Spontaneous and Selective Formation of HSNO, a Crucial Intermediate Linking H₂S and Nitroso Chemistries

Abstract

Thionitrous acid (HSNO), a potential key intermediate in biological signaling pathways, has been proposed to link NO and H₂S biochemistries, but its existence and stability in vivo remain controversial. We establish that HSNO is spontaneously formed in high concentration when NO and H₂S gases are mixed at room temperature in the presence of metallic surfaces. Our measurements reveal that HSNO is formed by the reaction H₂S + N₂O₃ → HSNO + HNO₂, where N₂O₃ is a product of NO disproportionation. These studies also suggest that further reaction of HSNO with H₂S may form HNO and HSSH. The length of the S-N bond has been derived to high precision and is found to be unusually long: 1.84 Å, the longest S-N bond reported to date for an R-SNO compound. The present structural and, particularly, reactivity investigations of this elusive molecule provide a firm foundation to better understand its potential physiological chemistry and propensity to undergo S-N bond cleavage in vivo.