Two-electron electrochemical oxidation of quercetin and kaempferol changes only the flavonoid C-ring.
Bulk electrolysis of the antioxidant flavonoids quercetin and kaempferol in acetonitrile both yield a single oxidation product in two-electron processes. The oxidation products are more polar than their parent compounds, with an increased molecular weight of 16g/mol, and were identified as 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone and 2-(4-hydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone for quercetin and kaempferol, respectively. Two-electron oxidation of the parent flavonoid is suggested to yield a 3,4-flavandione with unchanged substitution pattern in the A- and B-ring, which may rearrange to form the substituted 3(2H)-benzofuranone through the chalcan-trione ring-chain tautomer. The acidity of the 3-OH group is suggested to determine the fate of the flavonoid phenoxyl radical, originally formed by one-electron oxidation, as no well-defined oxidation product of luteolin (lacking the 3-OH group) could be isolated despite rather similar half-peak potentials: Ep/2 = 0.97V, 0.98 V and 1.17 V vs. NHE for quercetin, kaempferol and luteolin, respectively, as measured by cyclic voltammetry in acetonitrile.
Jørgensen
L V
LV
Institute of Food Safety and Toxicology, Danish Veterinary and Food Administration, Ministry of Food, Agriculture and Fisheries, Søborg.
Cornett
C
C
Justesen
U
U
Skibsted
L H
LH
Dragsted
L O
LO
D001572:Benzofurans / Q000737:chemistry / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D004572:Electrolysis; D004583:Electrons; D005419:Flavonoids / Q000737:chemistry / Q000378:metabolism; D044949:Kaempferols; D047311:Luteolin; D013058:Mass Spectrometry; D019906:Nuclear Magnetic Resonance, Biomolecular; D010084:Oxidation-Reduction; D011794:Quercetin / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism*; D013056:Spectrophotometry, Ultraviolet; D013997:Time Factors
C000601184:2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone; D001572:Benzofurans; D005419:Flavonoids; D044949:Kaempferols; C006552:kaempferol; D011794:Quercetin; D047311:Luteolin