Metalloporhyrins as Biomimetical MOdels of Cytochrome P450 in the Oxidation of Pesticides / Metaloporfirinas como modelos biomiméticos do citocromo P450 na oxidação de pesticidas"

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

In this work we investigated the ability of metalloporphyrin model systems to mimic the action of cytochrome P450 in the oxidation of a herbicide, atrazine. To this end, we employed the second generation commercially available metalloporphyrins metal (III) 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin chloride [M(TDCPP)Cl] and metal (III) 5,10,15,20- tetrakis(pentafluorophenyl)porphyrin chloride [M(TFPP)Cl] (metal = iron or manganese), all soluble in organic solvents, as well as the water soluble metalloporphyrins iron (III) 5,10,15,20-tetrakis(N-methyl-4-pyridyl)porphyrin chloride [Fe(TMPy)Cl] and iron (III) 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin chloride [Fe(TCPP)Cl]. These metalloporphyrins were used both in homogeneous solution and supported on montmorillonite K-10. Iodosylbenzene, metachloroperbenzoic acid, and hydrogen peroxide were tested as oxidants, using one of the following reaction media: water, methanol, and acetonitrile. Products generated during atrazine oxidation were identified by high performance liquid chromatography. Our results show that the metalloporphyrins are able to oxidize atrazine, a highly persistent herbicide in the environment, as well as mimic the action of P450 enzymes both in vivo and in vitro, with formation of two metabolites, namely DEA and DIA, which result from the N-dealkylation of the ethyl and propyl chains of the substrate, respectively. We also detected the formation of five unknown products, and we were able to identify only one of them by means of mass spectrometry, which corresponds to the formation of an amide on the atrazine ethyl chain (COA) and was the compound obtained in highest yields in most of the reactions. The other four unknown products were obtained in very low concentrations, which prevented us from determining their structures. By monitoring the reactions at different time intervals and varying the reactional conditions, we were able to see that the main herbicide oxidation products, DEA and COA, are generated via distinct mechanisms and different active catalytic species. DEA is formed via the species Me(V)OP [Mn(V)OP or Fe(IV)OP.+], while COA results from the action of the species Me(IV)OP [Mn(IV)OP or Fe(IV)OP]. Studies of the reaction intermediates by UV-VIS and EPR showed that the ferryl species is the main reaction intermediate in the case of Fe(TFPP)Cl/ACN systems and the oxidants, iodosylbenzene and metachloroperbenzoic acid. Studies of herbicide oxidation were difficult to carry out in the case of the metalloporphyrins Fe(TCPP)Cl and Fe(TMPy)Cl due to the low solubility of atrazine in water, which led to its precipitation and catalyst destruction. With respect to the metalloporphyrins supported on montmorillonite K-10, no reaction products were obseved because of the difficult diffusion of the inert substrate into the catalytic site. All these results demonstrate the potential application of these biomimetic model systems in studies that pursue the elucidation of herbicide metabolism in vivo, especially when one bears in mind the difficulty in working with enzymes in vitro. Our data enabled the proposition of a scheme for metalloporphyrin-catalyzed atrazine oxidation under the conditions used herein.

ASSUNTO(S)

citocromo p450 pesticides pesticidas metalloporphyrins cytochrome p450 metaloporfirinas

ACESSO AO ARTIGO

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