Antioxidant Mechanism of Curcumin in the
Presence of Linoleate. From our elucidation of the
chemical structures of the six isolated compounds, we
propose the antioxidant mechanism of curcumin in the
presence of ethyl linoleate as illustrated in Figure 9.
As shown in Figure 9, curcumin traps a radical at the
phenolic group and is converted to a curcumin radical.
The curcumin radical reacts with a peroxyl radical of
the ethyl linoleate at the 3¢-position, affording a coupling
product through a peroxyl linkage. The coupling product
is not very stable because the aromatic stability of the
original benzene ring is disrupted by the coupling
reaction. Thus, the subsequent Diels-Alder reaction
occurs smoothly in the coupling product. It is accepted
that linoleic acid gives four isomeric peroxyl radicals
during its autoxidation (20), which were presumed by
the chemical structures of the corresponding four hydroperoxides
including the 9-trans-11-trans-diene 13-
hydroperoxide, 9-cis-11-trans-diene 13-hydroperoxide,
10-trans-12-cis-diene 9-hydroperoxide, and 10-trans-12-
trans-diene 9-hydroperoxide. Coupling of the isomeric
peroxyl radical 7 with the curcumin radical at the 3¢-
position would produce the corresponding peroxide. The
unstable peroxide could then be converted by the
intramolecular Diels-Alder reaction. Reaction of the
double bond at the 11-position of linoleate part of the
peroxide with the conjugated diene from the 1¢,5¢-
position of the curcumin would afford compound 3. On
the other hand, reaction of the double bond at the 1¢-
position of the curcumin part with the diene from the
linoleate part would give compound 1. By similar
reaction procedures, compounds 2 and 5 would be
produced from the peroxyl radical 10, compound 4 from
the peroxyl radical 8, and compound 6 from the peroxyl
radical 9, respectively. It is predictable that one of the
two types of the Diels-Alder reactions, as illustrated
in Figure 9, does not occur in the case of the peroxides