Statistical thermodynamics of protein stability in mixed aqueous solutions

Biomolecules (such as proteins, DNAs, membranes) are surrounded in most cases by water molecules. Water molecules play crucial role in their function. When ligands bind proteins, water molecules must be expelled from the binding sites, and this is often a bottleneck of binding reactions. When a protein folds, dehydration of hydrophobic (non-polar) side chains is one of the major driving forces. Therefore, counting the number of water molecules released is crucial to the understanding of the mechanism of these processes. Recently, I have developed a rigorous statistical thermodynamic theory on biomolecular solvation in mixed solvents, and have shown that these numbers can be obtained from experiments, by combining volume changes upon these processes with data on structural changes. Based upon these numbers, I will characterise the structure of water in the close vicinity of biomolecules. This will help understand how water molecules contribute to the biochemical reactions. In addition, the presence of cosolvents, such as salts, osmolytes and other biomolecules, affect the binding and folding of biomolecules. I have shown recently that the numbers of water and cosolvents involved in these processes can be counted by combining the data on volume change, cosolvent-induced shift of equilibrium, and structural information. This theory is applied to explain how proteins are protected from denaturing stresses by osmolytes, how protein-ligand binding is affected by metabolite and salt molecules, and how the presence of other biomolecules affect the stability of proteins.