title: Flavonoid oxidation by the radical generator AIBN: a unified mechanism for quercetin radical scavenging.
abstract: Four oxidized flavonoid derivatives generated from reacting quercetin (a pentahydroxylated flavone) with the peroxyl radical generator 2,2'-azobis-isobutyronitrile (AIBN) were isolated by chromatographic methods and identified by NMR and MS analyses. Compounds included 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone (2); 1,3,11a-trihydroxy-9-(3,5,7-trihydroxy-4H-1-benzopyran-4-on-2-yl)-5a-(3,4-dihydroxyphenyl)-5,6,11-hexahydro-5,6,11-trioxanaphthacene-12-one (3); 2-(3,4-dihydroxybenzoyloxy)-4,6-dihydroxybenzoic acid (4); and methyl 3,4-dihydroxyphenylglyoxylate (5). Product ratios under different hydrogen ion concentrations and external nucleophiles revealed that two of the products, namely the substituted benzofuranone (2) and the depside (4), are generated from a common carbocation intermediate. Indirect evidence for the operation of a cyclic concerted mechanism in the formation of the dimeric product (3) is provided. The identification of these products supports the model that the principal site of scavenging reactive oxygen species (ROS) in quercetin is the o-dihydroxyl substituent in the B-ring, as well as the C-ring olefinic linkage.
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, USA.
authors.forename: Venkat
authors.initials: V
authors.lastname: Krishnamachari
authors.forename: Lanfang H
authors.initials: LH
authors.lastname: Levine
authors.forename: Paul W
authors.initials: PW
authors.lastname: Paré
mesh_terms: D005419:Flavonoids / Q000737:chemistry*; D016166:Free Radical Scavengers; D005609:Free Radicals; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D009570:Nitriles / Q000737:chemistry*; D010084:Oxidation-Reduction; D010545:Peroxides / Q000737:chemistry; D011794:Quercetin / Q000737:chemistry*; D017382:Reactive Oxygen Species / Q000737:chemistry; D013481:Superoxides / Q000737:chemistry
chemical_list: D005419:Flavonoids; D016166:Free Radical Scavengers; D005609:Free Radicals; D009570:Nitriles; D010545:Peroxides; D017382:Reactive Oxygen Species; D013481:Superoxides; C049375:perhydroxyl radical; D011794:Quercetin; C004526:azobis(isobutyronitrile)