Crystal-state 3D-structural characterization of novel, Aib-based, turn and helical peptides

The crystal-state conformations of the hexapeptide amide Pht-(Aib)6-NH-C(CH3)2-O-OtBu (7), the hexapeptide Ac-L-aIle-(Aib)5-OtBu (6), the pentapeptide Z-(Aib)3-L-Glu(OtBu)-Aib-O-(CH2)2-(1)Nap (5), the tetrapeptides Z-(Aib)2-L-His(Nτ-Trt)-Aib-OMe (4 I) and Z-(Aib)2-L-Nva-Aib-OtBu (4 II), the tripeptide Pyr-(Aib)3-OtBu (3 I), the dipeptide amides Pyr-(Aib)2-(4)NH-TEMPO (3 II) and Piv-(Aib)2-NH-C(CH3)2-O-OtBu (3 III), and the dipeptides Pht-Aib-βAc6c-OtBu (2 I), Pht-Aib-NH-C(CH3)2-O-OtBu (2 II) and Boc-gGly-mAib-OH (2 III) have been determined by X-ray diffraction analyses. All peptides investigated are characterized by one or more turn/helix forming Aib residues. Except the three short dipeptides, all are folded into CO···HN intramolecularly H-bonded 310-helices, or into various types of β-turns. In the structure of 6, two independent molecules of opposite screw sense were observed in the asymmetric unit, generating diastereomeric 310-helices