Structure analysis on complexes of xanthone glycosides and metal ions using electrospray ionization tandem mass spectrometry

The structure and fragmentation mechanism of metal (Mn(II), Al(III) and Cu(II)) complexes of xanthone glycosides as well as metal coordination sites were investigated by electrospray ionization tandem mass spectrometry. Four Mn(II) complexes of xanthone glycoside with different stoichiometric ratios were observed from swertianin 7-O-rhamnosyl(l -> 2)xyloside, swertianin 7-O-xylosy](1 -> 2)xyloside and swertianolin, while only two complexes were formed for 3,7,8-trimethoxy-1-O-primeverosyloxanthone, 2,3,5-trimethoxy-1-O-primeverosyloxanthone and 2,3,4,5-tetramethoxy-1-O-primeverosyloxanthone, respectively. For xanthone glycoside-Cu(II) complexes, three complexes with different stoichiometric ratios were observed from swertianin 7-O-rhamnosyl(l -> 2)xyloside, swertianin 7-O-xylosyl(l -> 2) xyloside and swertianolin, but only one Al(III) complexes were formed for above three xanthone glycosides. Based on the analysis of metal complexes by tandem mass spectrometry, a fragmentation mechanism for 1:1 swertianin 7-O-rhamnosyl(l -> 2)xyloside-Mn(II) complex was proposed and the structures of four swertianin 7-O-rhamnosyl(1 -> 2)xyloside-Mn(II) complexes were interpreted. In addition, the hydroxyl groups in the oligosaccharide chains, the hydroxyl groups substituted at C-1 and C-8, and the carbonyl group in the C-ring of the aglycone, were proved to be the dominant coordination sites in the xanthone glycosidemetal complexes.