Biomimetic Modeling of Copper Nitrite Reductase: Acid-Catalyzed Reduction of Nitrite to Nitric Oxide at Cu(I)-center via a Cu(II)-hydroxide Intermediate

Abstract

Nitric oxide (NO), a key signaling molecule, is generated by Cu/Fe-based nitrite reductases (NiRs) in the biological system under hypoxic conditions. The conversion of nitrite (NO₂⁻) to NO, a crucial step of the biological nitrogen cycle, utilizes two protons and one electron. Herein, we structurally characterized a functional mimic of Cu-NiR, a Cu^I-NO₂⁻ complex ([(PMDT)Cu^I(NO₂⁻)], (2) synthesized by reacting [(PMDT)Cu^I(CH₃CN)] (1) with one equivalent of NaNO₂⁻, supported by a tridentate N₃-ligand scaffold. To emulate the Cu-NiR reaction pathway, we investigated the reaction of complex 2 with an acid (HClO₄ as H⁺ source), yielding NO and a Cu^II-complex, [(PMDT)Cu^II(CH₃CN)₂](ClO₄)₂ (3), along with H₂O, consistent with a 2H⁺ and one-electron reduction process. Mechanistic studies of this reaction support the formation of a proposed Cu^II-hydroxo intermediate ([(PMDT)Cu^II(OH)]]⁺, as evident by electronic absorption, FT-IR, and EPR spectroscopic measurements. In addition, studies employing ¹⁵N-labeled ¹⁵NO₂⁻ undoubtedly established that the N-atom in NO originates from the NO₂⁻ ligand, coordinated to the Cu^I-center of 2, as demonstrated by the detection of ¹⁵NO in the headspace. This chemistry represents the functional model of CuNiRs, showing the formation of Cu^II-hdyroxo intermediate species in the H⁺-induced reduction of Cu^I-NO₂⁻ to NO, offering new mechanistic insights into the reductive transformation of NO₂⁻ to NO in biological systems.