Thermodynamics of Unfolding of Lysozyme in Aqueous Alcohol Solutions

The thermal denaturation of lysozyme in aqueous alcohol solutions has been investigated through ultraviolet difference spectrophotometry and optical rotation measurements. Alcohols used were CH3OH, C2H5OH, and C3H7OH; concentrations up to 50% (v/v) of alcohol in water were employed. It was found that upon increasing the size or the concentration of the aliphatic alcohol, the denaturation temperature is gradually depressed. At low alcohol concentration, the α helix content of the protein decreases through the thermal transition. However, on increasing alcohol size or concentration, the α helix content goes through a maximum during denaturation. Apparent equilibrium constants and corresponding van't Hoff heats of denaturation were determined. The data indicate that the depression of the denaturation temperature due to addition of alcohols is essentially entropy dependent. It is suggested that these results may be explained in terms of a theoretical expression containing a binding constant for the alcohol to the nonpolar sites of the protein plus a diluent term accounting for changes in the dilution parameters of different conformers.