Structural characterization of isoprenylated flavonoids from Kushen by electrospray ionization multistage tandem mass spectrometry

摘要

Eighteen isoprenylated flavonoids (8 flavanones, 3 flavanols, and 7 chalcones) isolated from Kushen or synthesized were studied by positive and negative ion electrospray ionization multistage tandem mass spectrometry (ESI-MSn). Plausible fragmentation patterns were obtained by comparing their MSn spectra with each other, which were further supported by high-resolution MS data and two model compounds. It was shown that the 2 '-OH group would make the C-ring of flavonoids studied more labile through a six-membered mechanism, resulting in base peaks of (1,3)A+ (positive mode) and (1,4)A(-) (negative mode). In addition, the 2'-OH is also responsible for the neutral loss of water in (+)ESI/MS2 of flavanones. The neutral loss of water (or methanol) in (-)ESI/MS2 of flavanols was elucidated by a E2 elimination mechanism. Different relative abundances (RA) of (1,3)A(+) and S+ in (+)ESI/MS2 spectra were used to discriminate flavanones with their open-ring products, chalcones, since the equilibrium for flavanone <-> chalcone isomerization in ESI ion source could not be obtained in positive mode.