Scavenging mechanisms of (-)-epigallocatechin gallate and (-)-epicatechin gallate on peroxyl radicals and formation of superoxide during the inhibitory action

The scavenging effects of (-)-epigallocatechin gallate (EGCG) and (-)-epicatechin gallate (ECG) on peroxyl radicals and their mechanisms were studied by investigating the products formed during the first stages by 2,2′-azobis(2-aminopropane) hydrochloride (AAPH)-induced oxidation, without any isolation, using LC/MS, spectrophotometry, chemiluminescence analyses, and semiempirical molecular orbital (MO) calculations. The results show that EGCG can be converted to an anthocyaninlike compound followed by cleavage of the gallate moiety by oxidation. On the other hand, ECG can be converted to an anthocyaninlike compound after cleavage of the gallate moiety. The calculated C-H bond dissociation enthalpies (BDEs) for EGCG and ECG at the C-2 position were quite low (62.7 and 66.8 kcal/mol, respectively) compared with O-H BDEs at the phenolic sites (ca. 70 kcal/mol), suggesting that the C-2 hydrogen can be abstracted by free radicals. The addition of superoxide dismutase (SOD) decreased the chemiluminescence in EGCG by one-half during the inhibitory action. Active oxygen including superoxide (O2−) would be produced in EGCG, but not in ECG. The authors proposed the antioxidative mechanisms of EGCG and ECG depending on the experimental results and theoretical calculations.