Preferential Solvation Changes upon Lysozyme Heat Denaturation in Mixed Solvents

On the basis of scanning microcalorimetry data from literature and our own measurements, we have calculated the changes in preferential solvation of lysozyme upon heat denaturation in six solvent systems: water + methanol, ethanol, propanol [data from Velicelebi, G., & Sturtevant, J. M. (1979) Biochemistry 18, 1180], acetone, p-dioxane [data from Fujita, Y., & Noda, Y. (1983) Bull. Chem. Soc. Jpn. 56, 233], and dimethylsulfoxide [our data Kovrigin, E. L., Kirkitadze, M. D., & Potekhin, S. A. (1996) Biofizika 41, 549−553; Kovrigin, E. L., & Potekhin, S. A. (1996) Biofizika 41, 1201−1206]. These preferential solvation changes are (in effect) the numbers of cosolvent molecules entering or leaving the solvation shell of the protein upon denaturation. It has been shown that for a group of five substances in the initial activity range (approximately up to 0.3) the denaturational changes of preferential solvation of lysozyme do not depend on the nature of the solvent and depend only on its activity. This suggests that lysozyme does not distinguish these substances in the initial activity range and preferential solvation has a nonspecific character. It has been shown also that preferential solvation ΔΓ23 does not depend on the pH value at least for dimethylsulfoxide−water solutions. This indicates that the chargeable groups exposed on denaturation do not contribute significantly to preferential interaction of the protein surface with the solution components