Preferential solvation of lysozyme by dimethyl sulfoxide in binary solutions of water and dimethyl sulfoxide

Abstract

To reveal the denaturation mechanism of lysozyme by dimethyl sulfoxide (DMSO), thermal stability of lysozyme and its preferential solvation by DMSO in binary solutions of water and DMSO was studied by differential scanning calorimetry (DSC) and using densities of ternary solutions of water (1), DMSO (2) and lysozyme (3) at 298.15 K. A significant endothermic peak was observed in binary solutions of water and DMSO except for a solution with a mole fraction of DMSO (x2) of 0.4. As x2 was increased, the thermal denaturation temperature Tm decreased, but significant increases in changes in enthalpy and heat capacity for denaturation, ΔHcal and ΔCp, were observed at low x2 before decreasing. The obtained amount of preferential solvation of lysozyme by DMSO (∂g2/∂g3) was about 0.09 g g−1 at low x2, indicating that DMSO molecules preferentially solvate lysozyme at low x2. In solutions with high x2, the amount of preferential solvation (∂g2/∂g3) decreased to negative values when lysozyme was denatured. These results indicated that DMSO molecules do not interact directly with lysozyme as denaturants such as guanidine hydrochloride and urea do. The DMSO molecules interact indirectly with lysozyme leading to denaturation, probably due to a strong interaction between water and DMSO molecules.