Regeneration of phenolic antioxidants from phenoxyl radicals: An ESR and electrochemical study of antioxidant hierarchy

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Abstract
Radicals from the flavonoids quercetin, (+)-catechin, (±)-taxifolin and luteolin, and from all-rac-α-tocopherol have been generated electrochemically by one-electron oxidation in deaerated dimethylformamide (DMF), and characterised by electron spin resonance spectroscopy (ESR) after spin-trapping by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Simulations of the ESR spectrum based on estimated coupling constants of the spin-trapped quercetin radical, confirmed that this antioxidant radical is oxygen-centered. The complex mixture of radicals, quinoid intermediates and stable two-electron oxidation products, were for each antioxidant allowed to react with each of the four other antioxidants, and the progression of reaction followed by ESR after addition of DMPO, and the product solution further analysed by HPLC. All-rac-α-tocopherol was found to be most efficient in regenerating each of the other antioxidants from their oxidation products with a regeneration index (defined as moles regenerated of the oxidised phenolic antioxidant divided with moles of all-rac-α-tocopherol consumed) of 0.90 ± 0.16 for quercetin, 0.48 ± 0.11 for (+)-catechin, 0.48 ± 0.06 for (±)-taxifolin and 0.50 ± 0.10 for luteolin in equimolar 1.00 mM solution. Quercetin was found to have the highest regeneration index among the flavonoids: 0.88 ± 0.13 for (+)-catechin, 0.41 ± 0.03 for (±)-taxifolin and 0.41 ± 0.02 for luteolin. The antioxidant hierarchy based on the reduction potentials determined by cyclic voltammetry under similar conditions (0.93 V for all-rac-α-tocopherol, 1.07 V for quercetin, 1.15 V for luteolin, 1.16 V for (+)-catechin and 1.20 V for (±)-taxifolin) is compared with the observed over-all regeneration (34% for quercetin, 34% for (+)-catechin, 52% for (±)-taxifolin and 43% for luteolin by all-rac-α-tocopherol).