Study of the iron catechin complexes in dimethyl sulphoxide. Redox chemistry and interaction with superoxide radical anion in this medium.

The redox chemistry of catechin (catH(2)) and its iron complexes has been studied in dimethyl sulphoxide. In the absence of base a one-to-one iron (II)-catechin complex is formed which exhibits oxidation processes at 0.28, 0.66, and 0.92 V vs SCE. These processes correspond to the oxidation of Fe(II) to Fe(III), the formation of the quinonic form of the catechol moiety and the oxidation of another hydroxy group to a radical. In the presence of base a stable 1:1 complex is formed with oxidation processes that show up at +0.25, +0.64, and +0.88 V vs SCE. The voltammetric and spectroscopic characterization of the species produced after the oxidation processes is described. Upon interaction of the complex with superoxide radical anion in dimethyl sulphoxide, the basic character of this radical anion causes the formation of the monoanion of catechin leading to a more stable complex of iron(II). The protonated superoxide disproportionates to molecular oxygen and peroxide, causing the oxidation of the metal ion. The addition of a second equivalent of superoxide oxidizes bound catechin to the corresponding semiquinone. The formation of hydroxy radicals through Fenton chemistry does not take place because peroxide is consumed and the metal ion remains as a stable iron(III) complex.