Antioxidant activity of eugenol and its acetyl and nitroderivatives: the role of quinone intermediates-a DFT approach of DPPH test.

ABSTRACT

This work investigated the antioxidant potential of acetylated and nitrated eugenol derivatives through structural analysis and the mechanism of hydrogen atomic transfer (HAT) by density functional theory (DFT). The structures were optimized by the hybrid functional M06-2X with basis set 6-31 + G(d,p), and the HAT mechanism was evaluated with HO, HOO, CH3O, DPPH radicals. In agreement with experimental data from previous studies, two steps of hydrogen transfer were tested. The thermodynamic data showed the need for two hydrogen atomic transfer steps from antioxidants, followed by the formation of p-quinomethanes (27, 28, and 29) to make the reaction spontaneous with DPPH. Furthermore, theoretical kinetic data showed that the preferred antioxidant site depends on the instability of the attacking radical and confirmed the antioxidant profile for eugenol (1, 4-allylbenzene-1,2-diol), and nitro-derivative 7 (5-allyl-3-nitrobenzene-1,2-diol) in the DPPH assay. Finally, this study showed that nitro compound 6 (4-allyl-2-methoxy-6-nitrophenol) also has anti-radical activity with smaller radicals but is not observed in the experiment due to structural characteristics and chemoselectivity of DPPH.