REDUCTION POTENTIALS OF FLAVONOID AND MODEL PHENOXYL RADICALS - WHICHRING IN FLAVONOIDS IS RESPONSIBLE FOR ANTIOXIDANT ACTIVITY

Model phenoxyl and more complex flavonoid radicals were generated by a zide radical induced one-electron oxidation in aqueous solutions. Spec tral, acid-base and redox properties of the radicals were investigated by the pulse radiolysis technique. The physicochemical characteristic s of the flavonoid radicals closely match those of the ring with the l ower reduction potential. In flavonoids which, have a 3,5-dihydroxyani sole (catechins), or a 2,4-dihydroxyacetophenone (hesperidin, rutin, q uercetin)-like A ring and a catechol- or 2-methoxyphenol-like B ring, the antioxidant active moiety is clearly the B ring [reduction potenti al difference between the model phenoxyls is Delta E(A-B ring models) > 0.1 V]. In galangin, where the B ring is unsubstituted phenyl, the a ntioxidant active moiety is the A ring. Even though the A ring is not a good electron donor, E(7) > 0.8/NHE V, it can still scavenge alkyl p eroxyl radicals, E(7) = 1.06 V, and the superoxide radical, E(7) > 1.0 6 V. Quercetin is the best electron donor of all investigated flavonoi ds (measured E(10.8) = 0.09 V, and calculated E(7) = 0.33 V). The favo urable electron-donating properties originate from the electron donati ng O-3 hydroxy group in the C ring, which is conjugated to the catecho l (B ring) radical through the 2,3-double bond. The conjugation of the A and B rings is apparently minimal, amounting to less than 2.5% of t he substituent effect in either direction. Thus, neglecting the acid-b ase equilibria of the A ring, and using those of the B ring and the me asured values of the reduction;potentials at pH 3, 7 and 13.5, the pH dependence of the reduction potentials of the flavonoid radicals can b e calculated. In neutral and slightly alkaline media (pH 7-9), all inv estigated flavonoids are inferior electron donors to ascorbate. Querce tin, E(7) = 0.33 V, and gallocatechins, E(7) = 0.43 V, can reduce vita min E radicals (assuming the same reduction potential as Trolox C radi cals, E(7) = 0.48 V). Since all investigated flavonoid radicals have r eduction potentials lower than E(7) = 1.06 V of alkyl peroxyl radicals , the parent flavonoids qualify as chain-breaking antioxidants in any oxidation process mediated by these radicals.