Kinetics, CW and Pulsed EPR characterization of CrV saccharide complexes

FEDERICO SALA, LUIS; JUAN CARLOS GONZALEZ

Manchester

Congreso; European Inorganic Chemistry Congress 1; 2011

Manchester University

Hea Kinetics, CW and Pulsed EPR characterization of CrV saccharide complexes   Sala, LF(*)a; González, JCa; Van Doorsslaerb, S; Mangiameli, MFa; Garcia, Sa; Frascarolli, MIa. a Instituto de Química de Rosario-CONICET, Universidad Nacional de Rosario, UNR, Rosario, Argentina - b Departament of Physics-SIBAC Laboratory, University of Antwerp, Belgium   Heavy metals, like chromium, are common pollutants in soil and  water (1). Due to its stability it cannot be degraded and removed easily from the environment and turn out to be very harmful for the living organism (2).The element chromium has several oxidation states, two of which are the most stable: CrIII, an essential nutritional factor, and CrVI, mainly present as CrO42-, widely used in several industries, which was report to be toxic and carcinogenic(3). The presence of strong oxidants can change CrIII into CrVI(4). This latest oxidation state, CrVI, is a potential hazard also in biological and ecological context (5). The observation of CrV intermediates in the selective oxidation of organic substrates by CrVI and their implication in the mechanism of Cr-induced cancer has generated a considerable amount of interest in the chemistry and biochemistry of this Cr oxidation state(5). The reduction of CrVI occurs with a wide variety of reductants (6).This is why removal of Chromium turns out a very important issue. In order to do this, is necessary to understand its kinetics redox behavior and to determine the structure and stability of its intermediate redox species. Saccharides oxidation by CrVI has a very complex mechanism and in the last years only a few were develop. Our group has studied this process with several substrates like, alditols, uronic acids, aldaric acids, aldonic acids and aldoses (7-9). Our kinetics studies shows that the mechanism involved in all this cases, include various intermediates species: CrII, CrIV, CrV and free radicals. In particular, kinetics result of CrV, shows that this species are bischelate or monochelate; which gives by redox reaction, the organic oxidation and CrIII. Since CrV complexes are paramagnetic, we carried out EPR (Electron Paramagnetic Resonance) studies over this species, to support the kinetic results. EPR is an ideal tool to characterize these compounds. It can be used as a finger print to identify CrV complexes formed in biological systems. So, in vitro studies on the physic chemistry of CrV complexes will provide structural information on the nature of the species that are likely to be stabilized in vivo. Recent developments in EPR (introduction of pulse and high-field EPR) have increased enormously the potential of this technique. Our work in this specific area (CW and Pulsed EPR at room and low temperature) allow us to determine EPR spectroscopy parameters, such as g values, hyperfine coupling interactions between 53Cr nuclei and ligand nuclei (1H, 13C), of CrV species formed with different saccharides of biological importance, in order to obtain detailed structural information, HYSCORE, Mims and Davies ENDOR methods were used. We have found that, depending on the experimental conditions, this species are bischelates or monochelates, oxo-CrV-pentacoordinated (or hexacoordinated). Coordination preferential sites depended on pH and ligand involved. In acid pH, for acid ligands the preferential sites are 2-hydroxyacid; while for non acid ligands are 2-OH vic. Some of this species could be relatively stable for several hours. Thus, the EPR characterization of these CrV saccharides complexes is in agreement with our previous kinetics results.   1- Hawari, AH; Mulligan, CN. Process Biocehm  2006, 41, 187-198; 2- Xuejiang, W; Ling, C; Siqing, X; Jianfu, Z; Chovelon, JM; Renault, NJ. Miner. Eng. 2006, 19, 968-971; 3- Krishna, KR; Philip, L. J Hazard Mater. 2005, 121, 109-117; 4- Tadesse, I; Isoaho, SA; Geen, FB; Pukhakka, JA. Bioresour. Technol. 2006, 4, 529; 5- Levina, A; Lay, PA. Coord. Chem. Review 2005, 249, 281-298; 7-Gonzalez, JC; Garcia, S; Bellu, S; Salas, Peregrin, JM; Atria, AM; Sala, LF; Signorella, S. Dalton Trans. 2010, 39 1-4; 8- Mangiameli, MF; González, JC; García, S; Bellú, S;Santoro, M; Caffaratti, E; Frascaroli, MI; Salas Peregrín, JM; Atria, AM; Sala, LF. J. Phys. Org. Chem. 2010, DOI 10.1002/poc.1745; 9- Gonzalez, JC; Daier, V; Garcia, S; Goodman, BA; Atria, AM; Sala, LF; Signorella, S. Dalton Trans. 2004, 2288-2296.