Oxygen atom transfer promoted nitrate to nitric oxide transformation: a step-wise reduction of nitrate → nitrite → nitric oxide

Abstract

Nitrate reductases (NRs) are molybdoenzymes that reduce nitrate (NO3−) to nitrite (NO2−) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO2−from NO3−, which further produces nitric oxide (NO) either in acid-induced NO2−reduction or in the presence of nitrite reductases (NiRs). Here, we report a new approach of VCl3(V3+ion source) induced step-wise reduction of NO3−in a CoII-nitrato complex, [(12-TMC)CoII(NO3−)]+(2,{CoII-NO3−}), to a CoIII-nitrosyl complex, [(12-TMC)CoIII(NO)]2+(4,{CoNO}8), bearing anN-tetramethylated cyclam (TMC) ligand. The VCl3inspired reduction of NO3−to NO is believed to occur in two consecutive oxygen atom transfer (OAT) reactions,i.e.,OAT-1= NO3−→ NO2−(r1) andOAT-2= NO2−→ NO (r2). In these OAT reactions, VCl3functions as an O-atom abstracting species, and the reaction of2with VCl3produces a CoIII-nitrosyl ({CoNO}8) with VV-Oxo ({VVO}3+) species,viaa proposed CoII-nitrito (3, {CoII-NO2−}) intermediate species. Further, in a separate experiment, we explored the reaction of isolated complex3with VCl3, which showed the generation of4with VV-Oxo, validating our proposed reaction sequences of OAT reactions. We ensured and characterized3using VCl3as a limiting reagent, as the second-order rate constant ofOAT-2(k2/) is found to be ∼1420 times faster than that of theOAT-1(k2) reaction. Binding constant (Kb) calculations also support our proposition of NO3−to NO transformation in two successive OAT reactions, asKb(CoII-NO2−)is higher thanKb(CoII-NO3−), hence the reaction moves in the forward direction (OAT-1). However,Kb(CoII-NO2−)is comparable toKb{CoNO}8, and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using15N-labeled-15NO3−and15NO2−revealed that the N-atom in the {CoNO}8is derived from NO3−ligand. This work highlights the first-ever report of VCl3induced step-wise NO3−reduction (NRs activity) followed by the OAT induced NO2−reduction and then the generation of Co-nitrosyl species {CoNO}8