CRYSTAL-STRUCTURE AND MOLECULAR-CONFORMATION OF THE CYCLIC HEXAPEPTIDE CYCLO-(GLY-L-PRO-GLY)2

The structure of cyclo-(Gly-L-Pro-Gly)2 has been determined by X-ray diffraction methods on a single crystal of symmetry P21. There are two formula weights of C18H26N6O6.4H2O in a unit cell having parameters a = 7.6194 (6), b = 21.0709 (19), c = 7.7018 (6) Å and ² = 108.904 (9)°. An orientation-translation search utilizing a mode! derived from the known geometry of cyclo-(Gly-L-Pro-D-Ala)2 yielded the structure, which refined to R = Σ|F0| - |Fc|ΣF0 = 0.060 for 2040 reflections. This cyclic hexapeptide deviates from internal twofold symmetry by forming one 4-1 transannular hydrogen bond in a type II ² turn, and in the other half of the molecule a type I ² turn without an internal hydrogen bond. In contrast, two earlier NMR studies have shown that this molecule has C2 symmetry on the NMR time scale. We also describe here the hydrogen bonds to water of crystallization. © 1979, American Chemical Society. All rights reserved.