Influence of the reaction medium on the activity of Manganese-Catalase mimics

C.M PALOPOLI; N. BRUZZO; C. BELTRAMINO; L. GHINAMO; M. VICTORIA COVOLO; S. SIGNORELLA

Angra do Reis

Encuentro; XV Brazilian Meeting on Inorganic Chemistry (XV BMIC) , II Latin American Meeting on Biological Inorganic Chemistry; 2010

Mn catalases catalyze the disproportionation of H2O2  into H2O and O2  by using a Mn2(ì-O2CR)(ì-O/OH/H2O)2 structural unit that cycles between the MnII2   and  Mn oxidation  states.  Because of the fast kinetics of this enzymatic reaction each independent step of the catalytic cycle has not yet been characterized. In this context, biomimetic compounds provide a unique way for testing mechanisms in these enzymes.1 The fine-tuning of Mn redox states is a critical feature when using artificial compounds to model the enzymatic activity. We have evidenced some of the key structural factors that control the oxidation states of Mn during H2O2 decomposition by comparing the catalase activity of diMn water soluble complexes with that of diMn compounds including other dinucleating ligands used in non aqueous solvents. New complexes that possess a triply bridged diMnIII core were synthesized  and  characterized,  and their catalase-like  activity was evaluated. 1,5-Bis(5-X(-3-Y-)salicilydenamino)pentan-2-ol (X(Y)-salpentOH) polydentate  ligands  afford  diMnIII   complexes  of  general  formula  [Mn2(m- OAc)(m-OMe)(X(Y)-salpentO)]+/-  that reproduce the geometrical motif of the enzyme  active site.2  Varying the  aromatic substituent  it was possible  to obtain complexes suitable for mechanistic studies in different solvents. Thus, when X = SO3-, the catalytic activity of these complexes could be evaluated in water. Comparative  studies  performed  in  DMF,  methanol  and  water showed that these compounds are able to disproportionate H2O2  with saturation  kinetics  on  [H2O2]  and  first-order  dependence  on  [catalyst]. Kinetics and spectroscopic monitoring of the reaction converge at a catalytic mechanism involving redox cycling between Mn2III/Mn2IV levels with retention of dinuclearity during catalysis. It was found that the presence of electron- donor substituents in the phenol ring and the addition of external base improved the catalytic efficiency and turnover rates of these compounds in both protic and non-protic solvents.