The Gibbs Energies of Activation of Lysozyme for Viscous Flow in Water + Dimethyl Sulfoxide Solutions at 298.15 K

Gibbs energies of activation for viscous flow of binary water (1) + dimethyl sulfoxide (2) mixtures, Δμ12○, and of lysozyme (3) in corresponding ternary mixtures, Δμ3○, were determined at 298.15 K. The binary mixtures have a maximum in the value of the excess quantity for Δμ12○ at a dimethyl sulfoxide mole fraction of x2≈0.31. The values of Δμ3○ are larger than Δμ12○ at all values of x2, even when normalized by their molar volumes, suggesting that the solvents interact more strongly with lysozyme than with themselves. The values of Δμ3○ significantly increased in the range of x2=0.3 to 0.4 because of an increase in solvent-lysozyme interactions, which resulted from an increase in the accessible surface area of lysozyme that was exposed by its unfolding. The mean value obtained for Δμ3○ per amino acid of lysozyme at x2=0.2 is greater than that for hydrophobic amino acids, indicating that the solvent interacts with hydrophilic amino acids more strongly than with hydrophobic ones.