Kinetics and Mechanisms of Antioxidant Activity using the DPPH.Free Radical Method

The reaction mechanisms of three antioxidants are proposed in order to explain experimental results obtained from a kinetic study using the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH.) method, previously adapted in our laboratory. In its radical form, DPPH.shows an absorbance maximum at 515 nm which disappears upon reduction by an antiradical compound. BHT, a synthetic antioxidant, slowly reacts with DPPH.reaching a steady state within 5 h. This 2.8-stoichiometric complete reaction follows a 1.5-order with respect to DPPH.and 0.5 to BHT. The kinetic rate constant, k, is estimated to be 5.0 L/(mol·s) at 20 °C and the energy of activation,Ea, is equal to 35 kJ/mol in methanol. Eugenol reacts with DPPH.reaching a steady state within 2 h. This 1.9-stoichiometric reaction follows a 2-order with respect to both DPPH.and eugenol, k andEaare estimated to be 5.4 × 1010L3/(mol3·s) at 20 °C and 30 kJ/mol, respectively. The eugenol mechanism may involve a dimerization between two phenoxyl radicals. The reaction with isoeugenol is rapid and reversible, with a stoichiometry of 1.1. It is first order with respect to isoeugenol with k (direct reaction) equal to 8.9 × 10−2s−1at 10 °C. This reaction is consistent with a pseudo-monomolecular mechanism.