Effect of Hydration on the Thermal Stability of Protein as Measured by Differential Scanning Calorimetry. Lysozyme–D2O System

The thermal denaturation of the deuterated lysozyme has been investigated by differential scanning calorimetry in the range of deuterium oxide (D2O) content from 0.03 to 1.6 g of D2O per g of protein. At D2O contents above 0.55 g/g, the temperature, Td, and enthalpy change, ΔHd, of denaturation were almost independent of the degree of hydration. At lower D2O contents, however, both Td and ΔHd showed marked dependence on the degree of hydration. The values of Td increased with a decrease in the D2O content. Whereas the values of ΔHd decreased with a decrease in the D2O content in the same region. The degree of hydration dependency of ΔHd exhibited a break at approximately 170 mol/mol, which indicates that at least two types of hydration contributes to the thermal stability of the protein. The conformational enthalpy change of the protein and the enthalpy change of the hydration layer have also been estimated from the degree of hydration dependency of ΔHd. The results have been compared with those of the Iysozyme–H2O system.