Reduction of nitric oxide to HNO by sodium dithionite: kinetics and mechanism

Sodium dithionite is a widely used reductant in biochemical and industrial applications, yet its intrinsic instability and complex redox chemistry continue to pose challenges for mechanistic interpretation. One relatively underexplored aspect is its reactivity with nitric oxide (NO˙), a small redox-active signalling molecule. While dithionite is commonly employed to reduce metal centres in enzymes, its potential interaction with NO˙ may influence experimental outcomes in aqueous redox systems. Here, we show that under anaerobic, near-neutral aqueous conditions, dithionite reacts with NO˙ leading to the formation of azanone (HNO, nitroxyl), the one-electron-reduced and protonated congener of nitric oxide. Formation of HNO is supported by direct trapping experiments using Mn(III) porphyrins and by indirect detection of N₂O, a characteristic product of HNO dimerization. These findings reveal a previously overlooked route for HNO generation in dithionite-containing systems and highlight potential artefacts in biochemical experiments involving NO˙ and strong reductants, particularly in studies probing thiol reactivity or metalloprotein function.