NO/H 2 S “Crosstalk” Reactions. The Role of Thionitrites (SNO – ) and Perthionitrites (SSNO – )

VENÂNCIO, MATEUS F.; OLABE, J A; ROCHA, WILLIAN R.; MARCOLONGO, JUAN P.; DOCTOROVICH, FABIO

INORGANIC CHEMISTRY

AMER CHEMICAL SOC

Año: 2019 vol. 58 p. 14981 - 14997

0020-1669

The redox chemistry of H2S with NO and other oxidantscontaining the NO group is discussed on a mechanistic basis because of theexpanding interest in their biological relevance, with an eye open to thechemical differences of H2S and thiols RSH. We focus on the properties of two?crosstalk? intermediates, SNO− (thionitrite) and SSNO− (perthionitrite,nitrosodisulfide) based in the largely controversial status on their identity andchemistry in aqueous/nonaqueous media, en route to the final products N2O,NO2−, NH2OH/NH3, and S8. Thionitrous acid, generated either in the directreaction of NO + H2S or through the transnitrosation of RSNO?s(nitrosothiols) with H2S at pH 7.4, is best described as a mixture of rapidlyinterconverting isomers, {(H)SNO}. It is reactive in different competitive modes, with a half-life of a few seconds at pH 7.4 forhomolytic cleavage of the N−S bond, and could be deprotonated at pH values of up to ca. 10, giving SNO−, a less reactivespecies than {(H)SNO}. The latter mixture can also react with HS−, giving HNO and HS2− (hydrogen disulfide), a S0(sulfane)-transfer reagent toward {(H)SNO}, leading to SSNO−, a moderately stable species that slowly decomposes in aqueous sulfidecontainingsolutions in the minute−hour time scale, depending on [O2]. The previous characterization of HSNO/SNO− andSSNO− is critically discussed based on the available chemical and spectroscopic evidence (mass spectrometry, UV−vis, 15NNMR, Fourier transform infrared), together with computational studies including quantum mechanics/molecular mechanicsmolecular dynamics simulations that provide a structural and UV−vis description of the solvatochromic properties of cis-SSNO− acting as an electron donor in water, alcohols, and aprotic acceptor solvents. In this way, SSNO− is confirmed as theelusive ?yellow intermediate? (I412) emerging in the aqueous crosstalk reactions, in contrast with its assignment to polysulfides,HSn−. The analysis extends to the coordination abilities of {(H)SNO}, SNO−, and SSNO− into heme and nonheme ironcenters, providing a basis for best unraveling their putative specific signaling roles.