Abstract
The stabilization of proteins by a variety of coβsolvents can be related to their property of increasing the surface tension of water. It is demonstrated that, during the thermal unfolding of proteins, this increase of the surface tension can be overcome by the increase in the temperature of the solution at the midpoint of the transition, T~m~, and the weak binding of coβsolvent molecules. Three such coβsolvents were studied: trehalose, lysine hydrochloride (LysHCl), and arginine hydrochloride (ArgHCl). Trehalose and LysHCl increase the midpoint of T~m~. The increase of the surface tension by addition of trehalose is completely compensated by its decrease due to the increase in T~m~. However, for LysHCl, the increase of the surface tension by the coβsolvent is partly reduced by its binding to the protein. For trehalose, preferential interaction measurements with RNaseA demonstrate that it is totally excluded from the protein. In contrast, LysHCl gives evidence of binding to RNaseA. ArgHCl also increases the surface tension of water. Nevertheless, T~m~ of RNaseA decreases on addition of ArgHCl to the solution. Preferential interaction measurements showed very small values of preferential hydration of the native protein, indicating extensive binding of ArgHCl to the protein. During unfolding, the amount of additional ArgHCl binding is sufficiently large to counteract the surface tension effects, and the protein is destabilized. Therefore, although surface tension appears to be a critical factor in the stabilization of proteins, its increase by coβsolvent does not ensure increased stabilization. The binding of ligands can reduce significantly, or even overwhelm, its effects.