Self-assembled polypeptide-surfactant complexes in organic solvents and in the solid-state: A new class of comb-shaped polypeptides
Abstract (Summary)We describe herein the preparation and physical characterization of novel water-insoluble complexes formed by synthetic polypeptides, sodium poly($\alpha$,L-glutamate) and poly(L-lysine) hydrobromide, and oppositely charged low molecular weight surfactants, alkyltrimethylammonium bromides and sodium alkyl sulfates with chain lengths from twelve to eighteen methylene groups. The complexes of nearly stoichiometric compositions were prepared by mixing equimolar amounts of the components in water. The goal of the research was to understand the influence of the electrostatically bound 'side chains' on properties of polypeptide chains (solubility and conformation) and the effect of polymer chains on organization of the complexed surfactants. The behavior of the complexes was compared to that of their covalent analogs, alkyl esters of poly($\alpha$,L-glutamic acid) and acyl derivatives of poly(L-lysine). Conformational and structural properties of the complexes in the solid state were studied via circular dichroism, infrared spectroscopy, X-ray diffraction and differential scanning calorimetry. Poly($\alpha$,L-glutamate) chains in the complexes adopt $\alpha$-helical conformations at room temperature and disordered conformations at elevated temperatures. Poly(L-lysine) chains in the complexes adopt either $\beta$-sheet conformation (as isolated after synthesis) or $\alpha$-helical conformation (in the solid films cast from chloroform-trifluoroacetic acid solutions). Organization of surfactants in the complexes depends on the surfactant chain length. Shorter chains (eight-sixteen carbon atoms) are packed with a short range order, while the longer chains (eighteen carbon atoms) crystallize on a hexagonal lattice. In complexes with mixed octyl and octadecyl sulfates, organization of the surfactants depends on the composition: the minimum octadecyl chain content required for crystallization is about 20 molar per cent. All complexes studied are organized in lamellar structures consisting of alternating layers of polypeptide chains separated by layers of surfactants. Dilute solution properties of poly(L-lysine)-dodecyl sulfate complexes in organic solvents were studied via viscometry, $\sp1$H NMR and $\sp1$H NMR relaxation techniques. Poly(L-lysine) chains in the complexes in chloroform-trifluoroacetic solution adopt either $\alpha$-helical (1-2 volume per cent trifluoroacetic acid) or disordered (4-10 volume per cent trifluoroacetic acid) conformations.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/1997