Thermodynamic Binding and Site Occupancy in the Light of the Schellman Exchange Concept

An analysis of Schellman's treatment of preferential interactions is presented, as viewed by a laboratory practitioner of the art. Starting with an intuitive description of what binding is in terms of the distribution of molecules of water and of a weakly interacting ligand (co-solvent), Schellman proceeded to a rigorous thermodynamic definition in which he showed that classical, dialysis equilibrium, binding is a purely thermodynamic quantity. Putting water and the co-solvent on an equivalent footing, he showed that the classical binding treatment is inadequate for weakly interacting systems, in which the replacement of water by ligand and exclusion of co-solvent are symmetrical concepts. Analyzing specifically the simple model of a single independent site, Schellman demonstrated how a positive binding constant can give rise to a measured negative binding stoichiometry. He showed that the origin of the complicated binding isotherms is the non-idealities of water and co-solvent, and went further to analyze critically the effect of site heterogeneity on the ligand concentration dependencies of site occupancy, preferential binding and the thermodynamic quantities, enthalpy, entropy and Gibbs free energy. This exposition of the Schellman treatment is accompanied by illustrations drawn from the experimental results obtained in this author's laboratory.