Unusual intramolecular hydrogen bonding in cycloamanide A, cyclic (LPro-LVal-LPhe-LPhe-LAla-Gly). A crystal structure analysis.

The naturally occurring cyclic hexapeptide, cycloamanide A, has only one intramolecular hydrogen bond. It is a 4 leads to 1 type that encompasses the L Phe-L Ala sequence in which the experimentally determined phi, psi values are +54 degrees, -118 degrees and -88 degrees, -4 degrees, respectively. Even though the chirality is L, L, the phi, psi values are characteristic for a D, L beta-bend, Type II'. The conformation of the molecule was established by a crystal structure determination using X-ray diffraction analysis. Cycloamanide A (C33H42N6O6 . 4H2O) crystallizes in space group P2(1)2(1)2(1) with cell parameters a = 13.307(2) A, b = 24.820(4)A and c = 11.231(1)A.