Specific solvent effects on the thermal denaturation of ribonuclease. Effect of dimethyl sulfoxide and p-dioxane on thermodynamics of denaturation

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Abstract
Differential scanning calorimetry was used to study the thermal denaturation of [bovine] RNase in aqueous dimethyl sulfoxide and aqueous p-dioxane. A 2-state reversible denaturation occurs in aqueous dimethyl sulfoxide. The denaturation is irreversible in aqueous p-dioxane. The transition temperature decreases in both organic solvents, with p-dioxane producing a considerably greater effect. The enthalpy of the denaturation initially increases with increasing dimethyl sulfoxide concentration and then decreases at high concentrations. Similar behavior is observed in the entropy in the dimethyl sulfoxide solutions. The enthalpy of the denaturation decreases markedly with low p-dioxane concentrations. Changes in pH due to the presence of organic solvent cannot account for the changes in enthalpy of denaturation. Addition of organic solvent tends to increase the pH of the RNase solution. Higher pH in aqueous solutions gives a higher transition temperature, whereas addition of organic solvent results in the opposite behavior. The relative changes in the 2 solvent systems suggest that specific solvent effects occur and destabilization of the native state relative to the denatured state is greater with p-dioxane than with dimethyl sulfoxide.