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)CuI(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 σ = 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)CuI(MeCN)]+ and 2 equiv of acid to give [(F8)FeIII]+, [(tmpa)CuII(solvent)] 2+, and N2O(g), fitting the stoichiometric •NO(g) reductive coupling reaction: 2•NO(g) + FeII + CuI + 2H+ → fN2O(g) + FeIII + CuII + 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)FeIII]SbF 6 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. © 2010 American Chemical Society.