Structure of cyclo(-L-prolylglycyl-)2 trihydrate.

(C14H20N4O4)2.3H2O, Mr = 670.721, monoclinic, P2(1), a = 7.353 (2), b = 21.921 (7), c = 9.878 (2) A, beta = 96.77 (2) degrees, V = 1581.1 (1) A3, Z = 2, Dx = 1.409 g cm-3, lambda(Cu Kalpha) = 1.54178 A, mu = 8.22 cm-1, F(000) = 716, T = 293 K, R = 0.034 for 2456 unique observed reflections. The two independent copies of the tetrapeptide found in the asymmetric unit have similar structures, which are both consistent with the results of NMR studies of cyclo-(-L-Pro-Gly-)2 in solution. The structures are asymmetric and have a trans-cis-trans-cis peptide backbone, in which the two L-Pro-Gly peptide bonds are trans and the two Gly-L-Pro peptide bonds are cis. A detailed comparison with other cyclic tetrapeptides is given, and a brief comparison with the results of single-crystal X-ray structures of other cyclic oligopeptides containing L-proline alternating with glycine is presented.