STANDARD FREE-ENERGIES OF BINDING OF SOLUTE TO PROTEINS IN AQUEOUS-MEDIUM .1. THERMODYNAMIC ANALYSIS FOR MULTICOMPONENT SYSTEM

Using an equilibrium dialysis technique, moles (Gamma(2)(1)) of cation ic and anionic surfactants bound per kilogram of proteins of various t ypes in aqueous media have been measured previously in this laboratory under different physicochemical conditions. From a thermodynamic anal ysis in the present paper, Gamma(2)(1) has been shown to be equal to t he Gibbs relative excess of surfactant per kilogram of protein at a me asured value of solute activity, a(2). The values of relative solvent excesses, Gamma(1)(2) (which are negative for surfactants) can be esti mated from values of Gamma(2)(1) and a(2). Using the Gibbs-Duhem relat ionship for protein solution inside the dialysis bag and dialysate sol utions respectively at equilibrium, an integrated expression for the s tandard free energy change, Delta G degrees (in kilojoules per kilogra m of protein for binding with ligand as a result of the change of a(2) from zero to unity) can be calculated from experimental data. The iso piestic vapour pressure technique was used extensively for evaluation of negative binding (-Gamma(2)(1)) of inorganic salts to proteins of d ifferent types for various values of a(2) of salts present in the bulk media. With some modifications of our derived equations for free ener gy of binding in such a system, Delta G degrees has been evaluated for the change of mean activity of electrolyte from zero to unity in the rational scale. Delta G degrees is positive since Gamma(2)(1) is negat ive and Gamma(1)(2) is positive for such ionic systems. Delta G degree s in all cases, however, are expressed in terms of the standard state of reference of unit activity so that their magnitudes and sign can be related to the relative affinities of a solute for binding with prote ins in aqueous media.