Electron paramagnetic resonance spectra of flavonoid anion-radicals

Antioxidant effects of flavonoids

“…Similar alkaline autoxidation studies have been performed more recently [14,57]. Coupling constants were reported in the literature [14,56,58] and could basically be confirmed in our studies with in situgeneration of the radicals by horseradish peroxidase/hydrogen peroxide in slightly alkaline solutions (pH < 9; [59]). The most easily recognized features of the EPR spectra of the radical species of flavonoids are those of the respective B-ring o-semiquinones [14,56,58,59].…”

“…To obtain structural information on radical species, EPR spectroscopy is a superior method. In order to achieve sufficient levels of a radical, the first investigations of flavonoid EPR spectra were carried out in strongly alkaline DMSO/water mixtures and autoxidizing conditions [56]. Similar alkaline autoxidation studies have been performed more recently [14,57].…”

Stopped-flow-optical and -ESR Study on Oxidative Reaction of Quercetin by Nitrosodisulfonate Radical as a Model of Reactive Oxygen Species

“…13 On the other hand, the oxidation product is postulated to be a benzofuranone derivative, which was formed by well known chalcan-trione ring-chain tautomeric rearrangement occurring at C-ring, followed by the water addition at 2-position. 14 These observations provide experimental evidence that NDS readily oxidize quercetin to the quinone form. 12 In order to detect the paramagnetic intermediate, the stopped-flow-ESR measurements coupled with the field-sweep method were performed for the same reaction solutions of NDS and quercetin.…”

“…Based on the g-and hfcc values, the unknown species is assigned to be the o-semiquinone anion radical of quercetin B-ring (Figure 2). 14 The time-sweep stopped-flow ESR measurements were made by the same reaction condition, for the both NDS and the quercetin radical. As illustrated in Figure 2c, the concentration of NDS quickly diminished and completely disappeared within about 100 ms. After the trial and error curve fitting, the apparent second order rate constants defined for the decay of NDS was evaluated to be %2 Â 10 3 M À1 s À1 .…”

Kinetics of inhibition of peroxidase activity of myeloperoxidase by quercetin

“…Besides, flavonoids undergo autoxidation—the nonenzymatic reaction with atmospheric oxygen. Autoxidation of flavonoids typically occurs in strongly alkaline aqueous or nonaqueous solutions and is accompanied by the production of flavonoid radical anions, superoxide, and hydrogen peroxide 23,24. With highly oxidizable flavonoids (e.g., quercetin), autoxidation can take place and has been investigated in physiological conditions (pH 7.5 aqueous buffers) 25,26.…”