Electrochemical properties and CAT/SOD activity of water soluble manganese complexes. Comparison with their iron counterparts

SIGNORELLA, SANDRA; PALOPOLI CLAUDIA; DAIER, VERÓNICA; GHINAMO, LUCIANA; MICHELOUD MARIANA

TESALONICA

Conferencia; 10th European Biological Inorganic Chemistry Conference (EUROBIC10); 2010

Catalases (CAT) and superoxide dismutases (SOD) efficiently catalyze disproportionation of intracellular O2= and O2·- and provide a vital biological defense against these toxic oxygen metabolites through a mechanism involving cyclic oxidation and reduction of the metal cofactor. In MnSOD the active site contains one penta-coordinated Mn ion in a N3O2 environment, whereas MnCAT catalyze the disproportionation of H2O2 by using a bis(m-oxo(hydroxo)-m-carboxylate dimanganese structural unit as the active site. MnSOD have their Fe analogues but are inactive when substituted with Fe (Mn) despite the metal active site of Fe- and MnSOD has been proven to be structurally identical. On the other hand, enzymes with dinuclear Fe centers and carboxylate-oxo/hydroxide bridges have a function different from CAT. A number of di- and mononuclear manganese-based complexes has been investigated as low molecular weight catalytic scavengers of H2O2 and O2·-, respectively. However, only a few studies have been performed on the ability of the ligand to control the redox potential, a physical property that makes the metal ion to act as catalytic center, and are still fewer those that have explored the divergence between Mn and Fe redox potentials in bioinorganic models despite their high biological relevance. In this context, we have synthesized and evaluated the redox potential and catalytic activity of water soluble mono- and dinuclear Mn complexes with the ligands: 1,3-bis(5-SO3-salicylidenamino)propan-2-ol, 1,3-bis(5-SO3-salicylidenamino)propane and 1,5-bis(5-SO3-salicylidenamino)pentan-3-ol, and their Fe counterparts. The redox potentials were determined by cyclic, linear and square-wave voltammetry. The catalase activity was evaluated by measuring the oxygen concentration of H2O2 + catalyst mixtures with a Clark-type oxygen electrode. The SOD activity was assayed by measuring the inhibition of the photoreduction of nitro blue tetrazolium in the presence of the catalyst. Our results show that although DE(Mn/Fe) is small (~ 100 mV, for either the mono- and dinuclear complexes), Mn complexes are much more efficient to catalyze O2= and O2·- disproportionation than the Fe analogues, and that kinetic, more than thermodynamic, factors should be responsible for the observed different activity.