Di-imine copper(II) complexes as redox mediator and modulator in 2-deoxy-D-ribose oxidative damage

Redox properties of copper complexes are important for their catalytic functions in vitro and in biological systems, and can contribute to their reactivity toward selected targets. In order to evaluate the influence of different ligands on the reactivity of copper ions, comparative studies were carried out with some copper(II) complexes containing a tridentate imine, or a tetradentate di-Schiff base ligand with a mixed pyridine, pyrazine, or imidazole donor set, acting as catalysts in the oxidation of 2-deoxy-D-ribose. Addition of the reducing agent glutathione (gamma-glutamylcysteinylglycine; GSH), which can also act as a good ligand for copper(I), mediated the oxidation of the substrate. For some of these compounds, a reductive activation followed by competition for the metal ion was verified, with formation of copper(I)-glutathione complex monitored by fluorescence measurements. For others, however, the reduction of the metal by the glutathione seems to not occur. In the presence of hydrogen peroxide, the oxidative damage is significantly enhanced for all the complexes tested. Redox potential measurements by cyclic voltammetry corroborated partially these results, indicating that the most reactive complexes are those with more positive redox potential. Evidence for site-specific attack to 2-deoxy-D-ribose was also observed, consistent with the intermediary formation of a copper-hydroxyl species, [LCuII(OH)], rather than 'free' hydroxyl radical.