A combined spectroscopic and theoretical approach to investigate structural properties of Co(II)/Co(III) tris-cysteinato complexes in aqueous medium

Physiological and toxicological effects of metallic ions depend on their speciation and on the structure of their associated bioligand complexes. In the field of chemical and/or nuclear toxicological studies, we are investigating cobalt complexes with biorelevant ligands such as amino acids or peptides. The aqueous reaction of cobalt dichloride with an excess of cysteine (CYS, C3H5NSO22-) in a basic medium under an anaerobic atmosphere and subsequent oxidation by 02, afforded the mononuclear complexes Co(II):3Cys and Co(II):3Cys, respectively. A combination of X-ray absorption spectroscopy (XAS) measurements and Car-Parrinello molecular dynamics (CPMD) simulations allowed us to assess structural features of the already explored Co(ni):3Cys complex. Inclusion of the temperature effects in the CPMD calculations gives an implicit access to disorder effects in the extended X-ray absorption fine structure (EXAFS) equation. The very good agreement between the measured and the simulated data showed the accuracy of these models provided by CPMD. The present investigation is completed by new UV-visible, X-ray absorption near edge structure (XANES) and electron paramagnetic resonance (EPR) data of Co(II):3Cys. These data are consistent with a Co(II) high-spin d(7) complex in a distorted octahedral geometry. This work contributes to the knowledge of topics such as metal-bioligand interaction which is of major interest in the field of biomorganic chemistry. Among the various transition metals that are involved in biological processes, the cobalt cation is of major interest. At low concentrations, cobalt is a biologically essential trace element for humans. 4 However, cobalt may be toxic at high concentrations, leading to adverse health effects. 5 Of particular radio- or toxicological concern is, for example, the case of skin contamination with 60CO of workers exposed to specific occupational conditions in the nuclear industry.(6) Indeed (CO)-C-59, the natural stable isotope of cobalt, is present in alloys in nuclear power plants and its activation by thermal neutrons produces 60CO. This isotope is of particular importance because of its long half-life (5.3 years) and its high energy emission of gamma photons. Attention has also been paid to 60 Co due to its presence in radioactive waste discharges, which can represent biogeochemical problems. 7 In the case of individual contamination, the nature of the chemical forms of cobalt that are present in vivo, as well as the molecular bases of its toxicity, transport, accumulation or detoxification, are far from being understood.8 In order to elucidate these phenomena, as well as to understand the biological role and functions of cobalt and its potential radiological impact, preliminary structural investigations of cobalt complexes containing biorelevant ligands are of great interest. Among the different spectroscopic techniques that can directly probe the environment of the metallic cations, XAS has long been widely and successfully applied to inorganic or bioinorganic systems, such as metalloproteins,9 because of its