Characterization of C-glycosyl flavones O-glycosylated by liquid chromatography–tandem mass spectrometry

Fifty three O-glycosyl-C-glycosyl flavones with O-glycosylation on phenolic hydroxyl or on the C-glycosyl residue or combination of both forms have been studied by liquid chromatography-UV diode array detection-electrospray ionisation mass spectrometry ion trap in the negative mode. The study of the relative abundance of the main ions from the MS preferential fragmentation on –MS2 and/or –MS3 events allows the differentiation of the position of the O-glycosylation, either on phenolic hydroxyl or on the sugar moiety of C-glycosylation. In addition, it is possible to discriminate between O-glycosylation at 2″ and at 6″ positions. The occurrence of an abundant ion Y0− ([(M − H)-132/-146/-162]−, mono-O-pentosyl/rhamnosyl/hexosyl-C-glycosyl derivatives) after –MS2 fragmentation characterizes the O-glycosylation on phenolic hydroxyls. The preferential fragmentation leading to a relevant Z1− ([Y1-18]−) fragment is characteristic of 2″-O-glycosyl-C-glycosyl derivatives. The 6″-O-glycosyl-C-glycosyl derivatives are characterized by 0,2X0−, which is generated by a global loss of the sugar moiety from the O-glycosylation at 6″ and the glycosidic fraction that involves the carbons 6″-3″ of the C-glycosyl residue ([(M − H)-162-120]−, in the case of 6″-O-hexosyl-C-hexosyl derivatives). Regarding the combined O-glycosylated compounds (both on phenolic hydroxyl and on sugar moiety at C-glycosylation), the main fragmentation on –MS2 events produces a Y0− characterizing the O-glycosylation on the phenolic hydroxyl, and the –MS3[(M − H) → Y0]− fragmentation of the O-glycosylation on the C-glycosyl residue.