Assigning glucose or galactose as the primary glycosidic sugar in 3-O-mono-, di- and triglycosides of kaempferol using negative ion electrospray and serial mass spectrometry.
Kaempferol 3-O-beta-glucopyranoside, kaempferol 3-O-beta-galactopyranoside and higher glycosides of these two flavonoids with alpha-rhamnose at C-2 and/or C-6 of the primary sugar were studied by negative ion electrospray ionisation and serial mass spectrometry in a three-dimensional (3D) ion trap mass spectrometer. Kaempferol 3-O-beta-glucopyranoside and kaempferol 3-O-alpha-rhamnopyranosyl(1-->6)-beta-glucopyranoside could be distinguished from their respective galactose analogues by differences in the ratio of the radical aglycone ion [Y(0) - H](*-) to the rearrangement aglycone ion Y(0) (-) following MS/MS of the deprotonated molecules. Kaempferol 3-O-rhamnopyranosyl(1-->2)-beta-glucopyranoside and kaempferol 3-O-alpha-rhamnopyranosyl(1-->2)[alpha-rhamnopyranosyl(1-->6)]-beta-glucopyranoside could be distinguished from their respective galactose analogues by differences in the product ion spectra of the [(M - H) - rhamnose](-) ion following serial mass spectrometry. In the triglycoside, it was deduced that this ion resulted from the loss of the rhamnose substituted at 2-OH of the primary sugar by observing that MS/MS of deprotonated kaempferol 3-O-beta-glucopyranosyl(1-->2)[alpha-rhamnopyranosyl(1-->6)]-beta-glucopyranoside showed the loss of glucose and not rhamnose. Thus the class of sugar (hexose, deoxyhexose, pentose) at C-2 and C-6 of the primary sugar can be determined. These observations aid the assignment of kaempferol 3-O-glycosides, having glucose or galactose as the primary glycosidic sugar, in LC/MS analyses of plant extracts, and this can be done with reference to only a few standards.
Kite
Geoffrey C
GC
Royal Botanic Gardens, Kew, Richmond TW9 3AB, UK. g.kite@kew.org
Veitch
Nigel C
NC
D005690:Galactose / Q000737:chemistry*; D005947:Glucose / Q000737:chemistry*; D006027:Glycosides / Q000737:chemistry*; D044949:Kaempferols / Q000737:chemistry*; D010936:Plant Extracts / Q000737:chemistry; D010944:Plants / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
D006027:Glycosides; D044949:Kaempferols; D010936:Plant Extracts; C006552:kaempferol; D005947:Glucose; D005690:Galactose