The Effect of Salt Stoichiometry on Protein−Salt Interactions Determined by Ternary Diffusion in Aqueous Solutions

• Published in: The journal of physical chemistry. B Vol. 110; no. 32; pp. 16139 - 16147
• Main Authors: Annunziata, Onofrio, Paduano, Luigi, Albright, John G
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 17.08.2006
• Subjects: Diffusion; Magnesium Chloride - chemistry; Muramidase - chemistry; Sodium Chloride - chemistry; Solutions - chemistry; Stereoisomerism; Thermodynamics; Water - chemistry
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp061632w

We report the four diffusion coefficients for the lysozyme−MgCl2−water ternary system at 25 °C and pH 4.5. The comparison with previous results for the lysozyme−NaCl−water ternary system is used to examine the effect of salt stoichiometry on the transport properties of lysozyme−salt aqueous mixtures. We find that the two cross-diffusion coefficients are very sensitive to salt stoichiometry. One of the cross-diffusion coefficients is examined in terms of common-ion, excluded-volume, and protein-preferential hydration effects. We use the four ternary diffusion coefficients to extract chemical-potential cross-derivatives and protein-preferential interaction coefficients. These thermodynamic data characterize the protein−salt thermodynamic interactions. We demonstrate the presence of the common-ion effect (Donnan effect) by analyzing the dependence of the preferential-interaction coefficient not only with respect to salt concentration but also with respect to salt stoichiometry. We conclude that the common-ion effect and the protein-preferential hydration are both important for describing the lysozyme−MgCl2 thermodynamic interaction.