Multiple pathways of the reaction of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH·) with (+)-catechin: evidence for the formation of a covalent adduct between DPPH· and the oxidized form of the polyphenol.

The pathways of the reaction of 2,2-diphenyl picrylhydrazyl radicals (DPPH·) with (+)-catechin were studied in alcoholic solvents. The reaction mixtures were analysed by using reversed-phase liquid chromatography (HPLC) and electrospray ionization mass spectrometry (ESI-MS). The intermediate o-quinone of catechin, yellow dimers, trimers and, interestingly, an adduct of the oxidized form of catechin with DPPH radicals were identified. The mass of this adduct was 681 Da, suggesting that one molecule of the DPPH radical complexes with the oxidized form of catechin. It is concluded that once the intermediate o-quinone is formed, the reaction proceeds in two pathways, either the o-quinone reacts with catechin to form a hydrophilic dimer (type B), which is further oxidized to hydrophobic dimers (type A) and consequently to oligomers of higher molecular weights; or the A-ring of the o-quinone is further oxidized by a DPPH radical and that this oxidized intermediate then reacts with another DPPH radical to form the observed adduct. The identification of the latter mechanism could explain the contradictory results reported in the literature for the reaction of polyphenols with DPPH radicals.