The contribution of flavonoid C-ring on the DPPH free radical scavenging efficiency.: A kinetic approach for the 3′,4′-hydroxy substituted members

5,7,3',4'-hydroxy-substituted flavonoids are considered as very efficient radical scavengers. This study examines the effect of the structural elements of the C-ring, on the radical scavenging activity of these compounds. The impact of di-hydroxy substitution of A- or B-ring on the activity of fully substituted C-ring flavonoids was also studied. Quercetin, luteolin, taxifolin, eriodictyol, rutin, (+)-catechin, (-)-epicatechin, fisetin and kaempferol were studied during the reaction with the DPPH radical; they revealed two distinctive steps of reaction, a first rapid and a second slower. DPPH scavenging followed second order kinetics during the rapid step; stoichiometric factors and rate constants were determined. Quercetin and fisetin scavenged four radicals by each molecule, while the other flavonoids scavenged two radicals with a consequent production of B-ring diquinone. Also the rate constants were affected by C-ring structure. The kinetics of the slow step was much more complicated and the contribution of C-ring was based on stoichiometry. (c) 2005 Elsevier Ltd. All rights reserved.