Structure and conformation of polypeptide-cationic surfactant complexes inorganic solvents

Hydrodynamic, electrooptical, and conformational properties of stoichiometr ic polyelectrolyte complexes composed of poly(L-glutamic) acid and a cation ic surfactant in chloroform and isopropyl and methyl alcohols were studied. In all these solvents, the molecules of the complex are individual compoun ds, neither dissociating into separate components nor forming intermolecula r associates. The optical rotation spectra of the solutions of the studied complex and its covalent analog, poly(gamma -benzyl-alpha ,L-glutamate), ar e characteristic of polypeptides occurring in the form of an alpha -helix. Comparison of the intrinsic viscosity [eta] and the times of orientational relaxation tau (d) of molecules shows that the hydrodynamic sizes of the co mplex molecules are markedly smaller than those of poly(gamma -benzyl-alpha ,L-glutamate) with the same degree of polymerization. A model is advanced according to which steric repulsion between bulky surfactant ions is respon sible for the local disturbance of the secondary structure of the complex m acromolecules; as a result, flexible fragments appear along with rigid heli cal regions. The presence of such fragments determines the compact convolut ed conformation of the complex macromolecules in the examined solvents.