RESUMEN
The nature of the ligand is an important aspect of controlling the structure and reactivity in coordination chemistry. In connection with our study of heme-copper-oxygen reactivity relevant to cytochrome c oxidase dioxygen-reduction chemistry, we compare the molecular and electronic structures of two high-spin heme-peroxo-copper [Fe(III)O(2)(2-)Cu(II)](+) complexes containing N(4) tetradentate (1) or N(3) tridentate (2) copper ligands. Combining previously reported and new resonance Raman and EXAFS data coupled to density functional theory calculations, we report a geometric structure and more complete electronic description of the high-spin heme-peroxo-copper complexes 1 and 2, which establish mu-(O(2)(2-)) side-on to the Fe(III) and end-on to Cu(II) (mu-eta(2):eta(1)) binding for the complex 1 but side-on/side-on (mu-eta(2):eta(2)) mu-peroxo coordination for the complex 2. We also compare and summarize the differences and similarities of these two complexes in their reactivity toward CO, PPh(3), acid, and phenols. The comparison of a new X-ray structure of mu-oxo complex 2a with the previously reported 1a X-ray structure, two thermal decomposition products respectively of 2 and 1, reveals a considerable difference in the Fe-O-Cu angle between the two mu-oxo complexes ( angleFe-O-Cu = 178.2 degrees in 1a and angleFe-O-Cu = 149.5 degrees in 2a). The reaction of 2 with 1 equiv of an exogenous nitrogen-donor axial base leads to the formation of a distinctive low-temperature-stable, low-spin heme-dioxygen-copper complex (2b), but under the same conditions, the addition of an axial base to 1 leads to the dissociation of the heme-peroxo-copper assembly and the release of O(2). 2b reacts with phenols performing H-atom (e(-) + H(+)) abstraction resulting in O-O bond cleavage and the formation of high-valent ferryl [Fe(IV)=O] complex (2c). The nature of 2c was confirmed by a comparison of its spectroscopic features and reactivity with those of an independently prepared ferryl complex. The phenoxyl radical generated by the H-atom abstraction was either (1) directly detected by electron paramagnetic resonance spectroscopy using phenols that produce stable radicals or (2) indirectly detected by the coupling product of two phenoxyl radicals.
Asunto(s)
Cobre/química , Complejo IV de Transporte de Electrones/química , Hemo/química , Oxígeno/química , Cristalografía por Rayos X , Espectroscopía de Resonancia por Spin del Electrón , Ligandos , Modelos Moleculares , Oxidación-Reducción , Teoría Cuántica , Espectrometría RamanRESUMEN
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.
Asunto(s)
Cobre/química , Hemo/química , Óxido Nítrico/química , Sitios de Unión , Complejo IV de Transporte de Electrones/metabolismo , Compuestos Férricos/química , Hierro/química , Modelos Químicos , Oxígeno/química , Piridinas/síntesis química , Piridinas/química , TemperaturaRESUMEN
An oxy-heme complex, the heme-superoxo species (tetrahydrofuran)(F(8))Fe(III)-(O(2)(*-)) (2) (F(8) = an ortho-difluoro substituted tetraarylporphyrinate), reacts with nitrogen monoxide (*NO; nitric oxide) to produce a nitrato-iron(III) compound (F(8))Fe(III)-(NO(3)(-)) (3) (X-ray). The chemistry mimics the action of *NO Dioxygenases (NODs), microbial and mammalian heme proteins which facilitate *NO detoxification/homeostasis. A peroxynitrite intermediate complex is implicated; if 2,4-di-tert-butylphenol is added prior to *NO reaction with 2, o-nitration occurs giving 2,4-di-tert-butyl-6-nitrophenol. The iron product is (F(8))Fe(III)-(OH) (4). The results suggest that heme/O(2)/*NO chemistry may lead to peroxynitrite leakage and/or exogenous substrate oxidative/nitrative reactivity.
Asunto(s)
Biomimética , Hemo/metabolismo , Óxido Nítrico/metabolismo , Oxígeno/metabolismo , Oxigenasas/metabolismo , Ácido Peroxinitroso/metabolismo , Animales , Hemo/química , Modelos MolecularesRESUMEN
A iron-dinitrosyl species ((6)L)Fe(NO)(2) (2), generated from nitrogen monoxide (*NO) binding to its related iron(II)-mononitrosyl complex ((6)L)Fe(NO) (1), efficiently effects reductive coupling of two *NO molecules to release nitrous oxide (N(2)O), when Cu(+) ion and 2 equiv acid are added; the heme/Cu product is [((6)L)Fe(III)...Cu(II)(D)](3+) (D = H(2)O or MeCN). In a control experiment where only ((6)L)Fe(NO)(2) (2) is exposed to 2 equiv acid, no UV-vis change is observed; upon warming, *NO((g)) is released and ((6)L)Fe(NO) is reformed. The copper ion complex within the (6)L ligand framework is required for the *NO coupling chemistry. In a further control experiment Cu(+) ion is added to ((6)L)Fe(NO)(2) without acid present, [((6)L)Fe(NO)...Cu(II)(NO(2)(-))](+) is obtained, with the amount of N(2)O((g)) released fitting with copper(I) ion promoted disproportionation chemistry, 3*NO + ligand-Cu(I) --> N(2)O + ligand-Cu(II)(NO(2)(-)). The chemical system described represents a (stoichiometric) functional model for heme/Cu protein nitric oxide reductase activity.