Thermodynamics of polyol-induced thermal stabilization of chymotrypsinogen

In order to clarify the mechanism of polyol-induced stabilization of protein, the thermodynamic parameters (delta G degree, delta H degree, and delta S degree) of thermal denaturation of chymotrypsinogen have been measured in aqeous solutions of some polyols (ethylene glycol, erythritol, adonitol, sorbitol, mannitol, and inositol) by a differential spectrophotometric method. On increasing the alcohol concentration and the number of hydroxymethyl groups of the alcohols, delta G degree increased as a result of a large decrease in delta S degree compensating for a decrease in delta H degree. This result means that the stabilization of this protein by polyols is due to the entropy effect, and that the free energy change of transfer of the denatured protein from water to aqueous media containing these alcohols must be larger than that of the native protein. This strongly supports the previous proposal that the driving force of protein stabilization induced by polyols is a solvent medium effect or a solvent ordering effect. The decreases in delta H degree and delta S degree with polyols are expected to be more due to the effects of polyols on peptide-water interactions than to exposed nonpolar groups of denatured protein.