Contribution of 2-Propenesulfenic Acid to the Antioxidant Activity of Allicin.

We re-examined the antioxidative mechanism of allicin as a radical scavenger on the basis of the reactivity toward 2, 2-diphenyl-1-picrylhydrazyl (DPPH) and peroxyl radicals. Initially, it was found that allicin decomposed more rapidly in n-hexane and chlorobenzene than in acetonitrile, ethanol, and ethanol/water solutions and decomposed into ajoene and vinyldithiins in these solvents. Furthermore, the decomposition of allicin and the following formations of ajoene and vinyldithiins from allicin were accelerated by the reaction of allicin with DPPH and peroxyl radicals. These results show that 2-propenesulfenic acid, which arises by Cope elimination from allicin, is proposed to contribute to scavenge these radicals because ajoene and vinyldithiins were produced from allicin through the use of 2-propenesulfenic acid. Next, allicin was more effective at inhibiting the linoleic acid oxidation at 50 °C than at 30 °C and in cyclohexane than in acetonitrile. These results indicate that allicin decomposed rapidly at high temperatures in a hydrogen-bond-acceptor solution to 2-propenesulfenic acid. In addition, 2-propene-1-sulfinothionic acid S-methyl ester, which does not produce sulfenic acid through Cope elimination, has no activity against the radicals. On the other hand, methanesulfinothionic acid S-2-propenyl ester, which produces methanesulfenic acid through Cope elimination, has the same or increased activity as its allicin against the radicals. Based on these results, the Cope elimination product, sulfenic acid, from thiosulfinates with an α-sulfenyl proton was found to make a larger contribution to the radical scavenger than that of allicin itself. PRACTICAL APPLICATION: We examined the antioxidant activity of allicin on the oxidation of cumene and linoleic acid in homogeneous solutions. It is obvious from these results that 2-propenesulfenic acid was found to make a larger contribution to the radical scavenger than that of allicin itself.