DSC study on the Sym-dialkylsulfoxides on the thermal denaturation of lysozime solutions.

The knowledge of the influence of Sym-dialkylsulfoxides (DASO)s on the thermal denaturation and to the structural stability of protein is essential to understand their role and we approached this problem by studying the thermal denaturation of lysozyme by means of Differential Scanning Calorimetry (DSC). The maximum of the endothermic DSC peak show a linear Td decrease in the presence of increasing DASOs amounts. On the contrary the dependence of ΔHd on DASO concentration appear to be complex, showing an increase at lower and a lowering at higher DASO content. This result can be interpreted in terms of the hydrophobic interaction between the apolar part of the protein and the alkyl chains of the sulfoxides, thus relatively favouring the denaturated state. The pH dependence found on Td, high in the presence of low-hydrophobic dimethylsulfoxide (DMSO) and negligible in the presence of high-hydrophobic di-n-propylsulfoxide (DnPSO), confirm the noticeable role of the interaction between the alkyl chains and the apolar groups on the protein structure. We suggest that both the ‘water structure related’ interaction, due to the polar S=O group, as well the hydrophobic interaction are involved in the denaturation process.