Conformation of cyclo-bis(-L-valyl-L-prolyl-D-alanyl-), a synthetic cyclic hexapeptide

Abstract

C26H42N6O6, Mr = 534.7, monoclinic, C2, a = 20.526 (2), b = 4.923 (1), c = 17.092 (2) A, beta = 126.37 (1) degrees, V = 1390.9 A3, Z = 2, Dm not measured, Dx = 1.28 g cm-3, lambda (Cu K alpha) = 1.5418 A, mu = 7.1 cm-1, F(000) = 576, R = 0.050, wR = 0.049 for 1012 reflections [I greater than 2 sigma (I)], 1501 unique reflections measured at room temperature (296 K). The synthetic cyclic hexapeptide, cyclo-bis(-L-Val-L-Pro-D-Ala-), exhibits exact C2 symmetry in the crystalline state with cis peptide links [omega = -13.1 (7) degrees] between Val and Pro residues; there are no intramolecular hydrogen bonds. The cyclic ring consists of two type VIb cis proline turns fused at the D-Ala residue. The backbone dihedral angles are all in the extended range except for psi Val [72.9 (5) degrees] and phi Pro [-78.9 (5) degrees] on either side of the cis peptide link. The carbonyl O atoms and the amide N atoms in the extended portion of the cyclic peptide form intermolecular hydrogen bonds with another cyclic hexapeptide molecule translated by a cell edge along the crystallographic b axis, forming an infinite stretch of beta-sheets. The parallel beta-sheet structures are separated by about 3.15 A.