Preparation and Characterization of a Stable Semiquinone-Iron Complex

Dopamine oxidation by iron oxide (Fe2O3) was studied in the presence and absence of sodium thiosulfate in aqueous medium around pH 7 by UV-Vis spectroscopy. The pH changes from 6 to 8 indicate that the dopamine oxidation process has occurred producing an anionic semiquinone radical which appears after ca. 100 hours presenting bands at 309 and 337 nm. It forms a stable compound with Fe(III) released by the iron oxide. The complex [CTA][Fe(SQ)2(CAT)], where SQ=semiquinone, CAT=catecholate, and CTA=cetyltrimethylammonium cation, was isolated by precipitation with cetyltrimethylammonium bromide and was characterized through EPR, Raman and IR spectroscopies. The EPR spectrum presented two intense bands, one with g=2.003 assigned to o-semiquinone and the other with g=4.274 characteristic for high spin Fe(III) approaching an octahedral symmetry. The most intense Raman resonance band occurs at 1360 cm−1 assigned to ν(C1–C2) and at 1575 cm−1 to ν(C–C)ring of the o-semiquinone. The O2 dissolved in solution is mainly responsible for the dopamine oxidation when sodium thiosulfate is present. A thermal decomposition mechanism based on the thermogravimetric curves (TG) was proposed. These results suggest that iron can participate in the degenerative process of the dopaminergic nigral neurons. Its role seems to be its coordination with the dopamine oxidation products as o-semiquinone and catecholate which could damage neurons giving rise to parkinsonism.