Poly(alpha,L-glutamic acid): Synthesis of its monodisperse derivatives and interaction of its alkylated derivatives with phospholipid bilayer membranes

A general strategy has been developed to synthesize biologically monodisperse polypeptides with the major repeat unit 1. These polymers are derivatives of poly({dollar}alpha{dollar},L-glutamic acid) (PLGA).{dollar}{dollar}- rm Glusb{lcub}17{rcub}Asp-qquad{lcub}bf 1{rcub}{dollar}{dollar}; Such polymers should adopt an {dollar}alpha{dollar}-helical structure when the side chain carboxylate groups are protonated, since Glu has the highest helix forming propensity (P{dollar}sbalpha{dollar} = 1.59) of all twenty natural amino acids. Polymer 2 was synthesized as a fusion protein with glutathione S-transferase (GST) in a bacterial host, and was liberated from the GST fragment by CNBr cleavage.{dollar}{dollar}rm GluAsp(Glusb{lcub}17{rcub}Asp)sb4GluGluqquad{lcub}bf 2{rcub}{dollar}{dollar}; DNA sequencing and amino acid analysis confirmed the composition of 2. Polymer 2 undergoes a conformational transition in aqueous solution from a random coil to an {dollar}alpha{dollar}-helix as indicated by circular dichroism measurements. The {dollar}alpha{dollar}-helical structure persists when the solvent is removed as demonstrated by Fourier transform infrared (FTIR) spectroscopy. Electrophoresis shows that polymer 2 is much more homogeneous in terms of molecular weight than chemically synthesized PLGAs of comparable molecular weight.; A method was adopted to make the monodisperse rodlike molecule 3, a derivative of poly({dollar}gamma{dollar}-benzyl {dollar}alpha{dollar},L-glutamate) (PBLG), by reacting 2 with phenyl diazomethane. Quantitative conversion was indicated by nuclear magnetic resonance spectroscopy. In helicogenic solvents for PBLG, 3 also assumes an {dollar}alpha{dollar}-helical structure and transforms into a random coil when trifluoroacetic acid is added to the solution. Polymer 3 self-assembles into {dollar}alpha{dollar}-helical structure when cast as a film from tetrahydrofuran solution. Gel permeation chromatography shows that 3 has a much narrower molecular weight distribution than chemically synthesized PBLG of comparable molecular weight.{dollar}{dollar}eqalign{lcub}rm Glu(OBzl)Asp(OBzl)&{lcub}lbrack(rm Glu(OBzl)rbracksb{lcub}17{rcub} Aspcr&qquad(rm OBzl){rcub}sb4Glu(OBzl)Glu(OBzl)qquad{lcub}bf 3{rcub}cr{rcub}{dollar}{dollar}where OBzl denotes a benzyl ester.; High molecular weight PLGA was chemically synthesized from {dollar}gamma{dollar}-benzyl {dollar}alpha{dollar},L-glutamate N-carboxy anhydride. This polymer was modified with 8 mol % or 15 mol % hexylamine. The modified polymers can disrupt dilauroylphosphatidylcholine multilamellar vesicles and egg yolk phosphatidylcholine small unilamellar vesicles in a manner which is dependent on the solution pH. Fluorescent probes, specifically pyrene and S-anilino-naphthalene-1-sulfonic acid, ammonium salt, indicate that the modified polymers associate in a pH-dependent fashion and provide hydrophobic domains to solubilize lipid membrane vesicles.