Stopped-flow kinetic study of the aroxyl radical-scavenging action of catechins and vitamin C in ethanol and micellar solutions.
Kinetic study of the aroxyl radical-scavenging action of catechins (epicatechin (EC), epicatechin gallate (ECG), epigallocatechin (EGC), and epigallocatechin gallate (EGCG)) and related compounds (methyl gallate (MG), 4-methylcatechol (MC), and 5-methoxyresorcinol (MR)) has been performed. The second-order rate constant ( k s) for the reaction of these antioxidants with aroxyl radical has been measured in ethanol and aqueous Triton X-100 micellar solution (5.0 wt %). The k s values decreased in the order of EGCG > EGC > MC > ECG > EC > MG >> MR in ethanol, indicating that the reactivity of the OH groups in catechins decreased in the order of pyrogallol B-ring > catechol B-ring > gallate G-ring > resorcinol A-ring. The structure-activity relationship in the free radical-scavenging reaction by catechins has been clarified by the detailed analyses of the pH dependence of k s values. From the results, the p K a values have been determined for catechins. The monoanion form at catechol B- and resorcinol A-rings and dianion form at pyrogallol B- and gallate G-rings show the highest activity for free radical scavenging. It was found that the free radical-scavenging activities of catechins are 3.2-13 times larger than that of vitamin C at pH 7.0.
Mitani
Shuji
S
Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan.
Ouchi
Aya
A
Watanabe
Emi
E
Kanesaki
Yu
Y
Nagaoka
Shin-Ichi
S
Mukai
Kazuo
K
D001205:Ascorbic Acid / Q000737:chemistry*; D002392:Catechin / Q000031:analogs & derivatives / Q000737:chemistry*; D000431:Ethanol; D016166:Free Radical Scavengers / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008823:Micelles; D017382:Reactive Oxygen Species / Q000737:chemistry*; D012996:Solutions
D016166:Free Radical Scavengers; D008823:Micelles; D017382:Reactive Oxygen Species; D012996:Solutions; D000431:Ethanol; D002392:Catechin; C062669:epicatechin gallate; D001205:Ascorbic Acid