Stoichiometric Complexes of Synthetic Polypeptides and Oppositely Charged Surfactants in Organic Solvents and in the Solid State

Abstract

Properties of the stoichiometric complex formed by poly(l-lysine) cations and dodecyl sulfate anions in organic solvents and in the solid state were investigated via viscometry, 1H NMR, infrared, circular dichroism, and X-ray diffraction techniques. In dilute solutions in chloroform with up to 10 vol % of trifluoroacetic acid, the complex neither forms interchain aggregates nor dissociates to yield free surfactant. In chloroform solutions containing small amounts of trifluoroacetic acid (1−2 vol %), the polypeptide chains are in an α-helical conformation, while a transition to a disordered form occurs at higher trifluoroacetic acid contents (4−6 vol %). The helix−coil transition of the poly(l-lysine) chains is accompanied by a decrease in the 1H spin−lattice relaxation times of the polypeptide chain segments, while the relaxation times of the surfactant chains remain essentially unchanged. In chloroform−trifluoroacetic acid mixtures, the α-helical conformation of the poly(l-lysine) chains is stabilized by increasing temperature. Polypeptide chains in the solid complex can adopt either α-helical or β-sheet conformations, depending on the trifluoroacetic acid content of the chloroform solution used for film casting. The solid complex is organized into a lamellar structure consisting of alternating layers of poly(l-lysine) chains and double layers of surfactant, arranged tail to tail.