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