Despite its importance, little is known about the absolute performance and the mechanism for quercetin's antioxidant activity in water solution. We have investigated this aspect by combining differential oxygen-uptake kinetic measurements and B3LYP/6311+g (d,p) calculations. At pH = 2.1 (30 degrees C), quercetin had modest activity (k(inh) = 4.0 x 10(3) M-1 s(-1)), superimposable to catechol. On raising the pH to 7.4, reactivity was boosted 40-fold, trapping two peroxyl radicals in the chromen-4-one core and two in the catechol with k(inh) of 1.6 x 10(5) and 7.0 x 10(4) M-1 s(-1). Reaction occurs from the equilibrating mono-anions in positions 4 and 7 and involves firstly the OH in position 3, having bond dissociation enthalpies of 75.0 and 78.7 kcal/mol, respectively, for the two anions. Reaction proceeds by a combination of proton-coupled electron-transfer mechanisms: electron-proton transfer (EPT) and sequential proton loss electron transfer (SPLET). Our results help rationalize quercetin's reactivity with peroxyl radicals and its importance under biomimetic settings, to act as a nutritional antioxidant.

The Antioxidant Activity of Quercetin in Water Solution

Baschieri, Andrea;
2017-01-01

Abstract

Despite its importance, little is known about the absolute performance and the mechanism for quercetin's antioxidant activity in water solution. We have investigated this aspect by combining differential oxygen-uptake kinetic measurements and B3LYP/6311+g (d,p) calculations. At pH = 2.1 (30 degrees C), quercetin had modest activity (k(inh) = 4.0 x 10(3) M-1 s(-1)), superimposable to catechol. On raising the pH to 7.4, reactivity was boosted 40-fold, trapping two peroxyl radicals in the chromen-4-one core and two in the catechol with k(inh) of 1.6 x 10(5) and 7.0 x 10(4) M-1 s(-1). Reaction occurs from the equilibrating mono-anions in positions 4 and 7 and involves firstly the OH in position 3, having bond dissociation enthalpies of 75.0 and 78.7 kcal/mol, respectively, for the two anions. Reaction proceeds by a combination of proton-coupled electron-transfer mechanisms: electron-proton transfer (EPT) and sequential proton loss electron transfer (SPLET). Our results help rationalize quercetin's reactivity with peroxyl radicals and its importance under biomimetic settings, to act as a nutritional antioxidant.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11697/139201
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