Structure and properties of stoichiometric complexes formed by sodium poly(alpha,L-glutamate) and oppositely charged surfactants

The conformational and structural properties of the stoichiometric complexes formed by poly(alpha,L-glutamate) anions and alkyltrimethylammonium cations with chain lengths of twelve, sixteen, and eighteen carbon atoms were examined by circular dichroism, infrared, differential scanning calorimetry, and X-ray diffraction techniques. The polypeptide chains complexed with the surfactants adopt alpha-helical conformations. The shorter surfactant alkyl chains, consisting of twelve and sixteen carbon atoms, are disordered in the complexes, while the longer surfactant chains of eighteen carbon atoms crystallize on a hexagonal lattice. All of the complexes are organized in lamellar structures consisting of alternating layers of poly(alpha,L-glutamate) chains separated by bimolecular layers of the surfactants. The surfactant alkyl chains are interdigitated and perpendicular to the lamellar surfaces.