Direct stereochemical assignment of hexose and pentose residues in flavonoid O‐glycosides by fast atom bombardment and electrospray ionization mass spectrometry

Abstract

Mass spectrometric methods have been developed which allow the direct stereochemical assignment of terminal monosaccharide residues in flavonoid O-glycosides without the need for chemical hydrolysis. Standards containing a glucose, galactose, mannose, xylose, arabinose or apiose residue were examined because these monosaccharides are by far the most commonly encountered in flavonoid glycosides. Following acetylation, the major peracetylated sugar related fragments, generated by fast atom bombardment (FAB) or electrospray ionization (ESI), were selected for collisional activation employing a broad range of collision energies. Both FAB and ESI proved to be useful as ionization techniques. Stereoselective fragmentation was achieved and allowed us clearly to differentiate and characterize isomeric monosaccharide residues. The method developed was successfully applied to an unknown flavonoid containing a terminal pentose and hexose residue which was isolated from Farsetia aegyptia.

Anomeric distinction and oxonium ion formation in acetylated glycosides

“…The characteristic presence of the ion at m/z 271 in the CID spectra of ammonium-cationized isomeric 1 and 2 is also apparent in the MS 3 spectra of the corresponding ions at m/z 331 (not shown). 12 This confirms that the ion at m/z 271 is the product of acetic acid loss from the oxonium ions at m/z 331 in the CID of 1 and 2 that contain an equatorial acetyl group at C 4 . On the other hand, only consecutive losses of two acetic acid moieties, upon collisional activation of the oxonium ions produced from 3, occur.…”

Strategies for differentiation of isobaric flavonoids using liquid chromatography coupled to electrospray ionization mass spectrometry

“…Even though it has been shown that it is possible to distinguish aglycones with isobaric substituents [28][29][30] and the same substituent(s) at different positions [29] this is difficult when evaluating the results from compound 7, 8 and 22, 23, 24 with the settings used in method 1 in this study. To address this limitation a CE ramp was used in an attempt to distinguish compound 7, 8 and 22, 23, 24 (see discussion about method 3 below).…”

Determination of the glycosylation site of flavonoid monoglucosides by metal complexation and tandem mass spectrometry

“…Several groups have reported characterization of flavonoids by electron ionization (EI) [6 -10] or fast atom bombardment (FAB) mass spectrometry [7,[11][12][13][14][15][16][17][18][19]. More recently, electrospray ionization (ESI) [19 -29], atmospheric pressure chemical ionization (APCI) [27][28][29][30][31], and matrix-assisted laser desorption ionization (MALDI) [32,33] have been used to analyze flavonoids, typically in conjunction with tandem mass spectrometry (MS/MS) or high-performance liquid chromatography (HPLC).…”

“…Otherwise only tentative identifications can be made, particularly in terms of saccharide location and identity, unless supplementary analytical methods, such as UV, FTIR, or NMR spectroscopy, are employed. Some researchers have tried derivatization to enable unambiguous identification by mass spectrometry [11,19,34], but this is a difficult and time-consuming approach.…”

“…In addition, commonly encountered diglycosyl flavonoids can often be distinguished by their fragmentation path-ways [14,18,20,51,52]. For less common flavonoid glycosides, methods have recently been proposed for determining the saccharide identity [19], as well as the linkage order [14]. Furthermore, distinctive patterns have been reported for methoxylated flavonoids [15,25] and for chlorinated and nitrated isoflavonoids [26].…”