Several authors have attributed the
free radical scavenging ability, and inhibition of lipid peroxidation,
of curcumin to electron donation from the phenolic part
[4–10]. In contrast, Jovanovic et al. [11] have proposed that the
antioxidant mechanism of curcumin involves H-atom donation,
mainly from the central active CH2 group in the heptadienone
link of curcumin keto form. They also proposed that the Habstraction
from the phenolic group accounted only for about
15% of the overall reaction. On the other hand, Barclay et al.
[12] studied the reaction of curcumin with peroxyl radicals
(ROO) and concluded that curcumin is a phenolic chain-breaking
antioxidant, donating H atoms from the phenolic groups and not
from the CH2 group. They have shown that synthetic non-phenolic
curcuminoids exhibited no antioxidant activity. According to
these authors, the reaction medium has a strong influence on
the antioxidant activities of phenols and a relatively non-polar
medium is a preferred one for such antioxidant activities. Recently
an alternative mechanism has been proposed by Litwinienko
and Ingold [13], which is favored in polar
environments. It is known as sequential proton loss electron
transfer (SPLET) and assumes that the enol deprotonation is followed
by single electron transfer and proton loss, in that order.
This mechanism is favored in ionizing solvents, which means
that the antioxidant activity is then expected to be higher in polar
media.