Left-Handed Helix of Three-Membered Ring Amino Acid Homopeptide Interrupted by an N-H···Ethereal O-Type Hydrogen Bond.

A chiral three-membered ring C-disubstituted α-amino acid ( R, R)-Acc, in which the α-carbon is not a chiral center, but two side chain β-carbons are chiral centers, was synthesized from dimethyl l-(+)-tartrate, and its homopeptides were prepared. X-ray crystallographic analysis of ( R, R)-Acc pentapeptide showed bent left-handed ( M) 3-helical structures with an unusual intramolecular hydrogen bond of the N-H···O (ethereal) type. The left-handedness of the bent helices was exclusively controlled by the side-chain β-carbon chiral centers.

Synthesis of chiral five-membered carbocyclic ring amino acids with an acetal moiety and helical conformations of its homo-chiral homopeptides.

Chiral five-membered carbocyclic ring amino acids bearing various diol acetal moieties were synthesized starting from l-malic acid, and homo-chiral homopeptides composed of cyclic amino acid (S)-Ac5 c(3EG) bearing an ethylene glycol acetal, up to an octapeptide, were prepared. A conformational analysis revealed that (S)-Ac5 c(3EG) homopeptides formed helical structures. (S)-Ac5 c(3EG) homopeptides, up to hexapeptides, formed helical structures without controlling the helical screw direction, while (S)-Ac5 c(3EG) hepta- and octapeptides formed helical structures with a preference for the left-handed (M) helical-screw direction.