W. Bors et al., INTERACTION OF FLAVONOIDS WITH ASCORBATE AND DETERMINATION OF THEIR UNIVALENT REDOX POTENTIALS - A PULSE-RADIOLYSIS STUDY, Free radical biology & medicine, 19(1), 1995, pp. 45-52
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.