Oxygen Atom Transfer from Nitrite Mediated by Fe(III) Porphyrins in Aqueous Solution

Abstract

Aqueous solutions of the iron(III) porphyrin complex FeIII(TPPS) (1, TPPS = tetra(4-sulfonatophenyl)-porphyrinato) and nitrite ion react with various substrates S to generate the ferrous nitrosyl complex FeII(TPPS)(NO) (2) plus oxidized substrate. When S is a water-soluble sulfonated phosphine, the product is the resulting monoxide. When air is introduced to the product solutions, 2 is rapidly reoxidized to 1; however, even in the absence of air, there is a slow regeneration of the ferric species with concomitant production of nitrous oxide. Thus, in an anaerobic aqueous environment, FeIII(TPPS) catalyzes oxygen atom transfer from nitrite ion to substrates with the eventual formation of N2O.

Mechanistic studies of nitrite reactions with metalloproteins and models relevant to mammalian physiology

“…More surprising was the observation by Khin et al [141] that the reduced system undergoes a slow spontaneous regeneration of the ferric complexes under anoxic conditions as well. Analysis of the gas phase of the system utilizing TPPTS as the substrate indicated stoichiometric formation of nitrous oxide, Thus, the Fe(TPPS) acts as a catalyst for oxidation of the substrate by nitrite (Eq.…”

“…This system is more applicable to physiological conditions and consists of NaNO 2 , the water-soluble porphyrin complex Fe III (TPPS)(H 2 O) 2 [141], and various substrates in phosphate buffer. The species oxidized included water-soluble sulfonated triphenylphosphine (TPPTS), dimethylsulfide (DMS) and the biological thiols cysteine (CysSH) and glutathione (GSH).…”

“…(30)) [142]. Upon [141]. However, the introduction of small quantities of air to the reaction media regenerated the spectrum of the initial ferric complexes.…”

“…Scheme 10 illustrates a mechanism proposed [141] for this reaction, proceeding first through the Fe III nitro-complex followed by OAT to the substrate. The relatively stable ferrous-nitrosyl Fe II (TPPS)(NO) would be the resting state of the system, so long as there is excess reductant, but the system eventually returns to the ferric state once the reductant is expended.…”

Mechanisms of Nitric Oxide Reactions Mediated by Biologically Relevant Metal Centers

“…Although, such ferrous porphyrinato nitrosyls are often considered to be quite unreactive, this solution slowly underwent spontaneous decay to give the ferric species Fe III (TPPS) (23). The possible formation of HNO was first suggested by the observation of N 2 O as a reaction product [157] (free HNO readily dimerizes to nitrous oxide) [158,159] and was later demonstrated by direct observation using an HNO-specific electrochemical technique [61]. The proposal that this occurs via protonation (effectively an oxidative addition of H + ) followed by dissociation of HNO was based on the pH-dependence of the reaction.…”