Mechanisms of transformation of the antioxidant kaempferol into depsides. Gamma-radiolysis study in methanol and ethanol.
In this study, we irradiated the antioxidant kaempferol in ethanol and methanol solutions with gamma rays at doses ranging from 0.2-20 kGy. NMR and ES-MS spectroscopy were used to identify radiolysis products. Two depsides, [2-[(4'-hydroxybenzoyl)oxy]-4,6-dihydroxyphenyl](oxo) methyl acetate and [2-[(4'-hydroxybenzoyl)oxy]-4,6-dihydroxyphenyl](oxo) ethyl acetate, were the major compounds of kaempferol degradation in methanol and in ethanol, respectively. Other products formed in low concentrations were identified as [4-hydroxyphenyl](oxo) methyl acetate, [4-hydroxyphenyl](oxo) ethyl acetate, and depside [2-[(4'-hydroxybenzoyl)oxy]-4,6-dihydroxyphenyl](oxo) acetic acid. The formation of the latter was observed in both solvents. We propose degradation mechanisms that suggest that (.)CH(2)OH and CH(3)(.)CHOH, produced by solvent radiolysis, react with the 3-OH kaempferol group because of its high H-donor capacity. pi-Electron delocalization in the flavonoxy formed after the first H-transfer leads to C-ring opening and consequently to the formation of depsides. G calculation of the degradation products and of (.)CH(2)OH and CH(3)(.)CHOH radicals confirmed the proposed mechanism of kaempferol radiolysis. The rate constants for the reaction between kaempferol and these free radicals were also calculated. Formation of depside has also been observed in many studies of the oxidation of flavonoids; those studying human metabolism have suggested similar redox transformation of flavonols. The antioxidant activities of radiolysis products were evaluated and compared to those of kaempferol.
Marfak
A
A
UPRES EA 1085, Biomolécules et Cibles Cellulaires Tumorales Laboratoire de Biophysique, Faculté de Pharmacie, 87025 Limoges Cedex, France.
Trouillas
P
P
Allais
D P
DP
Calliste
C A
CA
Cook-Moreau
J
J
Duroux
J L
JL
D000975:Antioxidants / Q000494:pharmacology*; D001713:Biphenyl Compounds; D002851:Chromatography, High Pressure Liquid; D053630:Depsides; D004307:Dose-Response Relationship, Radiation; D000431:Ethanol / Q000737:chemistry / Q000494:pharmacology*; D005419:Flavonoids / Q000494:pharmacology*; D005609:Free Radicals; D005720:Gamma Rays; D062385:Hydroxybenzoates / Q000737:chemistry*; D044949:Kaempferols; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D000432:Methanol / Q000737:chemistry / Q000494:pharmacology*; D008956:Models, Chemical; D010100:Oxygen / Q000378:metabolism; D010851:Picrates / Q000737:chemistry; D013481:Superoxides / Q000737:chemistry; D013997:Time Factors; D014466:Ultraviolet Rays
D000975:Antioxidants; D001713:Biphenyl Compounds; D053630:Depsides; D005419:Flavonoids; D005609:Free Radicals; D062385:Hydroxybenzoates; D044949:Kaempferols; D010851:Picrates; D013481:Superoxides; D000431:Ethanol; C006552:kaempferol; C004931:1,1-diphenyl-2-picrylhydrazyl; D010100:Oxygen; D000432:Methanol