Infinite dilution activity coefficients for benzene and toluene in water and in aqueous solutions of the protein denaturants urea and guanidine hydrochloride

Experimental infinite dilution activity coefficients gamma (infinity)(1) fo r benzene and toluene in water and in aqueous solutions of two protein dena turants, urea and guanidine hydrochloride, are reported. The quantities wer e measured in the temperature range (273 to 323) K. Four denaturant concent rations were used from (0.5 to 5.0) mol.dm(-3), covering the range where pr oteins undergo chemical denaturation. For both solutes in water, from the f reezing temperature of water, gamma (infinity)(1) rises with increasing tem perature, passes through a maximum close to room temperature, and then cont inuously decreases with increasing temperature. Based on the results report ed here and selected literature values, recommended correlations of gamma ( infinity)(1)(T) for benzene and toluene in water were established. They cle arly show that from the freezing temperature to the normal boiling temperat ure of water, the origin of the hydrophobicity of the non-polar solutes cha nges from being entropic to enthalpic in nature. The presence of a denatura nt (modifier) produces a significant decrease of gamma (infinity)(1) values , their maximum being displaced towards lower temperatures. The effect is m ore pronounced for guanidine hydrochloride, which acts as a better solubili zing agent than urea. The solute in (water + modifier) gamma (infinity)(1) data were analysed by a classical thermodynamic scheme employing solution a nd transfer {water to (water + modifier)} quantities. The accuracy of the d ata allowed the derivation of enthalpies, entropies, and heat capacities an d a detailed discussion of the several observed trends. A remarkable featur e is that there is only a narrow temperature interval where both the enthal pic and entropic contributions to the Gibbs energy are favourable to solute transfer. At lower and higher temperatures, the enthalpic and entropic com ponents compete, solute transfer being favoured by entropy or by enthalpy, respectively. Extrapolation of this behaviour suggests that the effect of t he modifier on the solubility of hydrophobic solutes should eventually inve rt at both lower and higher temperatures. The data presented here might be used to understand better, through the application of different models, the exposure of non-polar amino acid side chains from the protein interior to the aqueous environment, which characterizes protein denaturation. (C) 2000 Academic Press.