Mechanistic Studies of Catechins as Antioxidants against Radical Oxidation

The antioxidative mechanisms of catechins were studied by investigating products generated at the first stages by 2,2′-azobis(2-aminopropane)hydrochloride (AAPH)-induced radical oxidation, without any isolation, using LC/MS, spectrophotometry, and PM3 semiempirical molecular orbital (MO) calculations. Catechins were quite effective in scavenging peroxyl radicals in a liposomal system and in an aqueous system except for (−)-epigallocatechin (EGC). EGC was the least effective among four catechins tested. From the results of LC/MS and spectroscopic studies, (−)-epicatechin (EC) would be gently converted to an anthocyanin-like compound. According to the mechanisms, the compound produced from EC by radical oxidation can also function as an antioxidant. As a result, EC has a longer inhibition period (tinh= 9360 s). On the other hand, EGC decreased shortly after oxidation (tinh= 3420 s) and was transformed to a quinone-like compound. The addition of superoxide dismutase (SOD) reduced the chemiluminescence from EGC during oxidation. Active oxygen including superoxide anion radicals (O−2) may be produced in the case of EGC, but not in the case of EC. However, EGC has a more rapid scavenging effect on peroxyl radicals (kinh/kp= 232) than EC (kinh/kp= 41). The calculated C-H bond dissociation enthalpies (BDEs) for catechins at the C-2 position were unexpectedly low (65 kcal/mol) compared to O-H BDEs at phenolic sites (70 kcal/mol), suggesting that hydrogen at the C-2 position may be abstracted by free radicals. The authors propose the tentative antioxidative mechanisms of catechins depending on the experimental results and theoretical calculations.