Enthalpy of interaction between some cationic polypeptides and n-alkyl sulphates in aqueous solution

Abstract

The enthalpies of interaction of a homologous series of n-alkyl sulphates with poly(L-lysine)-hydrobromide, poly(L-arginine)hydrochloride and poly(L-histidine)hydrochloride have been measured at 25°C. A linear relationship between the enthalpy of interaction and carbon chain length has been found for alkyl chain lengths above C₈. The data support a model based on a stoichiometric interaction between the anionic head group of the n-alkyl sulphates and the cationic side chains of the polypeptides. The results are discussed in relation to the interaction between surfactants and proteins, and lead to the view that a major contribution to the enthalpy of interaction arises from the surfactant–cationic residue interactions, but that there remains an additional contribution from surfactant–apolar aminoacid residue interactions.