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The Biocoordination Chemistry of Nitric Oxide With Heme and Nonheme Iron Centers
... The FeÀ NO bond is of prime interest to the bio-inorganic chemists. [8,9] NO binding to relevant Fe centers in metalloenzymes has been the topic of research on both experimental and theoretical fronts. [10][11][12][13] Traditional inorganic chemists have also found the FeÀ NO bond very interesting, as reflected by the detailed study on Hieber's anion. ...
... [16,17] Hence, for a limited number of benchmark calculations, we will employ a complete active space with 9 electrons in 13 active orbitals, i. e., CAS(9,13) for accurate treatment of static correlation. However, we also evaluate the smaller active state CAS (9,8), which is used for sampling of large numbers of structures. Both active spaces include the essential valence orbitals required to account for the interaction between iron and the NO moiety. ...
... The nature of the electronic transitions, involved in the relevant peaks discussed above, was deduced by inspecting the natural population and natural orbitals of the respective excited states. For [Fe(H 2 O) 5 (NO)] 2 + , the peak at 440 nm from SA10-CASSCF/NEVPT2 (9,8) primarily arises as a result of transitions to the Q 9 and Q 10 states. These two states are reached through d-d transitions, involving excitations of an electron from either of the d xz and d yz orbitals to the d z 2 orbital, see Figure 4(a). ...
The chemistry of the brown‐ring test has been investigated for nearly a century. Though recent studies have focused on solid state structure determination and the measurement of spectra, mechanistic details and kinetics, the aspects of solution structure and dynamics remain unknown. We have investigated structural fluctuations of the brown‐ring complex in aqueous solution with ab initio molecular dynamics simulations, from which we identified that the classically established pseudo‐octahedral [Fe(H2O)5(NO)]2+ complex is present along with a square‐pyramidal [Fe(H2O)4(NO)]2+ complex. Based on the inability in multi‐reference calculations to reproduce the experimental UV‐vis spectra in aqueous solution by inclusion of thermal fluctuations of the [Fe(H2O)5(NO)]2+ complex alone, we propose the existence of an equilibrium between pseudo‐octahedral and square‐pyramidal complexes. Despite challenges in constructing models reproducing the solid‐state UV‐vis spectrum, the advanced spectrum simulation tool motivates us to challenge the established picture of a sole pseudo‐octahedral complex in solution.
... The mixed valence compound of Fe(NO) 2 eN-GQDs is due to the non-innocent ligand of NO that can become oxidized (NO þ ), radical (NO . ), or reduced (NO À ) when binding to a transition metal [61]. ...
Single-atom catalysts (SACs) supported on carbon-based materials are very promising for maximizing their electrocatalytic activity. However, carbon-based SACs are primarily bonded with carbons, resulting in inferior performance for oxygen evolution reaction (OER). Herein, we develop a novel coordination adsorption strategy for the synthesis of monodispersed Fe(NO)2 moiety anchored on the nitrogen sites of the nitrogen-doped graphene quantum dots (N-GQDs) to form a Fe(NO)2-N-GQDs complex as an efficient OER catalysts. The resultant Fe(NO)2-N-GQDs complex exhibits an highly stable overpotential of 270 mV at the current density of 10 mA cm⁻² and the Tafel of 48 mV dec⁻¹ together with long-term durability, which is greatly outperform the state-of-the-art RuO2. Our finding emphasizes the role of electron-transfer resistance changes during a simple synthesis method to enhance electrocatalytic efficiency. Therefore, this work will envision numerous opportunities for creating novel-type carbon-based SACs via nitrogen–metal coordination as highly robust OER catalysts.
Cyclic iron tetracarbenes are an emerging class of macrocyclic iron N-heterocyclic carbene (NHC) complexes. They can be considered as an organometallic compound class inspired by their heme analogs, however, their electronic properties differ, e.g. due to the very strong σ-donation of the four combined NHCs in equatorial coordination. The ligand framework of iron tetracarbenes can be readily modified, allowing fine-tuning of the structural and electronic properties of the complexes. The properties of iron tetracarbene complexes are discussed quantitatively and correlations are established. The electronic nature of the tetracarbene ligand allows the isolation of uncommon iron(III) and iron(IV) species and reveals a unique reactivity. Iron tetracarbenes are successfully applied in C-H activation, CO2 reduction, aziridination and epoxidation catalysis and mechanisms as well as decomposition pathways are described. This review will help researchers evaluate the structural and electronic properties of their complexes and target their catalyst properties through ligand design.
Small molecule activation & their transfer reactions in biological or catalytic reactions are greatly influenced by the metal-centers and the ligand frameworks. Here, we report the metal-directed nitric oxide (NO) transfer chemistry in low-spin mon-onuclear {Co(NO)} 8 , [(12-TMC)Co III (NO −)] 2+ (1-CoNO, S = 0), and {Cr(NO)} 5 , ([(BPMEN)Cr(NO)(Cl)] +) (4-CrNO, S =1/2), complexes. 1-CoNO transfer its bound NO moiety to a high-spin [(BPMEN)Cr II (Cl2)] (2-Cr, S = 2) and generates 4-CrNO via an associative pathway; however, we did not observe the reverse reaction, i.e., NO transfer from 4-CrNO to low-spin [(12-TMC)Co II ] 2+ (3-Co, S =1/2). Spectral titration for NO transfer reaction between 1-CoNO and 2-Cr confirmed 1:1 reaction stoichiometry. The NO transfer rate was found to be independent of 2-Cr, suggesting the presence of an intermediate species, which was further supported experimentally and theoretically. The experimental and theoretical observation supports the formation of µ-NO bridged intermediate species ({Cr-NO-Co} 4+). Mechanistic investigations using 15 N-labeled-15 NO and tracking the 15 N-atom established that the NO moiety in 4-CrNO is derived from 1-CoNO. Further, to investigate the factors deciding the NO transfer reactivity, we explored the NO transfer reaction between another high-spin Cr II-complex, [(12-TMC)Cr II (Cl)] + (5-Cr, S = 2), and 1-CoNO, showing the generation of the low-spin [(12-TMC)Cr(NO)(Cl)] + (6-CrNO, S = 1/2); however, again there was no opposite reaction, i.e., from Cr-center to Co-center. The above results advocates clearly that the NO transfer from Co-center generates thermally stable and low-spin & inert {Cr(NO)} 5 complexes (4-CrNO & 6-CrNO) from high-spin & labile Cr-complexes (2-Cr & 5-Cr), suggesting a metal-directed NO transfer (Cobalt to Chromium, not Chromium to Cobalt). These results explicitly highlight that the NO transfer is strongly influenced by the labile/inert behavior of the metal-centers and /or thermal stability rather than the ligand architecture.
The chemistry of the brown-ring test has been investigated for nearly a century. Though recent studies have focused on solid state structure determination and the measurement of spectra, mechanistic details and kinetics, the aspects of solution structure and dynamics remain unknown. From ab initio molecular dynamics simulations of the brown-ring complex in aqueous solution, we have identified that the classically established pseudo-octahedral [Fe(H2O)5(NO)]2+ complex is in equilibrium with a square-pyramidal [Fe(H2O)4(NO)]2+ complex through the exchange of one of the coordinated H2O molecules. We also find, using ab-initio multi-reference methods, that the mixture of these two complexes is what gives the distinctive brown coloration to the brown-ring test. We show that its UV-vis spectrum can be theoretically reproduced only by accounting these two species and not the [Fe(H2O)5(NO)]2+ complex alone. The energetics of the two complexes are also investigated with multi-reference methods.
The brown ring test is one of the most popular and visually appealing reagent tests, commonly known to chemistry undergrads and familiar even to school students. The exact composition, mechanism and structure of the complex has been investigated for nearly a century. Recent studies have elucidated its UV-vis, EPR and Mossbauer spectra, mechanistic details and kinetics, followed by crystallization and structure determination in solid state. Nonetheless these studies were unable to address the aspects of solution structure and dynamics of the brown ring complex. We have conducted ab initio molecular dynamics simulations of the classic brown ring complex in aqueous solution. In the process from the simulation trajectory, we have identified that the classically established pseudo-octahedral [Fe(H2O)5(NO)]2+ complex is in chemical equilibrium with the square-pyramidal [Fe(H2O)4(NO)]2+ complex through the exchange of one of the coordinated H2O molecules. The dynamics in aqueous solution between the penta-aqua and tetra-aqua complexes in the brown ring system has to our knowledge never been suggested earlier. Interestingly we find, using ab initio multi-reference quantum chemical methods i.e. CASSCF/NEVPT2 and CASPT2 calculations, that the mixture of these two complexes is what gives the distinctive brown coloration to the brown ring test. We show that its UV-vis spectrum can be theoretically reproduced only by accounting these two species, and not solely the classically established [Fe(H2O)5(NO)]2+ complex. The energetics of the penta-aqua and tetra-aqua complexes is also investigated at the level of multi-reference quantum chemical methods.
Cytochrome cd1 is a key enzyme in bacterial denitrification and catalyzes one-electron reduction of nitrite (NO2–) to nitric oxide (NO) at the heme d1 center under anaerobic conditions. The heme d1 has a unique dioxo-isobacteriochlorin structure and is present only in cytochrome cd1. To reveal the functional role of the unique heme d1 in the catalytic nitrite reduction, we studied effect of the porphyrin macrocycle on each reaction step of the catalytic cycle of cytochrome cd1 using synthetic model complexes. The complexes investigated are iron complexes of dioxo-octaethylisobacteriochlorin (1), mono-oxo-octaethylchlorin (2) and octaethylporphyrin (3). We show here that the reduction potential for the transition from the ferric state to the ferrous state and the binding constant for binding of NO2– to the ferrous complex increases with a trend of 3 < 2 < 1. However, the reactivity of the ferrous nitrite complex with protons increases in the reversed order, 1 < 2 < 3. We also show that the iron bound NO of the ferric NO complex is readily replaced by addition of 1equiv of p-nitrophenolate. These results indicate that the dioxo-isobacteriochlorin structure is superior to porphyrin and mono-oxo-chlorin structures in the first iron reduction step, the second nitrite binding step, and the NO dissociation step, but inferior in the third nitrite reduction step. These results suggest that the heme d1 has evolved as the catalytic site of cytochrome cd1 to catalyze the nitrite reduction at the highest possible redox potential while maintaining its catalytic activity.
Denitrifying bacteria and fungi efficiently detoxify the toxic metabolite nitric oxide (NO) through reduction to nitrous oxide (N2O) using nitric oxide reductase (NOR) enzymes. In fungi, for example Fusarium oxysporum, NO is reduced by a Cytochrome P450 NOR (P450nor). This enzyme contains a heme b center coordinated to a proximal cysteinate ligand in the active site. In the proposed mechanism of P450nor, the ferric heme binds NO first to form a ferric heme-nitrosyl complex, which is subsequently reduced by NAD(P)H to generate a ferrous HNO species as the next key intermediate. Recently, key progress has been made in our understanding of the electronic structures and fundamental reactivity of these important intermediates, using suitable model complexes. In this review, model complexes of ferric heme-nitrosyls with varied axial anionic ligands (such as N-donors, O-donors, and S-donors) are discussed first. Then, the generation and reactivity of ferrous heme-HNO complexes is summarized and related back to the mechanism of P450nor.
Significance
Soluble guanylate cyclase (sGC) is the primary nitric oxide (NO) receptor in mammals and a central component of the NO-signaling pathway. Disruptions in NO signaling have been linked to hypertension, neurodegeneration, and heart disease. The mechanistic details underlying the modulation of sGC activity remain largely unknown. Determining the structure of full-length sGC is a prerequisite to understanding its function and for the design and improvement of therapeutics for treatment of related diseases. We use electron microscopy to determine the quaternary structure of the protein. Furthermore, we found that both ligand-free and ligand-bound sGC are highly flexible. This structural information provides a significant step forward in understanding the mechanism of sGC activation and will ultimately empower the development of next-generation therapeutics.
"Endothelium-derived relaxing factor (EDRF) must be nitric oxide." Although there was much experimental evidence that effects on or actions of EDRF were identical to those of nitric oxide, proposing this hypothesis certainly meant going out on a limb. However, once this discovery was made increased investigation indicated the physiological significance of nitric oxide in vascular biology.
The presence of air bubbles, and thus the unintentional removal of endothelial cells, may have prevented the discovery of endothelium-dependent vasodilation at an earlier date. In investigations with perfused rabbit central ear artery the infusion of acetylcholine, a known potent prejunctional inhibitor of adrenergic neurotransmission, did not inhibit vasoconstriction produced by infused norepinephrine. It was later determined that air bubbles in the perfusion line had mechanically removed the endothelial cells, and as a result no release of endothelium-derived relaxing factor (EDRF) could take place.
We have determined a convenient method for the bulk synthesis of high-purity ferric heme-nitrosyl complexes ({FeNO}(6) in the Enemark-Feltham notation); this method is based on the chemical or electrochemical oxidation of corresponding {FeNO}(7) precursors. We used this method to obtain the five- and six-coordinate complexes [Fe(TPP)(NO)](+) (TPP(2-) = tetraphenylporphyrin dianion) and [Fe(TPP)(NO)(MI)](+) (MI = 1-methylimidazole) and demonstrate that these complexes are stable in solution in the absence of excess NO gas. This is in stark contrast to the often-cited instability of such {FeNO}(6) model complexes in the literature, which is likely due to the common presence of halide impurities (although other impurities could certainly also play a role). This is avoided in our approach for the synthesis of {FeNO}(6) complexes via oxidation of pure {FeNO}(7) precursors. On the basis of these results, {FeNO}(6) complexes in proteins do not show an increased stability toward NO loss compared to model complexes. We also prepared the halide-coordinated complexes [Fe(TPP)(NO)(X)] (X = Cl(-), Br(-)), which correspond to the elusive, key reactive intermediate in the so-called autoreduction reaction, which is frequently used to prepare {FeNO}(7) complexes from ferric precursors. All of the complexes were characterized using X-ray crystallography, UV-vis, IR, and nuclear resonance vibrational spectroscopy (NRVS). On the basis of the vibrational data, further insight into the electronic structure of these {FeNO}(6) complexes, in particular with respect to the role of the axial ligand trans to NO, is obtained.
Significance
The enzymatic reactions that occur during nitrification are nature’s means to use ammonia as cellular fuel. Complete understanding of nitrification and related processes are vital to sustainable agriculture and renewable energy technologies. The prevailing view of the first phase of nitrification is that ammonia oxidizing bacteria use two enzymes, ammonia monooxygenase and hydroxylamine oxidoreductase, to oxidize ammonia to nitrite via hydroxylamine as an obligate intermediate. Our work reveals nitric oxide as an additional obligate intermediate. The presented findings necessitate revision of a key biogeochemical process, identify a new bioenergetic role for nitric oxide, predict participation of a third enzyme in the biological oxidation of ammonia to nitrite, and will inform models toward sustainable agriculture.
In this perspective, we highlight recent studies that use X-ray spectroscopic methods to identify and elucidate the molecular and/or electronic structures of intermediate and resting transition-metal species implicated in homogeneous catalysis. We begin with a brief survey of the data that can be accessed with these spectroscopic methods, followed by case studies that showcase advances in the interpretation of X-ray spectroscopic data, demonstrate how these methods complement other types of spectroscopy, and call attention to common misconceptions associated with X-ray spectroscopic techniques. In some cases, we suggest research areas in which alternative spectroscopic tools could aid advances. We conclude with an unorthodox perspective on the reactivity of “high-valent” late transition metals on the basis of recent X-ray spectroscopic insights into their chemical bonding.
As part of microbial denitrification, NO is reduced to N2O in the membrane bound enzyme nitric oxide reductase, NOR. The N-N coupling occurs in the diiron binuclear active site, BNC, and different mechanisms for this reaction step have been suggested. Computational studies have supported a so called cis:b3-mechanism, in which the hyponitrite product of the reductive N-N bond formation coordinates with one nitrogen to the heme iron and with both oxygens to the non-heme iron in the BNC. In contrast, experimental results have been interpreted to support a so called trans-mechanism, in which the hyponitrite intermediate coordinates with one nitrogen atom to each of the two iron-ions. Hybrid density functional theory is here used to perform an extensive search for possible intermediates of the NO reduction in the cNOR enzyme. It is found that hyponitrite structures coordinating with their negatively charged oxygens to the positively charged iron-ions are the most stable ones. The hyponitrite intermediate involved in the suggested trans-mechanism, which only coordinates with the nitrogens to the iron-ions, is found to be prohibitively high in energy, leading to a too slow reaction, which should rule out this mechanism. Furthermore, intermediates binding one NO molecule to each iron-ion in the BNC, which have been suggested to initiate the trans-mechanism, are found to be too high in energy to be observable, indicating that the experimentally observed EPR signals, taken to support such an iron-nitrosyl dimer intermediate, should be reinterpreted.
Flavo-diiron nitric oxide reductases (FNORs) are a subclass of nonheme diiron proteins in pathogenic bacteria that reductively transform NO to N2O, thereby abrogating the nitrosative stress exerted by macrophages as part of the immune response. Understanding the mechanism and intermediates in the NO detoxification process might be crucial for the development of a more efficient treatment against these bacteria. However, low molecular weight models are still rare, and only in a few cases have their reductive transformations been thoroughly investigated. Here, we report on the development of two complexes, based on a new dinucleating pyrazolate/triazacyclononane hybrid ligand L(-), which serve as model systems for nonheme diiron active sites. Their ferrous nitrile precursors [L{Fe(R'CN)}2(μ-OOCR)](X)2 (1) can be readily converted into the corresponding nitrosyl adducts ([L{Fe(NO)}2(μ-OOCR)](X)2, 2). Spectroscopic characterization shows close resemblance to nitrosylated nonheme diiron sites in proteins as well as previous low molecular weight analogues. Crystallographic characterization reveals an anti orientation of the two {Fe(NO)}(7) (Enemark-Feltham notation) units. The nitrosyl adducts 2 can be (electro)chemically reduced by one electron, as shown by cyclic voltammetry and UV/vis spectroscopy, but without the formation of N2O. Instead, various spectroscopic techniques including stopped-flow IR spectroscopy indicated the rapid formation, within few seconds, of two well-defined products upon reduction of 2a (R = Me, X = ClO4). As shown by IR and Mössbauer spectroscopy as well as X-ray crystallographic characterization, the reduction products are a diiron tetranitrosyl complex ([L{Fe(NO)2}2](ClO4), 3a') and a diacetato-bridged ferrous complex [LFe2(μ-OAc)2](ClO4) (3a″). Especially 3a' parallels suggested products in the decay of nitrosylated methane monooxygenase hydroxylase (MMOH), for which N2O release is much less efficient than for FNORs.
In biology, a heme-Cu center in heme-copper oxidases (HCOs) is used to catalyze the four-electron reduction of oxygen to water, while a heme-nonheme diiron center in nitric oxide reductases (NORs) is employed to catalyze the two-electron reduction of nitric oxide to nitrous oxide. Although much progress has been made in biochemical and biophysical studies of HCOs and NORs, structural features responsible for similarities and differences within the two enzymatic systems remain to be understood. Here, we discuss the progress made in the design and characterization of myoglobin-based enzyme models of HCOs and NORs. In particular, we focus on use of these models to understand the structure-function relations between HCOs and NORs, including the role of nonheme metals, conserved amino acids in the active site, heme types, and hydrogen-bonding networks, in tuning enzymatic activities and total turnovers. Insights gained from these studies are summarized and future directions are proposed.
Biomedical devices are essential for patient diagnosis and treatment; however, when blood comes in contact with foreign surfaces or homeostasis is disrupted, complications including thrombus formation and bacterial infections can interrupt device functionality, causing false readings and/or shorten device lifetime. Here, we review some of the current approaches for developing antithrombotic and antibacterial materials for biomedical applications. Special emphasis is given to materials that release or generate low levels of nitric oxide (NO). Nitric oxide is an endogenous gas molecule that can inhibit platelet activation as well as bacterial proliferation and adhesion. Various NO delivery vehicles have been developed to improve NO's therapeutic potential. In this review, we provide a summary of the NO releasing and NO generating polymeric materials developed to date, with a focus on the chemistry of different NO donors, the polymer preparation processes, and in vitro and in vivo applications of the two most promising types of NO donors studied thus far, N-diazeniumdiolates (NONOates) and S-nitrosothiols (RSNOs).
The synthesis of the stable pair of {Fe(NO)2}9/10 DNICs [Fe(dmp)(NO)2]+/0 is reported. The Mössbauer and NRVS spectra of these complexes in conjunction with DFT calculations are used to show that reduction of the {Fe(NO)2}9 to the {Fe(NO)2}10 species is iron-centered and leads to a significant increase in the covalency of the Fe−(NO)2 unit due to enhanced π-back-bonding.
Although the interaction of low-spin ferric complexes with nitric oxide has been well studied, examples of stable high-spin ferric nitrosyls (such as those that could be expected to form at typical non-heme iron sites in biology) are extremely rare. Using the TMG3 tren co-ligand, we have prepared a high-spin ferric NO adduct ({FeNO}(6) complex) via electrochemical or chemical oxidation of the corresponding high-spin ferrous NO {FeNO}(7) complex. The {FeNO}(6) compound is characterized by UV/Visible and IR spectroelectrochemistry, Mössbauer and NMR spectroscopy, X-ray crystallography, and DFT calculations. The data show that its electronic structure is best described as a high-spin iron(IV) center bound to a triplet NO(-) ligand with a very covalent iron-NO bond. This finding demonstrates that this high-spin iron nitrosyl compound undergoes iron-centered redox chemistry, leading to fundamentally different properties than corresponding low-spin compounds, which undergo NO-centered redox transformations.
Nitrogen is the fourth most abundant element in cellular biomass, and it comprises the majority of Earth's atmosphere. The interchange between inert dinitrogen gas (N2) in the extant atmosphere and 'reactive nitrogen' (those nitrogen compounds that support, or are products of, cellular metabolism and growth) is entirely controlled by microbial activities. This was not the case, however, in the primordial atmosphere, when abiotic reactions likely played a significant role in the inter-transformation of nitrogen oxides. Although such abiotic reactions are still important, the extant nitrogen cycle is driven by reductive fixation of dinitrogen and an enzyme inventory that facilitates dinitrogen-producing reactions. Prior to the advent of the Haber-Bosch process (the industrial fixation of N2 into ammonia, NH3) in 1909, nearly all of the reactive nitrogen in the biosphere was generated and recycled by microorganisms. Although the Haber-Bosch process more than quadrupled the productivity of agricultural crops, chemical fertilizers and other anthropogenic sources of fixed nitrogen now far exceed natural contributions, leading to unprecedented environmental degradation.
The contested electronic structure of [Cu(CF3)4](1-) is investigated with UV/visible/near IR spectroscopy, Cu K-edge X-ray absorption spectroscopy, and 1s2p resonant inelastic X-ray scattering. These data, supported by density functional theory, multiplet theory, and multireference calculations, support a ground state electronic configuration in which the lowest unoccupied orbital is of predominantly trifluoromethyl character. The consensus 3d(10) configuration features an inverted ligand field in which all five metal-localized molecular orbitals are located at lower energy relative to the trifluoromethyl-centered σ orbitals.
This chapter focuses on the synthesis, physical properties, and chemical reactivity of the synthetic analogues of such dinitrosyl iron species. Though monomeric dinitrosyl iron complexes (DNICs) have gained interest due to their biological relevance, there are various types of iron nitrosyls that have historical or chemical significance. The aim of the chapter is to highlight the synthesis, structure, and reactivity of various known discrete DNICs currently in the literature, which can complement the existing reviews of their biological roles. Dinitrosyl iron complexes may also play roles outside of NO storage and transfer, as is suggested by the ability of neutral {Fe(NO)2}10 complexes to promote phenol nitration in the presence of dioxygen. Examining the trends in electronic and geometric structure in these complexes and how that contributes to their reactivity is vital in forming an understanding of how they behave in a biological setting.
The nitrogen cycle is among the most significant biogeochemical cycles on Earth because nitrogen is an essential nutrient for all forms of life. The largest contributor of nitrogen in the cycle is atmospheric dinitrogen (N_2), which is generally unavailable to plants by direct assimilation. Hence, access to fixed forms of nitrogen constitutes in many cases the most limiting factor for plant growth. This is in sharp contrast to carbon, which is easily taken up by plants from the atmosphere by fixing gaseous carbon dioxide (CO_2). The availability of nitrogen (and water) hence limits the ability to produce sufficient crops to feed our planet’s growing population.
In this Forum Article, an extensive discussion of the mechanism of six-electron, seven-proton nitrite reduction by the cytochrome c nitrite reductase enzyme is presented. On the basis of previous studies, the entire mechanism is summarized and a unified picture of the most plausible sequence of elementary steps is presented. According to this scheme, the mechanism can be divided into five functional stages. The first phase of the reaction consists of substrate binding and N-O bond cleavage. Here His277 plays a crucial role as a proton donor. In this step, the N-O bond is cleaved heterolytically through double protonation of the substrate. The second phase of the mechanism consists of two proton-coupled electron-transfer events, leading to an HNO intermediate. The third phase involves the formation of hydroxylamine, where Arg114 provides the necessary proton for the reaction. The second N-O bond is cleaved in the fourth phase of the mechanism, again triggered by proton transfer from His277. The Tyr218 side chain governs the fifth and last phase of the mechanism. It consists of radical transfer and ammonia formation. Thus, this mechanism implies that all conserved active-site side chains work in a concerted way in order to achieve this complex chemical transformation from nitrite to ammonia. The Forum Article also provides a detailed discussion of the density functional theory based cluster model approach to bioinorganic reactivity. A variety of questions are considered: the resting state of enzyme and substrate binding modes, interaction with the metal site and with active-site side chains, electron- and proton-transfer events, substrate dissociation, etc.
The question of why mammalian systems use nitric oxide (NO), a potentially hazardous and toxic diatomic, as a signaling molecule to mediate important functions such as vasodilation (blood pressure control) and nerve signal transduction initially perplexed researchers when this discovery was made in the 1980s. Through extensive research over the past two decades, it is now well rationalized why NO is used in vivo for these signaling functions, and that heme proteins play a dominant role in NO signaling in mammals. Key insight into the properties of heme-nitrosyl complexes that make heme proteins so well poised to take full advantage of the unique properties of NO has come from in-depth structural, spectroscopic, and theoretical studies on ferrous and ferric heme-nitrosyls. This Account highlights recent findings that have led to greater understanding of the electronic structures of heme-nitrosyls, and the contributions that model complex studies have made to elucidate Fe–NO bonding are highlighted. These results are then discussed in the context of the biological functions of heme-nitrosyls, in particular in soluble guanylate cyclase (sGC; NO signaling), nitrophorins (NO transport), and NO-producing enzymes.
The nitrosylation of inorganic protein cofactors, specifically that of [Fe-S] clusters to form iron nitrosyls, plays a number of important roles in biological systems. In some of these cases, it is expected that a repair process reverts the nitrosylated iron species to intact [Fe-S] clusters. The repair of nitrosylated [2Fe-2S] cluster, primarily in the form of protein-bound dinitrosyl iron complexes (DNICs), has been observed in vitro and in vivo, but the mechanism of this process remains uncertain. The present work expands upon a previous observation (Fitzpatrick et al. J. Am. Chem. Soc. 2014, 136, 7229) of the ability of mononitrosyl iron complexes (MNICs) to be converted into [2Fe-2S] clusters by the addition of nothing other than a cysteine analogue. Herein, each of the critical elementary steps in the cluster repair has been dissected to elucidate the roles of the cysteine analogue. Systematic variations of a cysteine analogue employed in the repair reaction suggest that (i) the bidentate coordination of a cysteine analogue to MNIC promotes NO release from iron, and (ii) deprotonation of the α carbon of the ferric-bound cysteine analogue leads to the C-S cleavage en route to the formation of [2Fe-2S] cluster. The [2Fe-2S] cluster bearing a cysteine analogue has also been synthesized from thiolate-bridged iron dimers of the form [Fe2(μ-SR)2(SR)4](0/2-), which implies that such species may be present as intermediates in the cluster repair. In addition to MNICs, mononuclear tetrathiolate ferric or ferrous species have been established as another form of iron from which [2Fe-2S] clusters can be generated without need for any other reagent but a cysteine analogue. The results of these experiments bring to light new chemistry of classic coordination complexes and provides further insight into the repair of NO-modified [2Fe-2S] clusters.
This review summarizes our current understanding of the geometric and electronic structures of ferrous and ferric heme–nitrosyls, which are of key importance for the biological functions and transformations of NO. In-depth correlations are made between these properties and the reactivities of these species. Here, a focus is put on the discoveries that have been made in the last 10 years, but previous findings are also included as necessary. Besides this, ferrous heme–nitroxyl complexes are also considered, which have become of increasing interest recently due to their roles as intermediates in NO and multiheme nitrite reductases, and because of the potential role of HNO as a signaling molecule in mammals. In recent years, computational methods have received more attention as a means of investigating enzyme reaction mechanisms, and some important findings from these theoretical studies are also highlighted in this chapter.
Nitric oxide (NO) produced by the vascular endothelium is an important biologic messenger that regulates vessel tone and permeability and inhibits platelet adhesion and aggregation. NO exerts its control of vessel tone by interacting with guanylyl cyclase in the vascular smooth muscle to initiate a series of reactions that lead to vessel dilation. Previous efforts to investigate this interaction by mathematical modeling of NO diffusion and reaction have been hampered by the lack of information on the production and degradation rate of NO. We use a mathematical model and previously published experimental data to estimate the rate of NO production, 6.8 × 10-14μmol ⋅ μm-2⋅ s-1; the NO diffusion coefficient, 3,300 μm2s-1; and the NO consumption rate coefficient in the vascular smooth muscle, 0.01 s-1(1st-order rate expression) or 0.05 μM-1⋅ s-1(2nd-order rate expression). The modeling approach is discussed in detail. It provides a general framework for modeling the NO produced from the endothelium and for estimating relevant physical parameters.
Herein, we review the preparation and coordination chemistry of cis and trans isomers of hyponitrite, [N2O2](2-). Hyponitrite is known to bind to metals via a variety of bonding modes. In fact, at least eight different bonding modes have been observed, which is remarkable for such a simple ligand. More importantly, it is apparent that the cis isomer of hyponitrite is more reactive than the trans isomer because the barrier of N2O elimination from cis-hyponitrite is lower than that of trans-hyponitrite. This observation may have important mechanistic implications for both heterogeneous NOx reduction catalysts and NO reductase. However, our understanding of the hyponitrite ligand has been limited by the lack of a general route to this fragment, and most instances of its formation have been serendipitous.
Dinitrosyl iron complexes (DNICs) have been recognized as storage and transport agents of nitric oxide capable of selectively modifying crucial biological targets via its distinct redox forms (NO+, NO• and NO–) to initiate the signaling transduction pathways associated with versatile physiological and pathological responses. For decades, the molecular geometry and spectroscopic identification of {Fe(NO)2}9 DNICs ({Fe(NO)x}n where n is the sum of electrons in the Fe 3d orbitals and NO π* orbitals based on Enemark–Feltham notation) in biology were limited to tetrahedral (CN = 4) and EPR g-value ∼2.03, respectively, due to the inadequacy of structurally well-defined biomimetic DNICs as well as the corresponding spectroscopic library accessible in biological environments.
The role of NO in biology is well established. However, increasing body of evidence suggests that azanone (HNO), could also be involved in biological processes, some of which are attributed to NO. In this context, one of the most important and yet unanswered questions is whether and how HNO is produced in vivo. A possible route concerns the chemical or enzymatic reduction of NO. In the present work, we have taken advantage of a selective HNO sensing method, to show that NO is reduced to HNO by biologically relevant alcohols with moderate reducing capacity, such as ascorbate or tyrosine. The proposed mechanism involves a nucleophilic attack to NO by the alcohol, coupled to a proton transfer (PCNA: proton-coupled nucleophilic attack) and a subsequent decomposition of the so produced radical, to yield HNO and an alcoxyl radical.
The global biogeochemical nitrogen cycle is essential for life on Earth. Many of the underlying biotic reactions are catalyzed by a multitude of prokaryotic and eukaryotic life forms whereas others are exclusively carried out by microorganisms. The last century has seen the rise of a dramatic imbalance in the global nitrogen cycle due to human behavior that was mainly caused by the invention of the Haber-Bosch process. Its main product, ammonia, is a chemically reactive and biotically favorable form of bound nitrogen. The anthropogenic supply of reduced nitrogen to the biosphere in the form of ammonia, for example during environmental fertilization, livestock farming, and industrial processes, is mandatory in feeding an increasing world population. In this chapter, environmental ammonia pollution is linked to the activity of microbial metalloenzymes involved in respiratory energy metabolism and bioenergetics. Ammonia-producing multiheme cytochromes c are discussed as paradigm enzymes.
Inducible NO synthase in mammals helps to produce up to micromolar concentration of nitric oxide (NO) which acts as a key immune defense agent to kill invading pathogens. In order to counter the toxic effects of NO, the pathogens have expressed flavodiiron nitric oxide reductases (FNORs). The FNORs reduce the toxic NO into much less toxic N2O and thus help the pathogens to survive under nitrosative stress. As a consequence, these pathogens proliferate in the human body and cause harmful infections. An appreciable amount of research work has been performed to discover the true mechanism of the FNORs. Different mechanisms involving both mononitrosyl and dinitrosyl diiron complexes as key intermediates are proposed. Evidences for the involvement of new intermediates and more and more experimental evidences for existing ones in the proposed catalytic cycle of FNORs are coming up. These interesting biochemical events have recently boosted the biomimetic chemistry of the FNOR activity as well. This article discusses the importance and the currently understood mechanistic aspects of FNORs. Structural and functional models for the active site of FNORs are discussed along with their success and limitations. Possible future prospects of the modeling chemistry are also suggested.
Significance
Nitric oxide (NO) influences diverse biological processes, ranging from vasodilation in mammals to communal behavior in bacteria. Heme-nitric oxide/oxygen (H-NOX) binding domains, a recently discovered family of heme-based gas sensor proteins, have been implicated as regulators of these processes. Crucial to NO-dependent activation of H-NOX proteins is rupture of the heme–histidine bond and formation of a five-coordinate NO complex. To delineate the molecular details of NO binding, high-resolution crystal structures of a bacterial H-NOX protein in the unligated and intermediate six- and five-coordinate NO-bound states are reported. From these structures, it is evident that NO-induced scission of the heme–histidine bond elicits a pronounced conformational change in the protein as a result of structural rearrangements in the heme pocket.
Flavo-diiron proteins (FDPs) function as anaerobic nitric oxide scavengers in some microorganisms, catalyzing reduction of nitric to nitrous oxide. The FDP from Thermotoga maritima can be prepared in a deflavinated form with an intact diferric site (deflavo-FDP). Hayashi, et al. ((2010) Biochemistry 49, 7040-7049) reported that reaction of NO with reduced deflavo-FDP produced substoichiometric N2O. Here we report a multi-spectroscopic approach to identify the iron species in the reactions of deflavo-FDP with NO. Mössbauer spectroscopy identified two distinct ferrous species after reduction of the antiferromagnetically coupled diferric site. Approximately sixty percent of the total ferrous iron was assigned to a diferrous species associated with the N2O-generating pathway. This pathway proceeds through successive diferrous-mononitrosyl (S = ½ FeII{FeNO}7) and diferrous-dinitrosyl (S = 0 [{FeNO}7]2) species that form within ~100 ms after mixing the reduced protein with NO. The diferrous-dinitrosyl converted to an antiferromagnetically coupled diferric species which was spectroscopically indistinguishable from that in the starting deflavinated protein. These diiron species closely resembled those reported for the flavinated FDP (Caranto et al. (2014) J. Am. Chem. Soc. 136, 7981-7992), and the time scales of their formation and decay were consistent with the steady state turnover of the flavinated protein. The remaining ~40% of ferrous iron was inactive in N2O generation but reversibly bound NO to give an S = 3/2 {FeNO}7 species. The results demonstrate that N2O formation in FDPs can occur via conversion of S = 0 [{FeNO}7]2 to diferric without participation of the flavin cofactor.
This essay for EurJIC's cluster issue on cooperative and redox non-innocent ligands introduces the reader to redox-active ligands, which range from the small archetypical NO+/•/– and O20/•–/2–systems via the classical 1,4-dihetero-1,3-diene chelates (e.g. α-diimine, dithiolene, or o-quinone redox series) to π-conjugated macrocycles. The increased attention paid recently to the redox activity of ligands in coordination chemistry has now prompted wider successful searches, resulting in the establishing of less-conventional examples such as cyanide, carbon monoxide, thioethers, or acetylacetonate derivatives as non-innocently behaving ligands. By considering situations with significantly covalent metal–ligand bonding, the cases of metal–oxo, metal–hydrido, and organometallic compounds will also be addressed, with a perspective on how pervasive non-innocent ligand behavior is. The materials and reactivity potential of redox-active ligands will be pointed out.
The detoxification of nitric oxide (NO) by bacterial NO reductase (NorBC) represents a paradigm of how NO can be detoxified anaerobically in cells. In order to elucidate the mechanism of this enzyme, model complexes provide a convenient means to assess potential reaction intermediates. In particular, there have been many proposed mechanisms that invoke the formation of a hyponitrite bridge between the heme b3 and nonheme iron (FeB) centers within the NorBC active site. However, the reactivity of bridged iron hyponitrite complexes has not been investigated much in the literature. The model complex {[Fe(OEP)]2(μ-N2O2)} offers a unique opportunity to study the electronic structure and reactivity of such a hyponitrite-bridged complex. Here we report the detailed characterization of {[Fe(OEP)]2(μ-N2O2)} using a combination of IR, nuclear resonance vibrational spectroscopy, electron paramagnetic resonance, and magnetic circular dichroism spectroscopy along with SQUID magnetometry. These results show that the ground-state electronic structure of this complex is best described as having two intermediate-spin (S = (3)/2) iron centers that are weakly antiferromagnetically coupled across the N2O2(2-) bridge. The analogous complex {[Fe(PPDME)]2(μ-N2O2)} shows overall similar properties. Finally, we report the unexpected reaction of {[Fe(OEP)]2(μ-N2O2)} in the presence and absence of 1-methylimidizole to yield [Fe(OEP)(NO)]. Density functional theory calculations are used to rationalize why {[Fe(OEP)]2(μ-N2O2)} cannot be formed directly by dimerization of [Fe(OEP)(NO)] and why only the reverse reaction is observed experimentally. These results thus provide insight into the general reactivity of hyponitrite-bridged iron complexes with general relevance for the N-N bond-forming step in NorBC.
Driven by its biological importance, the coordination chemistry of nitric oxide (NO) has undergone a renaissance over the past 10 years. This is especially true for the late first row transition metals, including iron, cobalt, nickel, and copper. This article summarizes the literature from 2003 until the present pertaining to the synthesis and reactivity of nitrosyl complexes of these metals. Recent notable advancements in this area include the synthesis of several iron nitroxyl (NO–) complexes, as well as the development of the chemistry of dinitrosyl iron complexes (DNICs) and trinitrosyl iron complexes (TNICs). Additionally, the first structural study of a {Cu(NO)}10 nitrosyl complex was reported, and in the last 10 years several interesting transformations of bound NO involving both three- and five-coordinated nickel nitrosyl complexes have been discovered. Considerable progress has also been made toward our understanding of the redox non-innocence of the nitrosyl ligand. In particular, the electronic structure of linear metal nitrosyls has proven far more complicated than the traditional “NO+” description given to these species.
The unique active site of flavodiiron proteins (FDPs) consists of a non-heme diiron-carboxylate site proximal to a flavin mononucleotide (FMN) cofactor. FDPs serve as the terminal components for reductive scavenging of dioxygen or nitric oxide to combat oxidative or nitrosative stress in bacteria, archaea, and some protozoan parasites. Nitric oxide is reduced to nitrous oxide by the four-electron reduced (FMNH2-FeIIFeII) active site. In order to clarify the nitric oxide reductase mechanism, we undertook a multi-spectroscopic pre-steady state investigation, including the first Mössbauer spectroscopic characterization of diiron redox intermediates in FDPs. A new transient intermediate was detected and determined to be an antiferromagnetically coupled diferrous-dinitrosyl (S = 0, [{FeNO}7}2) species. This species has an exchange energy, J ≥ 40 cm-1 (JS_1 "∘" S_2), which is consistent with a hydroxo or oxo bridge between the two irons. The results show that the nitric oxide reductase reaction proceeds through successive formation of diferrous-mononitrosyl (S = ½, FeII{FeNO}7) and the S = 0 diferrous-dinitrosyl species. In the rate-determining process, the diferrous-dinitrosyl converts to diferric (FeIIIFeIII) and, by inference, N2O. The proximal FMNH2 then rapidly re-reduces the diferric site to diferrous (FeIIFeII), which can undergo a second 2NO → N2O turnover. This pathway is consistent with previous results on the same deflavinated and flavinated FDP, which detected N2O as a product (Hayashi, T. et al. Biochemistry 2010, 49, 7040). Our results do not support other proposed mechanisms, which proceed either via "superreduction" of [{FeNO}7]2 by FMNH2 or through FeII{FeNO}7 directly to a diferric-hyponitrite intermediate. The results indicate that an S = 0 [{FeNO}7}]2 complex is a proximal precursor to N-N bond formation and N-O bond cleavage to give N2O, and that this conversion can occur without redox participation of the FMN cofactor.
Nature's wisdom in enzyme design: Compounds I and II in the catalytic cycle of the Cytochrome P450 enzymes have been trapped and characterized recently. This work has provided further insight into the electronic structure and reactivity of these crucial intermediates, and key questions regarding the mechanism of these enzymes have finally been answered.
A controllable and inexpensive electrochemical nitric oxide (NO) release system is demonstrated to improve hemocompatibility and reduce bacterial biofilm formation on biomedical devices. Nitric oxide is produced from the electrochemical reduction of nitrite using a copper(II)-tri(2-pyridylmethyl)amine (Cu(II)TPMA) complex as a mediator, and the temporal profile of NO release can be modulated readily by applying different cathodic potentials. Single lumen and dual lumen silicone rubber catheters are employed as initial model biomedical devices incorporating this novel NO release approach. The modified catheters can release a steady, physiologically-relevant flux of NO for more than 7 days. Both single and dual lumen catheters with continuous NO release exhibit greatly reduced thrombus formation on their surfaces after short-term 7-h intravascular placement in rabbit veins (p<0.02, n=6). Three day in vitro antimicrobial experiments in which the catheters are "turned on" for only 3-h of NO release each day exhibit more than a 100-fold decrease in the amount of surface attached live bacteria (n=5). These results suggest that this electrochemical NO generation system could provide a robust and highly effective new approach to improving the thromboresistance and antimicrobial properties of intravascular catheters and potentially other biomedical devices.
Researchers have completed extensive studies on heme and non-heme iron-nitrosyl complexes, which are labeled {FeNO}7 in the Enemark-Feltham notation, but they have had very limited success in producing corresponding, one-electron reduced, {FeNO}8 complexes where a nitroxyl anion (NO−) is formally bound to an iron(II) center. These complexes, and their protonated iron(II)-NHO analogues, are proposed key intermediates in nitrite (NO2–) and nitric oxide (NO) reducing enzymes in bacteria and fungi. In addition, HNO is known to have a variety of physiological effects, most notably in the cardiovascular system. HNO may also serve as a signaling molecule in mammals. For these functions, iron-containing proteins may mediate the production of HNO and serve as receptors for HNO in vivo. In this Account, we highlight recent key advances in the preparation, spectroscopic characterization, and reactivity of ferrous heme and non-heme nitroxyl (NO–/HNO) complexes that have greatly enhanced our understanding of the potential biological roles of these species.
Cytochrome P450 NO reductase is an unusual member of the cytochrome P450 superfamily. It catalyzes the reduction of nitric oxide to nitrous oxide. The reaction intermediates were studied in detail by a combination of experimental and computational methods. They have been characterized experimentally by UV/Vis, EPR, Mössbauer, and MCD spectroscopy. In conjunction with quantum mechanics/molecular mechanics (QM/MM) calculations, we sought to characterize the resting state and the two detectable intermediates in detail and to elucidate the nature of the key intermediate I of the reaction. Six possible candidates were taken into account for the unknown key intermediate in the computational study, differing in protonation state and electronic structure. Two out of the six candidates could be identified as putative intermediates I with the help of the spectroscopic data: singlet diradicals Fe(III) -NHO(.) (-) and Fe(III) -NHOH(.) . In a companion publication (C. Riplinger, F. Neese, ChemPhysChem- 2011, 12, 3192) we have used QM/MM models based on these structures and performed a kinetic simulation. The combination of these two studies shows the nature of the key intermediate to be the singlet diradical, Fe(III) -NHOH(.) .
Denitrifying NO reductases are transmembrane protein complexes that are evolutionarily related to heme/copper terminal oxidases. They utilize a heme/nonheme diiron center to reduce two NO molecules to N2O. Engineering a nonheme FeB site within the heme distal pocket of sperm whale myoglobin has offered well-defined diiron clusters to investigate the mechanism of NO reduction in these unique active sites. In this study, we use FTIR spectroscopy to monitor the production of N2O in solution, and to show that the presence of a distal FeBII is not sufficient to produce the expected product. However, the addition of a glutamate side chain peripheral to the diiron site allows for 50% of productive single-turnover reaction. Unproductive reactions are characterized by resonance Raman spectroscopy as dinitrosyl complexes, where one NO molecule is bound to the heme iron to form a five-coordinate low-spin {FeNO}7 species with v(FeNO)heme and v(NO)heme at 522 and 1660 cm-1, and a second NO molecule is bound to the nonheme FeB site with a v(NO)FeB at 1755 cm-1. Stopped-flow UV-vis absorption coupled with rapid-freeze-quench resonance Raman spectroscopy provide a detailed map of the reaction coordinates leading to the unproductive iron-nitrosyl dimer. Unexpectedly, NO binding to FeB is kinetically favored and occurs prior to the binding of a second NO to the heme iron, leading to a (six-coordinate low-spin heme-nitrosyl/FeB-nitrosyl) transient dinitrosyl complex with characteristic v(FeNO)heme at 570 ± 2 cm-1 and v(NO)FeB at 1755 cm-1. Without the addition of a peripheral glutamate, the dinitrosyl complex is converted to a dead-end product after the dissociation of the proximal histidine of the heme iron, but the added peripheral glutamate side chain in FeBMb2 lowers the rate of dissociation of the proximal histidine which in turn allows the (six-coordinate low-spin heme-nitrosyl/FeB-nitrosyl) transient dinitrosyl complex to decay with production of N2O at a rate of 0.7 s-1 at 4 °C. Taken together, our results support the proposed trans mechanism of NO reduction in NORs.
The study focused on the mechanisms of redox-active metalloenzymes, while computational results on structural and spectroscopic issues which were directly related to the mechanisms were rarely treated. There were also some results for metalloenzymes that were not redox-active, such as the section on zinc-containing enzymes, since the modeling aspects were similar. The most difficult situations to describe with the cluster model were the cases where the model changes its charge, such as in the calculations of redox potentials and pKa values. It was observed that the group of Noodleman had the largest experience during the early years of DFT-cluster modeling for the calculation of redox potentials of metalloenzyme systems.
Stable but able: Chemical and electrochemical reduction of a five-coordinate high-spin non-heme {FeNO}(7) complex (see structure: N blue, Fe orange, and O red) generated the first stable high-spin (S=1) non-heme {FeNO}(8) model complex. The finding that the reduction is metal-centered and causes a decrease in FeNO covalency indicates that in biological systems, reduction activates stable non-heme FeNO units for further transformations.
The electrochemistry of Fe(P)(NO), where P was tetraphenylporphyrin (TPP), tetraphenylchlorin (TPC) and protoporphyrin dimethyl ester (PPDME), was studied in the presence of substituted pyridines and various amines. Both in the presence and absence of the ligand, the first reduction wave of Fe(P)(NO) was reversible. Weak complexes between the iron porphyrin nitrosyls and the pyridines or amines were observed. Upon reduction, the pyridine or amine was lost, and there was no evidence of complexation of the ligand with Fe(P)(NO)−. There were also no significant differences in the Fe(P)(NO)–ligand formation constants between P = TPP and TPC. The formation constants for the Fe(P)(NO)–ligand complex, KNOL, varied linearly with the pKa of the ligand. The slopes of the pKaversus log KNOL curves were 0.22, 0.19 and 0.20 for P equal to TPP, TPC and PPDME, respectively. These slopes were significantly smaller than the values previously observed for FeII(P) and FeIII(P), but only slightly smaller than the complexes where NO was replaced by CS or CSe (0.31 and 0.23, respectively).The visible spectra of Fe(P)(NO) in the presence of the nitrogenous bases were also obtained. With the addition of ligand, the Soret band shifted to longer wavelengths (405 to 419 nm for pyridine), while the long wavelength region shifted to shorter wavelengths (532 to 520 nm for pyridine). Spectra of Fe(P)(NO)(L) at high concentrations of L were not stable indefinitely, but slowly lost NO to generate the bis-ligand complex, Fe(P)(L)2.
High-spin non-heme iron–nitrosyls are of direct interest to both the chemical and biological communities as these species exhibit interesting chemical properties and act as direct models for enzymatic intermediates. The electronic ground state of the ferrous NO complexes, {Fe–NO}7, is best described as high-spin FeIII antiferromagnetically coupled to NO−, generating the spectroscopically observed S = 3/2 ground state. These species have been identified as catalytically relevant to a variety of NO-reducing enzymes such as bacterial nitric oxide reductase (NorBC) and flavo(rubredoxin) nitric oxide reductase (FNOR). Recently, the corresponding one-electron reduced {Fe–NO}8 (nitroxyl) complexes have also been implicated as biologically significant species. In this review the available spectroscopic data for {Fe–NO}7 and {Fe–NO}8 mono- and dinuclear non-heme iron–nitrosyls are summarized, and the implications of these results with respect to the electronic structures and reactivities of these species, in particular towards NO reduction, are discussed.
In the last six years it has become apparent that nitric oxide (NO) has a crucial and extensive role in human physiology. It acts as a messenger molecule effecting muscle relaxation, as a cytotoxic agent in the non-specific immune system, as a carcinogen, and as a neurotransmitter in the brain and peripheral nervous system. This article explores some of the chemistry of NO in an attempt to understand how such a modest molecule can play so many diverse roles.
Flavo-diiron enzymes are among the most recently recognized sub-class of non-heme diiron proteins. The active sites of flavo-diiron enzymes consist of a unique juxtaposition of a flavin cofactor and a histidine, glutamate, aspartate-ligated, solvent-bridged diiron site. Flavo-diiron enzymes were initially thought to function as scavenging dioxygen reductases (s-O2Rs), based on their ability to catalyze the reduction of dioxygen to water, thus protecting air-sensitive bacteria and archaea against “oxidative stress”. However, genetic and biochemical evidence strongly suggests that, at least in some bacteria, flavo-diiron enzymes function as scavenging nitric oxide reductases (s-NORs), catalyzing the reduction of nitric oxide to nitrous oxide, thus, protecting against “nitrosative stress” under anaerobic growth conditions. Key unsettled questions include: should flavo-diiron enzymes be divided into s-NOR and s-O2R categories, and, if so, what features of the active sites distinguish the two activities? If not, how does the active site accommodate and optimize the relative levels of these activities? Systematic investigations of the structures and catalytic mechanisms of several flavo-diiron enzymes constitute an approach to answer these questions. The current state of knowledge and progress toward this end is reviewed here.
NO forms reversible complexes with non-heme ferrous enzymes and model complexes which exhibit unusual S = 3/2 ground states. These nitrosyl derivatives can serve as stable analogs of possible oxygen intermediates in the non-heme iron enzymes. Two complexes, Fe(Me[sub 3]TACN)(NO)(N[sub 3])[sub 2] and FeEDTA-NO, have been studied in detail using X-ray absorption, resonance Raman, absorption, magnetic circular dichroism. and electron paramagnetic resonance spectroscopies and SQUID magnetic susceptibility. These studies have been complemented by spin restricted and spin unrestricted SCF-X[Alpha]-SW electronic structure calculations. As these calculations have been strongly supported by experiment for the nitrosyl complexes, they have been extended to possible oxygen intermediates. In parallel with the Fe[sup 3+]-NO[sup [minus]] complexes, the description of the intermediate obtained involves superoxide antiferromagnetically coupled to a high spin ferric center with a strong [sigma] donation of charge from the superoxide to the iron. These studies allow spectral data on the nitrosyl complexes to be used to estimate bonding differences in possible oxygen intermediates of different non-heme iron proteins and provide insight into the activation of superoxide by coordination to the ferric center for reaction or further reduction. 75 refs., 17 figs., 9 tabs.
The 50-year old debate regarding the true electronic structure of transition metal bis(dithiolene) complexes has been revolutionized recently by involvement of sulfur K-edge X-ray absorption spectroscopy (XAS) to directly probe the sulfur composition of the frontier orbitals. In concert with other spectroscopic methods, and increasingly theoretical calculations, a more accurate electronic structure description has been delivered. The methodology developed has also been applied to mono(dithiolene) and tris(dithiolene) coordination complexes whereby the electronic structure of these systems has been defined in terms of physical oxidation levels of both metal and ligand, ultimately providing direct experimental evidence for the noninnocence of dithiolene ligands.