Multicomponent Diffusion of Lysozyme in Aqueous Calcium Chloride. The Role of Common-Ion Effects and Protein-Salt Preferential Interactions.

To investigate the effect of calcium salts on the thermodn. and transport properties of aq. solns. of proteins, we report ternary diffusion coeffs. for the lysozyme-CaCl2-water ternary system at 25 C and pH 4.5. We have used our ternary diffusion coeffs. to calc. preferential-interaction coeffs. as a function of salt concn. This has allowed us to characterize protein-salt thermodn. interactions. We have obsd. the presence of large common-ion effects by examg. the dependence of the diffusion coeffs. on salt concn. Our results are compared to those previously reported for the lysozyme-MgCl2-water ternary system. We have found that the common-ion effect is essentially the same for both salt cases. On the other hand, by examg. the dependence of the preferential-interaction coeff. on salt concn., we have found that salt preferentially interacts with the protein in the lysozyme-CaCl2-water system, whereas water preferentially interacts with the protein in lysozyme-MgCl2-water system. This is consistent with the known generally larger affinity of Mg2+ for water, as compared to Ca2+, and the different roles that these two divalent metal ions play in biochem. processes. We remark that neglecting the common-ion contribution of the preferential-interaction coeff. can lead to qual. inaccurate descriptions of protein-salt aq. systems, even at high salt concns. Indeed, for the lysozyme-CaCl2 system, this approxn. would lead to interpretations inconsistent with the known destabilizing effect of calcium ions on proteins.