Design, Synthesis, And Conformational Studies Of Peptides Containing α,α-Disubstituted amino acids

Abstract

Cα,α-disubstituted amino acids (ααAAs) are widely utilized to conformationally constrain peptides. Several pentapeptides containing dipropylglycine (Dpg) at alternating positions and their α-amino acid counterpart L-norvaline (Nva) analogues were synthesized to fully investigate the impact of Dpg on peptide backbone structure in aqueous solution. CD, VCD and NMR spectral analysis suggest that Dpg containing peptides adopt more ordered structures relative to their Nva containing analogues. The central residues (Ala, Thr, Tyr, Val) and the charged side-chains of Glu and Lys play important roles in the degree of peptide folding. Hydrophobic and branched residues (Val, Tyr) at the central position of the peptide produce greater folding as judged by CD and NMR. Temperature-dependent NMR analysis (Δδ/ ΔΤ ΝΗ) of Ac-Glu-Dpg-Tyr-Dpg-Lys-NH2 suggests a series of i→i+3 hydrogen bonds between the N-terminal acetyl carbonyl and the Tyr3NH, and the Glu1 carbonyl and the Dpg4NH. The solution conformation of Ac-Glu-Dpg-Tyr-Dpg-Lys-NH2 calculated from NMR-derived constraints shows a 310-helical structure (two repetitive type-III β-turns) at residues 1-4, which is supported by 2D NMR, CD and VCD spectra. Analysis of NMR-derived models of these peptides suggest that there is a strong hydrophobic interaction of the pro-S propyl side chain of Dpg2 and the Tyr3 side-chain that may be a strong stabilizing force of the peptide folding in water. Cα,α-diisobutylglycine(Dibg) was synthesized via palladium catalyzed allylation reaction with an excellent overall yield and incorporated into various positions of a model β-hairpin peptide GHP in order to determine the effectiveness of the ααAAs as design elements in both the strand and turn portions of β-hairpins. CD and NMR data of Dibg containing peptides showed Dibg residue can contribute to the stability of the strand portion of a β-hairpin peptide and destabilize the β-turn in the GHP. The sheet stabilizing effect of Dibg may be due to the strong propensity of Dibg to have a fully extended conformation(φ/ψ = 180°). Several chiral amino esters were prepared with high enantioselectivity by alkylation of the corresponding Schiff bases under chiral phase-transfer condition. The enantiomeric excess of these chiral amino esters was efficiently determined by 19F-NMR analysis of the corresponding diastereomeric Mosher amides.