Second-Sphere Interaction Allows Selective Reduction of Nitrite to NO or Ammonia by Synthetic Iron Porphyrins

Heme nitrite reductases (NiRs) are key enzymes in the assimilatory and dissimilatory reduction of nitrite (NO₂^-) by 6e^-/8H⁺ to NH₄⁺ and by 1e^-/2H⁺ to NO during denitrification in the global nitrogen cycle. The different heme cofactors in different NiRs are flanked by highly conserved second-sphere basic residues in their active sites, e.g., histidine, lysine, and arginine. Biochemical and computational investigations have indicated that these basic residues offer hydrogen bond stabilization and provide protons to the intermediate species formed during NO₂^- reduction. Iron porphyrins with one or two pendent pyridines, mimicking the basic residues in the active site of NiRs, are synthesized, and their electrochemical NO₂^- reduction is investigated. The second-sphere hydrogen bonding residues allow the iron porphyrin with two pendent pyridines to successfully emulate the reactivity of cytochrome c NiR, where it can generate >95% NO (1e^-/2H⁺) at high potentials and >75% NH₄⁺ (6e^-/8H⁺), rest being NO, at lower potentials without releasing any other partially reduced species. These pendent pyridines stabilize the NO₂^- binding and very selective reduction of NO₂^- to either NO or NH₄⁺ in the presence of a modest proton donor. While the reduction to NO can be satisfactorily achieved using one basic group in the second sphere, the 6e^-/8H⁺ reduction to NH₄⁺ can be achieved satisfactorily only when two such basic groups are present. The quantitative understanding of the roles played by these basic residues in these reactions, obtained from these functional synthetic models, highlights the importance of these basic residues in multiproton multielectron reduction reactions in nature.