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

Abstract

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-CaCl 2 -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 lysozymeMgCl 2 -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-CaCl 2 -water system, whereas water preferentially interacts with the protein in lysozyme-MgCl 2 -water system. This is consistent with the known generally larger affinity of Mg 2+ for water, as compared to Ca 2+, 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 lysozymeCaCl 2 system, this approximation would lead to interpretations inconsistent with the known destabilizing effect of calcium ions on proteins.

Quaternary Diffusion Coefficients in a Protein−Polymer−Salt−Water System Determined by Rayleigh Interferometry

“…A similar expression can be also derived for (D 32 Effect of Cross-Diffusion on Protein Crystallization. The effect of cross-diffusion coefficients on protein crystallization processes has been examined in model calculations.…”

“…23,[29][30][31][32][33][34] Diffusion measurements at 25°C on lysozymesalt-water 31,32 and lysozyme-poly(ethylene glycol)-water 33,34 by optical interferometry have shown that cross-diffusion terms…”

Biopolymer Surfactant Interactions

“…The presences of neutral salts [50][51][52][53] showed a marked shift in phase transitions. It has been suggested that the protein unfolds in the cooperative binding region [54] and sodium chloride is reported to act as a denaturing agent in many protein systems [55][56][57]. According to Curtis et al [58] in classical salting-out behavior, the protein-salt preferential interaction is unfavorable as addition of salt raises the surface free energy of the protein, thereby protein-protein attraction increases leading to a reduction in solubility.…”

Determination of Preferential Interaction Parameters by Multicomponent Diffusion. Applications to Poly(ethylene glycol)−Salt−Water Ternary Mixtures

“…[1][2][3] Macromolecule-osmolyte thermodynamic interactions are also central for understanding diffusive mass transport of macromolecules. [9][10][11][12][13] Indeed a gradient of osmolyte concentration can generate a gradient of macromolecule chemical potential and vice versa. This is responsible for coupled diffusion in systems with two or more solutes.…”