Charge-dependent and charge-independent contributions to ion-protein interaction.

The thermodynamic framework and recent molecular descriptions of protein-salt interactions are critically reviewed. A reanalysis of earlier and recent data describing the role of salts on the thermal stability of collagen I over a wide range of concentration evidences the complex encroachment of charge-dependent and charge-independent interactions. Charge-independent interactions, operationally quantified by dilution parameters and association constants, are found to be the overriding factor for the large stabilization observed with nonbinding salts and for the large destabilization observed when strong salt binding occurs. Charge-dependent interactions, namely screening and selective ion adsorption, are instead the prevailing components at low ionic strength and nonisoelectric pH.