Variability in the backbone conformation of cyclic pentapeptides

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Abstract
All the peptide bonds in cyclic (Gly-L-Pro-D-Phe-Gly-L-Ala) are in the trans conformation; however, the peptide bond C''5-N1 is twisted by 19.degree. from planarity (.omega.5s = -161.degree.). A Type II .beta.-turn encompasses the L Pro-D Phe residues. Carbonyl oxygen O2, O4 and O5 are directed to the same side of the average plane through the backbone ring and they form hydrogen bonds with N3, N5 and N1, respectively, in adjacent molecules in a stacked column where the adjacent molecules are related by one translational unit. The conformation of the backbone is different from that established in other molecules with the .**GRAPHIC**. chirality sequence. The P21 cell contains two molecules of C21H26N5O5 with a = 4.836(2) A, b = 18.346(8) A, c = 12.464(5) A and .beta. = 100.05(4).degree.. The R factor for 1382 data with F0 > 1 .sigma. is 7.0%.