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 thermodynamic and transport properties of aqueous solutions of proteins, we report ternary diffusion coefficients for the lysozyme−CaCl2−water ternary system at 25 °C and pH 4.5. We have used our ternary diffusion coefficients to calculate preferential-interaction coefficients as a function of salt concentration. This has allowed us to characterize protein−salt thermodynamic interactions. We have observed the presence of large common-ion effects by examining the dependence of the diffusion coefficients on salt concentration. 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 examining the dependence of the preferential-interaction coefficient on salt concentration, 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 Mg2for water, as compared to Ca2, and the different roles that these two divalent metal ions play in biochemical processes. We remark that neglecting the common-ion contribution of the preferential-interaction coefficient can lead to qualitatively inaccurate descriptions of protein−salt aqueous systems, even at high salt concentrations. Indeed, for the lysozyme−CaCl2system, this approximation would lead to interpretations inconsistent with the known destabilizing effect of calcium ions on proteins. [ABSTRACT FROM AUTHOR]