Structural characterization of flavonol 3,7-di-O-glycosides and determination of the glycosylation position by using negative ion electrospray ionization tandem mass spectrometry

Flavonol 3,7-di-O-glycosides were investigated by negative ion electrospray ionization tandem mass spectrometry using a quadrupole linear ion trap (LIT) mass spectrometer. The results indicate that the fragmentation behavior of flavonol 3,7-di-O-glycosides is substantially different from that of their isomeric mono-O-diglycosides. In order to characterize a flavonoid as a flavonol 3,7-di-O-glycoside, both [Y-0(3) - H](-center dot)and [Y-0 - 2H](-) ions should be present in [M - H](-) product ion spectrum. The MS3 product ion spectra of Y-0(3)-, [Y-0(3) - H](-center dot) and y(0)(7)(-) ions generated from the [M - H](-) ion provide sufficient structural information for the determination of glycosylation position. Furthermore, the glycosylation positions are determined by comparing the relative abundances of Y-0(3)- and y(0)(7)(-) ions and their specific fragmentation patterns with those of flavonol mono-O-glycosides. In addition, a [Y-0(3) - H](-center dot) ion formed by the homolytic cleavage of 3-O glycosidic bond with high abundance points to 3-O glycosylation, while a [Y-0 - 2H](-) ion formed by the elimination of the two sugar residues is consistent with glycosylation at both the 3-O and 7-O positions. Investigation of negative ion ESI-MS2 and MS3 spectra of flavonol O-glycosides allows their rapid characterization as flavonol 3,7-di-O-glycoside and their differentiation from isomeric mono-O-diglycosides, and also enables their direct analysis in crude plant extracts. Copyright (c) 2006 John Wiley & Sons, Ltd.