Catalase activity of new diMn(III) complexes of heptadentate (N4O3) ligands with the [Mn2(m-OR)2(m-OAc)]3+ core

SIGNORELLA, SANDRA; PALOPOLI CLAUDIA; BIAVA HERNAN; MORENO DIEGO

Los Cocos

Congreso; 9th Latin American Conference on Physical Organic Chemistry; 2007

UNC

Catalases are enzymes that protect cells from deleterious effects caused by H2O2, by disproportionating it into water and dioxygen. Besides heme type catalases, there is a class of manganese catalases (MnCAT) that has been found in several bacterial organisms. These enzymes possess a diMn active site and disproportionate H2O2 by cycling between the MnII2 and MnIII2 oxidation states. The crystal structure of MnCAT from L. plantarum and T. thermophilus reveal that the two Mn ions are triply bridged through a carboxylate from a glutamate residue and two solvent-derived single atom bridges. Owing to the intrinsic chemical interest of H2O2 disproportionation reaction and the paucity of mechanistic information, many dimanganese model complexes that show catalase activity have been reported. However, the best reactivity models are still 2-3 orders of magnitude slower than the catalase enzymes in terms of kcat and kcat/KM. This limitation stimulates the interest in understanding the structural and electronic factors that control the catalysis of H2O2 dismutation by dinuclear manganese complexes with the intention to obtain more effective catalysts. Within this context, we have evaluated the catalase-like activity of two families of diMn complexes formed with 1,5-bis[(2-hydroxybenzyl)(2-pyridylmethyl)amino]pentan-3-ol, 1,3-bis[(2-hydroxybenzyl)(2-pyridylmethyl])amino]propan-2-ol and their phenyl-ring substituted derivatives, with the goal of assessing the magnitude by which the bite of the donor sites may influence the reactivity of the dimanganese center. Further, we have correlated the turnover numbers with the redox potentials of the complexes of each family as a means to evaluate the influence of electronic factors on the H2O2 dismutation by this class of complexes. Additionally, we have compared the activity of the present diMn compounds with that of complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol and 1,3-bis(X-salicylidenamino)propan-2-ol (X = phenyl-ring substituent) in order to ascertain the influence of the Mn coordination environment on the catalase activity. As a result of these studies we have found that, besides the redox potentials, the ligand-imposed Mn···Mn separation is a factor that controls the Mn oxidation states during the H2O2 dismutation. Thus, diMn complexes with Mn···Mn ~ 2.9 Ã shuttle between Mn2III/Mn2IV oxidation states during the catalytic cycle, while diMn complexes with similar redox potentials but intermetallic separation longer than 3.2 Ã employ Mn2II/Mn2III oxidation states. The Mn coordination sphere affects the kinetic parameters (kcat, KM) but has not incidence on the oxidation states involved in the catalysis. Among complexes that cycle between Mn2II/Mn2III oxidation states, those with longer intermetallic separation show lower activity.