Microcalorimetric Study of the Effect of Dimethylsulphoxide on the Heat Denaturing of Lysozyme

The effect of dimethylsulfoxide on heat denaturation of lysozyme has been studied by scanning microcalorimetry. Measurements have been performed in a wide range of apparent pH values and organic component concentrations and have revealed a destabilizing effect of dimethylsulfoxide on the structure of lysozyme. The whole range of dimethylsulfoxide concentrations can be subdivided into three regions according to the effect exerted on the enthalpy of denaturation: in one region the enthalpy increases, in the second falls and in the third the denaturational transition is not revealed. In a narrow range of dimethylsulfoxide concentrations (55-60% of the volume) at pH 2.5 the so-called anomalous heat denaturation has been revealed. This phenomenon is characterized by a sharp decrease of denaturation enthalpy and a considerable excess of effective enthalpy over calorimetric one while denaturation remains absolutely reversible. It has been shown that this is not connected with oligomerization of the protein; it has been suggested that this phenomenon is of kinetic origin