Synthesis of phosphonopeptides and alpha, alpha-dialkylated amino acid-rich peptides

Peptides and peptide analogs are central to the understanding of biological and biochemical phenomena as well as in the development of new pharmaceutical agents. This research encompasses two classes of peptide mimetics: phosphonopeptides and short, amphipathic peptides rich in α,α-disubstituted amino acids (ααAA). The focus of the first part of the peptide analog project was on the preparation of sterically-hindered phosphonamide and thiophosphonamide dipeptides. Conditions were developed, based on our P(III) synthetic approach, for the preparation of the phosphonamide dipeptides, Cbz-CyhGly ψ [P (O) (OpNb) NH] Trp-NH₂ and Cbz-CyhGly ψ [P (O) (OpNb) NH] Trp-OMe, which were previously unattainable by P(V) methods. The use of reduced phosphorus intermediates also allowed preparation of the related thiophosphonamides, Cbz-CyhGly ψ [P (S) (OpNb) NH] Trp-NH₂ and Cbz-CyhGly ψ [P (S) (O pNb) NH] TrpOMe. With the P(III) synthetic strategy we started from an amine-protected H-phosphinate amino acid ester and activated, in the presence of 2 equivalents of pyridine, with dichlorotriphenylphosphorane to generate a phosphonochloridite, which was treated with a diisopropylethylamine to form a proposed, very reactive, oxazaphospholine. The oxazaphospholine was coupled to an amine nucleophile followed by oxidation or sulfurization. This method was also used to prepare several simpler phosphonamide and thiophosphonamide dipeptides and thiophosphonates.; In the second part of this project a homologous series of 9-mer, ααAA-rich peptides (and their acetylated versions), which were designed to be amphipathic and 3₁₀-helical, were prepared using solid-phase synthetic techniques. The peptides were composed of 78% ααAAs (one 4-aminopiperidine-4-carboxylic acid (Api) and six aminoisobutyric acid (Aib)). The in situ coupling agent, PyAOP, was used in the synthesis. The use of Fmoc-Aib-Aib-OH in the syntheses caused a significant improvement in the coupling and overall yields of the peptides. However, the monomer Aib had to be incorporated, along with double coupling, for the first two Aibs in each peptide's sequence.; The helix preference of each peptide in different solvent environments were investigated as well as each peptide's antimicrobial activity and cytotoxicity. Significant populations of both α- and 3₁₀-helices were observed for the non-acetylated peptides under the solvent conditions tested. The 3 ₁₀-helical, amphipathic design was observed best for the acetylated peptides. Most of the peptides exhibited modest activity against E. coli and no activity against S. aureus. The non-acetylated peptides (concentrations ≤100 μM) and the acetylated peptides (concentrations ≤200 μM) did not exhibit, based on a trypan blue stain procedure, any significant cytotoxicity to normal macrophages.