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Evaluation of the antioxidative activity by measuring the rate of the homogeneous oxidation reaction with ferroceniumdimethanol cation. Comparative analysis of of glutathione and ascorbic acid

A simple methodology for estimation of the antioxidative activity by measuring the kinetics of the homogeneous electron transfer reaction between an antioxidant and ferroceniumdimathanol cation (Fc+) is proposed. The method is based on the well-known regenerative ErC’ catalytic reaction scheme, where the antioxidant is catalytically oxidized by Fc+, which is in situ formed by the reversible oxidation of ferrocenedimethanol at glassy carbon electrode. Comparing the rate of the homogeneous oxidation of different antioxidant with the same oxidant, i.e., Fc+, which serves as a referent system, a more consistent comparison of the antioxidative activity can be done than comparing the heterogeneous oxidation reactions at the electrode surface. The applicability of the method is demonstrated in a comparative analysis of glutathione (GSH) and ascorbic acid (AA), the two important and widely abundant low molecular weight antioxidants, by means of cyclic voltammetry (CV), square-wave voltammetry (SWV), and chronoamperometry (CA). At physiological conditions (pH  7) the rate of AA oxidation ( 104 mol-1 L s-1) is two orders of magnitude higher than GSH ( 102 mol-1 L s-1) implying superior antioxidative activity of the former. From a methodological point of view it is important to stress that the kinetic data estimated with CV and CA are comparable, while SWV gives much higher estimates for the catalytic rate constants. This implies that different techniques lead to different kinetic data, which must be taken into account in estimating the antioxidative activity, as well as in comparing the kinetic data reported in the literature.

antioxidant; voltammetry; rate constant; glutathione; ascorbic acid

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