Helical structures of homo-chiral isotope-labeled α-aminoisobutyric acid peptides

The chiral deuterium- and 13C-isotope-labeled α-aminoisobutyric acids CD3-Aib and 13CH3-Aib were enantioselectively synthesized from L-Ala aldimine using simplified Maruoka chiral phase-transfer catalysts. Homo-chiral (S)-CD3-Aib homopeptides, up to decamers, were prepared. A (R)-CD3-Aib polymer and (S)-13CH3-Aib polymer were also prepared. Conformational studies on homopeptides using CD spectra and an X-ray crystallographic analysis revealed that the preferred conformations were 310-helical structures comprising equal amounts of right-handed (P) and left-handed (M) helical-screw structures. The α-carbon chiral centers induced by the D- or 13C-isotope substitution of Aib were incapable of controlling the helical-screw directions of their oligopeptides and short polymers. [Display omitted]