title: 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, (F(8))Fe (F(8) = 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 (F(8))Fe(NO) (lambda(max) = 399 (Soret), 541 nm in acetone). Variable-temperature (1)H and (2)H NMR spectra reveal a broad peak at delta = 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 degrees C. A new heme dinitrosyl species, (F(8))Fe(NO)(2), obtained by bubbling (F(8))Fe(NO) with *NO((g)) at -80 degrees C, could be reversibly formed, as monitored by UV-vis (lambda(max) = 426 (Soret), 538 nm in acetone), EPR (silent), and NMR spectroscopies; that is, the mono-NO complex was regenerated upon warming to RT. (F(8))Fe(NO)(2) reacts with [(tmpa)Cu(I)(MeCN)](+) and 2 equiv of acid to give [(F(8))Fe(III)](+), [(tmpa)Cu(II)(solvent)](2+), and N(2)O((g)), fitting the stoichiometric *NO((g)) reductive coupling reaction: 2*NO((g)) + Fe(II) + Cu(I) + 2H(+) --> N(2)O((g)) + Fe(III) + Cu(II) + H(2)O, 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 (F(8))Fe(NO) complex, while the remaining nitrogen monoxide undergoes copper complex promoted disproportionation chemistry. As part of this study, [(F(8))Fe(III)]SbF(6) was synthesized and characterized by X-ray crystallography, along with EPR (77 K: g = 5.84 and 6.12 in CH(2)Cl(2) 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.
authors.affiliation: Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: Jun
authors.initials: J
authors.lastname: Wang
authors.forename: Mark P
authors.initials: MP
authors.lastname: Schopfer
authors.forename: Simona C
authors.initials: SC
authors.lastname: Puiu
authors.forename: Amy A N
authors.initials: AA
authors.lastname: Sarjeant
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Karlin
mesh_terms: D001665:Binding Sites; D003300:Copper / Q000737:chemistry*; D003576:Electron Transport Complex IV / Q000378:metabolism; D005290:Ferric Compounds / Q000737:chemistry; D006418:Heme / Q000737:chemistry*; D007501:Iron / Q000737:chemistry; D008956:Models, Chemical; D009569:Nitric Oxide / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D011725:Pyridines / Q000138:chemical synthesis / Q000737:chemistry; D013696:Temperature
chemical_list: D005290:Ferric Compounds; D011725:Pyridines; C431843:tris(2-pyridylmethyl)amine; D009569:Nitric Oxide; D006418:Heme; D003300:Copper; D007501:Iron; D003576:Electron Transport Complex IV; D010100:Oxygen