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

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 beta-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 beta-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 beta-turns, the first of type VIa and the second intermediary between types I and III at the solid state and a gamma-turn in solution. (C) 2008 Elsevier Ltd. All rights reserved.