Calorimetric study on thermal denaturation of lysozyme in polyol-water mixtures

In order to clarify the mechanism of polyol-induced stabilization of protein, the thermal denaturation of lysozyme was studied at pH 4 in aqueous mixtures of some polyols (ethylene glycol, glycerol, erythritol, xylitol, and sorbitol) by a differential scanning calorimetry (DSC). The denaturation temperature, Td, increased with increasing the polyol concentration and the number of hydroxymethyl groups per polyol molecule. The calorimetric enthalpy or denaturation, delta H cal, increased with the increase in polyol concentration, but it was not significantly affected by the chain length of the polyol: delta H cal was about 30 kcal/mol larger in 30% (w/w) aqueous polyols than in water. The standard thermodynamic parameters for denaturation, delta G degrees, delta S degrees, and delta H degrees, which were calculated for glycerol and sorbitol systems using Td and delta H cal and assuming a constant heat capacity change, were an increasing function of polyol concentration. According to the thermodynamics of three component systems, it appeared that one or two polyol molecules are preferentially excluded from the domain of this protein on thermal denaturation. These thermodynamic data support the hypothesis that the thermal stabilization of lysozyme by polyols is due to a preferential solvent interaction effect which strengthens the hydrophobic interaction of the protein.