Low energy induced homolytic fragmentation of flavonol 3-O-glycosides by negative electrospray ionization tandem mass spectrometry

RATIONALENegative ESI-QIT-MS of several subtypes of flavonoid O-glycosides is known to produce deprotonated molecular ions which undergo homolytic fragmentation at the glycosidic bond upon collision-induced dissociation (CID). However, these subtypes have never been simultaneously compared under unified MS conditions. METHODSThe (-)-ESI-MSn fragmentations of 69 flavonoid O-glycosides, involving eight subtypes, were analyzed using a quadrupole ion-trap mass spectrometer with collision energies varying from 18-44%. Factors influencing the homolytic glycosidic bond fragmentation, such as collision energy, hydroxylation of aglycone, and glycosylation pattern, were comprehensively studied. RESULTSUnder the unified CID-QIT-MS2 conditions, the precursor deprotonated molecular ions [M-H](-) for 3-O-glycosyl, 3,7-di-O-glycosyl and 3,6,7-tri-O-glycosyl flavonols experienced homolytic fragmentation at the glycosidic bond and generated the radical aglycone ion [Y-0-H](-center dot). This gas-phase CID fragmentation behavior was not observed for the other subtypes. A general trend was found that hydroxyl substitution at C-6, glycosylation at C-6/C-7, and acetylation of the saccharide moiety remarkably suppressed this fragmentation. In addition, flavonol 3-O-diglycosides (disaccharides) possessing a 12 glycosidic bond generated more abundant [Y-0-H](-center dot) product ions than those with a 13 or 16 bond. The terminal sugar triggered the homolytic fragmentation in the order Rha>Xyl>Glc. Moreover, new counterexamples were found for previously reported fragmentation rules. CONCLUSIONSThe low-energy CID homolytic fragmentation was diagnostic for structural identification of flavonol 3-O-glycosides. We have summarized key factors affecting this fragmentation. The results could be useful for rapid characterization of flavonoid O-glycosides in complicated herbal extracts. Copyright (c) 2014 John Wiley & Sons, Ltd.