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

The chiral deuterium- and 13 C-isotope-labeled α-aminoisobutyric acids CD 3 -Aib and 13 CH 3 -Aib were enantioselectively synthesized from L-Ala aldimine using simplified Maruoka chiral phase-transfer catalysts. Homo-chiral ( S )-CD 3 -Aib homopeptides, up to decamers, were prepared. A ( R )-CD 3 -Aib polymer and ( S )- 13 CH 3 -Aib polymer were also prepared. Conformational studies on homopeptides using CD spectra and an X-ray crystallographic analysis revealed that the preferred conformations were 3 10 -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 13 C-isotope substitution of Aib were incapable of controlling the helical-screw directions of their oligopeptides and short polymers. [ABSTRACT FROM AUTHOR]