UNUSUAL CONFORMATIONS ADOPTED BY STANDARD AMINO-ACIDS IN AIB-CONTAINING OLIGOPEPTIDES

The structures of the protected tetrapeptides Z-Aib-Aib-Aib-Val-OtBu ( I) and Z-Val-Aib-Aib-Gln-OtBu (II), which contain the conformationally constrained residue a-aminoisobutyric acid (Aib), have been studied b y X-ray crystallography. Both molecules in the asymmetric unit of I ad opt left-handed 3(10)-helical conformation, stabilized both by two 4 - -> 1 intramolecular hydrogen bonds. The structure of II consists of a beta-turn of type II and a consecutive beta-turn of type III' and is e xtended at the C-terminus. This structure is stabilized by three intra molecular hydrogen bonds. All non-Aib residues (Val, Gin) in I and II adopt rather unusual backbone conformations compared with other helica l structures in proteins.