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Over the past 30 years, the significance of nitric oxide (NO) has become increasingly apparent in mammalian physiology. It is biosynthesized by three isoforms of nitric oxide synthases (NOS): neuronal (nNOS), endothelial (eNOS), and inducible (iNOS). Neuronal and eNOS both produce low levels of NO (nM) as a signaling agent and vasodilator, respectively. Inducible (iNOS) is present in activated macrophages at sites of infection to generate acutely toxic (μM) levels of NO as part of the mammalian immune defense mechanism. These discoveries have led to numerous animal and clinical studies to evaluate the potential therapeutic utility of NO in various medical operations/treatments, primarily using NO gas (via gas-cylinders) as the NO source. In this review, we focus specifically on recent advances in the electrochemical generation of NO (E-NOgen) as an alternative means to generate NO from cheap and inert sources, and the fabrication and testing of biomedical devices that utilize E-NOgen to controllably generate NO for medical applications.

Flavodiiron nitric oxide reductases (FNORs) protect microbes from nitrosative stress under anaerobic conditions by mediating the reduction of nitric oxide (NO) to nitrous oxide (N2O). The proposed mechanism for the catalytic reduction of NO by FNORs involves a dinitrosyldiiron intermediate with a [hs-{FeNO}7]2 formulation, which produces N2O and a diferric species. Moreover, both NO and hydrogen sulfide (H2S) have been implicated in several similar physiological functions in biology and are also known to cross paths in cell signaling. Here we report the synthesis, spectroscopic and theoretical characterization, and N2O production activity of an unprecedented monohydrosulfidodinitrosyldiiron compound, with a [(HS)hs-{FeNO}7/hs-{FeNO}7] formulation, that models the key dinitrosyl intermediate of FNORs. The generation of N2O from this unique compound follows a semireduced pathway, where one-electron reduction generates a reactive hs-{FeNO}8 center via the occupation of an Fe-NO antibonding orbital. In contrast to the well-known reactivity of H2S and NO, the coordinated hydrosulfide remains unreactive toward NO and acts only as a spectator ligand during the NO reduction process.

Flavodiiron nitric oxide reductases (FNORs) protect microbes from nitrosative stress under anaerobic conditions by mediating the reduction of nitric oxide (NO) to nitrous oxide (N

The chemistry of nonheme iron centers with NO in different oxidation states has experienced a renaissance in the last 10 years because of recent findings of their involvement in N2O generation (in ...

The reduction of NO to N2O by flavodiiron nitric oxide reductases (FNORs) is related to the disruption of the defense mechanism in mammals against invading pathogens. The proposed mechanism for this catalytic reaction involves both nonheme mono- and dinitrosyl diiron(II) species as the key intermediates. Recently, we reported an initial account for NO reduction activity of an unprecedented mononitrosyl diiron(II) complex, [Fe2(N-Et-HPTB)(NO)(DMF)3](BF4)3 (1) (N-Et-HPTB is the anion of N,N,N',N'-tetrakis(2-(l-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane; DMF = dimethylformamide) with [FeII{FeNO}7] formulation [Jana et al. J. Am. Chem. Soc. 2017, 139, 14380]. Here we report the full account for the selective synthesis, characterization, and reactivity of FNOR model complexes, which include a dinitrosyl diiron(II) complex, [Fe2(N-Et-HPTB)(NO)2(DMF)2](BF4)3 (2) with [{FeNO}7]2 formulation and a related, mixed-valent diiron(II, III) complex, [Fe2(N-Et-HPTB)(OH)(DMF)3](BF4)3 (3). Importantly, whereas complex 2 is able to produce 89% of N2O via a semireduced mechanism (1 equiv of CoCp2 per dimer = 50% of NO reduced), complex 1, under the same conditions (0.5 equiv of CoCp2 per dimer = 50% of NO reduced), generates only ∼50% of N2O. The mononitrosyl complex therefore requires superreduction for quantitative N2O generation, which constitutes an interesting dichotomy between 1 and 2. Reaction products obtained after N2O generation by 2 using 1 and 2 equiv of reductant were characterized by molecular structure determination and electron paramagnetic resonance spectroscopy. Despite several available literature reports on N2O generation by diiron complexes, this is the first case where the end products from these reactions could be characterized unambiguously, which clarifies a number of tantalizing observations about the nature of these products in the literature.

The nitrogen cycle is one of the most important biogeochemical cycles on Earth because nitrogen is an essential nutrient for all life forms. To supplement natural nitrogen fixation, farmers add large amounts of nitrogen-containing fertilizer to their soils such that nitrogen never becomes a limiting nutrient for plant growth. However, of the nitrogen added to fields — most of which is in the form of NH3 and NO3− — only 30–50% is taken up by plants, while the remainder is metabolized by soil microorganisms in processes with detrimental environmental impacts. The first of these processes, that is, nitrification, refers to the biological oxidation of NH3 to NO2− and NO3−, which have low retention in soil and pollute waterways, leading to downstream eutrophication and ultimately ‘dead zones’ (low oxygen zones) in coastal waters, for example, the Gulf of Mexico. In a second process, namely, denitrification, NO3− and NO2− undergo stepwise reduction to N2O and N2. Substantial amounts of the N2O produced in this process escape into the atmosphere, contributing to climate change and ozone destruction. Recent results suggest that nitrification also affords N2O. This Review describes the enzymes involved in NH3 oxidation and N2O production and degradation in the nitrogen cycle. We pay particular attention to the active site structures, the associated coordination chemistry that enables the chemical transformations and the reaction mechanisms. Nitrification and denitrification are responsible for the processing of ammonia fertilizer, ultimately leading to the generation of environmental pollutants that accumulate in waterways and the atmosphere. This Review describes the enzymes involved in these processes, which fascinate with their unusual active sites and the surprising reactions that they catalyse.

Heme and non-heme iron–nitrosyl complexes are important intermediates in biology. While there are numerous examples of low-spin heme iron–nitrosyl complexes in different oxidation states, much less is known about high-spin (hs) non-heme iron–nitrosyls in oxidation states other than the formally ferrous NO adducts ({FeNO}7 in the Enemark–Feltham notation). In this study, we present a complete series of hs-{FeNO}6–8 complexes using the TMG3tren coligand. Redox transformations from the hs-{FeNO}7 complex [Fe(TMG3tren)(NO)]2+ to its {FeNO}6 and {FeNO}8 analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. Here, we present new MCD, NRVS, XANES/EXAFS, and Mossbauer data, demonstrating that these redox transformations are metal based, which allows us to access hs-Fe(II)–NO–, Fe(III)–NO–, and Fe(IV)–NO– complexes. Vibrational data, analyzed by NCA, directly quantify changes in Fe–NO bonding along this series. Optical data allow for the ident...

Flavodiiron nitric oxide reductases (FNORs) are a subclass of flavodiiron proteins (FDPs) capable of preferential binding and subsequent reduction of NO to N2O. FNORs are found in certain pathogenic bacteria, equipping them with resistance to nitrosative stress, generated as a part of the immune defense in humans, and allowing them to proliferate. Here, we report the spectroscopic characterization and detailed reactivity studies of the diiron dinitrosyl model complex [Fe2(BPMP)(OPr)(NO)2](OTf)2 for the FNOR active site that is capable of reducing NO to N2O [Zheng et al., J. Am. Chem. Soc. 2013, 135, 4902–4905]. Using UV–vis spectroscopy, cyclic voltammetry, and spectro-electrochemistry, we show that one reductive equivalent is in fact sufficient for the quantitative generation of N2O, following a semireduced reaction mechanism. This reaction is very efficient and produces N2O with a first-order rate constant k > 102 s–1. Further isotope labeling studies confirm an intramolecular N–N coupling mechanism, co...

The nitrogen cycle is a key biogeochemical cycle on Earth, as nitrogen is an essential nutrient required for all life-forms. The largest source of bioavailable nitrogen originates from biological and synthetic nitrogen (N2) fixation, whereby N2 is converted to ammonia (NH3), which can then be incorporated into biomass by plants. In the developing world, soils often do not contain enough nitrogen to give large crop yields, so fertilizers, produced by synthetic nitrogen fixation, are critically important to supplement this lack of nitrogen. On the other hand, in the developed world, overfertilization is a common problem (1). In the soil, NH3-oxidizing bacteria (AOB) and archaea compete for the uptake of NH3 with plants, aerobically oxidizing it to nitrite ( NO 2 - ) or nitrate ( NO 3 - ). The first step of this process is catalyzed by ammonia monooxygenase, producing hydroxylamine (NH2OH). Hydroxylamine oxidoreductase (HAO) then further oxidizes NH2OH to NO 2 - (2). These oxidation reactions provide soil microbes with reducing equivalents for ATP synthesis. Complementary to this process is denitrification, which ultimately reduces NO 2 - back down to N2 during anaerobic respiration, producing nitric oxide (NO) and nitrous oxide (N2O) in intermediate steps (3). Release of both NO and N2O during these transformations is of global consequence, as NO participates in the depletion of the ozone layer, whereas N2O has become the third most significant greenhouse gas, with a global warming potential that is 300 times that of CO2 (4). With the increase in the use of fertilizer since the preindustrial era, global N2O emissions have increased substantially. Understanding the processes that generate N2O from fertilizer in soil and seawater is therefore essential in devising practical strategies … [↵][1]1To whom correspondence should be addressed. Email: lehnertn{at}umich.edu. [1]: #xref-corresp-1-1

We present the synthesis, properties, and characterization of [Fe(T1Et4iPrIP)(NO)(H(2)O)(2)](OTf)(2) (1) (T1Et4iPrIP = Tris(1-ethyl-4-isopropyl-imidazolyl)phosphine) as a model for the nitrosyl adduct of gentisate 1,2-dioxygenase (GDO). The further characterization of [Fe(T1Et4iPrIP)(THF)(NO)(OTf)](OTf) (2) which was previously communicated (Inorg. Chem. 2014, 53, 5414) is also presented. The weighted average Fe-N-O angle of 162° for 1 is very close to linear (≥ 165°) for these types of complexes. The coordinated water ligands participate in hydrogen bonding interactions. The spectral properties (EPR, UV-vis, FTIR) for 1 are compared with 2 and found to be quite comparable. Complex 1 closely follows the relationship between the Fe−N−O angle and NO vibrational frequency which was previously identified for 6-coordinate {FeNO}(7) complexes. Liquid FTIR studies on 2 indicate that the ν(NO) vibration position is sensitive to solvent shifting to lower energy (relative to the solid) in donor solvent THF and shifting to higher energy in dichloromethane. The basis for this behavior is discussed. The K(eq) for NO binding in 2 was calculated in THF and found to be 470 M(−1). Density functional theory (DFT) studies on 1 indicate donation of electron density to the iron center from the π* orbitals of formally NO(−). Such a donation accounts for the near linearity of the Fe−N−O bond and the large ν(NO) value of 1791 cm(−1).

Flavodiiron nitric oxide reductases (FNORs), a common enzyme family found in various types of pathogenic bacteria, are capable of reducing nitric oxide (NO) to nitrous oxide (N2O) as a protective detoxification mechanism. Utilization of FNORs in pathogenic bacteria helps them survive and proliferate in the human body, thus causing chronic infections. In this paper, we present a new diiron model complex, [Fe2((Py2PhO2)MP)(OPr)2](OTf), with bridging propionate ligands (OPr−) that is capable of directly reducing NO to N2O in quantitative yield without the need to (super)reduce the complex. We first prepared the diferric precursor and characterized it by UV–vis, IR, NMR and Mossbauer spectroscopies, cyclic voltammetry, and mass spectrometry. This complex can then conveniently be reduced to the diferrous complex using CoCp2. Even though this diferrous complex is highly reactive, we have successfully isolated and characterized this species using X-ray crystallography and various spectroscopic techniques. Most i...

Reaction of [Fe

Reaction of [Fe2(N-Et-HPTB)(CH3COS)](BF4)2 (1) with (NO)(BF4) produces a nonheme mononitrosyl diiron(II) complex, [Fe2(N-Et-HPTB)(NO)(DMF)3](BF4)3 (2). Complex 2 is the first example of a [FeII{Fe(NO)}7] species and is also the first example of a mononitrosyl diiron(II) complex that mediates the reduction of NO to N2O. This work describes the selective synthesis, detailed characterization and NO reduction activity of 2 and thus provides new insights regarding the mechanism of flavodiiron nitric oxide reductases. peerReviewed

The recent discovery of significant hydropersulfide (RSSH) levels in mammalian tissues, fluids and cells has led to numerous questions regarding their possible physiological function. Cysteine hydropersulfides have been found in free cysteine, small molecule peptides as well as in proteins. Based on their chemical properties and likely cellular conditions associated with their biosynthesis, it has been proposed that they can serve a protective function. That is, hydropersulfide formation on critical thiols may protect them from irreversible oxidative or electrophilic inactivation. As a prelude to understanding the possible roles and functions of hydropersulfides in biological systems, this study utilizes primarily chemical experiments to delineate the possible mechanistic chemistry associated with cellular protection. Thus, the ability of hydropersulfides to protect against irreversible electrophilic and oxidative modification was examined. The results herein indicate that hydropersulfides are very reactive towards oxidants and electrophiles and are modified readily. However, reduction of these oxidized/modified species is facile generating the corresponding thiol, consistent with the idea that hydropersulfides can serve a protective function for thiol proteins.

Recent reports indicate the ubiquitous prevalence of hydropersulfides (RSSH) in mammalian systems. The biological utility of these and related species is currently a matter of significant speculation. The function, lifetime and fate of hydropersulfides will be assuredly based on their chemical properties and reactivity. Thus, to serve as the basis for further mechanistic studies regarding hydropersulfide biology, some of the basic chemical properties/reactivity of hydropersulfides were studied. The nucleophilicity, electrophilicity and redox properties of hydropersulfides were examined under biological conditions. These studies indicate that hydropersulfides can be nucleophilic or electrophilic, depending on the pH (i.e. the protonation state) and can act as good one- and two-electron reductants. These diverse chemical properties in a single species make hydropersulfides chemically distinct from other, well-known sulfur containing biological species, giving them unique and potentially important biological function.