INTERACTION OF FLAVONOIDS WITH ASCORBATE AND DETERMINATION OF THEIR UNIVALENT REDOX POTENTIALS - A PULSE-RADIOLYSIS STUDY

Concurrent pulse-radiolytic generation of flavonoid aroxyl radicals an d ascorbyl radicals causes a complex kinetic interplay of competing an d parallel reactions. Evaluation by ''kinetic modelling,'' that is, ta king into account all possible reactions by a set of differential equa tions, allowed us to determine equilibria constants for the univalent steps by a novel method. From these kinetic data we were able to calcu late the redox potentials for dihydroquercetin, quercetin, rutin (a qu ercetin 3-glycoside), kaempferol, fisetin, and luteolin. Despite the l imited 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 consequentl y able to oxidize it to the ascorbyl radical. With fisetin and kaempfe rol having values very similar to ascorbate, only the flavanone dihydr o-quercetin was capable of reducing the ascorbyl radical, thus fulfill ing the so-called ''ascorbate-protective'' function, originally propos ed by Szent-Gyorgyi. While flavonoids are effective radical scavengers , these rather high redox potentials for most flavonols may explain th eir occasional prooxidative behavior.