Enthalpy and entropy of transfer of amino acids and diglycine from water to aqueous polyol solutions.

Abstract

The enthalpies of transfer of several amino acids and diglycine from water to aqueous polyols (glycerol, xylitol, sorbitol, and inositol) have been calorimetrically determined in order to clarify the mechanism of polyol-induced stabilization of proteins. The obtained enthalpy data were combined with the data on free energy of transfer previously determined to calculate the corresponding entropy of transfer. The enthalpies of transfer of nonpolar side chains of amino acids and peptide group were positive and negative, respectively, depending on the polyol concentration and hydrophobicity of the side chains. This indicates that the non-spontaneous transfer of nonpolar side chains to aqueous polyols is due to an enthalpy effect while that of peptide group is due to an entropy effect. These thermodynamic data are discussed in terms of changes in water structure or solvent ordering around the solute molecules. It was found that the polyol-induced stabilization of proteins is a result of a complicated enthalpy-entropy compensation phenomenon closely related to the solvent ordering around the solute molecules; consequently, the polyol effects on nonpolar side chains do not necessarily dominate those on peptide groups in an enthalpic sense.