STRUCTURE OF Z-(AIB)(9)OBUT

The structure of the synthetic protected oligopeptide Z-(Aib)(9)OBu', tert-butoxynona(alpha-aminoisobutyric acid), which contains the unusua l cr-aminoisobutyric acid (Aib), was determined by X-ray crystallograp hy. The two independent molecules in the asymmetric unit fold into 3(1 0)-helices, each stabilized by seven intramolecular hydrogen bonds. Th e C terminus of one of the molecules is disordered and adopts a semi-e xtended conformation, which is rather unusual for Aib residues. This i s the first observation of such a conformation involved in a disorder in Aib-containing oligopeptides. The existence of a second conformatio n for the C-terminal residue might explain the difficulties in crystal lizing the title compound and a different behaviour of the title compo und in thin layer chromatography compared with the other homopeptides.