o-Quinone involvement in the prooxidant tendency of a mixture of quercetin and caffeic acid

The oxidation products of a previously tested prooxidant mixture of quercetin (Q) and caffeic acid (CA) at 1:2 ratio were analysed by LC–MS. The UV–Vis and MS spectra of three chromatographic peaks eluting at tR = 9.11 min, tR = 14.36 min and tR = 30.30 min were studied further. The structures of the tentatively identified compounds indicate polymeric molecules. A pentamer formed by 3 units of quercetin and 2 units of caffeic acid was attributed in the case of peaks tR = 9.11 min and tR = 14.36 min. The quercetin quinone (QQ) – a polymerization intermediate – is a fragment of the compound identified in the MS spectrum with the m/z = 323 coming from Q o-quinone m/z = 300 plus 23 from Na+. According to the UV–Vis spectrum, we suggest a different intermolecular arrangement which gives a more extended e-delocalisation. At tR = 30.30 min, the spectra helped us to tentatively identify this oxidation product as being a polymer of 4 CA units and 1 QQ.