Study on the spatial structure of annomuricatin A, a cyclohexapeptide from the seeds of Annona muricata

A cyclic hexapeptide, annomuricatin A (the molecular formula: C27H38N6O7), was isolated from the seeds of Annona muricata. The types and sequence of the amino acids were confirmed by X-ray diffraction analysis. The stereochemistry of the title cyclopeptide was clarified by X-ray crystallographic study. The backbone contains two β-turns, one is type I β-turn and the other is type II, which are stabilized by two transannular 4 → 1 backbone hydrogen bonds between Ala and Phe. There are intermolecular hydrogen bonds between the cyclopeptide and the solvent molecules which maintained the steady spatial arrangement in crystal.