To aid in the identification and quantification of biologically and agriculturally significant natural products, tandem mass spectrometry can provide accurate structural information with high selectivity and sensitivity. In this study, diagnostic fragmentation patterns of isoflavonoids were examined by liquid chromatography-ion trap-time of flight-mass spectrometry (LC-IT-TOF-MS). The fragmentation scheme for [M+H-2CO]+ ions derived from isoflavones and [M+H-B-ring-CO]+ ions derived from 5-hydroxyisoflavones, were investigated using different isotopically labeled isoflavones, specifically [1',2',3',4',5',6',2,3,4-13C9] and [2',3',5',6',2-D5] isoflavones. Specific isotopically labeled isoflavones were prepared through the biosynthetic incorporation of pharmacologically applied 13C- and D-labelled L-phenylalanine precursors in soybean plants following the application of insect elicitors. Using this approach, we empirically demonstrate that the [M+H-2CO]+ ion is generated by an intramolecular proton rearrangement during fragmentation. Furthermore, [M+H-B-ring-CO]+ ion is demonstrated to contain a C2H moiety derived from C-ring of 5-hydroxyisoflavones. A mechanistic understanding of characteristic isoflavone fragmentation patterns contributes to the efficacy and confidence in identifying related isoflavones by LC-MSn.
Keywords: CID: collision induced dissociation; ESI: electrospray ionization; HPLC: high performance liquid chromatography; Isoflavone; LC-IT-TOF-MS; LC-IT-TOF-MS: liquid chromatography-ion trap-time of flight-mass spectrometry; LC-MSn: liquid chromatography-multiple mass spectrometry; MS/MS: tandem mass spectrometry; MSn: multiple mass spectrometry; plant-insect interaction; soybean.