Date of Award
Doctor of Philosophy (PhD)
Mark L. McLaughlin
The synthesis of a series of polyfunctional Calpha,C alpha-disubstituted glycines is described. The lysine-like amino acid analog, Api, was prepared by regioselective hydrolysis of a triBoced hydantoin, followed by Nalpha-protection to yield the first orthogonally protected ionizable Calpha,Calpha-disubstituted amino acid which is alicyclic. The synthesis of three orthogonally protected tetrafunctional amino acids, Bap, Bglu, and Basp was envisioned by alkylation of the organic synthon ethyl nitroacetate. Subsequent regioselective modification should allow for the isolation of the first tetrafunctional amino acid derivatives suitable for solid-phase synthesis. These amino acids are designed to induce peptide secondary structure by salt-bridge stabilization. A series of peptides, incorporating 80% Calpha,C alpha-disubstituted glycines were synthesized to establish the helix stabilizing effect of amphipathicity in short helices. The peptides were prepared as permutation isomer pairs, Pi-10 & Ipi-10; Ach-10alpha & Ach-10; and Cyh-10 & Ich-10. The peptides within each pair contain the same amino acid content, with different sequences, each designed to preferentially adopt a 310- or alpha-helix. Circular dichroism confirms that amphipathicity is a significant factor in shifting the 310-/alpha-helix equilibrium, notably so in micellar environments. The bioactivity of the aforementioned peptides was established by minimum inhibitory concentrations (MICs) against representative Gram-positive and Gram-negative bacteria. The more hydrophobic peptides showed higher levels of cytotoxicity than the less hydrophobic peptides. In vitro studies using a strain of Brucella abortus expressing Green Fluorescent Protein (GFP) show that all of the peptides exhibit moderate to high selectivity towards the destruction of murine macrophages infected with the intracellular pathogen. The spectroscopic properties of the o-nitrobenzenesulfonyl (oNBS) group have been established to confirm the practical use of this protecting group in solid-phase synthesis. Synthesis of the deprotection product, resulting from treatment of the oNBS-protected amino acid with mercaptoacetic acid/DBU, shows a linear correlation between concentration and absorption at 390 nm. Molar absorptivity, &egr;390 was determined to be 2950 cm-1M-1. This number was verified by solution-phase cleavage of several oNBS-protected Calpha,Calpha-disubstituted amino acids, which were synthesized by a modified Bolin procedure to allow for synthesis under non-aqueous conditions.
Hammarstrom, Lars Gustav johan, "Synthetic Peptides: Design, Structure and Biological Function." (2001). LSU Historical Dissertations and Theses. 289.