Radical scavenging by dietary flavonoids. A kinetic study of antioxidant efficiencies

The rate of scavenging of peroxyl radicals and of diphenylpicrylhydrazyl radicals by flavanones, flavones, flavanols and flavonols commonly occurring in foods was found, when determined by ESR spin trapping and by stopped-flow spectroscopy, respectively, to be the highest for the most reducing of the flavonoids. Among 12 flavonoids investigated a threshold potential of approximately +0.4V seems to exist, above which the flavonoids become ineffective radical scavengers, as determined by ESR spectroscopy using 5,5-dimethyl-1-pyrroline-N-oxide as spin trap. Scavenging of diphenylpicrylhydrazyl radicals showed (pseudo) first order kinetics for excess of flavonoids, and the reaction half-life could also be determined for the less effective flavonoids. For kaempferol and eriodictyol, flavonoids with very similar reduction potentials for their phenoxyl radicals, rate constants for the bimolecular scavenging could be determined in methanol and had the value (7.0 +/- 0.7) x 10(2) s(-1) M-1 and 33 +/- 1 s(-1) M-1 at 25 degrees C, respectively, showing that other factors than the driving force, such as hydrophilic/lipophilic balance are important, as was further confirmed in an oxygen consumption assay based on a linoleic acid emulsion, where decreasing antioxidant efficiency followed the order: (+)-catechin>(+/-)-taxifolin greater than or equal to luteolin > kaempferol > quercetin >> naringenin.