Antioxidant chemistry of green tea catechins. New oxidation products of (-)-epigallocatechin gallate and (-)-epigallocatechin from their reactions with peroxyl radicals

Citation
S. Valcic et al., Antioxidant chemistry of green tea catechins. New oxidation products of (-)-epigallocatechin gallate and (-)-epigallocatechin from their reactions with peroxyl radicals, CHEM RES T, 13(9), 2000, pp. 801-810
Citations number
46
Categorie Soggetti
Pharmacology & Toxicology
Journal title
CHEMICAL RESEARCH IN TOXICOLOGY
ISSN journal
0893228X → ACNP
Volume
13
Issue
9
Year of publication
2000
Pages
801 - 810
Database
ISI
SICI code
0893-228X(200009)13:9<801:ACOGTC>2.0.ZU;2-I
Abstract
The green tea catechins (-)-epigallocatechin gallate (EGCG) and (-)-epigall ocatechin (ECC) react with peroxyl radicals generated by thermolysis of the azo initiator 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN) to produce sev eral oxidation products. Structure elucidation of these products can provid e insights into specific mechanisms of antioxidant reactions. We isolated a nd identified a previously unreported reaction product of EGCG and three re action products of EGC. In the EGCG product, the B-ring was transformed int o a ring-opened unsaturated dicarboxylic acid moiety. The EGC products incl ude a seven-membered B-ring anhydride and a symmetrical EGC dimer, both ana logues of previously described EGCG oxidation products. The third EGC produ ct was an unsymmetrical dimer. In all identified products, changes occurred solely in the B-ring of EGCG or EGC. This confirmed our previous observati on that the principal site of antioxidant reactions in EGCG and EGC is the trihydroxyphenyl B-ring, regardless of the presence of a 3-galloyl moiety. A stoichiometric factor n of 4.16 +/- 0.51 was measured for EGCG, whereas f actors of 2.20 +/- 0.26 was found for EGC and 2.33 +/- 0.18 measured for me thyl gallate. These values represent the net peroxyl radical trapping per c atechin molecule by several competing reactions. EGCG and EGC oxidation inv olves addition of oxygen, which is not derived from water, but most likely from atmospheric oxygen via peroxyl radicals. Characteristic oxidation prod ucts may be useful markers for antioxidant actions in living systems.