Structure of Z-(Aib)9OBut

The structure of the synthetic protected oligopeptide Z-(Aib)(9)OBu', tert-butoxynona(alpha-aminoisobutyric acid), which contains the unusual cr-aminoisobutyric acid (Aib), was determined by X-ray crystallography. The two independent molecules in the asymmetric unit fold into 3(10)-helices, each stabilized by seven intramolecular hydrogen bonds. The C terminus of one of the molecules is disordered and adopts a semi-extended conformation, which is rather unusual for Aib residues. This is the first observation of such a conformation involved in a disorder in Aib-containing oligopeptides. The existence of a second conformation for the C-terminal residue might explain the difficulties in crystallizing the title compound and a different behaviour of the title compound in thin layer chromatography compared with the other homopeptides.