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

In an earlier publication by Chattoraj et al, [Biophysical Chemistry 63 (19 96) 37], a generalized equation for standard free energy of (Delta G(0)) in teraction 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 Delta G(0) in kilojoules per kilogram of different proteins for the change of bulk su rfactant activity from zero to unity in the mole fraction scale. Values of binding interactions of CTAB, MTAB, DTAB and SDS to BSA, beta-lactoglobulin , gelatin, casein, myosin, lysozyme and their binary and ternary mixtures h ad already been determined in this laboratory at different surfactant conce ntrations, pH, ionic strength and temperature using an equilibrium dialysis technique. Values of Delta G(0) for saturated protein-surfactant complexes as well as unsaturated complexes are found to be equal. Delta G(0) is also found to vary linearly with maximum moles of surfactants bound to a kilogr am of protein or protein mixture and the slope of this linear plot represen ts standard free energy Delta G(B)(0) for the transfer of 1 mol of surfacta nt from the bulk for binding reaction with protein; -Delta G(0) values for different systems vary widely and the order of their magnitudes represents relative affinities of surfactants to proteins. Magnitude of -Delta G(B)(0) on the other hand varies within a narrow range of 32-37 kJ/mol of surfacta nt. For interaction of SDS with BSA, close to the CMC, values of Delta G(0) are very high due to the formation of micelles of protein-bound surfactant s. Values of Delta 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 t he data obtained from our previous isopiestic experiments. Delta G(0) value s in these cases are positive due to the excess hydration of proteins. Nega tive values of Delta G(0) in surfactant interaction and positive values of Delta G(0) for hydration of proteins in the presence of neutral salts repre sent relative affinities of proteins for solute and solvent since in all ca ses, the reference state for Delta G(0) is the unit mole fraction of solute in the aqueous phase. (C) 1999 Elsevier Science B.V. All rights reserved.