Stabilization of protein structure by sugars.

The preferential interaction of proteins with solvent components was measured in aqueous lactose and glucose systems by using a high precision densimeter. In all cases, the protein was preferentially hydrated; i.e., addition of these sugars to an aqueous solution of the protein resulted in an unfavorable free-energy change. This effect was shown to increase with an increase in protein surface area, explaining the protein stabilizing action of these sugars and their enhancing effect of protein associations. Correlation of the preferential interaction parameter with the effect of the sugars on the surface tension of water, i.e., their positive surface tension increment, has led to the conclusion that the surface free energy perturbation by sugars plays a predominant role in their preferential interaction with proteins. Other contributing factors are the exclusion volume of the sugars and the chemical nature of the protein surface.