ANTIOXIDANT ACTIVITY OF GALLATES - AN ELECTROCHEMICAL STUDY IN AQUEOUS-MEDIA

The electron-donating ability of gallates, which are food and pharmace utical 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 electroc hemically oxidized on the glassy carbon electrode by using differentia l pulse voltammetry, cyclic voltammetry and hydrodynamic voltammetry o n the rotating disk electrode. All the compounds under study were easi ly oxidized in acidic and neutral solutions. Electrochemical oxidation occurs via two electron-transfer steps; however good resolution for t he second wave was obtained only by using hydrodynamic conditions. The oxidation process results to be irreversible, diffusion controlled an d pH-dependent. The introduction of the alkyl groups seems to affect t he intensities of the semiquinone gallate radicals as can be ascribed from the observed differences in i(d)(II)/i(d)(I) ratio obtained from hydrodynamic voltammetric experiments for the different derivatives. W e have found that the intensity of the gallate radicals follows the se quence GA greater than or equal to 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 t hat the antioxidant behaviour of these compounds are important in the stomach acid. (C) 1998 Published by Elsevier Science Ireland Ltd. All rights reserved.