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

Abstract Two new cyclooctapeptides, cherimolacyclopeptide A, cyclo (Pro 1 -Gln 2 -Thr 3 -Gly 4 -Met 5 -Leu 6 -Pro 7 -Ile 8 -) ( 1 ) and the related cherimolacyclopeptide B, cyclo (Pro 1 -Gln 2 -Thr 3 -Gly 4 -Mso 5 -Leu 6 -Pro 7 -Ile 8 -) ( 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 1 H NMR data and molecular modelling is characterised by the presence of two β turns and a new type of β-bulge.