Reductive Coupling of Nitrogen Monoxide (•NO) Facilitated by Heme/Copper Complexes

Abstract

The interactions of nitrogen monoxide (•NO; nitric oxide) with transition metal centers continue to be of great interest, in part due to their importance in biochemical processes. Here, we describe •NO_(g) reductive coupling chemistry of possible relevance to that process (i.e., nitric oxide reductase (NOR) biochemistry), which occurs at the heme/Cu active site of cytochrome c oxidases (CcOs). In this report, heme/Cu/•NO_(g) activity is studied using 1:1 ratios of heme and copper complex components, (F₈)Fe (F₈ = tetrakis(2,6-difluorophenyl)porphyrinate(2-)) and [(tmpa)Cu^I(MeCN)]⁺ (TMPA = tris(2-pyridylmethyl)amine). The starting point for heme chemistry is the mononitrosyl complex (F₈)Fe(NO) (λ_max = 399 (Soret), 541 nm in acetone). Variable-temperature ¹H and ²H NMR spectra reveal a broad peak at δ = 6.05 ppm (pyrrole) at room temperature (RT), which gives rise to asymmetrically split pyrrole peaks at 9.12 and 8.54 ppm at −80 °C. A new heme dinitrosyl species, (F₈)Fe(NO)₂, obtained by bubbling (F₈)Fe(NO) with •NO_(g) at −80 °C, could be reversibly formed, as monitored by UV−vis (λ_max = 426 (Soret), 538 nm in acetone), EPR (silent), and NMR spectroscopies; that is, the mono-NO complex was regenerated upon warming to RT. (F₈)Fe(NO)₂ reacts with [(tmpa)Cu^I(MeCN)]⁺ and 2 equiv of acid to give [(F₈)Fe^III]⁺, [(tmpa)Cu^II(solvent)]²⁺, and N₂O_(g), fitting the stoichiometric •NO_(g) reductive coupling reaction: 2•NO_(g) + Fe^II + Cu^I + 2H⁺ → N₂O_(g) + Fe^III + Cu^II + H₂O, equivalent to one enzyme turnover. Control reaction chemistry shows that both iron and copper centers are required for the NOR-type chemistry observed and that, if acid is not present, half the •NO is trapped as a (F₈)Fe(NO) complex, while the remaining nitrogen monoxide undergoes copper complex promoted disproportionation chemistry. As part of this study, [(F₈)Fe^III]SbF₆ was synthesized and characterized by X-ray crystallography, along with EPR (77 K: g = 5.84 and 6.12 in CH₂Cl₂ and THF, respectively) and variable-temperature NMR spectroscopies. These structural and physical properties suggest that at RT this complex consists of an admixture of high and intermediate spin states.