Free radical scavenging activity of vanillin and o-vanillin using 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical

Vanillin, a plant derived natural product, used as food flavoring agent and its positional isomer o-vanillin, have been tested for their ability to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical using high performance liquid chromatography (HPLC). Trolox, a water-soluble analogue of vitamin E and a well-known antioxidant was used as a reference compound. The DPPH radical was monitored at 517 nm and its retention time was 8.6 min. From the decrease in optical density of DPPH radical in the presence of the test compounds, it was observed that o-vanillin was a more effective scavenger than vanillin. At equimolar concentrations (1 mM), vanillin and o-vanillin exhibited 22.9% and 66.4% DPPH radical scavenging activity, respectively. The kinetics of the reaction of vanillin and o-vanillin with DPPH radical was studied using stopped flow spectrophotometry and their rate constants were estimated to be 1.7+/-0.1 M-1 s(-1) and 10.1+/-0.8 M-1 s(-1), respectively. In comparison, the rate constant for the reaction of trolox with DPPH was estimated to be 360.2+/-10.1 M-1 s(-1). These scavenging reactions involve electron/H-atom transfer from antioxidant to DPPH. To confirm this, one electron reduction potentials of these compounds were estimated using cyclic voltammetry which showed that o-vanillin was more easily oxidized than vanillin. The reduction potential for o-vanillin was about 1.5 times that of trolox. These results demonstrate that o-vanillin is a more potent antioxidant than vanillin.