The kinetics and mechanisms of the complex formation and antioxidant behaviour of the polyphenols EGCg and ECG with iron(III)

(-)-Epigallocatechin-gallate ((-)-EGCg) and (-)-epicatechin-gallate ((-)-ECG) are important antioxidants which are found in green tea. The kinetics and mechanisms of the reactions of a pseudo-first order excess of iron(III) with EGCg and ECG have been investigated in aqueous solution at 25 degrees C and an ionic strength of 0.5 M NaClO4. Mechanisms have been proposed which account satisfactorily for the kinetic data. These are consistent with a mechanism in which the 2:1 metal:ligand complex initially formed on reaction of iron(III) with the ligand subsequently decomposes in an electron transfer step. Complex formation takes place at two separate binding sites via coupled reactions. Rate constants of 4.28(+/- 0.06) x 10(6) M-2 s(-1) and 2.83(+/- 0.04) x 10(6) M-2 s(-1) have been evaluated for the reaction of monohydroxy Fe(OH)(2+) species with EGCg and ECG, respectively while rate constants for of 2.94(+/- 0.4)x 10(4) M-2 s(-1) and 2.41(+/- 0.25) x 10(4) M-2 s(-1) have been evaluated for the reaction of Fe3+ species with EGCg and ECG, respectively. The iron(III) assisted decomposition of the initial iron(III) complex formed was also investigated and the rate constants evaluated. Both the complex formation and subsequent electron transfer reactions of iron(III) with EGCg and ECG were monitored using UV-visible spectrophotometry. All of the suggested mechanisms and calculated rate constants are supported by calculations carried out using global analysis of time dependant spectra. The results obtained show that one molecule of either EGCg or EGC is capable of reducing up to four iron(III) species, a fact which is consistent with the powerful antioxidant properties of the ligands. (c) 2006 Elsevier Inc. All rights reserved.