THE RELATION BETWEEN THE FREE-ENERGY OF INTERACTION AND BINDING

For ordinary stoichiometric interactions the extent of binding and the free energy of interaction follow parallel and well understood courses as a function of ligand concentration. This simple picture changes when interactions are very weak and the solutions are concentrated and non-ideal. In these cases, which include most instances of protein denaturation or unfolding, binding must be understood as a competition between the ligand and the principal solvent. Under these circumstances unusual phenomena can arise such as a changes in sign of the thermodynamic binding as concentration is varied. The most striking instances occur when strongly interacting reagents apparently do not bind at all. This paper discusses the paradoxical fact that the free energy of interaction is strongest when thermodynamic binding (selective interaction) goes to zero. Examples are cited where these phenomena have been observed experimentally in agreement with the theory presented here.