Antioxidant activity of eugenol and related monomeric and dimeric compounds

Since the inhibitory effect of eugenol (a), which was isolated as an antiox idative component from plant, Caryopylli flos, on lipid peroxidation was le ss than that of alpha-tocopherol, we synthesized the eugenol-related compou nds dieugenol (b), tetrahydrodieugenol (c), and dihydroeugenol (d), to find new strong antioxidants and assessed them for their inhibitory effect on l ipid peroxidation and scavenging ability for superoxide and hydroxyl radica ls. The antioxidative activities were in the order: (b)>(c)>(d)>(a) for the thiobarbituric acid reactive sui;stance (TBARS) formation. These results s uggest that the dimerized compounds have higher antioxidant activities than that of the monomers, Electron spin resonance (ESR) spin trapping experime nts revealed that eugenol and its dimer, having allyl groups in the structu re, scavenged superoxide, and that only eugenol trapped hydroxyl radicals u nder the conditions used. These finding suggest that eugenol and dieugenol have a different mechanism of antioxidation, Le. eugenol may inhibit lipid peroxidation at the level of initiation, however, the related dimeric compo unds may inhibit lipid peroxidation at the level of propagation of free rad ical chain reaction like alpha-tocopherol.