Isothermal titration calorimetry and molecular dynamics study of ion-selectivity in mixtures of hydrophobic polyelectrolytes with sodium halides in water

Ion-specificity and surface water dynamics in protein solutions

Ion-specific effects at the protein surface are investigated here in light of the changes they infer to surface water dynamics, as observed by 1H NMR relaxation (at 20 MHz). Two well-known proteins, hen egg-white lysozyme (LZM) and bovine serum albumin (BSA), show qualitatively opposite trends in the transverse relaxation rate, R2(1H), along a series of different monovalent salt anions in the solution. Presence of salt ions increases R2(1H) in the case of lysozyme and diminishes it in the case of BSA. The effect magnifies for larger and more polarizable ions. The same contrasting effect between the two proteins is observed for protein-solvent proton exchange. This hints at subtle effects ion-binding might have on the accessibility of water surface sites on the protein. We suggest that the combination of the density of surface charge residues and surface roughness, at the atomic scale, dictates the response to the presence of salt ions and is proper to each protein. Further, a dramatic increase in R2(1H) is found to correlate closely with the formation of protein aggregates. The same ordering of salts in their ability to aggregate lysozyme, as seen previously by cloud point measurements, is reproduced here by R2(1H). 1H NMR relaxation data is supplemented by 35Cl and 14N NMR relaxation for selected salt ions to probe the ion-binding itself.

Calorimetric studies of interactions between low molecular weight salts and bovine serum albumin in water at pH values below and above the isoionic point

Isothermal titration calorimetry was used to determine the temperature and salt concentration dependence of the enthalpy of mixing, Δ_mix H, of bovine serum albumin (BSA) in aqueous buffer solutions with several low molecular weight salts. Three buffers were used: acetate (pH = 4.0), MOPS (7.2), and borate (9.2). Since the isoionic point of BSA is at pI ≈ 4.7, the net charge of BSA in acetate buffer was positive (≈ +20), while in the other two buffer solutions it was negative (≈ -15 in MOPS and ≈ -25 in borate). The majority of the recorded heat effects were exothermic, while only at pH = 9.2 a weak endothermic effect upon mixing BSA with LiCl, NaCl, and KCl was observed. For all buffer solutions the absolute values of Δ_mix H of sodium salts followed the order: NaCl < NaBr < NaNO₃ < NaI < NaSCN, which is the reverse Hofmeister series for anions. The magnitude of the effects was the largest in acetate buffer and decreased with an increasing pH value of the solution. While the effect of varying the anion of the added salts was strongly pronounced at all pH values, the effect of the cation (LiCl, NaCl, KCl, RbCl and CsCl salts) was weak. The most interesting feature of the results obtained for pH > pI was the fact that Δ_mix H were considerably more sensitive to the anion (co-ion to the net BSA charge) than to the cation species. This indicated that anions interacted quite strongly with the BSA even at pH values where the net charge of the protein was negative. We showed that Δ_mix H at high addition of salts correlated well with the enthalpy of hydration of the corresponding salt anion. This finding suggested, consistently with some previous studies, that a part of the exothermic contribution to Δ_mix H originated from the hydration changes upon the protein-salt interaction. Theoretical analysis, based on the primitive model of highly asymmetric electrolyte solutions solved within the mean spherical approximation, was used to estimate Coulomb effects upon mixing.

Salt-specific effects observed in calorimetric studies of alkali and tetraalkylammonium salt solutions of poly(thiophen-3-ylacetic acid)

Differences in hydration of counterions are blamed for strong salt-specific effects produced upon dilution and mixing of poly(thiophene-3-ylacetic acid) salts with simple salts.

Presence of hydrophobic groups may modify the specific ion effect in aqueous polyelectrolyte solutions

Enthalpies of dilution of aqueous solutions of aliphatic 6,12- and 12,12-ionene bromides and fluorides and enthalpies of mixing with low molecular-weight salts, such as sodium fluoride and bromide, are determined. In the second part of the study, the various x,y-ionenes (x, y are numbers of methylene groups between the adjacent charges) with fluoride, bromide, and iodide counterions are mixed with aqueous sodium sulfate solution. The polyelectrolytes examined in this part of the work are 3,3-, 6,9-, 6,12-, and 12,12-ionenes. A comparison with theoretical results, based on the Poisson-Boltzmann cell model, is presented. The theory predicts for the enthalpy of dilution to be exothermic and the enthalpy of mixing endothermic, while experiments show that signs of the heat effects depend on the nature of the counterion of the added salt, as also on the hydrophobicity (numbers x, y of methylene groups) of the ionene. We show that the salts when ordered by heat effects produced by mixing of NaF and NaBr with 3,3-, 6,9-, or 6,12-ionene fluorides and bromides follow the opposite ordering than in the case when the same alkali halide salts are mixed with more hydrophobic 12,12-ionene salts. The results for the enthalpy of mixing of ionenes under study with Na2SO4 follow the same order as obtained for monovalent salts.

Specific counter-ion and co-ion effects revealed in mixing of aqueous solutions of 3,3 and 6,6-ionenes with solutions of low molecular weight salts

Enthalpies of mixing of aliphatic 3,3 and 6,6-ionene fluorides with low molecular weight salts (sodium formate, acetate, nitrate, chlorate(v), and thiocyanate), all dissolved in water, were determined. In addition, to complement our previous study (Lukšičet al., Phys. Chem. Chem. Phys., 2012, 14, 2024), new measurements were performed where aqueous solutions of 3,3 and 6,6-ionene bromides were mixed with solutions of sodium fluoride, chloride, bromide, and iodide. Electrostatic theory, based on Manning's limiting law or the Poisson-Boltzmann equation, predicted the enthalpy of mixing to be endothermic in all the cases, while experiments showed that this is not always true. When an aqueous solution of 3,3-ionene fluoride was mixed with a solution of sodium fluoride (or formate and acetate) in water, the effect was indeed endothermic. For all other salts, i.e. sodium chlorate, nitrate, and thiocyanate, heat was released upon mixing. The situation was similar for 6,6-ionene fluoride solutions with an exception of mixing with sodium chlorate, where the effect was endothermic. The enthalpy of mixing was strongly correlated with the enthalpy of hydration of the counterion of the low molecular weight salt. A lyotropic series, similar to that of Hofmeister, was obtained. To examine also the effect of co-ions, ionene bromides were titrated with tetramethyl-, tetraethyl-, or tetrapropylammonium bromides. The enthalpy was exothermic for all mixtures while, somewhat unexpectedly, the co-ion specific effect was quite strong.