OP25 Mechanism of the reactions of the nitroprusside ion [formula omitted] with sulfide

• Published in: Nitric oxide Vol. 31; p. S30
• Main Authors: Olabe, José A., Amorebieta, Valentín T.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.09.2013
• ISSN: 1089-8603; 1089-8611
• DOI: 10.1016/j.niox.2013.06.055

The title reaction has been of long–standing interest (”Gmelin” reaction), and is still of main concern, as an example of the chemistry comprising the nitrosyl ligand at the different three redox–states, formally considered as M (NO+), M (NO) and M (NO-) [1–3]. The nitroprusside ion, described as a MII(NO+) species, is widely used in medical experimentation as a “gold–standard” NO–donor drug [4]. On the other hand, the chemistry of H2S/ HS- in bio-relevant situations is receiving an increased attention [5]. Indeed, in both cases, we are dealing with currently accepted hot topics in basic mechanistic chemistry, most relevant to animal– and plant–physiology [6,7]. The work has been done at 25.0°C, pH–range (6.7–12.5), at variable ratios of the reactant concentrations, I=1M, and in an anaerobic medium. We provide a deep analysis of the underlying mechanism, previously reported by us [1], and subsequently questioned by others [7]. We used an experimental set–up comprising time-resolved UV–Vis, FTIR, EPR, stopped–flow and computational techniques, as well as a chemical control of products formation. In the scale of seconds to hours, different species have been characterized, depending on the pH. The first addition–intermediate, containing bound thionitrous acid (nitrososulfane, HSNO) is the precursor of [FeII(CN)5(NOS)]4-, containing the thionitrite ligand, which forms in a few seconds, and evolves rapidly up to the previously described bound nitroxyl (HNO) species, [FeII(CN)5(HNO)]3-[2]. N2O has been detected in the minute–hours time scale (depending on the pH), as well as NH3, with a final production of hexacyanoferrate (II). The production and decay of the different intermediates, and their proper characterization, has a strong mechanistic significance. We provide evidence for the three nitrosyl redox–states appearing as a result of successive reductions of MII(NO+), comprising 1–electron, 2–electron or 6–electron processes. In this context, the studied system shows up noticeable similarities with the behavior of NO2-– and NO–reductase enzymes. Key mechanistic features associated with the bioinorganic aspects of bound–nitrosyl chemistry[3], namely thiolate– and other nucleophile–additions, transnitrosations, trans–labilizations, dinitrosyl–formation, donor abilities and differentiated physiological roles of NO/NO-,HNO, will be addressed. The influence of oxygen and/or light in the reaction course will also be briefly discussed.