Thermodynamics of the adsorption of small molecules by proteins

An analysis of the binding of small molecules to proteins in solution is given using the methods of adsorption thermodynamics. It is shown that heats of binding may be obtained as a function of the binding ratio directly for non-ionic molecules and of ionic molecules binding in the presence of excess electrolyte. Ion-exchange data are required in addition to binding curves to obtain heats of binding for ionic ligands in the absence of excess neutral electrolyte. A comparison is made of the theory presented here with that given by Klotz. Data are presented on the binding of sodium dodecyl sulphate to keratin and the theory, applicable only below the critical micelle concentration, is used to calculate heats of binding up to this point. The integral heat of binding at high coverage is only - 2 kcal mole^-1, corresponding to physical adsorption. Binding continues to a maximum at a concentration above the micelle point. The entropy change of the condensed phase rises with increasing coverage.