Antioxidant activity of gallates: an electrochemical study in aqueous media

The electron-donating ability of gallates, which are food and pharmaceutical antioxidants, is quantitatively assessed on the basis of their electrochemical characteristics. Gallic acid and the propyl, i-propyl, butyl, i-butyl, pentyl and i-pentyl gallate derivatives were electrochemically oxidized on the glassy carbon electrode by using differential pulse voltammetry, cyclic voltammetry and hydrodynamic voltammetry on the rotating disk electrode. All the compounds under study were easily oxidized in acidic and neutral solutions. Electrochemical oxidation occurs via two electron-transfer steps; however good resolution for the second wave was obtained only by using hydrodynamic conditions. The oxidation process results to be irreversible, diffusion controlled and pH-dependent. The introduction of the alkyl groups seems to affect the intensities of the semiquinone gallate radicals as can be ascribed from the observed differences in i(II)d/i(I)d ratio obtained from hydrodynamic voltammetric experiments for the different derivatives. We have found that the intensity of the gallate radicals follows the sequence GA > or = i-PG > PG > i-BG > BG > i-PeG > PeG. From the pH-dependence of the peak current it is possible to affirm that pH 2 is the better condition for the oxidative activity showing that the antioxidant behaviour of these compounds are important in the stomach acid.