Curcuminoids as antioxidants and theoretical study of stability of curcumin isomers in gaseous state

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 a dagger S and the change in enthalpy a dagger H) of the isomerization conversion form curcumin(II) to curcumin(I) spontaneous and endothermic reaction, of a dagger S and a dagger H are -3.136 cal mol(-1) K-1 and -0.673 kcal mol(-1), respectively. The results showed that the wavelength for greatest absorption (lambda 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.