Standard free energies of binding of solute to proteins in aqueous medium. Part 2. Analysis of data obtained from equilibrium dialysis and isopiestic experiments

Abstract

In an earlier publication by Chattoraj et al. [Biophysical Chemistry 63 (1996) 37], a generalized equation for standard free energy of (Δ G 0) interaction of surfactant, inorganic salts and aqueous solvent with protein, forming a single phase has been deduced on strict thermodynamic grounds. In the present paper, this equation has been utilized to calculate Δ G 0 in kilojoules per kilogram of different proteins for the change of bulk surfactant activity from zero to unity in the mole fraction scale. Values of binding interactions of CTAB, MTAB, DTAB and SDS to BSA, β-lactoglobulin, gelatin, casein, myosin, lysozyme and their binary and ternary mixtures had already been determined in this laboratory at different surfactant concentrations, pH, ionic strength and temperature using an equilibrium dialysis technique. Values of Δ G 0 for saturated protein–surfactant complexes as well as unsaturated complexes are found to be equal. Δ G 0 is also found to vary linearly with maximum moles of surfactants bound to a kilogram of protein or protein mixture and the slope of this linear plot represents standard free energy Δ G B 0 for the transfer of 1 mol of surfactant from the bulk for binding reaction with protein; −Δ G 0 values for different systems vary widely and the order of their magnitudes represents relative affinities of surfactants to proteins. Magnitude of −Δ G B 0 on the other hand varies within a narrow range of 32–37 kJ/mol of surfactant. For interaction of SDS with BSA, close to the CMC, values of Δ G 0 are very high due to the formation of micelles of protein-bound surfactants. Values of Δ G 0 for negative binding of inorganic salts to proteins and protein mixtures have been evaluated using our generalized equation in which excess binding values of water and salts have been calculated from the data obtained from our previous isopiestic experiments. Δ G 0 values in these cases are positive due to the excess hydration of proteins. Negative values of Δ G 0 in surfactant interaction and positive values of Δ G 0 for hydration of proteins in the presence of neutral salts represent relative affinities of proteins for solute and solvent since in all cases, the reference state for Δ G 0 is the unit mole fraction of solute in the aqueous phase.