-0.4 mM. Spectrophotometric measurements at 282 nm show that this effect was caused by protonation of bases in fragments of denatured DNA in neutral solutions. The Poisson-Boltzmann (PB) equation was solved for salt-free DNA at the melting point. The linear dependence of Tm vs lgCp was interpreted in terms of Manning's condensation theory. PB and Manning's theories fit the experimental data if charge density parameter (xi) of denatured DNA is in the range 1.8-2.1 (assuming for native DNA xi = 4.2). Specificity of Li ions in interactions with DNA is discussed.

Magnesium-DNA interactions from interpretation of 25Mg-nmr relaxation rates: field and coion dependence

We have used 25Mg-nmr to investigate the binding of magnesium ions to double-stranded DNA. We have measured line shapes for 25Mg in the presence of monodisperse calf thymus DNA (160 base pairs; b.p.) (magnesium: phosphate = 2.0) at two different field strengths, 11.75 T and 7.05 T, and used the isotropic model of two-site exchange developed by Westlund and Wennerstrom to simultaneously fit the line shapes at both field strengths. This model does not reproduce the observed field dependence. This is in contrast to a previous study [E. Berggren, L. Nordenskiold, and W.H. Braunlin (1992), Biopolymers, Vol. 32, pp. 1339-1350] in which a similar model of isotropic two-site exchange qualitatively reproduced the temperature dependence of the line widths. Relaxation rates were also measured as a function of magnesium: phosphate ratio and coion type. These measurements were used to assess the sensitivity of magnesium relaxation measurements to small changes in DNA structure induced by changes in the solvent environment. The temperature dependence of the line shape varies with the type of coion (chloride or sulfate) present. This coion dependence of the line shape is consistent with the coion dependence of the aggregation midpoint temperature reported by Bloomfield and co-workers [O.A. Knoll, M.G. Fried, and V.A. Bloomfield (1988) in Structure and Expression, Vol. 2, R.H. Sarma and M. H. Sarma, Eds., Adenine Press, New York] and attributed to a lyotropic effect. These results suggest that even at low magnesium: phosphate ratios, relaxation parameters are specific to each magnesium-coion-DNA system.

Monte Carlo and Poisson-Boltzmann calculations of the fraction of counterions bound to DNA

The counterion density and the condensation region around DNA have been examined as functions of both ion size and added-salt concentration using Metropolis Monte Carlo (MC) and Poisson-Boltzmann (PB) methods. Two different definitions of the "bound" and "free" components of the electrolyte ion atmosphere were used to compare these approaches. First, calculation of the ion density in different spatial regions around the polyelectrolyte molecule indicates, in agreement with previous work, that the PB equation does not predict an invariance of the surface concentration of counterions as electrolyte is added to the system. Further, the PB equation underestimates the counterion concentration at the DNA surface, compared to the MC results, the difference being greatest in the grooves, where ionic concentrations are highest. If counterions within a fixed radius of the helical axis are considered to be bound, then the fraction of polyelectrolyte charge neutralized by counterions would be predicted to increase as the bulk electrolyte concentration increases. A second categorization--one in which monovalent cations in regions where the average electrostatic potential is less than -kT are considered to be bound--provides an informative basis for comparison of MC and PB with each other and with counterion-condensation theory. By this criterion, PB calculations on the B form of DNA indicate that the amount of bound counterion charge per phosphate group is about .67 and is independent of salt concentration. A particularly provocative observation is that when this binding criterion is used, MC calculations quantitatively reproduce the bound fraction predicted by counterion-condensation theory for all-atom models of B-DNA and A-DNA as well as for charged cylinders of varying linear charge densities. For example, for B-DNA and A-DNA, the fractions of phosphate groups neutralized by 2 A hard sphere counterions are 0.768 and .817, respectively. For theoretical studies, the radius enclosing the region in which the electrostatic potential is calculated to be less than -kT is advocated as a more suitable binding or condensation radius than that enclosing the fraction of counterions given by (1 - epsilon-1). A comparison of radii calculated using both of these definitions is presented.

A study of the quadrupolar NMR splittings of 7Li+, 23Na+, and 133Cs+ counterions in macroscopically oriented DNA fibers

The hydration and temperature dependencies of the 23Na+, 133Cs+, and 7Li+ quadrupolar splitting have been determined in hydrated, macroscopically oriented DNA fibers. At low water contents the quadrupolar splitting is found to decrease as the water content increases, regardless of counterion, while at high water contents the hydration dependence is reversed. The 23Na+ and 133Cs+ quadrupolar splittings decrease as the temperature increases, while the 7Li+ splitting shows the opposite behavior. At high water contents the 23Na+ and 133Cs+ splittings decrease, and then, after passing zero splitting, increase as the temperature increases. The interpretation of the temperature dependence is discussed in terms of a two-site model (free and bound ions) and a three-site model (free ions and specifically or nonspecifically bound ions). It is suggested that a three-site model is more consistent with the data for the present system. At high water contents, the temperature dependence of the 7Li+ splitting vanishes, indicating counterion condensation. The behavior of the 7Li+ splitting is confirmed by measurements on DNA fibers in equilibrium with a C2H5OD-D2O-LiCl solution. The salt dependence in this system is weak. The counterion quadrupolar splitting is seen to be very sensitive to structural transitions in double-helical DNA.

Interpretation of 25Mg spin relaxation in Mg-DNA solutions: temperature variation and chemical exchange effects

The temperature dependencies of line shapes and spin-lattice relaxation times T1 have been measured for 25Mg in dilute solutions of Na-DNA/NaCl containing varying amounts of added magnesium(II) ions. The 25Mg spectrum is clearly non-Lorentzian, due to the presence of motions modulating the quadrupolar interaction that are slow compared to the inverse of the Larmor frequency. The weakly temperature-dependent line shapes and relaxation rates appear to be influenced by the relatively slow exchange of the Mg2+ ions between the DNA surface and the aqueous bulk phase. The observed temperature dependencies depend on the ratio of total magnesium to DNA phosphate, Mg/P. The line shape as well as the temperature dependence of the line width at half height can be qualitatively reproduced with a two-site discrete exchange model for the quadrupolar relaxation of a spin 5/2 nucleus in isotropic solution. The calculations give a value of the lifetime for magnesium bound to DNA of 4 ms at room temperature. Previously reported temperature-dependent 43Ca relaxation measurements in DNA solution can be reproduced under the assumption of a mean lifetime of bound calcium that is not larger than 2 ms but not smaller than 50 microseconds at room temperature. The temperature variation of T1 for 25Mg has been calculated, giving some qualitative agreement with the data. The correlation time for bound 25Mg has been found to be about 40 ns at room temperature.

High-resolution capillary electrophoretic analysis of DNA in free solution

Capillary electrophoretic separations of double-stranded DNA fragments in the size range of 20-2200 base pairs were achieved in less than 20 min with the use of a Tris-borate buffer containing hydroxyethylcellulose. Analyses were carried out in both uncoated and phenylmethyl-coated capillaries of fused silica with internal diameters of 50 and 100 microns, respectively, and an effective column length of 50 cm. The addition of ethidium bromide resulted in an improved resolution of double-stranded DNA fragments, thereby permitting even the separation of fragments differing only 1-2 base pairs in length. Moreover, resolution was found to be linearly proportional to the size of the cation used to adjust ionic strength Cs+ greater than RB+ greater than K+ greater than Na+ greater than Li+. However, the analysis times also increased with increasing cation size due to a decrease in electroosmotic flow. Elution order was verified by spiking restriction digests with slab gel electrophoretically purified components. Subsequently, the described system was applied to the detection and quantitation of an mRNA transcript of the androgen receptor, which had been amplified by polymerase chain reaction and purified by size-exclusion chromatography to avoid peak broadening due to conductivity differences between sample and running buffer. Since the actual amount of DNA introduced into the capillary cannot be defined, molar ratio-peak area ratios of the polymerase chain reaction product to various restriction fragments of known concentration were used to determine the amount of amplified DNA. The coefficient of variation was as small as 3.4% and the results were in good agreement with a spectrophotometric assay.

Physico-chemical properties of aqueous solutions of xanthan: an n.m.r. study

The conformations of xanthan in aqueous solution as a function of temperature have been studied. Measurements of optical activity indicate that the conformational transition, induced by varying the polymer concentration, is analogous to that induced by changes in ionic strength and pH. Within a certain range of concentrations, the low-temperature conformation has a molecular-weight-dependent stability, which shows the usual sigmoidal melting profile with increase in temperature. The 13C-n.m.r. data reflect the increase of the mobility of C-1 and the side-chain carbon atoms in the transition-temperature region. The 23Na relaxation behaviour changes on melting the ordered xanthan conformation. At least two correlation times are needed in order to describe the field-strength dependence of the longitudinal and transverse 23Na relaxation. At 25 degrees, a value of 6.8 ns is obtained for the largest correlation time for the fluctuation of the electric-field gradient. The high-temperature conformation also generates correlation times of the order of ns. From 17O relaxation measurements, a reduction of the mobility of water molecules in the presence of xanthan chains is also observed.

Sodium-23 and lithium-7 NMR spin-lattice relaxation measurements in the study intercalation in DNA

Sodium-23 spin-lattice relaxation rate (the reciprocal relaxation time) measurements have been used to study the intercalation of 9-aminoacridine in calf thymus DNA. The results are analyzed by a two state model based on the counterion condensation theory and a theory for the quadrupolar relaxation of counterions in polyelectrolyte solutions. It is shown that change of the solvent from H2O to D2O has a negligible effect on the intercalation process. Furthermore, an attempt is made to analyze the dependence of the 7Li spin-lattice relation rate on intercalation of 9-aminoacridine in LiDNA. It is shown that both quadrupolar and dipolar mechanisms contribute to the bound 7Li relation rate, and that both these contributions are reduced upon intercalation of 9-aminoacridine.

Use of NMR and MRI to study water relations in foods

Water is the most important component of a food system, because it influences so many process variables, product characteristics, and stability attributes. Some of the most successful techniques used to probe the behavior of water in food systems are Nuclear Magnetic Resonance (NMR) spectroscopy and, more recently, pulsed-field gradient NMR and Magnetic Resonance Imaging (MRI). The purpose of this chapter is to review the theory underlying these techniques and to present several examples of how they have been applied to study water relations in foods.

87Rb+ spin relaxation in enzymically purified and in untreated iota-carrageenan

The temperature dependences of the transverse (R2) and longitudinal (R1) n.m.r. relaxation rates of 87Rb+ in aqueous 5% iota-carrageenan have been compared with similar data for a sample purified by treatment with kappa-carrageenase. In each sample, the relaxation rates were sensitive to the conformation (helix or random coil). In the intact sample, the small (less than or equal to 5%) fraction of kappa-carrageenan (which, in its helical state, specifically binds rubidium ions) was solely responsible for the pronounced line-broadening that has been observed hitherto for 87Rb in iota-carrageenan gels. In the purified sample, the effects on the relaxation of 87Rb induced by iota-carrageenan are similar to those found in comparable systems in the absence of site-binding of the ions. Thus, there was a modest enhancement of the relaxation with R1 approximately R2 for the flexible coil conformation and a comparably larger effect, with significant contributions from dynamic processes on the time-scale of the inverse resonance frequency or longer, for the thicker, more highly charged and rigid helix conformation of iota-carrageenan.

A 23Na-NMR study of sodium-DNA interactions in concentrated DNA solutions at low-supporting electrolyte concentration

Aqueous solutions of DNA fragments with a contour length (500 A) near the persistence length at DNA concentrations ranging from 10 to 290 mg/mL solvent and a constant supporting electrolyte concentration of 0.01 M (predominantly NaCl) were examined by 23Na-nmr spectroscopy at temperatures of 20, 40, and 60 degrees C. Over the higher portion of this concentration range (greater than 100 mg/ml) the DNA solutions undergo a complex series of transitions between different anisotropic, liquid crystalline phases (T. E. Strzelecka and R. L. Rill, Biopolymers, in press). Counterions in solutions of strong polyelectrolytes are usually described in terms of a two-state model as free or "bound" (influenced by the electrostatic field of the polyanion). The longitudinal relaxation rate (R1 = 1/T1) at all DNA concentrations decreased with increasing temperature, demonstrating fast exchange between free and bound counterions. R1 increased nearly linearly with increasing DNA phosphate/sodium ratio in the isotropic domain until the onset of anisotropic phase formation, in agreement with similar nmr studies conducted at low DNA concentrations. The value of R1,b = 194 +/- 7 Hz obtained for the isotropic phase from 10 to 100 mg DNA/mL at 20 degrees C was in agreement with values reported previously. A nonlinear increase in R1 with DNA concentration was observed upon onset of anisotropic phase formation, indicating an increase in the product of the fraction of bond ions times their relaxation rate (r.R1,b). The spectral lineshape of all isotropic samples was Lorentzian. Spectra of anisotropic samples exhibited low magnitude quadrupole splitting of less than or equal to 400 Hz correlated with appearance of a cholesteric phase with pitch approximately 2 microns. The magnitude of the quadrupole splitting decreased with increasing DNA concentration at low temperatures and increased with concentration at high temperatures. At all concentrations the quadrupole splitting decreased then increased with temperature. These temperature- and concentration-dependent changes in quadrupole splitting are consistent with an angle between the DNA helix axis and the principal component (VZZ) of the local electric field gradient tensor near the "magic angle" of 54.7 degrees.