3D-structure of cycloreticulin C and glabrin A, cyclopeptides from the seeds of Annona reticulata

Abstract

Two cyclopeptides, the cycloheptapeptide cycloreticulin C, cyclo(Pro 1-Gly 2-Gln 3-Pro 4-Pro 5-Tyr 6-Val 7) ( 1), and the cyclohexapeptide glabrin A, cyclo(Pro 1-Gly 2-Leu 3-Val 4-Ile 5-Tyr 6) ( 2), have been isolated from the methanol extract of the seeds of Annona reticulata. Their structures were elucidated on the basis of the MS/MS fragmentation using a Q-TOF mass spectrometer equipped with an ESI source, chemical degradation and extensive 2D-NMR. The solution conformation of cycloreticulin C involves two β-turns, one of type II with trans-Pro 1 and Gly 2 at the corners, another of type VIa with trans-Pro 4 and cis-Pro 5 at the corners, and followed by a β-bulge at the Tyr 6-Val 7 level. The solid state and solution conformations of glabrin A have been analyzed by X-ray and 2D-NMR studies, respectively, and are characterized by the presence of two β-turns, the first of type VIa and the second intermediary between types I and III at the solid state and a γ-turn in solution.