Nitrite as a Synthon for Nitroxyl (HNO): Mechanistic Insights and Chemical Evidences of Transient Intermediates

Nitrite anion has emerged as a key mediator in various redox signaling processes. This study utilizes a crystallographically characterized tetrahedral complex (Me₂PzQu)Zn(thioacetate)₂ (1-Qu) to demonstrate that thiocarboxylate at zinc(II) modulates nitrite-thiol reactivity to promote the generation of nitroxyl (HNO), a distinct signaling species often overshadowed by nitric oxide (NO). Unambiguous characterization of HNO is achieved via mass spectrometry and FTIR spectroscopy, in combination with detailed ¹⁴N/¹⁵N-isotope labeling studies. A set of control experiments clearly demonstrates that the presence of both zinc(II) and thiol shifts the reaction trajectory toward HNO formation rather than NO in the presence of physiologically relevant weakly acidic proton sources like thiols. Furthermore, investigations using thiols with varying pK_a highlight the critical role of proton transfer in the nitrite-to-HNO transformation, proposed via transient thionitrous acid (HSNO). Moreover, (Me₂PzQu)Zn(selenocarboxylate)₂ (5-Qu) also exhibits HNO-generating reactivity, although the yield of NO predominates over HNO. Interestingly, the reactions of nitrite with selenocarbonyl compounds in the absence of zinc(II) and thiol also offer valuable insights, as the NO, HNO, and organic product distributions from the seleno- and thiocarbonyl reactions closely resemble each other, suggesting a similar mechanistic pathway. Thus, these findings highlight the potential subtle factors regulating NO versus HNO generation and reinforce the underexplored role of the physiologically ubiquitous nitrite anion in HNO signaling.