Redox behaviour and reactivity of some di-Schiff base copper(II) complexes towards reduced oxygen species

Abstract

Some new di-Schiff base copper(II) complexes, derived from 2,3-butanedione and 2-(2-aminophenyl)benzimidazole, 2-(aminomethyl)benzimidazole or 2-(2-aminoethyl)pyridine, have been prepared and characterised using UV/Vis, IR, and EPR spectroscopy. Their reactivity towards superoxide radicals was studied, in comparison to similar complexes, indicating that these species are usually good mimics of superoxide dismutase. Evidence of tetrahedral distortion around the copper ion, similar to that observed in the native enzyme, was also found through spectroscopic parameters. However, these new complexes can also generate hydroxyl radicals in appreciable quantities in the presence of hydrogen peroxide, as detected by spin-trapping EPR experiments. By using μmol dm⁻³ concentrations of these complexes, considerable lipid peroxidation of liposomes prepared from L-α-phosphatidylcholine was also confirmed, in vitro. Comparative studies with similar complexes pointed at a modulation of both antioxidant and pro-oxidant properties by the diimine ligands. Their redox behaviour was studied by CV, with the aim of establishing some correlation between estimated parameters, and observed reactivity versus reduced oxygen species (superoxide radical and hydrogen peroxide). Among five compounds, only one was shown to be resistant to molecular oxygen, after being reduced, in the range −1.50 to +0.50 V vs. NHE, in dimethylformamide solution containing tetraethylammonium perchlorate (0.100 mol dm⁻³), although all exhibited substantial reactivity toward hydrogen peroxide. In a wider range of potential, evidence of oxidation of the ligands was also detected. Tentative conclusions regarding the most important parameters affecting their reactivity are made.

p