Structure characterization of flavonoid O-diglycosides by positive and negative nano-electrospray ionization ion trap mass spectrometry.

Isomeric flavonoid O-diglycosides were analyzed by positive and negative nano-electrospray ionization (ESI) ion trap mass spectrometry (ITMS) in order to evaluate whether the two most common interglycosidic linkage types, i.e. 1 --> 2 and 1 --> 6, found for glycosides containing a rhamnosylglucose glycan part can be differentiated. In the positive ion mode the degree of internal glucose residue loss was found to be strongly dependent on the aglycone type and was very pronounced for aglycones of the flavanone type. The relative abundance of the Y-type ions formed by fragmentation at glycosidic bonds only allows one to infer the interglycosidic linkage types in the case of flavone O-diglycosides. In contrast, the negative ion mode makes a clear differentiation between a rutinoside (1 --> 6) and a neohesperidoside (1 --> 2) glycan residue possible for all aglycone types. The neohesperidose-containing compounds could be characterized by additional product ions. When the compounds were dissolved in pure methanol a molecular radical ion was found to be the base peak in nano-ESI. Copyright 2001 John Wiley & Sons, Ltd.