Copper complexes with sulfur containing ligands. Homogeneous and heterogeneous phase studies on the generation and detection of reactive oxygen species

Abstract

Coordination compounds have been widely used as model systems in attempts for elucidating and/or understanding the mechanism of a myriad of processes including in vivo metabolism and oxygen conversion for the generation of energy or reactive oxygen species (ROS). It is well-known that copper ions are frequently found in enzymes that oxidize organic molecules and take part in interconversion processes of oxygen to water. To study the mechanisms involved in such processes, longstanding efforts have been made for elucidating the geometries and electronic structures of intermediates to unravel the relationships among these aspects and the catalytic functions. A successful strategy that have been adopted by inorganic chemists lies on the synthesis, characterization and reactivity study of copper containing molecules that mimic the enzymatic activity in vitro. In this context, this work presents the results obtained for two novel Schiff bases, 1,3-bis(1,10-phenantrolyn-2-yloxy)-N-(4- (methylhtio)benzylidene)propan-2-amine (2CP-Bz-SMe) and (E)-1-(4'-methil-[2,2'- bipyridine]-4-yl)-N-(4(methylthio)phenyl) metanimine (4-mbipy-Bz-SMe), and for the copper complexes (states I and II) produced from the reaction of these bases with Cu(NO3)2. The isolated compounds were characterized by nuclear magnetic resonance (Schiff bases), vibrational and electronic (room temperature and 90 oC)spectroscopies, electrochemical techniques, and electron paramagnetic resonance (EPR). The thioether moiety of the Schiff bases allowed the immobilization of the copper compounds on gold electrodes and on gold-coated Fe3O4 (Fe3O4@Au) nanoparticles (core@shell structures, Fe3O4@Au@Cu) so that the reactivity of these compounds was also evaluated on surface. To reach such goal, surface techniques were used, citing, beside the conventional electrochemical techniques, surface plasmon resonance (SPR), scanning electrochemical microscopy (SECM), surface-enhanced Raman scattering (SERS), and electrochemical impedance. The characterization of the nanomaterials, in turn, was performed by means of transmission electron microscopy (TEM), X-rays diffraction (XRD), and vibrating sample magnetometry (VSM). All the isolated compounds presented nuclease activity towards plasmid DNA, either in solution or adsorbed on electrodes or nanoparticles. According to EPR and SECM data, this activity was assigned to the generation of hydroxyl radicals ( •OH). The measurements performed to elucidate the mechanism involved in the production of this species showed the formation of H2O2 in a Fenton-like reaction, and a bimetallic intermediate containing peroxo as bridge ligand, {[CuII(4-mbpy-Bz-SMe)2]2(µ-O2 2-)}2+ .