Interaction of Poly[S-(2-carboxyethyl)-l-cysteine] with Cationic Surfactants

Abstract

Abstract The induction of the β-structure of fully ionized poly[S-(2-carboxyethyl)-l-cysteine] in solution markedly depended on the kind of head group of cationic surfactants; it occurred by the addition of dodecylammonium chloride (DAC) but not by the addition of dodecyldimethylammonium chloride (DDAC) or dodecyltrimethylammonium chloride (DTAC), as concluded based on circular dichroism spectra. The induction of the β-structure by DAC was not inhibited by raising ionic strength of the medium but was enhanced when the polypeptide concentration was increased. The polypeptide took the β-structure in the precipitates produced by the addition of any one of these three surfactants, as indicated by the infrared absorption (IR) spectra. Intensity of a characteristic absorption band of carboxylate group around 1570 cm−1 for the complex with DDAC was vanishingly weak, while those with DAC and DTAC were medium and strong, respectively. The precipitates were solubilized in the presence of excess amount of surfactants. In the solubilized solution, the β-structure was again found in the case of DAC, irrespective of the polypeptide concentration, but it was found only at high polypeptide concentrations in the case of DDAC.