Preferential interactions in the H2O/lysozyme/AlCl3 system.

The preferential interactions of lysozyme with solvent components is studied in aqueous solutions of AlCl3. The interaction parameter is negative at large salt concentrations, indicating that the interaction process of both salt and protein is thermodynamically favorable. The transfer free-energy parameter and the solubility data show that aluminum chloride is a salting-in agent for lysozyme. Moreover, these preferential interactions also are correlated with both protein solubility in the solvent medium and the influence of salt on the lysozyme structure. Viscometric and refractometric studies show that lysozyme can undergo a conformational change at 1 mM of salt, and spectrophotometric studies indicate a protein activity of approximately 75% at 10 mM of salt. Therefore, neither the interaction of AlCl3 with the lysozyme nor the conformational change undergone directly affect the catalytic amino acid residues of the active site.