The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric Oxide Inhibition of Glycyl Radical Enzymes and Their Activases

Innate immune response cells produce high concentrations of the free radical nitric oxide (NO) in response to pathogen infection. The antimicrobial properties of NO include nonspecific damage to essential biomolecules and specific inactivation of enzymes central to aerobic metabolism. However, the molecular targets of NO in anaerobic metabolism are less understood. Here, we demonstrate that the Escherichia coli glycyl radical enzyme pyruvate formate lyase (PFL), which catalyzes the anaerobic metabolism of pyruvate, is irreversibly inhibited by NO. Using electron paramagnetic resonance and site-directed mutagenesis we show that NO destroys the glycyl radical of PFL. The activation of PFL by its cognate radical S-adenosyl-l-methionine-dependent activating enzyme (PFL-AE) is also inhibited by NO, resulting in the conversion of the essential iron-sulfur cluster to dinitrosyl iron complexes. Whole-cell EPR and metabolic flux analyses of anaerobically growing E. coli show that PFL and PFL-AE are inhibited by physiologically relevant levels of NO in bacterial cell cultures, resulting in diminished growth and a metabolic shift to lactate fermentation. The class III ribonucleotide reductase (RNR) glycyl radical enzyme and its corresponding RNR-AE are also inhibited by NO in a mechanism analogous to those observed in PFL and PFL-AE, which likely contributes to the bacteriostatic effect of NO. Based on the similarities in reactivity of the PFL/RNR and PFL-AE/RNR-AE enzymes with NO, the mechanism of inactivation by NO appears to be general to the respective enzyme classes. The results implicate an immunological role of NO in inhibiting glycyl radical enzyme chemistry in the gut.

Functional Nonheme Diiron(II) Complexes Catalyze the Direct Reduction of Nitrite to Nitric Oxide in Relevance to the Diiron Protein YtfE

The present work reports the functional modeling chemistry of YtfE, which features a nonheme diiron active site and mediates the direct reduction of NO2- to NO. The model complex, [Fe2(HPTP)Cl2]1+ (1), reduces NO2- to NO in a 100% yield within 12 h and generates [Fe4(HPTP)2(μ-O)3(μ-OH)]3+ (2). Similar to YtfE, the reaction involves stepwise oxidation of two Fe(II) centers and product (NO) inhibition, of which the latter produces [Fe2(HPTP)(NO)2Cl2]1+ (3). Complex 3 could also be synthesized by the reaction of [Fe2(HPTP)(NO)2(ClO4)]2+ (4) and chloride. Complex 1 catalyzes the reduction of NO2- to NO in the presence of PhS-, albeit with a low TON of 5, due to the formation of an insoluble product, [Fe2(HPTP)(μ-SPh)Cl2] (5). Another model complex [Fe2(HPTP)(OPr)]1+ (6), reduced NO2- to NO in an 80% yield after 24 h, generated [Fe2(HPTP)(OPr)(NO)2]1+ (7), and offered a TON of 19. The third model complex, [Fe2(HPTP)(ClO4)2]1+ (8), could reduce NO2- to NO in a 100% yield but only after 48 h. A comparison of these results establishes that easy oxidation of the Fe(II) centers, easy accessibility of the Fe(II) centers for the coordination of NO2-, and easy release of NO from the in situ generated dinitrosyl diiron complex increase the efficiency of the functional model complexes of YtfE.

Skeletal substituents and the distal environment determine the spin state of natural and synthetic iron porphyrins: role in the O2 reduction reaction

The poprhyrin structure, along with its axial ligands and surrounding environment determine its electronic structure which results in a wide range of reduction potentials and different spin states of the iron center in heme enzymes in nature. Tuning these electronic structure attributes is crucial for heme proteins to be able to efficiently catalyze multiproton and multielectron reduction of small molecules such as O2, NO2- and SO2, which have very different reduction potentials, and this is important in designing small-molecule catalysts for these energy- and environment-related transformations. However, deconvoluting the effects of porphyrin modifications and protein environments on the electronic structures of active sites is often difficult. Site-isolated imidazole-bound heme b, diacetyl heme and their synthetic analogue active sites are created atop self-assembled monolayers of thiols on Au electrodes. In situ surface-enhanced resonance Raman spectroscopy indicates that imidazole-bound heme b prefers a low-spin active site in both its redox states in contrast to the protein active sites with a histidine-bound heme b cofactor, which are all high spin. The imidazole-bound diacetyl heme, with electron-withdrawing groups like that of heme a, however, prefers a high-spin ground state under the same conditions. Imidazole-bound synthetic iron porphyrins show that the ground state gradually changes from low spin, in iron tetraphenyl porphyrin, to high spin as electron-withdrawing groups are attached to the porphyrin ligand. When the solvent-exposed site of a low-spin iron porphyrin is hydrophobic, it switches to its high-spin state. The electron-withdrawing groups and the spin state can tune the reduction potential of imidazole-bound iron porphyrins by more than 300 mV. The high-spin ground state allows faster electrocatalytic O2 reduction at a lower overpotential, while the low-spin ground state stays inhibited due to product inhibition.

Electrocatalytic Nitrite Reduction by a Monomeric NrfA: Commonality in Ammonification Mechanisms

Cytochrome c nitrite reductase (NrfA) is a pentaheme enzyme capable of the six-electron reduction of nitrite to ammonia, which is a key step in the nitrogen cycle. All NrfA enzymes appear to have a branched set of two heme-based pathways for electron transfer to a conserved active site, and until recently, NrfA enzymes from a variety of microorganisms were considered to possess a homodimeric structure; yet, recent efforts have shown that in solution, purified Geobacter lovleyi (Gl) NrfA is a monomer. Direct protein electrochemistry has been used in the past to characterize the dimeric NrfAs from Escherichia coli and Shewanella oneidensis, revealing features of maximal activity as a function of nitrite concentration, and redox poise, both of which were interpreted in terms of the dimeric structure providing multiple redox equivalents. Here, we examine Gl NrfA using protein film electrochemistry and find that all of the features that were associated with the dimeric enzymes are also found in the monomeric enzyme. Further, we probe the contribution of specific heme environments through investigation of two His to Met heme ligand mutants, each along a different branch of the electron transfer network, which demonstrates that each path is likely essential to support native-like catalysis.

Surface-like NOx Reduction at an Atomically-Precise Tricopper Cluster

The combustion of nitrogen-containing fuels and increasing use of nitrogen-rich fertilizers is accumulating NOx pollutants in the environment. Copper is an attractive catalyst material for reductive NOx remediation, yet ambiguity persists regarding the elementary bond-making and bond-breaking steps occurring at the catalyst interface. Starting from a molecular tricuprous μ3-oxo complex (1), an unusually reduced and highly reactive surface-like cluster (2) has been prepared. Characterization data and electronic structure calculations are consistent with 2 featuring σ-aromaticity that primes the tricopper core for two-electron chemistry. Cluster 2 mediates catalytic reductive coupling of NO to N2, proceeding through N2O, via sequential oxygen atom transfer steps. Stoichiometric reduction of NO3 - and NO2 - is also disclosed, mapping the complete denitrification cycle at an atomically-precise molecular cluster.

The Enemark-Feltham formalism at 50: An interview with John Enemark and a personal reflection

In an interview, one of us (JHE) recounts the state of NO chemistry around the middle of the twentieth century and the events that led to the development of Enemark and Feltham's {MNO}n notation. A personal perspective by one of us (AG) underscores the continued role of the notation as a source of electro-structural correlations. Interestingly, although recent ab initio calculations have on occasion resulted in somewhat different NO oxidation states relative to earlier, classic studies, the Enemark-Feltham notation remains as relevant as ever. Thus, for iron nitrosyls, there appears to be a one-to-one mapping between the Enemark-Feltham count and the NO oxidation state. Whether an analogous mapping exists for other transition metals has not been resolved at this point.

Intermolecular N-N Coupling of a Dinitrosyl Iron Complex Induced by Hydrogen Bond Donors in the Secondary Coordination Sphere

The intermolecular N-N coupling of NO in a dinitrosyl iron complex (DNIC) induced by hydrogen bond donors in the secondary coordination sphere to form N2O is reported. A family of complexes containing pendant anilines in the secondary coordination sphere were synthesized and characterized. Reduction of the {Fe(NO)2}9 complex [Fe(PhNHPDI)(NO)2][BPh4] (3) to the {Fe(NO)2}10 Fe(PhNHPDI)(NO)2 (4) results in intermolecular N-N coupling to form N2O. Similar reactions of the control {Fe(NO)2}9 complex [Fe(PhNMePDI)(NO)2][BPh4] (7), which does not have H-bonding groups in the secondary coordination sphere, do not result in N2O formation. The hydrogen bonding capabilities of the complexes were explored spectroscopically and computationally.

Advances in Electrochemical Nitrite Reduction toward Nitric Oxide Synthesis for Biomedical Applications

Nitric oxide (NO) is an essential molecule in biomedicine, recognized for its antibacterial properties, neuronal modulation, and use in inhalation therapies. The effectiveness of NO-based treatments relies on precise control of NO concentrations tailored to specific therapeutic needs. Electrochemical generation of NO (E-NOgen) via nitrite (NO2 -) reduction offers a scalable and efficient route for controlled NO production, while also addressing environmental concerns by reducing NO2 - pollution and maintaining nitrogen cycle balance. Recent developments in catalysts and E-NOgen devices have propelled NO2 - conversion, enabling on-demand NO production. This review provides an overview of NO2 - reduction pathways, with a focus on cutting-edge Fe/Cu-based E-NOgen catalysts, and explores the development of E-NOgen devices for biomedical use. Challenges and future directions for advancing E-NOgen technologies are also discussed.

Nitric oxide transfer between nominal Fe and Co biomimetics of the nitrile hydratase active site

Related to the inactive form of nitrile hydratase, NHase, that contains Fe(NO) within tripeptide N2S2 binding environment, the NO transfer reactivity of (bis-mercaptoethane diazacycloheptane)Fe(NO) and (bis-mercaptoethane diazadimethylethane)Fe(NO) is compared to Co(NO) analogs. Acceptors of NO include cobalt octaethylporphyrin and the [(N2S2)M] dimeric precursors in the synthesis of the Fe(NO) and Co(NO) biomimetics. Qualitative rates are augmented by a definitive kinetic study finding that rates of NO transfer from (N2S2)M(NO) to [(N2S2)M']2 are dependent on M and M' as well as the hydrocarbon N to N and N to S linkers. We conclude that while Fe(NO) and Co(NO) units are similar in chemical stability, minor first coordination sphere differences may favor the former, Fe(NO), consistent with the discovery of Fe(NO), but not Co(NO), in the as-isolated NHase active site.

Nitric Oxide Inhibition of Glycyl Radical Enzymes and Their Activases

Innate immune response cells produce high concentrations of the free radical nitric oxide (NO) in response to pathogen infection. The antimicrobial properties of NO include non-specific damage to essential biomolecules and specific inactivation of enzymes central to aerobic metabolism. However, the molecular targets of NO in anaerobic metabolism are less understood. Here, we demonstrate that the Escherichia coli glycyl radical enzyme pyruvate formate lyase (PFL), which catalyzes the anaerobic metabolism of pyruvate, is irreversibly inhibited by NO. Using electron paramagnetic resonance and site-directed mutagenesis we show that NO destroys the glycyl radical of PFL. The activation of PFL by its cognate radical S-adenosyl-L-methionine-dependent activating enzyme (PFL-AE) is also inhibited by NO, resulting in the conversion of the essential iron-sulfur cluster to dinitrosyl iron complexes. Whole-cell EPR and metabolic flux analyses of anaerobically growing Escherichia coli show that PFL and PFL-AE are inhibited by physiologically relevant levels of NO in bacterial cell cultures, resulting in diminished growth and a metabolic shift to lactate fermentation. The class III ribonucleotide reductase (RNR) glycyl radical enzyme and its corresponding RNR-AE are also inhibited by NO in a mechanism analogous to those observed in PFL and PFL-AE, which likely contributes to the bacteriostatic effect of NO. Based on the similarities in reactivity of the PFL/RNR and PFL-AE/RNR-AE enzymes with NO, the mechanism of inactivation by NO appears to be general to the respective enzyme classes. The results implicate an immunological role of NO in inhibiting glycyl radical enzyme chemistry in the gut.

Electrochemical and spectroelectrochemical investigation of Ru(por)(NO)(OAr) derivatives (por = octaethylporphyrin, tetraanisolylporphyrin; Ar = Ph, C6H4-2-NHC(O)CF3; C6H3-2,6-(NHC(O)CF3)2)

The electrochemistry and spectroelectrochemistry of Ru(porphyrin)(NO)(phenoxide) complexes Ru(por)(NO)(OPh) (por = OEP, 1a; TAP, 2a; Ph = C6H5), Ru(por)(NO)(OAr1) (por = OEP, 1b; TAP, 2b; OAr1 = -OC6H4-(2-NHC(O)CF3)), Ru(por)(NO)(OAr2) (por = OEP, 1c; TAP, 2c; OAr2 = OC6H3-(2,6-NHC(O)CF3)2; OEP = octaethylporphyrinato dianion, TAP = tetraanisolylporphyrinato dianion) indicate that initial one-electron oxidation results in structure-dependent net reactivity at the phenoxide ligand. Oxidation of 1a generates 1a+, which undergoes a relatively slow rate-limiting second-order follow-up reaction. In contrast, 2a undergoes a diffusion-limited follow-up reaction after oxidation. Oxidation of species 1b and 2b results in dissociation of the corresponding phenoxide radicals from the metal center following a relatively slow first-order kinetic process. The ˙OAr1 radical was detected by EPR spectroelectrochemistry. The follow-up reactions after oxidation of 1c and 2c are also very fast. In all cases, the ultimate fate of the ruthenium complex is to be trapped with adventitious water to generate the stable aqua complex. Further oxidation of each compound at higher potentials occurs at the porphyrin ligand, generating the π-radical cation observed by IR spectroelectrochemistry.

A single outer-sphere amino-acid substitution turns on the NO reactivity of a hemerythrin-like protein

Mycobacterial hemerythrin-like proteins (HLPs) are important for the survival of pathogens in macrophages. Their molecular mechanisms of function remain poorly defined but recent studies point to their possible role in nitric oxide (NO) scavenging. Unlike any nonheme diiron protein studied so far, the diferric HLP from Mycobacterium kansasii (Mka-HLP) reacts with NO in a multistep fashion to consume four NO molecules per diiron center. HLPs are largely conserved across mycobacteria and we argued that comparative studies of distant orthologs may illuminate the role of the protein scaffold in this reactivity and yield intermediates with properties more favorable for detailed spectroscopic characterization. Herein, we show that HLP from Azotobacter vinelandii (Avi-HLP) requires a single T47F point mutation in the outer sphere of its diferric center to adopt a bridging μ-oxo diferric structure as in Mka-HLP and makes it reactive toward NO. Radical combination of NO with the μ-oxo bridge yields nitrite and a mixed valent Fe(iii)Fe(ii) cluster that further react with NO to produce a stable magnetically coupled Fe(iii){FeNO}7 cluster. We report characterization of this stable cluster by electronic absorption, EPR, FTIR and resonance Raman spectroscopies and suggest ways Phe 46 (Mka numbering) might control the Fe(iii) reduction potential and the NO reactivity of HLPs.

NO Oxidation States in Nonheme Iron Nitrosyls: A DMRG-CASSCF Study of {FeNO}6-10 Complexes

Building upon an earlier study of heme-nitrosyl complexes (Inorg. Chem. 2023, 62, 20496-20505), we examined a wide range of nonheme {FeNO}6-10 complexes (the superscript represents the Enemark-Feltham count) and two dinitrosyl iron complexes using DMRG-CASSCF calculations. Analysis of the wave functions in terms of resonance forms with different [π*(NO)]i occupancies (where i = 0-4 for mononitrosyl complexes) identified the dominant electronic configurations of {FeNO}6 and {FeNO}7 complexes as FeIII-NO0 and FeII-NO0, respectively, mirroring our previous findings on heme-nitrosyl complexes. A trigonal-bipyramidal S = 1 {FeNO}8 complex with an equatorial triscarbene ligand set appears best described as a resonance hybrid of FeI-NO0 and FeII-NO-. Reduction to the corresponding S = 1/2 {FeNO}9 state was found to involve both the metal and the NO, leading to an essentially FeI-NO- complex. Further reduction to the {FeNO}10 state was found to be primarily metal-centered, leading to a predominantly Fe0-NO- configuration. Based on the weights wi of the [π*(NO)]i resonance forms, an overall DMRG-CASSCF-based π*(NO) occupation number could be derived, which was found to exhibit a linear correlation with both the NO bond distance and NO stretching frequency, allowing a readout of the NO oxidation state from the NO bond distance.

Reaction of a Nitrosyl Complex of Co(II)-Porphyrin with Hydrogen Peroxide: Formation of a Porphyrin Radical Cation

A nitrosyl complex of Co(II)-porphyrinate, [CoII(TTMPP2-)(NO)], 1 [TTMPPH2=5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl)porphyrin] having {Co(NO)}8 configuration has been synthesized and characterized spectroscopically as well as structurally. Complex 1 in dichloromethane:acetonitrile (1 : 4, v/v) solution at -40 °C reacts with hydrogen peroxide (H2O2) to result in the corresponding nitrite (NO2 -) complex, [CoIII(TTMPP2-)(NO2 -)], 2. The formation of a Co(III)-peroxynitrite intermediate is presumed in this reaction based on the characteristic phenol ring nitration test. From spectroscopic studies, formation of [CoIV=O] species followed by a Co(III)-porphyrin radical complex was observed during the course of the reaction.

Revealing Significant Electronic Effects on the Oxygen Reduction Reaction with Iron Porphyrins

Understanding electronic effects on catalysis from a mechanism point of view is of fundamental significance but is also challenging. We herein report on electronic effects on the oxygen reduction reaction (ORR) with Fe porphyrins. By using FeIII tetraphenylporphyrin (TPP-Fe) and FeIII tetra(pentafluorophenyl)porphyrin (TPFP-Fe), we showed their different electrochemical and chemical behaviors for ORR. Mechanism studies revealed that the FeIII-superoxo species of TPP-Fe can undergo smooth protonation with trifluoroacetic acid (TFA) but the electron-deficient FeIII-superoxo species of TPFP-Fe cannot be protonated with TFA. The FeIII-superoxo reactivity difference between TPP-Fe and TPFP-Fe is the origin of their different catalytic ORR behaviors.

Photo-induced nitroxyl anion/HNO release from a nitrosyl complex of Mn(II)-porphyrinate

A nitrosyl complex of MnII-porphyrinate, 1 has been synthesized and characterized. It was found to donate a nitroxyl anion (NO-) to suitable acceptors in dichloromethane solution in the presence of visible light. The evolution of N2O and the characteristic reaction with PPh3 in the presence of H+ confirms the NO-/HNO donation.

Reduction of iron porphyrin nitrate to the iron nitrosyl by H2S/thiol. studies in sublimed layers

Hemes play key roles in enzymatic production of the mammalian gasotransmitter NO by nitric oxide synthase as well as in conversion from inorganic nitrite. In the present study, we report a hitherto unknown pathway of nitrosyl formation via thiol reduction of a iron porphyrin nitrate complex in the solid state.

Electronic structure of light-induced nitrosyl linkage isomers revealed by X-ray absorption spectroscopy at Ru L3,2-edges

X-ray absorption spectroscopy (XAS) is a powerful tool for examining changes of the electronic and molecular structure following light-induced excitation of a molecule. Specifically, this method can be applied to investigate the ground (GS, RuNO) and metastable states (MS1, RuON and MS2, Ruη2(NO)) of the nitrosyl ligand (NO), which differ in their coordination mode to the metal. In this work, we report for the first time experimental and theoretical (DFT) Ru L3,2-edge XA spectra for the octahedral complex trans-[RuNOPy4F](ClO4)2 (1, Py = pyridine) in both ground and metastable states. The transition from GS to MS1 using 420 nm light excitation leads to a significant downshift of the 2p → LUMO(+1) peaks by about 0.5-0.8 eV, attributed to the destabilisation of 2p orbitals and stabilization of LUMO(+1). Subsequent irradiation of MS1 at 920 nm produces isomer MS2, for which even greater stabilization of LUMO occurs, though without a significant change in 2p energy. The change in 2p energy is attributed to a variation in the charge on the Ru atom after NO isomerization, while LUMO(+1) stabilization is related to changes in the Ru(NO) bond length and the composition of this orbital.

Tetrapodal iron complexes invoke observable intermediates in nitrate and nitrite reduction

This study investigates the mechanistic pathways of nitrate and nitrite reduction by the tetrapodal iron complex [Py2Py(afamcyp)2Fe]OTf2, revealing key intermediates to elucidate the reaction process. Using UV-Vis, IR, mass and NMR spectroscopies, stable binding of oxyanions to the iron centre was observed, supporting the formation of the iron(iii)-hydroxide intermediate [Py2Py(afamcyp)2Fe(OH)]OTf2. This intermediate is less stable than in previous systems, providing insights into the behaviour of metalloenzymes. A bimetallic mechanism is proposed for nitrogen oxyanion reduction where additional iron is required to drive the complete reaction, resulting in the formation of the final nitrosyl complex, Py2Py(pimcyp)2Fe(NO), and water. Our findings enhance the understanding of iron-based reduction processes and contribute to the broader knowledge of oxyanion reduction mechanisms.

Coordination and Bioorganometallic Chemistry: Exploring the Potential Applications of Metal Coordination and Organometallic Complexes in Medical and Microbiological Advancements

In the field of coordination and bioorganometallic chemistry, a notable shift is occurring. This review explores a new generation of carefully 3D-crafted coordination and organometallic complexes that differ from conventional structures. Emphasizing disease intervention and microbial control, these compounds, incorporate noble and transition metals and aim to enhance therapeutic efficacy while minimizing potential health risks. This review covers diverse applications, showcasing their effectiveness against bacteria, viruses, and fungi, and as potential tools in cancer treatment. Additionally, it sheds light on the inventive aspects of these complexes within biological systems. By highlighting advancements in bioorganometallic chemistry, the review offers insights and guidance for future developments in safer and more effective therapeutics.

Structure, properties, and decomposition in biological systems of a new nitrosyl iron complex with 2-methoxythiophenolyls, promising for the treatment of cardiovascular diseases

A new promising binuclear tetranitrosyl iron complex with 2-methoxythiophenolyl of the composition [Fe2(C7H7OS)2(NO)4] (complex 1), which acts on the therapeutic targets of cardiovascular diseases, guanylate and adenylate cyclase, has been synthesized. X-ray diffraction data show the presence of two isoforms of complex 1; according to quantum chemical calculations, the structure of only the trans isomer is stable in solutions. The processes of transformation of complex 1 in DMSO, in aqueous solutions, as well as in the presence of bovine serum albumin, reduced glutathione, and mucin were studied. DMSO promotes the decomposition of the original complex 1 into mononuclear products. In biological systems, the mechanisms of decomposition of the complex 1 differ from aqueous solutions. In albumin solution, a gradual formation of a high-molecular-weight dinitrosyl complex is observed, obtained by coordinating the [Fe(NO)2]+ fragment with the amino acid groups of the protein. In the presence of mucin, an EPR signal from stable mononitrosyl products is observed. The introduction of glutathione into the system of the complex 1 leads to the replacement of one initial thioligand with glutathione. In the model systems under study, a more efficient and prolonged generation of NO groups is observed compared to a buffer solution. The obtained data on the influence of the environment on the properties of the complex 1 in combination with a study of their effect on enzymes allow us to recommend it for further study as a potential drug with vasodilator, antianginal, and hypotensive properties.

Tuning the Reactivity of Copper(II)-Nitrite Core Towards Nitric Oxide Generation

Insights into the molecular mechanism and factors affecting nitrite-to-NO transformation at transition metal sites are essential for developing sustainable technologies relevant to NO-based therapeutics, waste water treatment, and agriculture. A set of copper(II)-nitrite complexes 1-4 have been isolated employing tridentate pincer-type ligands (quL, pyL, ClArOL-, PhOL-) featuring systematically varied donors. Although the X-ray crystal structures of the copper(II)-nitrite cores in 1-4 are comparable, electrochemical studies on complexes 1-4 reveal that redox properties of these complexes differ due to the changes in the σ-donor abilities of the phenolate/N-heterocycle based donor sites. Reactivity of these nitrite complexes with oxygen-atom-transfer (OAT) reagent (e. g. triphenyl phosphine Ph3P) and H+/e- donor reagent (e. g. substituted phenols ArOH) show the reduction of nitrite to NO gas. Detailed kinetic investigations including kinetic isotope effect (KIE), Eyring analyses for determining the activation parameters unfold that reduction of nitrite at copper(II) by Ph3P or ArOH are influenced by the CuII/CuI redox potential. Finally, this study allows mechanism driven development of catalytic nitrite reduction by ArOH in the presence of 10 mol % copper complex (1).

Photo-triggered NO release of nitrosyl complexes bearing first-row transition metals and therapeutic applications

In biological systems, nitric oxide (NO) is a crucial signaling molecule that regulates a wide range of physiological and pathological processes. Given the significance of NO, there has been considerable interest in delivering NO exogenously, particularly through light as a non-invasive therapeutic approach. However, due to the high reactivity and instability of NO under physiological conditions, directly delivering NO to targeted sites remains challenging. In recent decades, photo-responsive transition metal-nitrosyl complexes, especially based on first-row transition metals such as Mn, Fe, and Co, have emerged as efficient NO donors, offering higher delivery efficiency and quantum yields than heavy metal-nitrosyl complexes under light exposure. This review provides a comprehensive overview of current knowledge and recent developments in the field of photolabile first-row transition metal-nitrosyl complexes, focusing on the structural and electronic properties, photoreactivity, photodissociation mechanisms, and potential therapeutic applications. By consolidating the key features of photoactive nitrosyl complexes, the review offers deeper insights and highlights the potential of first-row transition metal-nitrosyl complexes as versatile tools for photo-triggered NO delivery.

Harnessing the Power of Our Immune System: The Antimicrobial and Antibiofilm Properties of Nitric Oxide

Nitric oxide (NO) is a free radical of the human innate immune response to invading pathogens. NO, produced by nitric oxide synthases (NOSs), is used by the immune system to kill microorganisms encapsulated within phagosomes via protein and DNA disruption. Owing to its ability to disperse biofilm-bound microorganisms, penetrate the biofilm matrix, and act as a signal molecule, NO may also be effective as an antibiofilm agent. NO can be considered an underappreciated antimicrobial that could be levied against infected, at-risk, and hard-to-heal wounds due to the inherent lack of bacterial resistance, and tolerance by human tissues. NO produced within a wound dressing may be an effective method of disrupting biofilms and killing microorganisms in hard-to-heal wounds such as diabetic foot ulcers, venous leg ulcers, and pressure injuries. We have conducted a narrative review of the evidence underlying the key antimicrobial and antibiofilm mechanisms of action of NO for it to serve as an exogenously-produced antimicrobial agent in dressings used in the treatment of hard-to-heal wounds.

A Cytochrome P450 TxtE Model System with Mechanistic and Theoretical Evidence for a Heme Peroxynitrite Active Species

The cytochrome P450 homolog, TxtE, efficiently catalyzes the direct and regioselective aromatic nitration of the indolyl moiety of L-tryptophan to 4-nitro-L-tryptophan, using nitric oxide (NO) and dioxygen (O2) as co-substrates. Pathways for such direct and selective nitration of heteroaromatic motifs present platforms for engineering new nitration biocatalysts for pharmacologically beneficial targets, among a medley of other pivotal industrial applications. Precise mechanistic details concerning this pathway are only weakly understood, albeit a heme iron(III)-peroxynitrite active species has been postulated. To shed light on this unique reaction landscape, we investigated the indole nitration pathway of a series of biomimetic ferric heme superoxide mimics, [(Por)FeIII(O2 -⋅)], in the presence of NO. Therein, our model systems gave rise to three distinct nitroindole products, including 4-nitroindole, the product analogous to that obtained with TxtE. Moreover, 15N and 18O isotope labeling studies, along with meticulously designed control experiments lend credence to a heme peroxynitrite active nitrating agent, drawing close similarities to the tryptophan nitration mechanism of TxtE. All organic and inorganic reaction components have been fully characterized using spectroscopic methods. Theoretical investigation into several mechanistic possibilities deem a unique indolyl radical based reaction pathway as the most energetically favorable, products of which, are in excellent agreement with experimental findings.

Synthesis and Structural Characterization of a Non-Heme Iron Hyponitrite Complex

Flavodiiron NO reductases (FNORs) are important enzymes in microbial pathogenesis, as they equip microbes with resistance to the human immune defense agent nitric oxide (NO). Despite many efforts, intermediates that would provide insight into how the non-heme diiron active sites of FNORs reduce NO to N2O could not be identified. Computations predict that iron-hyponitrite complexes are the key species, leading from NO to N2O. However, the coordination chemistry of non-heme iron centers with hyponitrite is largely unknown. In this study, we report the reactivity of two non-heme iron complexes with preformed hyponitrite. In the case of [Fe(TPA)(CH3CN)2](OTf)2, cleavage of hyponitrite and formation of an Fe2(NO)2 diamond core is observed. With less Lewis-acidic [Fe2(BMPA-PhO)2(OTf)2] (2), reaction with Na2N2O2 in polar aprotic solvent leads to the formation of a red complex, 3. X-ray crystallography shows that 3 is a tetranuclear iron-hyponitrite complex, [{Fe2(BMPA-PhO)2}2(μ-N2O2)](OTf)2, with a unique hyponitrite binding mode. This species provided the unique opportunity to us to study the interaction of hyponitrite with non-heme iron centers and the reactivity of the bound hyponitrite ligand. Here, either protonation or oxidation of 3 is found to induce N2O formation, supporting the hypothesis that hyponitrite is a viable intermediate in NO reduction.

The Effect of Intramolecular Proton Transfer on the Mechanism of NO Reduction to N2O by a Copper Complex: A DFT Study

DFT calculations were performed to explore the mechanism underlying the reduction of NO to N2O by a CuI complex. A nitrosyl complex reacts with another NO molecule and the CuI complex, leading to the formation of a dicopper-hyponitrite complex (Cu2N2O2). The first steps follow a common pathway until the formation of the intermediate [CuII-N2O2]+, after which the reaction pathway diverges into three Cu2N2O2 species: κ2-N,N', κ2-O,O', and κ3-N,O,O'. These species yield different products along their respective reaction pathways. In the case of the κ2-N,N' and κ3-N,O,O' species, the subsequent steps involve a methanol-mediated proton transfer and N-O bond cleavage, resulting in the generation of N2O and [CuII-OH]+. Conversely, for the κ2-O,O' species, two proton transfers occur without N-O bond cleavage, leading to the formation of H2N2O2 and [CuII]2+. H2N2O2 spontaneously converts into N2O and H2O. These computational results elucidate how the coordination mode of hyponitrite influences reactivity and provide insights into NO reduction via proton transfer. Notably, switching of the N2O2 coordination mode to metal ions from N to O was not required, offering insights for more efficient NO reduction strategies in the future.

In Situ FT-IR Spectroelectrochemistry Reveals Mechanistic Insights into Nitric Oxide Release from Ruthenium(II) Nitrosyl Complexes

Ruthenium(II) tetraamine nitrosyl complexes with N-heterocyclic ligands are known for their potential as nitric oxide (NO•) donors, capable of releasing NO• through either direct photodissociation or one-electron reduction of the Ru(II)NO+ center. This study delivers a novel insight into the one-electron reduction mechanism for the model complex trans-[RuII(NO)(NH3)4(py)]3+ (RuNOpy, py = pyridine) in phosphate buffer solution (pH 7.4). In situ FT-IR spectroelectrochemistry reveals that the pyridine ligand is readily released upon one-electron reduction of the nitrosyl complex, a finding supported by nuclear magnetic resonance spectroscopy (1H NMR) and electrochemistry coupled to mass spectrometry (EC-MS), which detect free pyridine in solution. However, direct evidence of NO• release from RuNOpy as the primary step following reduction was not observed. Interestingly, FT-IR results indicate that the isomers of the nitrosyl complex, cis-[Ru(NO)(NH3)4(OH)]+ and trans-[Ru(NO)(NH3)4(OH)]+, are formed following reduction and pyridine labilization, initiating an outer-sphere electron transfer process that triggers a chain electron transfer reaction. Finally, nitric oxide is liberated as an end product, arising from the reduction of the hydroxyl isomer complexes cis-[Ru(NO)(NH3)4(OH)]2+ and trans-[Ru(NO)(NH3)4(OH)]2+. This study provides new insights into the reduction mechanism and transformation pathways of ruthenium nitrosyl complexes, contributing to our understanding of their potential as NO• donors.

One-Pot Synthesis of Fe-Norepinephrine Nanoparticles for Synergetic Thermal-Enhanced Chemodynamic Therapy

Chemodynamic therapy (CDT) is an innovative and burgeoning strategy that utilizes Fenton-Fenton-like chemistry and specific microenvironments to produce highly toxic hydroxyl radicals (•OH), with numerous methods emerging to refine this approach. Herein, we report a coordination compound, Fe-norepinephrine nanoparticles (Fe-NE NPs), via a one-pot synthesis. The Fe-NE NPs are based on ferrous ions (Fe2+) and norepinephrine, which are capable of efficient Fe2+/Fe3+ delivery. Once internalized by tumor cells, the released Fe2+/Fe3+ exerts the Fenton reaction to specifically produce toxic •OH. Moreover, the internal photothermal conversion ability of Fe-NE NPs allows us to simultaneously introduce light to trigger local heat generation and then largely improve the Fenton reaction efficiency, which enables a synergetic photothermal and chemodynamic therapy to realize satisfactory in vivo antitumor efficiency. This proof-of-concept work offers a promising approach to developing nanomaterials and refining strategies for enhanced CDT against tumors.

Near-Infrared-II Photoactivated Iron(III) Complexes for Highly Efficient RNS and ROS Synergistic Therapy

Reactive nitrogen species (RNS) are more lethal than reactive oxygen species (ROS), which gives them a very promising future in the field of cancer treatment. However, there are still a few drugs available for RNS generation. In this work, two 5th-order nonlinear optical materials, FB-Fe(III)/SNP@PEG and FB-Fe(II)-FB/SNP@PEG, are synthesized. The outstanding nonlinear optical properties of FB-Fe(III)/SNP@PEG help to achieve generation of bounteous superoxide anions (O2•-) in deep tissues, while sodium nitroprusside (SNP) provides NO in the body, both of which are prerequisites for RNS generation. Meanwhile, type I and type II ROS were also generated under irradiation of a 1600 nm laser. Based on the synergistic effect of ROS and RNS, FB-Fe(III)/SNP@PEG induced mitochondrial damage and DNA fragmentation and inhibited tumor cells through apoptosis, possessing better therapeutic effects than FB-Fe(II)-FB/SNP@PEG. This work put forward an innovative strategy to achieve the cooperative release of RNS and ROS in deep tissues, which provides insights and ideas for applying nonlinear optical materials to RNS therapy.

Role of ancillary ligands in S-nitrosothiol and NO generation from nitrite-thiol interactions at mononuclear zinc(ii) sites

Generation of S-nitrosothiol (RSNO) and nitric oxide (NO) mediated by zinc(ii) coordination motifs is of prime importance for understanding the role of zinc(ii)-based cofactors in redox-signalling pathways. This study uniquely employs a set of mononuclear [L2ZnII]2+ cores (where L = Me4PzPz/Me2PzPy/Me2PzQu) for introducing subtle alterations of the primary coordination sphere and investigates the role of ligand tuning in the transformation of NO2 - in the presence of thiols. Single crystal X-ray diffraction (SCXRD) analyses on [L2ZnII-X](X) (where X = perchlorate/triflate) illustrate consistent changes in the bond distances, thereby showing variations of the metal-ligand interactions depending on the nature of the heterocyclic donor arms (pyrazole/pyridine/quinoline). Moreover, such tuning of the ligands affects the Lewis-acidity of the [L2ZnII]2+ cores as evaluated by 31P NMR and SCXRD studies on the 1 : 1 acid-base adducts [L2ZnII(OPEt3)]2+. Crystallographic and 15N NMR spectroscopic analyses on the nitrite complexes [L2ZnII(κ2-nitrite)](ClO4) reveal that the chemical environments of the nitrite anions in these complexes are nearly identical, despite the dissimilarity in the Lewis-acidity of the [L2ZnII]2+ cores. Interestingly, RSNO and NO generation from the reactions of [L2ZnII(κ2-nitrite)](ClO4) with 4-tert-butylbenzylthiol ( t BuBnSH) exhibits that the [(Me2PzQu)2ZnII]2+ core is the most efficient in promoting nitrite-thiol interactions due to the ease of available hemilabile coordination sites at the Lewis acidic [ZnII]. Detailed UV-vis studies in tandem with computational investigation, for the first time, provide an unambiguous demonstration of the nitrous acid (HNO2) intermediate generated through an intramolecular proton-transfer from thiol to nitrite at zinc(ii).

Copper sulfide mineral performs non-enzymatic anaerobic ammonium oxidation through a hydrazine intermediate

Anaerobic ammonium oxidation (anammox)-the biological process that activates ammonium with nitrite-is responsible for a significant fraction of N2 production in marine environments. Despite decades of biochemical research, however, no synthetic models capable of anammox have been identified. Here we report that a copper sulfide mineral replicates the entire biological anammox pathway catalysed by three metalloenzymes. We identified a copper-nitrosonium {CuNO}10 complex, formed by nitrite reduction, as the oxidant for ammonium oxidation that leads to heterolytic N-N bond formation from nitrite and ammonium. Similar to the biological process, N2 production was mediated by the highly reactive intermediate hydrazine, one of the most potent reductants in nature. We also found another pathway involving N-N bond heterocoupling for the formation of hybrid N2O, a potent greenhouse gas with a unique isotope composition. Our study represents a rare example of non-enzymatic anammox reaction that interconnects six redox states in the abiotic nitrogen cycle.

Crystal Structure and Electron Configuration of {MNO}8 Heme Complexes

Although research on nitrosyl (NO) heme complexes and their one-electron reduced form, nitroxyl (or nitroxyl anion, NO-) derivatives, has been going on for decades, there are still disagreements about the electrical configuration of nitroxyl complexes, and the majority of the work on this topic is based on theoretical calculations. Following the initial nitroxyl iron porphyrin crystal structure, we present two further polymorphic forms of [CoCp2][Fe(TFPPBr8)(NO)]. Using the same completely halogenated porphyrin ligand, we also present two polymorphic forms of nitrosyl cobalt(II) complexes, which are another sort of {MNO}8 structure. In addition to the EXANES and EPR studies of these {FeNO}7 and {CoNO}8 complexes, the {FeNO}8 [CoCp2][Fe(TFPPBr8)(NO)] complex is also investigated by temperature-dependent Mössbauer experiments for the first time with the {FeNO}7 precursor as a control sample. The analysis of the Mössbauer and crystal structural parameters between these two types of {MNO}8 (M = Fe or Co) species and previously reported analogous ones allow us to conclude that the electronic configuration of [Fe(TFPPBr8)(NO)]- is best described as an intermediate between low-spin Fe(II)-NO- and Fe(I)-NO•.

Vibrational analyses of the reaction of oxymyoglobin with NO using a photolabile caged NO donor at cryogenic temperatures

The NO dioxygenation reaction catalyzed by heme-containing globin proteins is a crucial aerobic detoxification pathway. Accordingly, the second order reaction of NO with oxymyoglobin and oxyhemoglobin has been the focus of a large number of kinetic and spectroscopic studies. Stopped-flow and rapid-freeze-quench (RFQ) measurements have provided evidence for the formation of a Fe(III)-nitrato complex with millisecond lifetime prior to release of the nitrate product, but the temporal resolution of these techniques is insufficient for the characterization of precursor species. Most mechanistic models assume the formation of an initial Fe(III)-peroxynitrite species prior to homolytic cleavage of the OO bond and recombination of the resulting NO2 and Fe(IV)=O species. Here we report vibrational spectroscopy measurements for the reaction of oxymyoglobin with a photolabile caged NO donor at cryogenic temperatures. We show that this approach offers efficient formation and trapping of the Fe(III)-nitrato, enzyme-product, complex at 180 K. Resonance Raman spectra of the Fe(III)-nitrato complex trapped via RFQ in the liquid phase and photolabile NO release at cryogenic temperatures are indistinguishable, demonstrating the complementarity of these approaches. Caged NO is released by irradiation <180 K but diffusion into the heme pocket is fully inhibited. Our data provide no evidence for Fe(III)-peroxynitrite of Fe(IV)=O species, supporting low activation energies for the NO to nitrate conversion at the oxymyoglobin reaction site. Photorelease of NO at cryogenic temperatures allows monitoring of the reaction by transmittance FTIR which provides valuable quantitative information and promising prospects for the detection of protein sidechain reorganization events in NO-reacting metalloenzymes.

Vibrational properties of heme-nitrosoalkane complexes in comparison with those of their HNO analogs, and reactivity studies towards nitric oxide and Lewis acids

C-Nitroso compounds (RNO, R = alkyl and aryl) are byproducts of drug metabolism and bind to heme proteins, and their heme-RNO adducts are isoelectronic to ferrous nitroxyl (NO-/HNO) complexes. Importantly, heme-HNO compounds are key intermediates in the reduction of NO to N2O and nitrite to ammonium in the nitrogen cycle. Ferrous heme-RNO complexes act as stable analogs of these species, potentially allowing for the investigation of the vibrational and electronic properties of unstable heme-HNO intermediates. In this paper, a series of six-coordinate ferrous heme-RNO complexes (where R = iPr and Ph) were prepared using the TPP2- and 3,5-Me-BAFP2- co-ligands, and tetrahydrofuran, pyridine, and 1-methylimidazole as the axial ligands (bound trans to RNO). These complexes were characterized using different spectroscopic methods and X-ray crystallography. The complex [Fe(TPP)(THF)(iPrNO)] was further utilized for nuclear resonance vibrational spectroscopy (NRVS), allowing for the detailed assignment of the Fe-N(R)O vibrations of a heme-RNO complex for the first time. The vibrational properties of these species were then correlated with those of their HNO analogs, using DFT calculations. Our studies support previous findings that RNO ligands in ferrous heme complexes do not elicit a significant trans effect. In addition, the complexes are air-stable, and do not show any reactivity of their RNO ligands towards NO. So although ferrous heme-RNO complexes are suitable structural and electronic models for their HNO analogs, they are unsuitable to model the reactivity of heme-HNO complexes. We further investigated the reaction of our heme-RNO complexes with different Lewis acids. Here, [Fe(TPP)(THF)(iPrNO)] was found to be unreactive towards Lewis acids. In contrast, [Fe(3,5-Me-BAFP)(iPrNO)2] is reactive towards all of the Lewis acids investigated here, but in most cases the iron center is simply oxidized, resulting in the loss of the iPrNO ligand. In the case of the Lewis acid B2(pin)2, the reduced product [Fe(3,5-Me-BAFP)(iPrNH2)(iPrNO)] was identified by X-ray crystallography.

Gas Therapy: Generating, Delivery, and Biomedical Applications

Oxygen (O2), nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S), and hydrogen (H2) with direct effects, and carbon dioxide (CO2) with complementary effects on the condition of various diseases are known as therapeutic gases. The targeted delivery and in situ generation of these therapeutic gases with controllable release at the site of disease has attracted attention to avoid the risk of gas poisoning and improve their performance in treating various diseases such as cancer therapy, cardiovascular therapy, bone tissue engineering, and wound healing. Stimuli-responsive gas-generating sources and delivery systems based on biomaterials that enable on-demand and controllable release are promising approaches for precise gas therapy. This work highlights current advances in the design and development of new approaches and systems to generate and deliver therapeutic gases at the site of disease with on-demand release behavior. The performance of the delivered gases in various biomedical applications is then discussed.

Reaction Discovery Using Spectroscopic Insights from an Enzymatic C-H Amination Intermediate

Engineered hemoproteins can selectively incorporate nitrogen from nitrene precursors like hydroxylamine, O-substituted hydroxylamines, and organic azides into organic molecules. Although iron-nitrenoids are often invoked as the reactive intermediates in these reactions, their innate reactivity and transient nature have made their characterization challenging. Here we characterize an iron-nitrosyl intermediate generated from NH2OH within a protoglobin active site that can undergo nitrogen-group transfer catalysis, using UV-vis, electron paramagnetic resonance (EPR) spectroscopy, and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) techniques. The mechanistic insights gained led to the discovery of aminating reagents─nitrite (NO2-), nitric oxide (NO), and nitroxyl (HNO)─that are new to both nature and synthetic chemistry. Based on the findings, we propose a catalytic cycle for C-H amination inspired by the nitrite reductase pathway. This study highlights the potential of engineered hemoproteins to access natural nitrogen sources for sustainable chemical synthesis and offers a new perspective on the use of biological nitrogen cycle intermediates in biocatalysis.

Recent mechanistic developments for cytochrome c nitrite reductase, the key enzyme in the dissimilatory nitrate reduction to ammonium pathway

Cytochrome c nitrite reductase, NrfA, is a soluble, periplasmic pentaheme cytochrome responsible for the reduction of nitrite to ammonium in the Dissimilatory Nitrate Reduction to Ammonium (DNRA) pathway, a vital reaction in the global nitrogen cycle. NrfA catalyzes this six-electron and eight-proton reduction of nitrite at a single active site with the help of its quinol oxidase partners. In this review, we summarize the latest progress in elucidating the reaction mechanism of ammonia production, including new findings about the active site architecture of NrfA, as well as recent results that elucidate electron transfer and storage in the pentaheme scaffold of this enzyme.

Nanocatalytic NO gas therapy against orthotopic oral squamous cell carcinoma by single iron atomic nanocatalysts

Oral squamous cell carcinoma (OSCC) has been being one of the most malignant carcinomas featuring high metastatic and recurrence rates. The current OSCC treatment modalities in clinics severely deteriorate the quality of life of patients due to the impaired oral and maxillofacial functions. In the present work, we have engineered the single-atom Fe nanocatalysts (SAF NCs) with a NO donor (S-nitrosothiol, SNO) via surface modification to achieve synergistic nanocatalytic NO gas therapy against orthotopic OSCC. Upon near-infrared laser irradiation, the photonic hyperthermia could effectively augment the heterogeneous Fenton catalytic activity, meanwhile trigger the thermal decomposition of the engineered NO donor, thus producing toxic hydroxyl radicals (•OH) and antitumor therapeutic NO gas at tumor lesion simultaneously, and consequently inducing the apoptotic cell death of tumors via mitochondrial apoptosis pathway. This therapeutic paradigm presents an effective local OSCC therapeutics in a synergistic manner based on the nanocatalytic NO gas therapy, providing a promising antitumor modality with high biocompatibility.

Diversity and evolution of nitric oxide reduction in bacteria and archaea

Nitrous oxide is a potent greenhouse gas whose production is catalyzed by nitric oxide reductase (NOR) members of the heme-copper oxidoreductase (HCO) enzyme superfamily. We identified several previously uncharacterized HCO families, four of which (eNOR, sNOR, gNOR, and nNOR) appear to perform NO reduction. These families have novel active-site structures and several have conserved proton channels, suggesting that they might be able to couple NO reduction to energy conservation. We isolated and biochemically characterized a member of the eNOR family from the bacterium Rhodothermus marinus and found that it performs NO reduction. These recently identified NORs exhibited broad phylogenetic and environmental distributions, greatly expanding the diversity of microbes in nature capable of NO reduction. Phylogenetic analyses further demonstrated that NORs evolved multiple times independently from oxygen reductases, supporting the view that complete denitrification evolved after aerobic respiration.

Effect of Nitrosyl Iron Complex with 3,4-Dichlorothiophenolyls on the Level of Cyclic Nucleotide In Vitro

The effect of a promising NO donor, a binuclear nitrosyl iron complex (NIC) with 3,4-dichlorothiophenolyls [Fe2(SC6H3Cl2)2(NO)4], on the adenylate cyclase and soluble guanylate cyclase enzymatic systems was studied. In in vitro experiments, this complex increased the concentration of important secondary messengers, such as cAMP and cGMP. An increase of their level by 2.4 and 4.5 times, respectively, was detected at NIC concentration of 0.1 mM. The ligand of the complex, 3,4-dichlorothiophenol, produced a less pronounced effect on adenylate cyclase. It was shown that the effect of this complex on the activity of soluble guanylate cyclase was comparable to the effect of anionic nitrosyl complex with thiosulfate ligands that exhibits vasodilating and cardioprotective properties.

Reduction of molecular oxygen in flavodiiron proteins - Catalytic mechanism and comparison to heme-copper oxidases

The family of flavodiiron proteins (FDPs) plays an important role in the scavenging and detoxification of both molecular oxygen and nitric oxide. Using electrons from a flavin mononucleotide cofactor molecular oxygen is reduced to water and nitric oxide is reduced to nitrous oxide and water. While the mechanism for NO reduction in FDPs has been studied extensively, there is very little information available about O2 reduction. Here we use hybrid density functional theory (DFT) to study the mechanism for O2 reduction in FDPs. An important finding is that a proton coupled reduction is needed after the O2 molecule has bound to the diferrous diiron active site and before the OO bond can be cleaved. This is in contrast to the mechanism for NO reduction, where both NN bond formation and NO bond cleavage occurs from the same starting structure without any further reduction, according to both experimental and computational results. This computational result for the O2 reduction mechanism should be possible to evaluate experimentally. Another difference between the two substrates is that the actual OO bond cleavage barrier is low, and not involved in rate-limiting the reduction process, while the barrier connected with bond cleavage/formation in the NO reduction process is of similar height as the rate-limiting steps. We suggest that these results may be part of the explanation for the generally higher activity for O2 reduction as compared to NO reduction in most FDPs. Comparisons are also made to the O2 reduction reaction in the family of heme‑copper oxidases.

Iron(I) Complex Bearing an Open-Shell Diazenido Ligand

Low-valent transition-metal diazenido species are important intermediates in transition-metal-mediated dinitrogen reduction reactions. Isolable complexes of the type unanimously feature closed-shell diazenido ligands. Those bearing open-shell diazenido ligands have remained elusive. Herein, we report the synthesis, characterization, and reactivity of a d7 iron(I) complex featuring an open-shell silyldiazenido ligand, [(ICy)Fe(NNSiiPr3)(η2:η2-dvtms)] (1, ICy = 1,3-dicyclohexylimidazole-2-ylidene, dvtms = divinyltetramethyldisiloxane). Complex 1 is prepared in good yield by silylation of the iron(-I)-N2 complex [K(18-crown-6)][(ICy)Fe(N2)(η2:η2-dvtms)] with iPr3SiOTf and has been fully characterized by various spectroscopic methods. Theoretical studies, in combination with characterization data, established an S = 1/2 ground spin-state for 1 that can best be described as a quartet iron(I) center featuring an antiferromagnetically coupled triplet silyldiazenido ligand. The diazenido and alkene ligands in 1 are labile, as indicated by the facile disproportionation reaction of 1 at ambient temperature to transform into the iron(II) bis(diazenido) species [(ICy)(NNSiiPr3)2Fe(dvtms)Fe(NNSiiPr3)2(ICy)] (2) and the iron(0) species [(ICy)Fe(η2:η2-dvtms)] and also the alkene-exchange reaction of 1 with PhCH═CHBC8H14 to form [(ICy)Fe(NNSiiPr3)(η2-trans-PhCH═CHBC8H14)] (3). Complex 1 is light-sensitive. Upon photolysis, it undergoes a SiiPr3 radical-transfer reaction to yield [(ICy)Fe(σ:η2-MeCHSiMe2OSiMe2CH═CHSiiPr3)] (4) and N2. The reactions of 1 with the trityl radical and organic bromides yield iron(II) complexes, which indicates its reducing nature. Moreover, 1 is a weak hydrogen-atom abstractor, as indicated by its inertness toward HSi(SiMe3)3 and cyclohexa-1,4-diene and the low calculated N-H bond dissociation energy (48 kcal/mol) of its corresponding iron(II) iso-hydrazenido species.

Reactivity of Thiolate and Hydrosulfide with a Mononuclear {FeNO}7 Complex Featuring a Very High N-O Stretching Frequency

Synthesis, characterization, electronic structure, and redox reactions of a mononuclear {FeNO}7 complex with a very high N-O stretching frequency in solution are presented. Nitrosylation of [(LKP)Fe(DMF)]2+ (1) (LKP = tris((1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methyl)amine) produced a five-coordinate {FeNO}7 complex, [(LKP)Fe(NO)]2+ (2). While complex 2 could accommodate an additional water molecule to generate a six-coordinate {FeNO}7 complex, [(LKP)Fe(NO)(H2O)]2+ (3), the coordinated H2O in 3 dissociates to generate 2 in solution. The molecular structure of 2 features a nearly linear Fe-N-O unit with an Fe-N distance of 1.744(4) Å, N-O distance of 1.162(5) Å, and Collapse

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Affiliation(s)

Soumik Karmakar School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Suman Patra School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Koushik Pramanik School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Amit Adhikary Department of Chemistry, Technology Campus, University of Calcutta, JD Block, Sector III, Salt Lake, Kolkata 700098, India
Abhishek Dey School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Amit Majumdar School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India

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Estimation of Electrostatic Interaction Energies on a Trapped-Ion Quantum Computer

We present the first hardware implementation of electrostatic interaction energies by using a trapped-ion quantum computer. As test system for our computation, we focus on the reduction of NO to N2O catalyzed by a nitric oxide reductase (NOR). The quantum computer is used to generate an approximate ground state within the NOR active space. To efficiently measure the necessary one-particle density matrices, we incorporate fermionic basis rotations into the quantum circuit without extending the circuit length, laying the groundwork for further efficient measurement routines using factorizations. Measurements in the computational basis are then used as inputs for computing the electrostatic interaction energies on a classical computer. Our experimental results strongly agree with classical noise-less simulations of the same circuits, finding electrostatic interaction energies within chemical accuracy despite hardware noise. This work shows that algorithms tailored to specific observables of interest, such as interaction energies, may require significantly fewer quantum resources than individual ground state energies would require in the straightforward supermolecular approach.

Mechanistic and Kinetic Insights into Cellular Uptake of Biomimetic Dinitrosyl Iron Complexes and Intracellular Delivery of NO for Activation of Cytoprotective HO-1

Dinitrosyl iron unit (DNIU), [Fe(NO)2], is a natural metallocofactor for biological storage, delivery, and metabolism of nitric oxide (NO). In the attempt to gain a biomimetic insight into the natural DNIU under biological system, in this study, synthetic dinitrosyl iron complexes (DNICs) [(NO)2Fe(μ-SCH2CH2COOH)2Fe(NO)2] (DNIC-COOH) and [(NO)2Fe(μ-SCH2CH2COOCH3)2Fe(NO)2] (DNIC-COOMe) were employed to investigate the structure-reactivity relationship of mechanism and kinetics for cellular uptake of DNICs, intracellular delivery of NO, and activation of cytoprotective heme oxygenase (HO)-1. After rapid cellular uptake of dinuclear DNIC-COOMe through a thiol-mediated pathway (tmax = 0.5 h), intracellular assembly of mononuclear DNIC [(NO)2Fe(SR)(SCys)]n-/[(NO)2Fe(SR)(SCys-protein)]n- occurred, followed by O2-induced release of free NO (tmax = 1-2 h) or direct transfer of NO to soluble guanylate cyclase, which triggered the downstream HO-1. In contrast, steady kinetics for cellular uptake of DNIC-COOH via endocytosis (tmax = 2-8 h) and for intracellular release of NO (tmax = 4-6 h) reflected on the elevated activation of cytoprotective HO-1 (∼50-150-fold change at t = 3-10 h) and on the improved survival of DNIC-COOH-primed mesenchymal stem cell (MSC)/human corneal endothelial cell (HCEC) under stressed conditions. Consequently, this study unravels the bridging thiolate ligands in dinuclear DNIC-COOH/DNIC-COOMe as a switch to control the mechanism, kinetics, and efficacy for cellular uptake of DNICs, intracellular delivery of NO, and activation of cytoprotective HO-1, which poses an implication on enhanced survival of postengrafted MSC for advancing the MSC-based regenerative medicine.

A Crucial Role of Proteolysis in the Formation of Intracellular Dinitrosyl Iron Complexes

Dinitrosyl iron complexes (DNICs) stabilize nitric oxide in cells and tissues and constitute an important form of its storage and transportation. DNICs may comprise low-molecular-weight ligands, e.g., thiols, imidazole groups in chemical compounds with low molecular weight (LMWDNICs), or high-molecular-weight ligands, e.g., peptides or proteins (HMWDNICs). The aim of this study was to investigate the role of low- and high-molecular-weight ligands in DNIC formation. Lysosomal and proteasomal proteolysis was inhibited by specific inhibitors. Experiments were conducted on human erythroid K562 cells and on K562 cells overexpressing a heavy chain of ferritin. Cell cultures were treated with •NO donor. DNIC formation was monitored by electron paramagnetic resonance. Pretreatment of cells with proteolysis inhibitors diminished the intensity and changed the shape of the DNIC-specific EPR signal in a treatment time-dependent manner. The level of DNIC formation was significantly influenced by the presence of protein degradation products. Interestingly, formation of HMWDNICs depended on the availability of LMWDNICs. The extent of glutathione involvement in the in vivo formation of DNICs is minor yet noticeable, aligning with our prior research findings.

Exploring the carbonic anhydrase-mimetic [(PMDTA)2ZnII2(OH-)2]2+ for nitric oxide monooxygenation

In biology, nitrite (NO2-) serves as a storage pool of nitric oxide (NO); however, the formation of NO2- from NO is still under investigation. Here, we report the NO monooxygenation (NOM) reaction of a ZnII-hydroxide complex (1), producing a ZnII-nitrito complex {2, (ZnII-NO2-)} + H2.

Development of (NO)Fe(N2S2) as a Metallodithiolate Spin Probe Ligand: A Case Study Approach

ConspectusThe ubiquity of sulfur-metal connections in nature inspires the design of bi- and multimetallic systems in synthetic inorganic chemistry. Common motifs for biocatalysts developed in evolutionary biology include the placement of metals in close proximity with flexible sulfur bridges as well as the presence of π-acidic/delocalizing ligands. This Account will delve into the development of a (NO)Fe(N2S2) metallodithiolate ligand that harnesses these principles. The Fe(NO) unit is the centroid of a N2S2 donor field, which as a whole is capable of serving as a redox-active, bidentate S-donor ligand. Its paramagnetism as well as the ν(NO) vibrational monitor can be exploited in the development of new classes of heterobimetallic complexes. We offer four examples in which the unpaired electron on the {Fe(NO)}7 unit is spin-paired with adjacent paramagnets in proximal and distal positions.First, the exceptional stability of the (NO)Fe(N2S2)-Fe(NO)2 platform, which permits its isolation and structural characterization at three distinct redox levels, is linked to the charge delocalization occurring on both the Fe(NO) and the Fe(NO)2 supports. This accommodates the formation of a rare nonheme {Fe(NO)}8 triplet state, with a linear configuration. A subsequent FeNi complex, featuring redox-active ligands on both metals (NO on iron and dithiolene on nickel), displayed unexpected physical properties. Our research showed good reversibility in two redox processes, allowing isolation in reduced and oxidized forms. Various spectroscopic and crystallographic analyses confirmed these states, and Mössbauer data supported the redox change at the iron site upon reduction. Oxidation of the complex produced a dimeric dication, revealing an intriguing magnetic behavior. The monomer appears as a spin-coupled diradical between {Fe(NO)}7 and the nickel dithiolene monoradical, while dimerization couples the latter radical units via a Ni2S2 rhomb. Magnetic data (SQUID) on the dimer dication found a singlet ground state with a thermally accessible triplet state that is responsible for magnetism. A theoretical model built on an H4 chain explains this unexpected ferromagnetic low-energy triplet state arising from the antiferromagnetic coupling of a four-radical molecular conglomerate. For comparison, two (NO)Fe(N2S2) were connected through diamagnetic group 10 cations producing diradical trimetallic complexes. Antiferromagnetic coupling is observed between {Fe(NO)}7 units, with exchange coupling constants (J) of -3, -23, and -124 cm-1 for NiII, PdII, and PtII, respectively. This trend is explained by the enhanced covalency and polarizability of sulfur-dense metallodithiolate ligands. A central paramagnetic trans-Cr(NO)(MeCN) receiver unit core results in a cissoid structural topology, influenced by the stereoactivity of the lone pair(s) on the sulfur donors. This {Cr(NO)}5 radical bridge, unlike all previous cases, finds the coupling between the distal Fe(NO) radicals to be ferromagnetic (J = 24 cm-1).The stability and predictability of this S = 1/2 moiety and the steric/electronic properties of the bridging thiolate sulfurs suggest it to be a likely candidate for the development of novel molecular (magnetic) compounds and possibly materials. The role of synthetic inorganic chemistry in designing synthons that permit connections of the (NO)Fe(N2S2) metalloligand is highlighted as well as the properties of the heterobi- and polymetallic complexes derived therefrom.

Recent advances on the development of NO-releasing molecules (NORMs) for biomedical applications

Nitric oxide (NO) is an important biological messenger as well as a signaling molecule that participates in a broad range of physiological events and therapeutic applications in biological systems. However, due to its very short half-life in physiological conditions, its therapeutic applications are restricted. Efforts have been made to develop an enormous number of NO-releasing molecules (NORMs) and motifs for NO delivery to the target tissues. These NORMs involve organic nitrate, nitrite, nitro compounds, transition metal nitrosyls, and several nanomaterials. The controlled release of NO from these NORMs to the specific site requires several external stimuli like light, sound, pH, heat, enzyme, etc. Herein, we have provided a comprehensive review of the biochemistry of nitric oxide, recent advancements in NO-releasing materials with the appropriate stimuli of NO release, and their biomedical applications in cancer and other disease control.