Modulation of molecular interactions of homologous series of amino acids with (sorbitol + glycine betaine) mixture compared to individual osmolytes: Thermodynamic analysis

Abstract

Sorbitol and glycine betaine (GB) are the important osmolytes which are known to stabilize proteins. To understand the conformational stability of these complex biomolecules and their interactions with solvents in fine details, physico-chemical properties of the homologous series of amino acids were studied. The values of densities (ρ) and speeds of sound (u) of five amino acids: glycine, dl-alanine, dl-α-amino-n-butyric acid, l-valine, and l-leucine were measured in mixtures of {water + sorbitol (0.53 mol·kg−1) + glycine betaine (0.51 mol·kg−1)} and {water + sorbitol (1.12 mol·kg−1) + glycine betaine (1.04 mol·kg−1)} at T = 298.15 K. Heats of dilution (q) of amino acids from water to mixture {water + sorbitol (0.53 mol·kg−1) + glycine betaine (0.51 mol·kg−1)} solution were also measured. These values have been used to find the infinite dilution standard partial molar volume (V2.m0), standard partial molar adiabatic compressibility (Ks,2.m0), enthalpy of dilution (ΔdilH0), standard partial molar volume of transfer (ΔtrV2,m0) and hydration number of amino acids. By linear regression fitting of V2.m0,contributions of zwitterionic and hydrophobic groups of amino acids have been calculated. All these results have been correlated and interpreted in terms of intermolecular interactions on the basis of co-sphere overlap model. In case of all the amino acids except valine and leucine, the dominance of ionic–ionic and hydrophilic–ionic interaction is indicated. The values of (ΔtrΔdilH0) obtained by isothermal titration calorimetry support the volumetric interpretations. An attempt has also been made to find the calorimetric evidences to preferential hydration of lysozyme and preferential exclusion of osmolytes.