Denaturation of lysozyme by n-alkyltrimethylammonium bromides in alkaline solution

Abstract

The interactions of a homologous series of n-alkyltrimethylammonium bromides (C₈–C₁₄ TABs) with hen egg lysozyme have been investigated using microcalorimetry. The C₈ TAB does not interact with lysozyme whereas the C₁₀–C₁₄ TABs interact endothermically and deactivate the enzyme. The endothermicity of the TAB–lysozyme interaction is in marked contrast to the exothermic interactions between n-alkyl sulfates and lysozyme which have been attributed to specific binding between the anionic sulfate head groups and cationic amino acid residues. The enthalpies of interaction between the cationic surfactants and lysozyme follow sigmoidal curves characteristic of an interaction dominated by the endothermic unfolding of the native structure. The enthalpy data have been used to obtain the Gibbs energy and entropy for surfactant-induced denaturation. At pH 10 the Gibbs energy, enthalpy and entropy of denaturation are 17.9 ± 4.2 kJ mol^–1, 148 ± 15 kJ mol^–1 and 436 J mol^–1 K^–1 at 25 °C.