Interaction of unfolded lysozyme with hexa(oxyethylene) dodecylether

Reduced lysozyme at pH 2.5 bound hexa(oxyethylene) dodecylether in two steps and the bound amount of the surfactant reached as much as 0.5–0.6 mole per mole amino acid residue in the cooperative binding step. Circular dichroism (CD) spectra suggested a change in the polypeptide main-chain conformation as a result of the surfactant binding, but little or no organization of the tertiary structure. The interaction most likely took place between the hydrocarbon tail of the surfactant and the hydrophobic domain of reduced lysozyme. Alkylated lysozyme, obtained from the reaction with iodoacetamide, gave an essentially identical binding isotherm to that of reduced lysozyme, but different CD results were obtained for each of them.This research was partially supported by a grant-in-aid for scientific research (No. 02403004) from the Ministry of Education, Science, and Culture, Japan, and also by Nippon Oil & Fats Co., Ltd.