Thermodynamics of Polyol-Induced Thermal Stabilization of Chymotrypsinogen1

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Abstract
In order to clarify the mechanism of polyol-induced stabilization of protein, the thermodynamic parameters (▵Go, ▵H°, and ▵S°) of thermal denaturation of chymotrypsinogen have been measured in aqueous solutions of some polyols (ethylene glycol, erythritol, adonitol, sorbitol, mannitol, and inositol) by a differential spectro-photometric method. On increasing the alcohol concentration and the number of hydroxymethyl groups of the alcohols, ▵G° increased as a result of a large decrease in ▵H° compensating for a decrease in ▵H°. 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 ▵H° and ▵S° 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.