Catalase-like activity of a diMn complex in aqueous medium

PALOPOLI, CLAUDIA; SIGNORELLA, SANDRA

Los Cocos

Congreso; 9th Latin-American Conference on Physical Organic Chemistry (CLAFQO9); 2007

UNC

Mn catalases are enzymes of bacterial origin that may effectively catalyze the disproportionation of H2O2 into H2O and O2 by using a Mn2(µ-O2CR)(µ-O/OH) structural unit as the active site. Although the structure of these enzymes has been determined, very limited information is available on their mechanism of action. Biochemical and spectroscopic studies have shown that during catalysis, the diMn core shuttles between the oxidation states MnII2/MnIII2. However, the fast kinetics of the enzymatic reaction (kcat ˜ 105 s-1) and the fact that the oxidized and reduced form react with the same substrate have precluded a precise description. As a consequence, most mechanistic proposals have been based on studies of biomimetic compounds. Most of these studies have been performed in non-aqueous solvents (DMF, acetonitrile, methanol). In the few cases where the effect of different solvents was evaluated, it was observed that the reaction rate and even the mechanism change with the solvent. For this reason, there is great interest in obtaining information on the mechanism of the catalytic disproportionation of H2O2 in aqueous medium. DiMn complexes of 1,5-bis(salycilidenamino)pentan-3-ol (salpentOH) and its phenyl-ring substituted derivatives X-salpentOH (X = NO2, Br, Cl, H, Me, OMe) posses catalase activity with saturation kinetics and are able to disproportionate more than 1000 equivalents of H2O2. However, the activity of these complexes was evaluated in DMF and methanol because of their slight solubility in water. These observations prompted us to obtain the diMn complex of 5-SO3-salpentOH and to study the kinetics of H2O2 disproportionation upon reaction of H2O2 with [Mn2(µ-OMe)(µ-OAc)(5-SO3-salpentO)]Na in aqueous medium. This complex shows catalase activity, with first-order dependence on catalyst and saturation kinetics on [H2O2], in water. Upon addition of base the complex converts to [Mn2(OMe)(5-SO3-salpentO)], which possesses improved efficiency (kcat/KM) and is able to disproportionate more than 1500 eq. of H2O2 without decomposition. The effect of the solvent and the nature of the base on the catalytic efficiency will be discussed.