The focus of this Forum review highlights work from our own laboratories and those of others in the area of biochemical and biologically inspired inorganic chemistry dealing with nitric oxide (nitrogen monoxide, ·NO (g) ) and its biological roles and reactions. The latter focus is on (i) oxidation of ·NO (g) to nitrate by nitric oxide dioxygenases (NOD's), and (ii) reductive coupling of two molecules of ·NO (g) to give N 2 O (g) . In the former case, NOD's are described and the highlighting of possible peroxynitrite-heme intermediates and consequences of this are given by discussion of recent works with myoglobin and a synthetic heme model system for NOD action. Summaries of recent copper complex chemistries with ·NO (g) and O 2(g) leading to peroxynitrite species are given. The coverage of biological reductive coupling of ·NO (g) deals with bacterial nitric oxide reductases (NOR's) with heme/non-heme diiron active sites, and on heme/Cu oxidases such as cytochrome c oxidase which can mediate the same chemistry. Recent designed protein and synthetic model compound (heme/non-heme diiron or heme/copper) as functional mimics are discussed in some detail. We also highlight examples from the chemical literature, not necessarily involving biologically relevant metal ions, which describe the oxidation of ·NO (g) to nitrate (or nitrite) and possible peroxynitrite intermediates, or reductive coupling of ·NO (g) to give nitrous oxide.

An oxy-heme complex, the heme-superoxo species (tetrahydrofuran)(F 8 )Fe III -(O 2 ·− ) (2) (F 8 = anortho-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-tbutylphenol is added prior to ·NO reaction with 2, o-nitration occurs giving 2,4-di-t-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.The reaction of nitric oxide (·NO; nitrogen monoxide) with oxygenated heme proteins is of considerable biological interest. Nitric oxide is generated in vivo by the oxidation of L-arginine to L-citrulline mediated by the enzyme Nitric Oxide Synthase (NOS). 1 Nitric oxide itself plays an important role in a number of physiological processes that include as a signaling agent leading to smooth muscle vasodilation, platelet disaggregation, neurotransmission, and immune response to bacterial infection. 2 Overproduction of ·NO can lead to toxicological processes that include DNA damage, protein nitration leading to cell death, and formation of peroxynitrite ( − OON=O) with the latter's further chemistry leading to highly reactive free radicals. 3 We report here the chemistry of a synthetic oxy-heme which exhibits NOD reactivity, where the intermediacy of a peroxynitrite species is implicated. (3) is never-the-less formed and unreacted DTBP is recovered. 18 However, we do observe effective nitration chemistry when DTBP (≥ 1 equiv) is added prior to addition of 1 equiv ·NO (g) to 2 (Scheme 2). 18 Workup of the reaction solution 18 reveals that the ferric hydroxo product (F 8 )Fe III -OH (4) forms (∼ 85% yield) along with high yields (> 82%) of 2,4-di-t-butyl-6-nitrophenol (NO 2 DTBP).The following control experiments indicate that a new heme-NO x intermediate forms and is able to effect a phenol nitration reaction faster than its own isomerization to the nitrate complex 3: (i) Use of excess ·NO (g) bubbled into solution had no effect on the products or their relative yields.(ii) Bubbling excess ·NO (g) into a solution containing 1 at −80 °C in the presence of DTBP and subsequent warming yielded less than 2% of the NO 2 DTBP and no other products In summary, we have here described a heme complex that acts as a nitrogen monoxide dioxygenase, facilitating the reaction of O 2 and ·NO to yield the nitrate anion NO 3 − . Generation of a heme-peroxynitrite species is implicated; it can be trapped by a phenolic substrate, leading to o-nitration. The results lead to the suggestion that in heme proteins peroxynitrite may leak and effect nitration of nearby residues or exogenous substrates. While Herold observed essentially no peroxynitrite leakage in ox...

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) (λ max = 399 (Soret), 541 nm in acetone). Variable temperature 1 H-and 2 H-NMR spectra reveal a broad peak at δ = 6.05 ppm (pyrrole) at RT, which gives rise to asymmetrically split pyrrole peaks at 9.12 and 8.54 ppm at −80°C. A new heme dinitrosyl species, ( 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.

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 two equiv acid are added; the heme/ Cu product is [( 6 While there is extensive recent activity in the design and study of discrete heme/copper synthetic complexes which resemble HCO active sites and/or effect dioxygen binding and reduction chemistry, 6,10 there are no cases where a synthetic small molecule heme/Cu complex reacts with •NO to effect reductive coupling leading to nitrous oxide. In this report, we describe such a system, employing the binucleating ligand 6 L and its iron and heme-copper derivatives which have been previously used in our investigations involving O 2 -chemistry. 10a The starting point is the reductive nitrosylation of ( 6 L)Fe III (OH) 11 to straightforwardlygive the iron(II)-nitrosyl compound ( 6 L)Fe(NO) (1) (Scheme 1), possessing the expected threeline hyperfine split EPR spectrum 12 (Fig. 1a). 13 As has been reported previously, 14 such complexes may react with additional •NO (g) to form a dinitrosyl species. This occurs here and evidence in support of this formulation, ( 6 L)Fe(NO) 2 are as follows: (a) As monitored by UVvis spectroscopy in acetone (−80 °C), 1 reversibly binds •NO (g) to form ( 6 L)Fe(NO) 2 (2); 2 is stable, but loses •NO (g) upon warming to RT. (b) With formation of 2 at −80 °C, the EPR signal due to ( 6 L)Fe-(NO) (1) disappears, as would be expected for 2, having an even number of electrons. (c) Titration of a −80 °C solution of ( 6 L)Fe(NO) 2 (2) with one equiv (F 8 )Fe II {F 8 = tetrakis(2,6-difluorophenyl)porphyrinate(2-)} leads to a clean conversion to ( 6 L)Fe(NO) (1) plus (F 8 )Fe(NO) (X-Ray structure determined); 2 possesses two equiv of bound •NO. 13The existence of ( 6 L)Fe(NO) 2 (2) . This is based on the observed UV-vis spectrum, appearing as a single species with λ max = 396, 515 nm in THF, matching that of a typical (porphyrinate)Fe III -X high-spin complex (X = a non-coordinating anion like PF 6 − or here B(C 6 F 5 ) 4 − ). Further supporting this formulation, an EPR spectrum (Fig. 1b) reveals that a high-spin heme-Fe III along with a Cu II (tetragonal complex) are both present. 13,16Wang et al.