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 Y(0)(-) ([(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 Z(1)(-) ([Y(1)-18](-)) fragment is characteristic of 2''-O-glycosyl-C-glycosyl derivatives. The 6''-O-glycosyl-C-glycosyl derivatives are characterized by (0,2)X(0)(-), 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 Y(0)(-) characterizing the O-glycosylation on the phenolic hydroxyl, and the -MS3[(M-H)-->Y(0)](-) fragmentation of the O-glycosylation on the C-glycosyl residue.