The effects of neutral salts on the stability of macromolecules. A new approach using a protein-ligand binding system.

A new approach to study the thermodynamics of salt-protein interaction is described using a protein-ligand binding system. The effects of salts on the thermodynamics of association or dissociation of the ligand are related to the effect on the hydrophobic side chains on the interior of the protein by these salts. The calculated free energies of transfer of a methylene group from an interior of the protein to salt solutions are +0.1, +0.06, -0.02, and -0.06 kcal/mol for Na2SO4, NaCl, NaSCN, and Cl3CCOONa at 1 M concentration, respectively. The relationships between the thermodynamic parameters and the partial molar entropies of the solutions are analyzed.