Crystal structure and conformation of the cyclic hexapeptide cyclo-(Gly-L-Pro-D-Ala)2

Abstract

The crystal structure and conformation of cyclo-(Gly-L-Pro-D-Ala)/sub 2/, C/sub 20/H/sub 30/N/sub 6/O/sub 6/, has been determined by single-crystal x-ray diffraction analysis. The crystals have the symmetry of the orthorhombic space group P2/sub l/2/sub l/2/sub l/ with a = 9.389 (6), b = 10.379 (8), and c = 22.006 (16) A. The structure was solved by direct methods and refined to a final R of 0.055 on 1575 unique reflections. The hexapeptide shows significant deviation from internal twofold symmetry. This asymmetric conformer apparently optimized one single 4 ..-->.. l transannular hydrogen bond at the expense of other possible interactions. Conformation angles are compared with those obtained by CD and NMR techniques, and bond distances are discussed. This hexapeptide contains two type II ..beta.. turns. All of the peptide bonds are in the transconformation and the C/sub ..gamma../ atom in one of the proline rings is disordered. 5 tables, 3 figures.