Nitrite Reduction in Aqueous Solution Mediated by Amavadin Homologues: N2O Formation and Water Oxidation

• Published in: Chemistry : a European journal 2018-02, Vol.24 (10), p.2474-2482
• Main Authors: Dias, Lúcia, Bekhti, Nihel, Kuznetsov, Maxim L., Ferreira, José A. B., Bacariza, Maria C., da Silva, José Armando L.
• Format: Article
• Language: English
• Subjects: Absorption spectroscopy; Anions; Aqueous solutions; Chemistry; density functional calculations; Fungi; Homology; Iridium; Ligands; Metabolism; natural products; Nitrite reductase; Nitrites; Nitrogen dioxide; Nitrogen metabolism; Nitrous oxide; Oxidation; Oxygen; Protonation; reaction mechanisms; Reduction; Vanadium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201705385

Reactions of two vanadium(IV) complex anions that are homologues of amavadin, [V(HIDPA)2]2− and [V(HIDA)2]2− (HIDPA=N‐oxyiminodipropionate, HIDA=N‐oxyiminodiacetate), with the nitrite ion (NO2−) in aqueous solution were investigated by experimental (absorption spectroscopy in the visible range, through measurements of dioxygen formed in solution from water oxidation and identification of nitrogen oxide species of a gaseous atmosphere from nitrite reduction by using an IR analyser) and theoretical methods. Two reactions, mediated by the vanadium complexes, with environmental and biological significance, were observed in this system, namely, reduction of nitrite to N2O and oxidation of water to molecular oxygen. The reduction of nitrite, as studied by DFT calculations, occurs through the formation of NO (ΔG≠=14.3 kcal mol−1), which is strongly dependent on pH and slightly endergonic, and is then easily converted into N2O, with an overall activation barrier of ΔG≠=11.8 kcal mol−1. The later process includes dimerisation of NO assisted by one molecule of the V complex, protonation and oxidation of the formed ONNO.− ligand by another amavadin molecule or by nitrite, and NO bond cleavage/proton transfer in the ONNOH− ligand. The results indicate that amavadin exhibits an unusual nitrite reductase type activity that could be involved in nitrogen metabolism of Amanita muscaria and other fungi containing this vanadium complex. Fungal metabolism: Reaction mechanisms of two vanadium(IV) complex anions that are homologues of amavadin, which is found in some fungi species, with NO2− in aqueous solution were investigated by experimental and theoretical methods. Direct nitrite reduction to N2O is unusual for reactions mediated by biomolecules and is a promoter of water oxidation (see scheme).