Relationship between the chemical structures of antioxidants and the differences in their Cupric Ion Reducing Antioxidant Capacity (CUPRAC) by electrochemical methods

• Electrochemical CUPRAC provides an approximate number of transferred electrons.
• This can be done by comparing slopes for calibration plots between antioxidants.
• Oxidation of Gallic acid, Catechin and Quercetin require over 2 electrons (pH = 7).
• Results were compared with data from bipotentiometric titrations using I3−/I−.
• The studied oxidation processes suggest the presence of quinone derivatives.

The number of transferred electrons during oxidation of molecules with antioxidant capacity was approximately obtained by means of the slope of CUPRAC calibration plots, compared with that for the Trolox, where a two-electron mechanism is presumed. This comparison led to the following relative number of electrons transferred: ∼2 for Ascorbic acid, ∼4 for Gallic acid, ∼4 for (+)-Catechin and approximately 6 for Quercetin. Independent determinations of these numbers by bipotentiometric titrations using KI3 revealed that Ascorbic acid required two electrons per mole for its oxidation, while for Gallic acid and (+)-Catechin, 6 electrons are required; finally, Quercetin oxidation consumes 8 electrons per mole of antioxidant. Both sets of data are in reasonable agreement between each other. A general analysis of the involved reactions is presented, in which further reactions following the formation of quinone intermediates by a first two-electron step are considered.