Natural Antioxidants : Their Activities and Action Mechanism

In the present work，we measured the rate constants of the reaction of peroxyl，tocopheroxyl，and aroxyl free radicals with natural antioxidants，such as catechins，caffeic acids and related compounds in homogeneous and micellar solutions，by using oxygen absorption method，stopped-flow spectrophotometer，and time-resolved ESR method。We have succeeded in obtaining the following interesting results：l)Detailed kinetic studies have been performed for the reaction of aroxyl(ArO·)and 5，7-diisopropyltocopheroxyl(Toc·)radicals with catechin，epicatechin，and related antioxidants in homogeneous and micellar solutions.The structure-activity relationship in the scavenging reaction of free radicals by catechin and epicatechin in solution was discussed.It has been found that catechin and epicatechin have high activity in vitamin E regeneration.2)The antioxidant activity of curcumin(1，7-bis(4-hydroxy-3-methoxyphenyl)-1，6-heptadiene-3，5-dione)was determined by inhibition of controlled initiation of styrene oxidation.Synthetic nonphenolic curcumins exhibited no antioxidant activity，therefore，curcumin is a classical phenolic chain-breaking antioxidant，donating H atoms from the phenolic groups not the CH_2group as has been suggested(Jovanovic et al.J.Am。Chem.Soc.，121，9677(1999).3)Studies of the kinetics of the proton-or hydrogen-transfer reactions concerning vitamin E in solutions and in micellar dispersions by means of stopped-flow and absorption spectroscopy indicated that proton tunneling plays an important role in the antioxidant and regeneration reactions that are advantageous in vivo but not as a part of the harmful prooxidant action.

In the present work, we measured the rate constants of the reaction of peroxyl, tocopheroxyl, and aroxyl free radicals with natural antioxidants, such as catechins, caffeic acids and related compounds in homogeneous and micellar solutions, by using oxygen absorption method, stopped-flow spectrophotometer, and time-resolved ESR method. We have succeeded in obtaining the following interesting results :l) Detailed kinetic studies have been performed for the reaction of aroxyl (ArO ・) and 5,7-diisopropyltocopheroxyl (Toc ・ ) radicals with catechin, epicatechin, and related antioxidants in homogeneous and micellar solutions. The structure-activity relationship in the scavenging reaction of free radicals by catechin and epicatechin in solution was discussed. It has been found that catechin and epicatechin have high activity in vitamin E regeneration.2) The antioxidant activity of curcumin (1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) was determined by inhibition of controlled initiation of styrene oxidation. Synthetic nonphenolic curcumins exhibited no antioxidant activity, therefore, curcumin is a classical phenolic chain-breaking antioxidant, donating H atoms from the phenolic groups not the CH_2 group as has been suggested (Jovanovic et al. J.Am. Chem. Soc., 121, 9677 (1999)).3) Studies of the kinetics of the proton- or hydrogen-transfer reactions concerning vitamin E in solutions and in micellar dispersions by means of stopped-flow and absorption spectroscopy indicated that proton tunneling plays an important role in the antioxidant and regeneration reactions that are advantageous in vivo but not as a part of the harmful prooxidant action.