Differentiation of isomeric malonylated flavonoid glyconjugates in plant extracts with UPLC-ESI/MS/MS

Introduction: Glycosylation at different hydroxyl groups of flavonoids and acylation of sugar moieties are ubiquitous modifications observed in plants. These modifications give rise to simultaneous presence of numerous isomeric and isobaric compounds in tissues and extracts thereof. Objective: To develop UPLC‐MS method capable for resolution of isomeric malonylated glycoconjugates of flavonoids and recognition of structural differences. Methodology: Flavonoid glycoconjugates were extracted from leaves of blue lupin (Lupinus angustifolius L.) plants with 80% methanol. Extracts were analysed using ultraperformance liquid chromatography (UPLC) combined with tandem (quadrupole–time of flight, QToF) mass spectrometry. Results: Differentiation of malonylated glycosides of isoflavones and flavones is demonstrated in this paper. The use of UPLC‐MS/MS enabled 38 flavonoid conjugates to be distinguished, including the discrimination of five different isomers of a single 3′‐O‐methylluteolin glycoside. Additionally, pseudo MS³ experiments (CID spectra registered at high cone voltages) enabled confirmation of the aglycone structures by comparison of their spectra with those obtained from aglycone standards. Conclusions: Application of UPLC‐MS/MS allows separation and identification numerous positional isomers of malonylated glycosides of flavonoids and isoflavonoids in plant material. Provided there is strict control of the MS ionisation parameters, this method may be useful for preparation of a flavonoids spectra database, enabling the inter‐laboratory comparison of analytical results. Copyright © 2008 John Wiley & Sons, Ltd.