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

by Zhang, Guanghui

Abstract (Summary)
A general strategy has been developed to synthesize biologically monodisperse polypeptides with the major repeat unit 1. These polymers are derivatives of poly($\alpha$,L-glutamic acid) (PLGA).$$- \rm Glu\sb{17}Asp-\qquad{\bf 1}$$ Such polymers should adopt an $\alpha$-helical structure when the side chain carboxylate groups are protonated, since Glu has the highest helix forming propensity (P$\sb\alpha$ = 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.$$\rm GluAsp(Glu\sb{17}Asp)\sb4GluGlu\qquad{\bf 2}$$ 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 $\alpha$-helix as indicated by circular dichroism measurements. The $\alpha$-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($\gamma$-benzyl $\alpha$,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 $\alpha$-helical structure and transforms into a random coil when trifluoroacetic acid is added to the solution. Polymer 3 self-assembles into $\alpha$-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.$$\eqalign{\rm Glu(OBzl)Asp(OBzl)&\{\lbrack(\rm Glu(OBzl)\rbrack\sb{17} Asp\cr&\qquad(\rm OBzl)\}\sb4Glu(OBzl)Glu(OBzl)\qquad{\bf 3}\cr}$$where OBzl denotes a benzyl ester. High molecular weight PLGA was chemically synthesized from $\gamma$-benzyl $\alpha$,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.
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Advisor:

School:University of Massachusetts Amherst

School Location:USA - Massachusetts

Source Type:Master's Thesis

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ISBN:

Date of Publication:01/01/1994

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