Thermodynamics of ion binding by proteins. Phenomenological linkage relations for binding of electrolyte and interpretation by double layer theory.

In this paper we report a new set of thermodynamic linkage relations for the binding of electrolyte by proteins. The relations are derived for protein solutions in membrane equilibrium with a reference solution, allowing a phenomenological definition of ion binding. This is an extension of Wyman's linkage theory. The theory is applied to the electrolyte dependence of proton titration curves for bovine serum albumin in KCl solution (C. Tanford, S. A. Swanson and W. S. Shore, J. Am. Chem. Soc. 77 (1955) 6414). The curves are re-analysed in terms of Esin-Markov coefficients. In addition, we discuss the interpretation of the phenomenological K+ and Cl- binding numbers in terms of a two-state binding model, in which part of the ions are thought to adsorb on specific sites at the protein surface and/or part in the diffuse layer. It is shown that the electrolyte binds largely in the diffuse layer, especially when the protein surface charge is high.