Addition and Redox Reactivity of Hydrogen Sulfides (H₂S/HS⁻) with Nitroprusside: New Chemistry of Nitrososulfide Ligands

Abstract

The nitroprusside ion [Fe(CN)(5)NO](2-) (NP) reacts with excess HS(-) in the pH range 8.5-12.5, in anaerobic medium ("Gmelin" reaction). The progress of the addition process of HS(-) into the bound NO(+) ligand was monitored by stopped-flow UV/Vis/EPR and FTIR spectroscopy, mass spectrometry, and chemical analysis. Theoretical calculations were employed for the characterization of the initial adducts and reaction intermediates. The shapes of the absorbance-time curves at 535-575 nm depend on the pH and concentration ratio of the reactants, R=[HS(-)]/[NP]. The initial adduct [Fe(CN)(5)N(O)SH](3-) (AH, λ(max) ≈570 nm) forms in the course of a reversible process, with k(ad)=190±20 M(-1)s(-1) , k(-ad)=0.3±0.05 s(-1) . Deprotonation of AH (pK(a)=10.5±0.1, at 25.0 °C, I=1 M), leads to [Fe(CN)(5)N(O)S](4-) (A, λ(max)=535 nm, ε=6000±300 M(-1) cm(-1) ). [Fe(CN)(5)NO](.)(3-) and HS(2)(.)(2-) radicals form through the spontaneous decomposition of AH and A. The minor formation of the [Fe(CN)(5)NO](3-) ion equilibrates with [Fe(CN)(4)NO](2-) through cyanide labilization, and generates the "g=2.03" iron-dinitrosyl, [Fe(NO)(2)(SH)(2)](-) , which is labile toward NO release. Alternative nucleophilic attack of HS(-) on AH and A generates the reactive intermediates [Fe(CN)(5)N(OH)(SH)(2)](3-) and [Fe(CN)(5)N(OH)(S)(SH)](4-) , respectively, which decompose through multielectronic nitrosyl reductions, leading to NH(3) and hydrogen disulfide, HS(2)(-) . N(2)O is also produced at pH≥11. Biological relevance relates to the role of NO, NO(-) , and other bound intermediates, eventually able to be released to the medium and rapidly trapped by substrates. Structure and reactivity comparisons of the new nitrososulfide ligands with free and bound nitrosothiolates are provided.

Three Redox States of Metallonitrosyls in Aqueous Solution

“…A more complex picture appears in the reaction between N 2 H 4 and [Fe(CN) 5 NO] 2À , with the intermediacy of bound linkage isomers of N 2 O, namely the "side-on" and "end-on" species as precursors of N 2 O release, together with the formation of NH 3 . A comprehensive mechanistic report of this reaction (93), reviewed later (91), allowed disclosing different stoichiometries and mechanistic routes for the adduct decompositions employing labeled 15 Very recently, we revisited the "Gmelin" reaction of SNP with HS À by performing a detailed kinetic and mechanistic study in a broad time scale, in the pH-range 8.5-12.5 (99). The experimental evidence led proposing the formation of a first adduct intermediate containing the NOSH ligand, which might deprotonate to yield NOS À with a pK a value estimated to be 10.…”

Chemical foundations of hydrogen sulfide biology

“…Here we present a few major concerns. First, sodium nitroprusside (SNP) was used as an • NO donor, which actually can directly react with H 2 S [182] through a mechanism that is still in debate [183,184]. This problem has also been brought to light by King [180].…”

Inorganic Reactive Sulfur-Nitrogen Species: Intricate Release Mechanisms or Cacophony in Yellow, Blue and Red?

“…The rate constant for SSNO − formation (from the reaction of sulfide with GSNO at the ratio 2:1) has been calculated as 640 ± 200 M −1 ·s −1 [45] and for HSNO formation (1:1 ratio) as 84 ± 7 M −1 ·s −1 [9]. At the same time, the rate constant for the interaction of nitroprusside and HSNO with HS 2 − is ~100-fold higher than with HS − [24,46]. Using the results of pulse radiolysis from Filipovic et al [9], Koppenol and Bounds have therefore estimated the rate constant of the reaction between HSNO and HS − (leading to formation of HSS − and HNO) to be 10 7 M −1 ·s −1 .…”