Non-proteinogenic amino acids - asymmetric synthesis, secondary structure, and biological activity -

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Optically active α,α-disubstituted amino acids were synthesized using (S,S)-cyclohexane-1,2-diol as a chiral auxiliary. Homo- and hetero-peptides containing α-ethylated α,α-disubstituted amino acid, αEtLeu in Aib and Deg sequences were prepared, and also heteropeptide constituted of diverse α-ethylated α,α-disubstituted amino acids was synthesized. The conformation of pentapeptides was studied by using CD, FT-IR, H NMR, and X-ray. In solution, the dominant conformation of heteropeptide Tfa-[(S)-αEtVal-(S)-αEtLeu-(S)-αEtNva-Deg-(S)-αEtNle]-OEt was a fully planar C_5-conformation, and those of the other peptides were 3_<10>-helical structures. In the crystalline state, all pentapeptides showed the 3_<10>-helical structures. Furthermore, we synthesized chiral cyclic α,α-disubstituted amino acids starting from dimethyl tartrate, and prepared its homo- and heteropeptides. The conformation of its peptides seem to be formed the 3_<10>-helical structure preferentially. The antibacterial activity of peptides was examined, but none of peptides showed strong activity.