| Abstract
| - The antiradical activities of some flavonols (kaempferol, quercetin, robinetin, quercetagetin, andmyricetin), flavones (apigenin, baicalein, and luteolin), flavanones (naringenin and dihydroquercetin),and flavanols [(+)-catechin and (−)-epicatechin] were determined by measuring the reaction kineticswith 2,2-diphenyl-1-picrylhydrazyl (DPPH) and α,γ-bisdiphenylene-β-phenylallyl (BDPA) radicals. Thereactions, which follow the mixed second-order rate law, were investigated under pseudo-first-orderconditions by use of a large excess of flavonoids, and their stoichiometry was determined byspectrophotometric titration. The results confirm stoichiometric factors of 1, 2, and 3 for flavonoidswith one, two, and three hydroxyl groups in the B-ring, respectively, excluding kaempferol, which,despite a single OH group in the B-ring, has a factor of 2, which is explained by the 3-OH groupsupporting the reaction with free radicals. Structure−activity considerations indicate for the presentseries of flavonoids the importance of multiple OH substitutions and conjugation. The logarithms ofreaction rate constants with the OH, DPPH, and BDPA radicals correlate well with the reductionpotential of the flavonoids. Keywords: Flavonoids; antiradical activity; reaction rate constants; DPPH and BDPA radicals;correlation with reduction potential; structure−activity relationship (SAR)
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