Effect of dimethylsulfoxide and its homologues on the thermal denaturation of lysozyme as measured by differential scanning calorimetry

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Abstract
The thermal denaturation of [hen egg white] lysozyme in aqueous sulfoxide solutions (pH 3) was investigated by differential scanning calorimetry (DSC). The sulfoxides employed were dimethylsulfoxide (DMSO), ethylmethylsulfoxide (EMSO), diethylsulfoxide (DESO) and butylmethylsulfoxide (BMSO). The temperature of denaturation, Td, decreased with an increase in the concentration of sulfoxide, the decrease becoming much more pronounced at higher sulfoxide concentrations. The lowering of Td was enhanced by an increase in the hydrocarbon content of the sulfoxide molecule. The enthalpy of denaturation, .DELTA.Hd, showed a complex dependence on the solvent composition; the .DELTA.Hd first increased with increasing sulfoxide concentration and then started decreasing at different concentrations for each sulfoxide. This behavior is analogous to that with monohydric alcohols but is different from that with guanidine hydrochloride. These results may be interpreted in terms of the interactions of the sulfoxide added with protein and water.