Effect of Alcohols on the Thermal Denaturation of Lysozyme as Measured by Differential Scanning Calorimetry

The thermal denaturation of lysozymc in aqueous alcohol solution has been investigated by differential scanning calorimetry. The alcohols employed were methanol, ethanol, isomeric propyl alcohols and butyl alcohols as monohydric alcohols, and ethylene glycol and glycerol as polyhydric alcohols. In monohydric alcohols, the temperature of denaturation, Td, of lysozyme decreased linearly with increasing alcohol concentration, which became pronounced with an increasing in the hydrophobic character. The enthalpy of denaturation, ΔHd, of lysozyme showed a complex dependence on the solvent composition; the ΔHd first increased with increasing alcohol concentration and then started decreasing at different concentrations for each alcohol. The branching of the alkyl chain decreased the destabilizing effect of the alcohol on the native conformation of the protein. In polyhydric alcohols, both Td and ΔHd increased with increasing alcohol concentration. The polyhydric alcohols stabilized the native conformation of the protein in contrast with the monohydric alcohols. The results are discussed in terms of the hydrophobic and hydrophilic characters of the alcohols.