Synopsis
The preferential binding of solvent components with a nonionic homopolypeptide, poly(N5-(3-hydroxypropyl)-~-glutamine), ([Gln((CH2)30H)],), has been determined in water/dioxane mixtures using differential refractometry. The degree of preferential binding was calculated from the difference between the refractive index increments of [Gln((CH2)30H)], obtained from experiments carried out under two conditions: experiments where the molality of dioxane was kept identical in both compartments of the differential cell, and experiments where the chemical potential was kept identical. The polypeptide was preferentially hydrated between 10 and 70 wt % of dioxane; the amount of preferential hydration per gram of the mixed solvent increases monotonically (with a plateau region between 40 and 60 wt %) with the dioxane concentration. A monotonic increase was also observed in the degree of helicity of the polypeptide.
The absolute amounts of water and dioxane bound by [Gln((CH2)30H)], were investigated in the frozen state by the method of nuclear magnetic resonance. Hydration was measured using a mixed solvent, waterldioxane-ds; dioxane solvation was measured using a mixed solvent, dioxane/D20. The polypeptide binds about 0.35 g of water per g of the polymer in aqueous solution, and hydration decreases gradually with an increase in dioxane concentration. On the other hand, the amount of dioxane solvation increases to 0.04 g per g of the polymer in the dioxane concentration range between 0 and 20 w t %, and then levels off. The rapid increase in solvation is observed before the conformational transition from random coil to a-helix occurs in [Gln((CH2)30H)],.
The dependence of the preferential and absolute binding of solvent components to [Gln((CH2)30H)], on dioxane concentration and the conformational change in the homopolypeptide suggest that addition of dioxane to aqueous solutions induces lowering of water activity and that the helical structure of the polypeptide is enhanced by the formation of intrachain hydrogen bonds. The validity of the frozen method is also discussed.