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The mechanism of conversion of globular native proteins into amyloid fibrils represents one of the most attractive research topics in biophysics, because of its involvement in the development of severe pathologies and in various biotechnological processes. Aqueous medium properties, such as pH and ionic strength, as well as interactions with other species in solution, play a key role in tuning the fibrillization process. Here, we describe a comparative study of the influence of different ions from the Hofmeister series on the thermal unfolding and aggregation propensity of MNEI, a model protein, selected because of its tendency to form amyloid aggregates at acidic pH, even at temperatures well below its melting temperature. By selecting a temperature at which only negligible amounts of protein are unfolded, we have focused on the effect of ions on fibril formation. ThT fluorescence experiments indicated that all the salts examined increased the rate and the extent of fibrillization. Moreover, we found that anions, particularly sulfate, strongly influence the process, which instead is only marginally affected by different cations. Finally, a specific link to the chloride concentration was detected.

The four diffusion coefficients of the ternary system human serum albumin (HSA,1)–sodium cholate (NaC,2)–water (W,0) were determined at three average compositions with constant protein concentration, C 1 o , and variable bile salt concentration, C 2. Various types of solute–solute interactions are present in this ternary system: electrostatic, co-ions–protein, and unspecific volume interactions due to the large difference in size of the two solutes. Each type of interaction has a different effect in determining the values and the sign of the experimental diffusion coefficients, exp D ij . To simplify the analysis of the dependence of the exp D ij s on C 2, we considered three model systems where different types of solute–solute interaction are present individually. The overall analysis allows highlighting which kinds of solute–solute interaction predominates in determining the value of each diffusion coefficient.

In commercial formulations, surfactants are often co-formulated with inorganic electrolytes, which are included as inexpensive thickeners. Salts affect the surfactant’s aggregative and functional behavior. However, while the electrolyte effect on the self-aggregation of ionic surfactants can be rationalized in terms of electrostatic interactions, in the case of nonionic surfactants the molecular determinants are still unclear. In this work, we investigate the effects of alkali and alkaline–earth metal chlorides on the micellization of the nonionic surfactant hexyl penta(oxyethylene) ether, C6E5, in aqueous solution. To this aim, the C6E5 intradiffusion (also named self-diffusion) coefficient in aqueous mixtures of various alkali and alkaline–earth metal chlorides was measured by pulsed gradient spin-echo NMR. The results show that all the considered electrolytes cause a decrease of the surfactant critical micellar concentration, cmc, while the micellar size is almost unaffected. The experimental evidence can be interpreted in terms of de-hydration of the apolar alkyl tails with a minor contribution arising from the dehydration of the poly(ethylene oxide) headgroups. The order of effectiveness of the different cations follows the Hofmeister series, some aspects of which are briefly discussed.

The infinite dilution partial molar volumes $\bar{V}^{\infty}$ of poly(acrylic acid), at different degrees of neutralization α N with sodium hydroxide, have been experimentally determined in aqueous solutions. Analysis of the data indicates that, to a good approximation, the observed decrease of $\bar{V}^{\infty}$ with α N can be interpreted in terms of the electrostriction of the water molecules involved in the hydration cospheres of the –RCOO− groups. The presence of PVA does not significantly affect the value of $\bar{V}^{\infty}$ for the undissociated acid, but causes a large decrease for the sodium salt. This aspect has been discussed in terms of direct interaction between the two polymers.

The molar conductivities of NaCl in several poly(ethylene glycol)(PEG)–water “mixed solvents” are presented as a function of the PEG content in solution. Three different PEG samples, a monodispersed one and two polydispersed ones, have been used. The comparison between the molar conductivity values in the mixed solvents and the corresponding ones in pure water shows that the only effect of PEG on the ions’ motions is an obstruction effect despite the variation of the macroscopic dielectric constant induced by PEG itself. A comparison between the molar conductivities and the corresponding mutual main diffusion coefficients shows a tight correlation between the two quantities. This correlation is possible only in the absence of electrostatic effects due to the presence of PEG.

Herein we report density measurements for the binary system 2-methyl-2,4-pentanediol(MPD,1)-water(0), and for the ternary system MPD(1)-NaCl(2)-water(0), at several fixed MPD/water ratios and at varying NaCl concentrations. The partial molar volume of MPD in the binary system has been discussed according to the McMillan-Meyer theory. At low MPD concentrations, the non-bonding MPD-MPD interactions are related to the overlapping of the hydrophobic parts of the hydration cospheres, whereas at higher concentrations the hydrophilic moieties are also involved in the solute-solute interactions. The ternary density data allowed us to determine the salt partial molar volume at infinite dilution. The trend of this quantity as a function of the MPD content in the MPD-water “mixed solvent” has been interpreted on the basis of the water partial molar volume in the binary MPD-water system.

Measurements of osmotic coefficients, mutual diffusion coefficients, and conductivity were performed on the binary system sodium n-octyl sulfonate (C8SO3Na)-water at 25degreesC both below and above the micellar composition range. The osmotic coefficient data were obtained through vapor-pressure osmometry, while the Taylor dispersion method was used to measure diffusion coefficients. The mass equilibrium model was applied to this self-aggregating system, taking into account the deviation of the activity coefficients from the Debye-Huckel limiting law by using the Guggenheim corrective terms for mixed electrolyte solutions. The expressions derived from the model fit the experimental osmotic and diffusion coefficient data well, when the same values of aggregation number, fraction of condensed counterions, and equilibrium constant are used. Osmotic coefficients were also used to determine the thermodynamic factor required to compute the solute mobility from diffusion data. Conductivity data were used to test two theoretical models, namely, the Onsager-Fuoss and the Mean Spherical Approximation theories. Both models have been found to yield unsatisfactory fits to our experimental data and some arbitrary terms had to be applied to the theoretical expressions to obtain good agreement between experiment and theory.