Interaction of flavonoids with ascorbate and determination of their univalent redox potentials: a pulse radiolysis study.
Concurrent pulse-radiolytic generation of flavonoid aroxyl radicals and ascorbyl radicals causes a complex kinetic interplay of competing and parallel reactions. Evaluation by "kinetic modelling," that is, taking into account all possible reactions by a set of differential equations, allowed us to determine equilibria constants for the univalent steps by a novel method. From these kinetic data we were able to calculate the redox potentials for dihydroquercetin, quercetin, rutin (a quercetin 3-glycoside), kaempferol, fisetin, and luteolin. Despite the limited number of substances, two structural criteria became apparent: all substances containing the B-ring catechol group and the 2,3-double bond have a higher redox potential than ascorbate and are consequently able to oxidize it to the ascorbyl radical. With fisetin and kaempferol having values very similar to ascorbate, only the flavanone dihydro-quercetin was capable of reducing the ascorbyl radical, thus fulfilling the so-called "ascorbate-protective" function, originally proposed by Szent-Györgyi. While flavonoids are effective radical scavengers, these rather high redox potentials for most flavonols may explain their occasional prooxidative behavior.
Bors
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Institut für Strahlenbiologie, GSF Forschungszentrum für Umwelt und Gesundheit Neuherberg, Oberschleissheim, Germany.
Michel
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Schikora
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D001205:Ascorbic Acid / Q000737:chemistry*; D005419:Flavonoids / Q000737:chemistry*; D044948:Flavonols; D006863:Hydrogen-Ion Concentration; D044949:Kaempferols; D007700:Kinetics; D047311:Luteolin; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011675:Pulse Radiolysis; D011794:Quercetin / Q000031:analogs & derivatives / Q000737:chemistry; D012431:Rutin / Q000737:chemistry
D005419:Flavonoids; D044948:Flavonols; D044949:Kaempferols; D012431:Rutin; C006552:kaempferol; D011794:Quercetin; C003377:taxifolin; D047311:Luteolin; C017875:fisetin; D001205:Ascorbic Acid