Nitrite Reduction in Aqueous Solution Mediated by Amavadin Homologues: N₂ O Formation and Water Oxidation

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 N₂ O 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 N₂ O, 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.