Calorimetric studies of hydrophobic interactions of alkanols in concentrated aqueous solutions of glucose - Implications for the mechanism of protein stabilization by sugars

Calorimetric measurements were carried out at 298 K on concentrated aqueous solutions of glucose, ranging from 0 to 5 mol kg(-1), containing alkan-1-ols, alkane- 1,2-diols and alkane-alpha,omega-diols from C-2 to C-7. The purpose of this study is to obtain more information about the influence of glucose on hydrophobic hydration and interactions. The pair-wise interaction coefficients of the virial expansion of the excess enthalpies were evaluated, and the results rationalized according to the preferential configuration model. At increasing glucose concentration, alkanols and 1,2-diols maintain almost unaltered the coefficients they have in water, while alpha,omega-diols show a smooth decrease only at 3 mol kg(-1) glucose. Urea and glycine have been studied in concentrated glucose, too. Their coefficients indicate that glucose is able to reduce hydrophilic interactions. Then, the invariance of the coefficients for alkanols means that both hydrophilic and hydrophobic interactions are attenuated in the presence of high concentrations of the cosolvent. The data show that the nature of the hydration cosphere of the alkyl chain does not change, so that the same interaction mechanism operates in water and in concentrated glucose solutions. The results obtained are compared with those found for diols in concentrated solutions of urea and ethanol: some comments are made on the possible mode of action of these substances on the stability of proteins in solution. (C) 2002 Elsevier Science B.V. All rights reserved.