ORIGINAL PAPER Igor Schepetkin Æ Andrei Potapov Æ Andrei Khlebnikov Elena Korotkova Æ Anna Lukina Æ Galina Malovichko Lilia Kirpotina Æ Mark T. Quinn Decomposition of reactive oxygen species by copper(II) bis(1-pyrazolyl)methane complexes Received: 14 December 2005 / Accepted: 17 March 2006 / Published online: 22 April 2006 Ó SBIC 2006 Abstract Two bis(1-pyrazolyl)alkane ligands, bis(3,5-di- methyl-1-pyrazolyl)methane and bis(4-iodo-3,5-dimethyl- 1-pyrazolyl)methane, and their copper(II) complexes, bis(3,5-dimethyl-1-pyrazolyl)methanedinitratocopper(II) [CuL 1 (NO 3 ) 2 ] and bis(4-iodo-3,5-dimethyl-1-pyrazolyl) methanedinitratocopper(II) [CuL 2 (NO 3 ) 2 ]Æ2H 2 O, were prepared. Physiochemical properties of the copper(II) complexes were studied by spectroscopic (UV–vis, IR, EPR) techniques and cyclic voltammetry. Spectroscopic analysis revealed a 1:1 stoichiometry of ligand:copper(II) ion and a bindentate coordination mode for the nitrate ions in both of the complexes. According to experimental and theoretical ab initio data, the copper(II) ion is lo- cated in an octahedral hexacoordinated environment. Both complexes were able to catalyze the dismutation of superoxide anion (O  2 ) (pH 7.5) and decomposition of H 2 O 2 (pH 7.5) and peroxynitrite (pH 10.9). In addition, both complexes exhibited superoxide dismutase (SOD) like activity toward extracellular and intracellular reac- tive oxygen species produced by activated human neu- trophils in whole blood. Thus, these complexes represent useful SOD mimetics with a broad range of antioxidant activity toward a variety of reactive oxidants. Keywords Pyrazole derivatives Æ Copper complexes Æ Superoxide dismutase mimetics Æ Reactive oxygen species Æ Peroxynitrite Introduction Reactive oxygen species (ROS) are produced during normal cellular metabolism and are essential in many biochemical processes, including intermolecular and intracellular signaling, cell growth and differentiation, and host defense mechanisms (reviewed in [1, 2]). Con- versely, excessive ROS production leads to oxidative stress and host tissue damage, and cells have developed both enzymatic and nonenzymatic antioxidant defense mechanisms to protect against the detrimental effects of ROS [3, 4]. Among the most prominent enzymatic anti- oxidant mechanisms are superoxide dismutases (SOD), which catalyze the dismutation of superoxide anion (O  2 ) to O 2 and hydrogen peroxide (H 2 O 2 ); catalases, which convert H 2 O 2 to H 2 O; and peroxidases, which also eliminate H 2 O 2 [4–6]. Because of the importance of antioxidant defenses in health and disease, a significant amount of research has focused on the understanding of these defense mechanisms, as well as on the development of potential therapeutic antioxidant compounds. One successful approach to understanding SOD function has been the development of model compounds that mimic SOD properties [7], and low molecular weight metal chelates have been shown to mimic SOD catalytic activity [8–10]. It is clear that the active site of SOD, as well as a number of other copper-containing proteins, such as laccase, galactose oxidase, and ascor- bate oxidases, could be partly mimicked by simple models containing pyrazole rings. For example, copper complexes coordinated to polydentate pyrazole-based ligands have been proposed as models for the type-3 active site of the copper proteins hemocyanin and tyrosinase [11]. The coordination properties of pyrazole ligands can be altered widely by introducing various substituents I. Schepetkin Æ L. Kirpotina Æ M. T. Quinn (&) Department of Veterinary Molecular Biology, Montana State University, Bozeman, MT 59717, USA E-mail: mquinn@montana.edu Tel.: +1-406-9945721 Fax: +1-406-9944303 A. Potapov Æ A. Khlebnikov Department of Chemistry, Altai State Technical University, Barnaul, Russia E. Korotkova Æ A. Lukina Department of Chemical Technology, Tomsk Polytechnic University, Tomsk, Russia G. Malovichko Department of Physics, Montana State University, Bozeman, MT 59717, USA J Biol Inorg Chem (2006) 11: 499–513 DOI 10.1007/s00775-006-0101-1