Utilization of microwave spectroscopy to identify and probe reaction dynamics of hsno, a crucial biological signaling molecule

Thionitrous acid (HSNO), a potential key intermediate in biological signaling pathways, has been proposed to link NO and H$_{2}$S biochemistries. Its existence and stability in vivo, however, remain controversial. By means of Fourier-transform microwave spectroscopy, we establish that HSNO is spontaneously formed in high concentration when NO and H$_{2}$S gases are simply mixed at room temperature in the presence of metallic surfaces. Our measurements reveal that HSNO is formed with high efficiency by the reaction H$_{2}$S and N$_{2}$O$_{3}$ to produce HSNO and HNO$_{2}$, where N$_{2}$O$_{3}$ is a product of NO disproportionation. These studies also suggest that further reaction of HSNO with H$_{2}$S may form HNO and HSSH. The length of the S--N bond has been derived to high precision from isotopic studies, and is found to be unusually long, 1.84 \AA\ -- the longest S--N bond reported to date for an SNO compound. The present structural and reactivity investigations of this elusive molecule provide a firm fundation to better understand its physiological chemistry and propensity to undergo S--N bond homolysis in vivo