Kinetic Study of Free-Radical-Scavenging Action of Flavonoids in Homogeneous and Aqueous Triton X-100 Micellar Solutions

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Kinetic study of the free radical-scavenging action of six kinds of flavonoids (flavone, chrysin, flavonol, apigenin, rutin, and quercetin) has been performed. The second-order rate constants for the reaction of flavonoids with the substituted phenoxyl (PhO) (k s ) and 5,7-diisopropyltocopheroxyl (Toe-) radicals (k r ) have been measured in ethanol, isopro-panol/water (5:1, v/v), and aqueous Triton X-100 micellar solution (5.0wt%). The rate constants (k s and k r ) observed for flavone, chrysin, and flavonol are very slow, indicating that the reactivities of 5- and 7-OH groups at the A-ring and the 3-OH group at the C-ring are very weak, almost negligible. Rutin and quercetin with 3′- and 4′-OH groups at the B-ring showed high reactivity, indicating that the o-dihydroxyl structure in the firing is the obvious radical target site for flavonoids. The rate constants obtained in the micellar solution showed notable pH dependence. For instance, both the k s and k r values of rutin increased as pH increased from 4 to 11. Rutin is a tetrabasic acid and can exist in five different molecular forms, depending on the pH value. By comparing the k s values with the mole fraction (f) of each molecular form of rutin, the reaction rate k sl for the undissociated form (RuH4), k s2 for the monoanion (RuH3 -), k s3 for the dianion (RuH2 2-), and k s4 for the trianion (RuH3-) were determined; the values are 9.5 × 10 M-1 s-1, 4.0 × 102 M-1 s-1, 3.8 × 103 M-1 s-1, and 4.0 × 103 M-1 s-1, respectively. The reaction rates (k si) increase remarkably with the increasing anionic character of rutin, that is, the electron-donating capacity of rutin.