Sequence and solution structure of cherimolacyclopeptides A and B, novel cyclooctapeptides from the seeds of Annona cherimola

Two new cyclooctapeptides, cherimolacyclopeptide A, cyclo(Pro¹-Gln²-Thr³-Gly⁴-Met⁵-Leu⁶-Pro⁷-Ile⁸-) (1) and the related cherimolacyclopeptide B, cyclo(Pro¹-Gln²-Thr³-Gly⁴-Mso⁵-Leu⁶-Pro⁷-Ile⁸-) (2), have been isolated from the methanol extract of the seeds of Annona cherimola Miller. The sequences 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-heteronuclear NMR. The three-dimensional solution structure of cherimolacyclopeptide A (1) determined by ¹H NMR data and molecular modelling is characterised by the presence of two β turns and a new type of β-bulge.