CONFORMATION OF CYCLIC OCTAPEPTIDES .6. STRUCTURE OF CYCLO-BIS-(-L-ALANYL-GLYCYL-L-PROLYL-L-PHENYLALANYL-) TETRAHYDRATE

C38H48N8O8.4H2O, M(r) = 816.9, monoclinic, P2(1), a = 10.381 (1), b = 13.273 (1), c = 15.742 (1) angstrom, beta = 101.83 (1)-degrees, V = 2123.1 angstrom 3, Z = 2, D(x) = 1.278 g cm-3, lambda-(Cu K-alpha) = 1.5418 angstrom, mu = 7.6 cm-1, F(000) = 872, R = 0.035, wR = 0.045 for 3497 reflections [I > 2-sigma-(I)], 4552 unique reflections measured. The synthetic cyclic octapeptide crystallizes from water/methanol solution as a tetrahydrate and the crystals are isomorphous to those of the disulfide-bridged cystine analog cyclo-bis(-L-Cys-Gly-L-Pro-L-Phe-) [Kopple, Wang, Cheng & Bhandary (1988). J. Am. Chem. Soc. 110, 4168-4176]. The coordinates of the Cys analog were taken as the starting coordinates for full-matrix least-squares refinement. The cyclic octapeptide ring has two beta turns encompassing the residues Pro-L-Phe, one type I and the other type II, with all peptide links trans. The conformation of the cyclic octapeptide backbone is similar to the Cys analog; all backbone dihedral angles in the two molecules agree to within 6-degrees. This suggests that the disulfide bridge of the Cys analog does not impose any conformational constraint on the octapeptide ring backbone.