Reduction potentials of flavonoid and model phenoxyl radicals. Which ring in flavonoids is responsible for antioxidant activity?

Abstract

Model phenoxyl and more complex flavonoid radicals were generated by azide radical induced oneelectron oxidation in aqueous solutions. Spectral, acid-base and redox properties of the radicals were investigated by the pulse radiolysis technique. The physicochemical characteristics of the flavonoid radicals closely match those of the ring with the lower reduction potential. In flavonoids which have a 3,s-dihydroxyanisole (catechins), or a 2,4-dihydroxyacetophenone (hesperidin, rutin, quercetin)-like A ring and a catechol-or 2-methoxyphenol-like B ring, the antioxidant active moiety is clearly the B ring [reduction potential difference between the model phenoxyls is AE(A-B ring models) > 0.1 V]. In galangin, where the B ring is unsubstituted phenyl, the antioxidant active moiety is the A ring. Even though the A ring is not a good electron donor, E7 > 0.8/NHE V, it can still scavenge alkyl peroxyl radicals, E7 = 1.06 V, and the superoxide radical, E7 > 1.06 V. Quercetin is the best electron donor of all investigated flavonoids (measured El,,, = 0.09 V, and calculated E, = 0.33 V). The favourable electron-donating properties originate from the electron donating 0 -3 hydroxy group in the C ring, which is conjugated to the catechol (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 the substituent effect in either direction. Thus, neglecting the acid-base equilibria of the A ring, and using those of the B ring and the measured values of the reduction potentials at pH 3,7 and 13.5, the pH dependence of the reduction potentials of the flavonoid radicals can be calculated. In neutral and slightly alkaline media (pH 7-9), all investigated flavonoids are inferior electron donors to ascorbate. Quercetin, E7 = 0.33 V, and gallocatechins, E, = 0.43 V, can reduce vitamin E radicals (assuming the same reduction potential as Trolox C radicals, E, = 0.48 V). Since all investigated flavonoid radicals have reduction potentials lower than E7 = 1.06 V of alkyl peroxyl radicals, the parent flavonoids qualify as chain-breaking antioxidants in any oxidation process mediated by these radicals. OH OH Flavan-3-01s RS'= HI (f)-Catechin RS'= OH, (-) -E p ig a I loca t ec h i n OH Flavanones R3=R7=R4'=OH, Di h ydroq ue rce t in (Tax if oli n)

Modulation of MPP+ uptake by procyanidins in Caco‐2 cells: Involvement of oxidation/reduction reactions

“…It is known that procyanidins have an elevated antioxidant effect, thereby being highly susceptible to oxidation, probably because they possess ortho-dihydroxyl groups on their structure, providing reaction sites [23]. We hypothesized a change in the procyanidins redox state during exposure time.…”

Chrysin and (±)-Taxifolin Electrochemical Oxidation Mechanisms

“…Their ability to act as chemical defence agents by donating an electron to an oxidant depends critically on the reduction potentials of their radicals. Structure-activity relationship studies of flavonoids have shown that the o-dihydroxy structure in the ring B and the 2,3-double bond in conjugation with the 4-oxo function in the ring C (as in flavones) is essential for effective free radical scavenging activity [6]. The presence of a 3-hydroxyl group in the heterocyclic ring also increases the radical scavenging activity while additional hydroxyl groups at positions 5 and 7 of the ring A appear to be less important [7].…”

Flavonoids through the eyes of a pharmacologist. Antioxidant and anti-inflammatory activities

“…В качестве доказательства приведенных за-кономерностей отметим, что при использовании циклической вольтметрии флавоноиды лютеолин и кверцетин, содержащие в молекуле катехольный фрагмент, оказались более мощными ингибитора-ми реакции Фентона, чем байцилеин и нарингенин, в структуре которых этот фрагмент отсутствует [26]. Ведущая роль в связывании железа катехольной группы у кольца B в сравнении с кольцом А была под-тверждена и другими исследователями [14,19,43]. Роль гидроксилов в 3-м и 5-м положениях в ком-плексе с 4-оксогруппой в процессе хелатирования железа также была продемонстрирована в экспери-менте [47].…”