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 degrees 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 Mg2+ for 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-CaCl2 system, this approximation would lead to interpretations inconsistent with the known destabilizing effect of calcium ions on proteins.
Annunziata
Onofrio
O
Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129, USA. O.Annunziata@tcu.edu
Paduano
Luigi
L
Albright
John G
JG
D002122:Calcium Chloride / Q000737:chemistry*; D004058:Diffusion; D004527:Egg Proteins / Q000737:chemistry; D009113:Muramidase / Q000737:chemistry*; D012996:Solutions; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
D004527:Egg Proteins; D012996:Solutions; D014867:Water; D009113:Muramidase; D002122:Calcium Chloride