title: Low energy induced homolytic fragmentation of flavonol 3-O-glycosides by negative electrospray ionization tandem mass spectrometry.
abstract: BACKGROUND
  Negative 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.
  
  
  METHODS
  The (-)-ESI-MS(n) 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.
  
  
  RESULTS
  Under the unified CID-QIT-MS(2) 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 [Y0-H](-•). 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 1 → 2 glycosidic bond generated more abundant [Y0-H](-•) product ions than those with a 1 → 3 or 1 → 6 bond. The terminal sugar triggered the homolytic fragmentation in the order Rha > Xyl > Glc. Moreover, new counterexamples were found for previously reported fragmentation rules.
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, 38 Xueyuan Road, Beijing, 100191, China.
authors.forename: Wen-Zhi
authors.initials: WZ
authors.lastname: Yang
authors.forename: Xue
authors.initials: X
authors.lastname: Qiao
authors.forename: Tao
authors.initials: T
authors.lastname: Bo
authors.forename: Qing
authors.initials: Q
authors.lastname: Wang
authors.forename: De-An
authors.initials: DA
authors.lastname: Guo
authors.forename: Min
authors.initials: M
authors.lastname: Ye
mesh_terms: D000107:Acetylation; D005591:Chemical Fractionation; D044948:Flavonols / Q000032:analysis* / Q000737:chemistry; D006027:Glycosides / Q000032:analysis* / Q000737:chemistry; D006031:Glycosylation; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D044948:Flavonols; D006027:Glycosides