Hydroxylamine iron complex intermediate

In the case of porphyrin models (24), it was proposed that for [Fe (TPPS)] the formation of HNO fi om hydroxylamine occurred by a two-electron tra/is-disproportionation of a bis-coordinated NIhOH complex producing a low spin ferric intermediate [Fe Por)(NH3)(HNO)]. Based on the experimental kinetic evidence, and the obtained N2/N2O ratios from the reaction mixtures, a different mechanism was proposed for Fe MPl 1, in which the formation of [Fe (Por)(NHis)(HNO)] was suggested to result from the reaction of free HA toward iron-boimd HA. 

Reaction mechanism Based on the observation of reaction intermediates in the crystal structure and on quantum chemical calculations Einsle et al. [148] propose an outline of the first detailed reaction mechanism of the cytochrome c Nir from W. succinogenes. Nitrite reduction starts with a het-erolytic cleavage of the weak N-O bond, which is facilitated by a pronounced backbonding interaction between nitrite and the reduced active site iron. The protons come firom a highly conserved histidine and tyrosine. Elimination of one of both amino acids results in a significant reduced activity. Subsequently, two rapid one-electron reductions lead to a FeNO form and, by protonation, to a HNO adduct. A further two-electron two-proton step leads to hydroxylamine. The iron in the hydroxylamine complex is in the Fe(III) state [149], which is unusual compared to synthetic iron-hydroxylamine complexes where the iron is mainly in the Fe(II) state. Finally, it readily loses water to give the product, ammonia. This presumably dissociates firom the Fe(III) form of the active site, whose re-reduction closes the reaction cycle. 

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