Products of the Intermediate Oxidation of Flavonoids in Aqueous Solutions and the Determination of Their Composition by High-Performance Liquid Chromatography–Mass Spectrometry

Abstract

Using high-performance liquid chromatography in combination with electrospray ionization mass spectrometry [HPLC–MS-ESI(NEG)], quercetin [Q] and luteolin [L] intermediate oxidation products were investigated. It was found that, in aqueous solutions at pH 8.0 under aerobic conditions, quercetin forms double and triple donor-acceptor complexes of the composition {[Q][Q–H₂]} and {[Q][Q–H₂]₂}, which can be separated by HPLC, but are rapidly converted to deeper oxidation products. In the presence of another flavonoid, luteolin (L), the oxidized form of quercetin also forms mixed complexes of {[L][Q–H₂]} and {[L][Q–H₂]₂} types. Luteolin in the oxidation does not form donor-acceptor {[L][L–H₂]} complexes. Quercetin in complexes with luteolin is always present in an oxidized form. It can be assumed that these complexes form as a result of the recombination of two radicals of flavonoids, which formed initially as a result of manifestation of the antioxidant properties (one-electron transfer). These transformations of flavonoids are possible if strict anaerobic conditions in their quantitative determination are not implemented.