EPR spin trapping studies of H2O2 activation in metaloporphyrin catalyzed oxygenation reactions: Insights on the biomimetic mechanism

Abstract

Oxygen radicals have been proposed to have a role in the metaloporphyrin based biomimetic mechanisms as they can be formed from metal-oxo or metal-hydroperoxy species. Herein EPR studies of biomimetic oxygenation reactions using BMPO as a spin trap are used to investigate the production of reactive oxygen species during the catalytic activation of H2O2 by FeIII and MnIII porphyrins. Firstly, reactions in control conditions were studied and the formation of BMPO-OH• adducts was observed. These provided a baseline system for further comparative evaluation of the effect of different components of the catalytic reactions, including metaloporphyrins, solvents, co-catalysts and substrates on ROS production and H2O2 consumption. In the absence of co-catalyst or using ammonium acetate as co-catalyst, the metaloporphyrin centered pathways do not generate additional hydroxyl or superoxide radicals. In contrast, in the presence of sodium hydroxide, the production of additional hydroxyl radicals by a metaloporphyrin-mediated process is observed. The EPR results also differentiate the action of intermediates ascribed to metaloporphyrin-H2O2 adducts and high-valent oxo-species by their relative rates of H2O2 consumption and their effect on the production of BMPO-OH•.