Binding rates, O--S substitution effects, and the pH dependence of chymotrypsin reactions.

The pH dependence for acylation of alpha-chymotrypsin by N-acetyltryptophan p-nitrophenyl-, p-nitrothiophenyl-, ethyl-, and thiolethyl esters has been studied by the stopped-flow technique. Values for the acylation rate constant, k2, and the binding constant, KS, were obtained by using measurements of phenolate release, for the p-nitrophenyl esters, and proflavin displacement, for the ethyl esters. The oxygen esters tested have slightly higher k2 values, and substantially higher KS values relative to the analogous thiol esters. Whereas k2/KS for the thiolethyl ester is higher than that for the analogous oxygen ester, the k2/KS values for oxy- and thio-p-nitrophenyl esters are nearly identical. These data are interpreted to indicate rate-determining formation of a tetrahedral intermediate in acylation of alpha-chymotrypsin by p-nitrophenyl esters, and rate-determining breakdown of such an intermediate in the case of the ethyl esters. It is also concluded that the oxygen to sulfur substitution causes a substantial increase in the proportion of nonproductive binding in these substrates. pH dependent k2 and KS values were used to calculate values for k1 and k-1, the binding and debinding rate constants for the two p-nitrophenyl compounds. This is the first such calculation based on experimentally determined acylation rate constants.     

Quantitative structure-activity relationships for the pre-steady state of Pseudomonas species lipase inhibitions by p-nirophenyl-N-substituted carbamates

The pre-steady states of Pseudomonas species lipase inhibitions by p-nitrophenyl-N-substituted carbamates (1-6) are composed of two steps: (1) formation of the non-covalent enzyme-inhibitor complex (E:I) from the inhibitor and the enzyme and (2) formation of the tetrahedral enzyme-inhibitor adduct (E-I) from the E:I complex. From a stopped-flow apparatus, the dissociation constant for the E:I complex, KS, and the rate constant for formation of the tetrahedral E-I adduct from the E:I complex, k2 are obtained from the non-linear least-squares of curve fittings of first-order rate constant (k(obs)) versus inhibition concentration ([I]) plot against k(obs)=k2+k2[I]/(KS+[I]). Values of pKS, and log k2 are linearly correlated with the sigma* values with the rho* values of -2.0 and 0.36, respectively. Therefore, the E:I complexes are more positive charges than the inhibitors due to the rho* value of -2.0. The tetrahedral E-I adducts on the other hand are more negative charges than the E:I complexes due to the rho* value of 0.36. Formation of the E:I complex from the inhibitor and the enzyme are further divided into two steps: (1) the pre-equilibrium protonation of the inhibitor and (2) formation of the E:I complex from the protonated inhibitor and the enzyme.     

Quantitative structure-activity relationships for the pre-steady state acetylcholinesterase inhibition by carbamates

4-Nitrophenyl-N-substituted carbamates (1) are characterized as pseudosubstrate inhibitors of acetylcholinesterase. The first step is formation of the enzyme-inhibitor tetrahedral intermediate with the inhibition constant (Ki), the second step is formation of the carbamyl enzyme with the carbamylation constant (kc), and the third step is hydrolysis of the carbamyl enzyme with decarbamylation constant (kd). According to pre-steady state kinetics the Ki step is divided further into two steps: (1) formation of the enzyme-inhibitor complex with the dissociation constant (KS) and (2) formation of the enzyme-inhibitor tetrahedral intermediate from the complex with the equilibrium constant (k2/k-2). Since the inhibitors are protonated in pH 7.0 buffer solution, the virtual dissociation constant (KS') of the enzyme-protonated inhibitor complex can be calculated from the equation, -log KS'=-log KS-pKa + 14. The -logKS, -log KS', log k2, and log k-2 values are multiply linearly correlated with the Jave equation (log(k/k0)=rho*sigma* + deltaEs + psi pi). For -log KS'-sigma*-Es)pi-correlation, the rho* value of -0.4 indicates that the enzyme-protonated inhibitor complexes have more positive charges than the protonated inhibitors, the delta value of 0.44 suggests that the bulkily substituted inhibitors lessen the reaction due to the difficulty of the inhibitors to enter the narrow enzyme active site gorge, and the psi value of 0.27 implies that the inhibitors with hydrophobic substituents accelerate the inhibitors entering the active site gorge of the enzyme. For log k2/k-2,-sigma*-Es-pi-correlation, the rho* value of 1.1 indicates that the enzyme-protonated inhibitor tetrahedral intermediates have more negative charges than the enzyme-protonated inhibitor complexes, the delta value of 0.15 suggests that the bulkily substituted inhibitors are difficult to bind into a small acyl binding site of the enzyme, and the psi value of -0.3 implies that the inhibitors with hydrophobic substituents resist binding to the hydrophilic acyl binding site of the enzyme.     

The osmotic coefficients of the sodium form of some biopolymers

The osmotic coefficients phi p,Na of dilute solutions of the sodium form of some weakly acidic polymers are theoretically predicted in this work. Based on the measured value 0.73 of gamma Na, the activity coefficient of free Na+, of the completely ionized humic acid (sodium salt) in a salt-free solution, the effective interligand distance b is calculated to be 11.34 A by using Manning's counterion condensation theory [Manning, G. S. (1969) J. Chem. Phys. 51(3), 924]. The corresponding values of gamma Na (measured experimentally) and b for the completely ionized exopolymer of Pseudomonas atlantica are 0.624 and 7.57 A when cultivated at a dilution rate D = 0.015 h-1, 0.647 and 8.19 A at D = 0.025 h-1, and 0.613 and 7.29 Aat D = 0.06 h-1. For alginic acid (in the completely ionized sodium form), gamma Na = 0.40 and b = 4.71 A. The osmotic coefficients phi p,Na for the partially and the completely ionized polymers are then predicted with Manning's theory as well.     

Application of conductivity studies and polyelectrolyte theory to the conformation and order-disorder transition of xanthan polysaccharide

The conductivity of xanthan (extracellular polysaccharide from Xanthomonas campestris) in the potassium salt form has been studied over the temperature range 5–80°C spanning the order-disorder conformational transition. In salt-free solution data analysis using Manning's polyelectrolyte-conductivity theory gives a charge spacing, b, of 0.58±0.04 nm for the low temperature ordered form, consistent with a single rather than a double helix (b=0.58 and 0.29 nm respectively). In solutions with 0.01 M added KBr the increase in counterion condensation on conformational ordering is found from conductivity studies to be — ^–1= 0.20 ± 0.02, in good agreement with the value 0.20±0.02 using polyelectrolyte-equilibrium theory for the variation of transition-midpoint temperature with added salt determined from opticalrotation data.     

Application of polyelectrolyte theories for analysis of DNA melting in the presence of Na+ and Mg2+ ions.

Numerical calculations, using Poisson-Boltzmann (PB) and counterion condensation (CC) polyelectrolyte theories, of the electrostatic free energy difference, DeltaGel, between single-stranded (coil) and double-helical DNA have been performed for solutions of NaDNA + NaCl with and without added MgCl2. Calculations have been made for conditions relevant to systems where experimental values of helix coil transition temperature (Tm) and other thermodynamic quantities have been measured. Comparison with experimental data has been possible by invoking values of Tm for solutions containing NaCl salt only. Resulting theoretical values of enthalpy, entropy, and heat capacity (for NaCl salt-containing solutions) and of Tm as a function of NaCl concentration in NaCl + MgCl2 solutions have thus been obtained. Qualitative and, to a large extent, quantitative reproduction of the experimental Tm, DeltaHm, DeltaSm, and DeltaCp values have been found from the results of polyelectrolyte theories. However, the quantitative resemblance of experimental data is considerably better for PB theory as compared to the CC model. Furthermore, some rather implausible qualitative conclusions are obtained within the CC results for DNA melting in NaCl + MgCl2 solutions. Our results argue in favor of the Poisson-Boltzmann theory, as compared to the counterion condensation theory.     

The LexA repressor and its isolated amino-terminal domain interact cooperatively with poly[d(A-T)], a contiguous pseudo-operator, but not with random DNA: a circular dichroism study

The interaction of the entire LexA repressor and its amino-terminal DNA binding domain with poly[d(A-T)] and random DNA has been studied by circular dichroism. Binding of both protein species induces an about 2-fold increase of the positive circular dichroism band at about 270 nm of both polynucleotides, allowing a precise determination of the principal parameters as a function of mono- and divalent salt concentration and pH. Both proteins interact much more strongly (about 2000-fold) with poly[d(A-T)] than with random DNA as expected from the homology with the specific consensus binding site of LexA (CTGTATATATATACAG). For both LexA and its DNA binding domain we find that the interaction with poly[d(A-T)] is cooperative with a cooperativity factor omega of about 50-70 for both proteins over a wide range of solvent conditions, suggesting that the carboxy-terminal domain of LexA is not involved in this type of cooperativity. On the contrary, no cooperativity could be detected for the interaction of the LexA DNA binding domain with a random DNA fragment. The overall binding constant K omega (or simply K in the case of random DNA) depends strongly on the salt concentration as observed for most protein-DNA interactions, but the behavior of LexA is unusual in that the steepness of this salt dependence (delta log K omega/delta log [NaCl]) is much more pronounced at slightly acidic pH values as compared to that at neutral or slightly alkaline pH. The behavior is not easily understood in terms of the current theories on the electrostatic contribution to protein-DNA interactions on the basis of polyelectrolyte theory. A comparison of the overall binding constant K omega of the entire LexA repressor and its DNA binding domain reveals that LexA binds only 20-50-fold stronger under a wide variety of salt and pH conditions. This result tends to demonstrate further that the additional energy due to the dimerization of LexA via the carboxy-terminal domain should be rather weak as expected from the small dimerization constant of LexA (2 X 10(-4) M-1).     

Synthesis, structure elucidation, determination of the lipophilicity and identification of antitumour activities in vitro of novel 3-(2-furanyl)-8-aryl-7,8-dihydroimidazo[2,1-c][1,2,4]triazin-4(6H)-ones with a low cytotoxicity towards normal human skin fibroblast cells

Eleven novel 3-(2-furanyl)-8-aryl-7,8-dihydroimidazo[2,1-c][1,2,4]triazin-4(6H)-ones (12-22) were designed and obtained from appropriate 1-aryl-2-hydrazonoimidazolidines (1-11) by condensation reaction with 2-oxo-2-furanacetic acid and subsequent cyclocondensation of intermediate chain derivatives. IR, (1)H NMR and (13)C NMR spectra and elemental analyses confirmed the chemical structure of all the synthesized compounds. The reversed-phase HPLC method was optimized and proved to be applicable and reliable for the analysis of these unknown small molecules (12-22). These compounds were chromatographed on octadecyl silica (ODS) stationary phase and their hydrophobic parameters expressed as the log k(w) values were determined by RP-HPLC, using mixtures of methanol and water as mobile phases with different methanol concentrations. Octane-1-sulfonic acid sodium salt (OSA-Na) and 20% acetate buffer (pH 3.5) was added to the mobile phase (eluent containing 0.01 M/L OSA-Na in organic modifier (MeOH)-buffered mobile phase). The high values of regression coefficients (r >0.9841) for all the compounds investigated proved the excellent fit between experimental data and the Snyder-Soczewiński equation. Results obtained from the reversed-phase HPLC were compared both with those theoretically calculated and with those obtained from an ALOGPS 2.1. software by the use of nine different computational methods for estimation of log P. The predicted values of log P by use of AB log P algorithm revealed the best correlation with the experimental log k(w) values for the investigated solutes, since a good correlation (r=0.7760) between these quantities was found. The majority of novel imidazotriazinones were found to be evidently effective in vitro against human cancerous cells (HeLa and T47D) in an effective concentration of 50 μg/mL. Five compounds (13, 15, 16, 18 and 22) revealed remarkable antiproliferative activities and selective cytotoxicities for cancer cells over normal HSF cells. Therefore these ones may be considered as a basis for the design of novel useful non-toxic (13, 15 and 16) and low toxic (18 and 22) anticancer agents.     

Dependence on ionic strength of charge-induced coil-helix transition of sodium poly(L-glutamate) in aqueous solution

Helix contents of sodium poly(L-glutamate) in aqueous NaCl solutions were estimated as functions of the degree of ionizalion, alpha. and the salt concentration by CD measurement. The helix content increases with increasing salt concentration but this helix stabilization decreases with decreasing alpha and at alpha<0.25 the helix conformation is destabilized by salt addition. The alpha dependence of the helix stabilization was qualitatively interpreted by Manning's theory in which electrostatic interactions between charges on alternatively arrayed coil and helix segments in a simplified polymer model were incorporated.     

Polyelectrolyte effects on site-binding equilibria with application to the intercalation of drugs into DNA

A theory of polyelectrolyte effects on site-binding equilibria is generalized to multivalent ligands, multivalent supporting salt, intercalation, and multiple-site exclusion. The theory, which contains no adjustable parameters, except the number of sites excluded by a bound ligand, gives the dependence of the equilibrium constant on the binding fraction and the salt concentration. The theory is compared with prior experimental data for the dissociation of poly(acrylic acid), the binding of magnesium to polyphosphate, and the binding of ethidium and actionomycin D to DNA. The theory predicts the binding fraction dependence of the dissociation constant of poly(acrylic acid) well. The theory predicts the binding fraction dependence of the association constant of the binding of Mg²⁺ to polyphosphate well, if either one or two phosphates are bound by a magnesium ion. We conclude that polyelectrolyte effects on drug-DNA equilibria must be substantial. It follows that an incorrect estimate of the number of sites excluded by a bound drug molecule (because of its size or some other nonpolyelectrolyte effect) can be obtained from binding data if polyelectrolyte effects are ignored. The estimate is also within the context of, and subject to the validity of, the model used to describe the nonpolyelectrolyte contribution to binding. Our results suggest that, subject to these conditions, the anticooperativity of the binding of ethidium to DNA might be explained solely in terms of polyelectrolyte effects, and without reference to multiple-site exclusion, if sequence-specificity effects can be safely ignored. Our results also suggest that as few as two base pairs might be excluded by an actinomycin molecule. The theory gives fairly good agreement for the salt-concentration dependence of the association constant of all of the systems studied, including the complex of the neutral drug actinomycin with DNA.     

On the application of polyelectrolyte “limiting laws” to the helix-coil transition of DNA. I. Excess univalent cations

A general theory of polyelectrolyte solutions is here used to calculate the differences in Gibbs free energy, enthalpy, and entropy between the coil and helix forms of DNA at any temperature and salt concentration. The salt has univalent cations and is assumed present in excess over the base concentration. The results are restricted to sufficiently dilute solutions. It is shown that the salt concentrations effect is entirely entropic in origin. When applied to the melting temperature, the calculations yield a relation between the enthalpy difference at the melting temperature and the slope of the plot of melting temperature vs. the logarithm of the salt concentration. In accord with observation, both the Gibbs free energy difference at any fixed temperature and the melting temperature are predicted to be linear functions of the log of the salt concentration. However, the theory is not in quantitative agreement with enthalpy data. Data on various colligative and transport properties of both helix and coil forms are reviewed in the text and in Appendix B, and good agreement is found with theory for both forms. No attempt is made to explain why the theory is quantitative for these properties but not for heat measurements. Finally, in Appendix A, an approximate calculation is made of the free energy contributions due to ionic effects not associated with the salt concentration.     

The charge effects of the self-diffusion constant of bovine mercaptalbumin

The charge effect on the translational self-diffusion constant, D, of polyelectrolytes has been quantitatively analyzed based on dynamic light scattering experiments. Perfectly monodisperse bovine mercaptalbumin has been used at low pH as a positively charged polyelectrolyte sample. Completely linear plots of log{g2(t)-1} vs. time t have been obtained for uncharged states of the protein, for the cor relation function of the scattered light intensity, g2(t). The plots deviate from linearity as polyions bear the charges. The D values for various ionic states, obtained from the initial slopes of the plots, have been analyzed using the simple theory of Imai and Mandel (N. Imai and M. Mandel, Macromolecules 15 (1982) 1562) derived based on the Onsager-Navier-Stokes equation for solvent flow with counterion distribution around a polyion. It has turned out that the experimental D values coincide well with the theory and that the characteristic nature of D can be elucidated principally from the charge effect.     

Salt dependence of DNA structural stabilities in solution. Theoretical predictions versus experiments

The predictions of currently available theories for treating DNA-diffuse ionic cloud free energy contributions to conformational stability have been tested against experimental data for salt induced B-Z and B-A transitions. The theories considered are (i) Manning's counterion condensation approach (CC), (ii) the idealized Poisson-Boltzmann approximation (PB), and (iii) the potentials of mean force (PMF) approach proposed by Soumpasis. As far as we can judge from comparison with the set of experimental data currently available, it is found that only the latter theory yields satisfactory quantitative results for the dependence of the B-Z and B-A relative stabilities on monovalent salt concentration. The correct application of the PB and CC theories does not yield very low salt Z-B transitions, in contradiction to earlier assertions. At low salt concentrations the PB theory is qualitatively correct in predicting that the B form is electrostatically more favorable than both the A and Z forms, whereas the CC theory is qualitatively wrong predicting that Z-DNA is more stable than both B and A DNA.     

Fluorescence energy transfer in one dimension: Frequency-domain fluorescence study of DNA–fluorophore complexes

The theory for the salt dependence of the free energy, entropy, and enthalpy of a polyelectrolyte in the PB (PB) model is extended to treat the nonspecific salt dependence of polyelectrolyte–ligand binding reactions. The salt dependence of the binding constant (K) is given by the difference in osmotic pressure terms between the react ants and the products. For simple 1-1 salts it is shown that this treatment is equivalent to the general preferential interaction model for the salt dependence of binding [C. Anderson and M. Record (1993) Journal of Physical Chemistry, Vol. 97, pp. 7116–7126]. The salt dependence, entropy, and enthalpy are compared for the PB model and one specific form of the preferential interaction coefficient model that uses counterion condensation/limiting law (LL) behavior. The PB and LL models are applied to three ligand–polyelectrolyte systems with the same net ligand charge: a model sphere–cylinder binding reaction, a drug–DNA binding reaction, and a protein–DNA binding reaction. For the small ligands both the PB and limiting law models give (ln K vs. In [salt]) slopes close in magnitude to the net ligand charge. However, the enthalpy/entropy breakdown of the salt dependence is quite different. In the PB model there are considerable contributions from electrostatic enthalpy and dielectric (water reorientation) entropy, compared to the predominant ion cratic (release) entropy in the limiting law model. The relative contributions of these three terms in the PB model depends on the ligand: for the protein, ion release entropy is the smallest contribution to the salt dependence of binding. The effect of three approximations made in the LL model is examined: These approximations are (1) the ligand behaves ideally, (2) the preferential interaction coefficient of the polyelectrolyte is unchanged upon ligand binding, and (3) the polyelectrolyte preferential interaction coefficient is given by the limiting law/counterion-condensation value. Analysis of the PB model shows that assumptions 2 and 3 break down at finite salt concentrations. For the small ligands the effects on the slope cancel, however, giving net slopes that are similar in the PB and LL models, but with a different entropy/enthalpy breakdown. For the protein ligand the errors from assumptions 2 and 3 in the LL model do not cancel. In addition, the ligand no longer behaves ideally due to its complex structure and charge distribution. Thus for the protein the slope is no longer related simply to the net ligand charge, and the PB model gives a much larger slope than the LL model. Additionally, in the PB model most of the salt dependence of the protein binding comes from the change in ligand activity, i.e. from nonspecific anion effects, in contrast to the small ligand case. While the absolute binding is sensitive to polyelectrolyte length, little length effect is seen on the salt dependence for the small ligands at 0.1M salt, and for lengths > 60 Å. Almost no DNA length dependenceis seen in the salt dependence of the protein binding, since this is determined primarily by the protein, not the DNA. © 1995 John Wiley & Sons, Inc.     

Evaluation of the predictive power of calculation procedure for molecular hydrophobicity of some estradiol derivates

Several calculation procedures for log P values based on the fragmental and atomic contributions are compared with experimental reversed-phase liquid chromatography (RPLC) retention of estradiol derivates. The RPLC experiments were performed on HPTLC and HPLC commercially available stationary phases. Binary solvent mixtures of methanol-water and acetonitrile-water were used as mobile phases. The correlation between log P and various chromatographically obtained hydrophobicity parameters (R(M)0, log k(w) and phi0) are quantified. The R(M)0, i.e., log k(w) were obtained by linear extrapolation of retention to 0% organic modifier. Phi0 values were obtained from the slopes and intercepts of such linear relationship. The mutual relationship between phi(0,MeOH) and phi(0,ACN) values of the compounds were discussed. The obtained statistical results can be summarized in the following order of reliabilities for different log P calculation methods: Broto>ACD/logP>Crippen>Rekker>Viswanadhan.     

Role of a conserved salt bridge between the PAS core and the N-terminal domain in the activation of the photoreceptor photoactive yellow protein

The effect of ionic strength on the conformational equilibrium between the I(2) intermediate and the signaling state I(2)' of the photoreceptor PYP and on the rate of recovery to the dark state were investigated by time-resolved absorption and fluorescence spectroscopy. With increasing salt concentration up to approximately 600 mM, the recovery rate k(3) decreases and the I(2)/I(2)' equilibrium (K) shifts in the direction of I(2)'. At higher ionic strength both effects reverse. Experiments with mono-(KCl, NaBr) and divalent (MgCl(2), MgSO(4)) salts show that the low salt effect depends on the ionic strength and not on the cation or anion species. These observations can be described over the entire ionic strength range by considering the activity coefficients of an interdomain salt bridge. At low ionic strength the activity coefficient decreases due to counterion screening whereas at high ionic strength binding of water by the salt leads to an increase in the activity coefficient. From the initial slopes of the plots of log k(3) and log K versus the square root of the ionic strength, the product of the charges of the interacting groups was found to be -1.3 +/- 0.2, suggesting a monovalent ion pair. The conserved salt bridge K110/E12 connecting the beta-sheet of the PAS core and the N-terminal domain is a prime candidate for this ion pair. To test this hypothesis, the mutants K110A and E12A were prepared. In K110A the salt dependence of the I(2)/I(2)' equilibrium was eliminated and of the recovery rate was greatly reduced below approximately 600 mM. Moreover, at low salt the recovery rate was six times slower than in wild-type. In E12A significant salt dependence remained, which is attributed to the formation of a novel salt bridge between K110 and E9. At high salt reversal occurs in both mutants suggesting that salting out stabilizes the more compact I(2) structure. However, chaotropic anions like SCN shift the I(2)/I(2)' equilibrium toward the partially unfolded I(2)' form. The salt linkage K110/E12 stabilizes the photoreceptor in the inactive state in the dark and is broken in the light-induced formation of the signaling state, allowing the N-terminal domain to detach from the beta-scaffold PAS core.     

Aligning LC peaks by converting gradient retention times to retention index of peptides in proteomic experiments

MOTIVATION: Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a powerful tool in proteomics studies, but when peptide retention information is used for identification purposes, it remains challenging to compare multiple LC-MS/MS runs or to match observed and predicted retention times, because small changes of LC conditions unavoidably lead to variability in retention times. In addition, non-contiguous retention data obtained with different LC-MS instruments or in different laboratories must be aligned to confirm and utilize rapidly accumulating published proteomics data. RESULTS: We have developed a new alignment method for peptide retention times based on linear solvent strength (LSS) theory. We found that log k(0) (logarithm of retention factor for a given organic solvent) in the LSS theory can be utilized as a 'universal' retention index of peptides (RIP) that is independent of LC gradients, and depends solely on the constituents of the mobile phase and the stationary phases. We introduced a machine learning-based scheme to optimize the conversion function of gradient retention times (t(g)) to log k(0). Using the optimized function, t(g) values obtained with different LC-MS systems can be directly compared with each other on the RIP scale. In an examination of Arabidopsis proteomic data, the vast majority of retention time variability was removed, and five datasets obtained with various LC-MS systems were successfully aligned on the RIP scale.     

Excessive counterion condensation on immobilized ssDNA in solutions of high ionic strength

We present experiments on the bias-induced release of immobilized, single-stranded (ss) 24-mer oligonucleotides from Au-surfaces into electrolyte solutions of varying ionic strength. Desorption is evidenced by fluorescence measurements of dye-labeled ssDNA. Electrostatic interactions between adsorbed ssDNA and the Au-surface are investigated with respect to 1), a variation of the bias potential applied to the Au-electrode; and 2), the screening effect of the electrolyte solution. For the latter, the concentration of monovalent salt in solution is varied from 3 to 1600 mM. We find that the strength of electric interaction is predominantly determined by the effective charge of the ssDNA itself and that the release of DNA mainly occurs before the electrochemical double layer has been established at the electrolyte/Au interface. In agreement with Manning's condensation theory, the measured desorption efficiency (etarel) stays constant over a wide range of salt concentrations; however, as the Debye length is reduced below a value comparable to the axial charge spacing of the DNA, etarel decreases substantially. We assign this effect to excessive counterion condensation on the DNA in solutions of high ionic strength. In addition, the relative translational diffusion coefficient of ssDNA in solution is evaluated for different salt concentrations.     

Polyelectrolyte microstructure in chitosan aqueous and alcohol solutions

This work deals with chain ordering in aqueous and water-alcohol solutions of chitosan. The so-called polyelectrolyte peak is investigated by small-angle synchrotron X-ray scattering. The polyelectrolyte microstructure was characterized by the position of the maximum of the polyelectrolyte scattering peak qmax, which scales with the polymer concentration cp as qmax approximately cp alpha. An evolution of the power law exponent alpha is observed as a function of the degree of acetylation (DA) of chitosan, which is responsible for changes of both the charge density (f) and the hydrophobicity of the polymer chains. The results highlighted the two organization regimes of the theory of Dobrynin and Rubinstein, investigated here for the first time for a natural polymer. At low DAs, alpha approximately 1/2, in agreement with a pearl necklace organization where the structure is controlled by the string between pearls. For higher DA, alpha approximately 1/3, and the correlation revealed by the polyelectrolyte peak is controlled by the pearls. This analysis offers a way to study quantitatively the balance between solvophobic-solvophilic interactions that play an important role in the solution properties of natural polymers. In addition, the role of several parameters acting on the interaction balance were evidenced, such as the nature of the counterion, the composition of the solvent (amount of alcohol in the aqueous solution), and the screening of Coulombic forces by salt addition. Finally, the nanostructure transition from a polyelectrolyte solution to a physical gel is discussed. The gel state is reached when the solvophobic interactions are favored, but depending on the gelation route the polyelectrolyte ordering could be preserved or not.