Electrochemical and spectroscopic studies of nitrite reductase model complexes
Abstract
The prosthetic group in assimilatory nitrite reductase has been identified as siroheme, an isobacteriochlorin. Nitrite is reduced by the enzyme to iron nitrosyl, hydroxylamine and then ammonia. The aim of this work is to examine this six electron enzymatic reduction using iron porphyrin model complexes. The electrochemical reduction of nitrite by iron porphyrins was investigated by cyclic voltammetry, UV-visible and infrared spectroelectrochemistry. Following the reduction of Fe(III)/Fe(II) in the presence of nitrite, iron porphyrin nitrosyl and $\mu$-oxo complexes were identified. Spectrochemical analysis indicated the reaction proceeds via the formation a nitrite bridged Fe(III) and Fe(II) intermediate. Hydroxylamine is presumed to be an intermediate in the assimilatory reduction of nitrosyl to ammonia. The reaction of hydroxylamine with a series of metal porphyrins was examined. For manganese and iron porphyrins, this was a very efficient method for the generation of metal nitrosyl complexes. Cobalt(II) porphyrin did not react with hydroxylamine under anaerobic conditions. With a trace amount of oxygen, the reaction of cobalt(II) porphyrin with hydroxylamine led to the formation of a stable cobalt(III)-bis-hydroxylamine complex. Two-electron reduction products of iron(III) porphyrins have been of current interest, because they are putative intermediates in the redox cycles of iron porphyrins and heme proteins. Two-electron reduced products of iron(III) porphinone and porphindione complexes were characterized by infrared spectroelectrochemistry. The results indicated the second electron reductions for Fe(OEPone)Cl (where OEP = octaethyl porphyrin) and Fe(OEPdione)Cl are centered at the porphyrin ring. Infrared spectroelectrochemistry was also used to study the reduction of iron nitrosyl complexes. Fe(OEP)NO has a strong NO IR absorbance at 1672 cm$\sp{-1}$. This band shifted to 1441 cm$\sp{-1}$ upon one-electron reduction. Reduction of Fe(OEPdione)NO showed similar change. The results suggest that the reduction of these nitrosyl complexes is centered at the Fe-NO moiety and is independent of the macrocycles.
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