Abstract The antioxidant activity of three extracted curcuminods, namely, curcumin
[(1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5dione] (C),
demethoxycurcumin (DMC), and bisdemethoxycurcumin (BDMC), were studied
with the DPPH, hydrogen peroxide and nitric oxide radical methods, and compared
with the known antioxidant ascorbic acid. Structures for the extracted curcuminods are
proposed on the basis of spectroscopic evidence. Curcumin molecule stability was
studied in the gaseous state, and it has two isomers [the diketone form, curcumin(I),
and the enol form, curcumin(II)] through the theoretical study by relying on the results
of density functions theory (DFT). The results of each of the total energy and the high
occupied molecular orbital (HOMO) to curcumin(I) are more stable than to curcumin(II).
The increase of the amount of total energy is -0.01301141 a.u., or equivalent,
-8,164.783 cal mol-1. The HOMO level is -0.35865 eV, also the thermodynamic
values (the change in entropy DS and the change in enthalpy DH) of the isomerization
conversion form curcumin(II) to curcumin(I) spontaneous and endothermic reaction,
of DS and DH are -3.136 cal mol-1 K-1 and -0.673 kcal mol-1, respectively. The
results showed that the wavelength for greatest absorption (k max) of the enol form
curcumin(II) is longer than curcumin(I). This is due to the formation of a new double
bond which leads to the association distribution of the electronic density along the
molecule in the enol form.
Keywords Antioxidant Curcumin DPPH HOMO LUMO.
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