Lewis Acid Activation of the Ferrous Heme-NO Fragment toward the N-N Coupling Reaction with NO To Generate N2O

Alternative Metal with an Alternative Mechanism for Metalloporphyrin-Enabled NO Reduction to N2O: A Combined Computational and Experimental Investigation of NO Reduction by Cr Porphyrin with Lewis Acid

Nitric oxide (NO) is an important molecule that regulates many physiological processes in humans and plants and contributes to the formation of greenhouse gases. Bacterial NO reductases utilize a di-Fe heme/nonheme active site to couple two NOs to generate nitrous oxide (N2O) via a two-electron mechanism. Here, we report a previously unexplored Cr porphyrin NO complex with a Lewis acid (LA) BF3 for the NO reduction reaction. Density functional theory calculations were first employed to reveal its reaction mechanism with a reasonable barrier for experimental realization. Subsequent experimental synthesis work confirms this reactivity and reports the first nitrosyl Cr porphyrin X-ray crystal structure. Theoretical analysis uncovered a distinctive reaction feature for the Cr system compared to Fe and Co porphyrins: the electron transfer from the metal to the bound NO occurs before LA binding. A comparative study of the NO coupling mechanisms with the three representative metals suggests that the metal reduction potential should be finely tuned, as found in previous studies of NOR enzymatic systems. Overall, this study offers new theoretical and experimental insights to further facilitate the development of alternative NO reduction compounds with biological, environmental, and industrial applications.

One-Electron NO to N2O Pathways via Heme Models and Lewis Acid: Metal Effects and Differences from the Enzymatic Reaction

Some pathogens use heme-containing nitric oxide reductases (NORs) to reduce NO to N2O as their defense mechanism to detoxify NO and reduce nitrosative stress. This reduction is also significant in the global N cycle. Our previous experimental work showed that Fe and Co porphyrin NO complexes can couple with external NO to form N2O when activated by the Lewis acid BF3. A key difference from conventional two-electron enzymatic reaction is that one electron is sufficient. However, a complete understanding of the entire reaction pathways and the more favorable reactivity for Fe remains unknown. Here, we present a quantum chemical study to provide such information. Our results confirmed Fe's higher experimental reactivity, showing advantages in all steps of the reaction pathway: easier metal oxidation for NO reduction and N-O cleavage as well as a larger size to expedite the N/O coordination mode transition. The Co system, with a similar product energy as the enzyme, shows potential for further development in catalytic NO coupling. This work also offers the first evidence that this new one-electron NO reduction is both kinetically competitive and thermodynamically more favorable than the native pathway, supporting future initiatives in optimizing NO reduction agents in biology, environment, and industry.

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.

Uncovering Nitrosyl Reactivity at N-Heterocyclic Carbene Center

N-heterocyclic carbenes (NHCs) have garnered much attention due to their unique properties, such as strong σ-donating and π-accepting abilities, as well as their transition-metal-like reactivity toward small molecules. In 2015, we discovered that NHCs can react with nitric oxide (NO) gas to form radical adducts that resemble transition metal nitrosyl complexes. To elucidate the analogy between NHC and transition metal NO adducts, here we have undertaken a systematic investigation of the electron- and proton-transfer chemistry of [NHC-NO]⋅ (N-heterocyclic carbene nitric oxide radical) compounds. We have accessed a suite of compounds, comprised of [NHC-NO]+ , [NHC-NO]- , [NHC-NOH]0 , and [NHC-NHOH]+ species. In particular, [NHC-NO]- was isolated as potassium and lithium ion adducts. Most interestingly, a monomeric potassium [NHC-NO]- compound was isolated with the assistance of 18-crown-6, which is the first instance of a monomeric alkali N-oxyl compound to the best of our knowledge. Our results demonstrate that [NHC-NO]⋅ exhibits redox behavior broadly similar to metal nitrosyl complexes, which opens up more possibilities for utilizing NHCs to build on the known reactivity of metal complexes.

Direct Reduction of NO to N2O by a Mononuclear Nonheme Thiolate Ligated Iron(II) Complex via Formation of a Metastable {FeNO}7 Complex

Addition of NO to a nonheme dithiolate-ligated iron(II) complex, FeII(Me3TACN)(S2SiMe2) (1), results in the generation of N2O. Low-temperature spectroscopic studies reveal a metastable six-coordinate {FeNO}7 intermediate (S = 3/2) that was trapped at -135 °C and was characterized by low-temperature UV-vis, resonance Raman, EPR, Mössbauer, XAS, and DFT studies. Thermal decay of the {FeNO}7 species leads to the evolution of N2O, providing a rare example of a mononuclear thiolate-ligated {FeNO}7 that mediates NO reduction to N2O without the requirement of any exogenous electron or proton sources.

A Borane Lewis Acid in the Secondary Coordination Sphere of a Ni(II) Imido Imparts Distinct C-H Activation Selectivity

Two borane-functionalized bidentate phosphine ligands that vary in tether length have been prepared to examine cooperative metal-substrate interactions. Ni(0) complexes react with aryl azides at low temperatures to form structurally unusual κ2-(N,N)-N3Ar adducts. Warming these adducts affords products of N2 extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN3) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp2 C-H bonds of arenes, even in the presence of benzylic C-H sites. Combined experimental and computational mechanistic studies demonstrate that the unique reactivity is a consequence of Lewis-acid-induced polarization of the Ni-NR bond, potentially providing a synthetic strategy for chemoselective reaction engineering.

NO Coupling at Copper to cis-Hyponitrite: N2O Formation via Protonation and H-Atom Transfer

Copper nitrite reductases (CuNIRs) convert NO2- to NO as well as NO to N2O under high NO flux at a mononuclear type 2 Cu center. While model complexes illustrate N-N coupling from NO that results in symmetric trans-hyponitrite [CuII]-ONNO-[CuII] complexes, we report NO assembly at a single Cu site in the presence of an external reductant Cp*2M (M = Co, Fe) to give the first copper cis-hyponitrites [Cp*2M]{[CuII](κ2-O2N2)[CuI]}. Importantly, the κ1-N-bound [CuI] fragment may be easily removed by the addition of mild Lewis bases such as CNAr or pyridine to form the spectroscopically similar anion {[CuII](κ2-O2N2)}-. The addition of electrophiles such as H+ to these anionic copper(II) cis-hyponitrites leads to N2O generation with the formation of the dicopper(II)-bis-μ-hydroxide [CuII]2(μ-OH)2. One-electron oxidation of the {[CuII](κ2-O2N2)}- core turns on H-atom transfer reactivity, enabling the oxidation of 9,10-dihydroanthracene to anthracene with concomitant formation of N2O and [CuII]2(μ-OH)2. These studies illustrate both the reductive coupling of NO at a single copper center and a way to harness the strong oxidizing power of nitric oxide via the neutral cis-hyponitrite [Cu](κ2-O2N2).

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

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

ZrO2/CeO2/polyacrylic acid nanocomposites with alkaline phosphatase-like activity for sensing

In the present work, we synthesized ZrO₂/CeO₂/polyacrylic acid (PAA) nanocomposites (nanozyme) with phosphatase-like activity. ZrO₂ evenly distributed in CeO₂ nanorods considered as lewis acids to enhance the phosphatase-like activity of CeO₂ nanorods. Furthermore, PAA was used to coat ZrO₂/CeO₂/ nanorods and improve the dispersion, stability and robustness. The ZrO₂/CeO₂/PAA nanocomposites had 100% enhanced phosphatase-like activity compared with CeO₂ nanorods and excellent adaptability in a wide pH range from 4.0 to 12.0. ZrO₂/CeO₂/PAA nanocomposites could hydrolyze methyl parathion (MP) to p-nitrophenol (p-NP) with bright yellow color for colorimetric detection. The developed colorimetric detection system showed a linear response from 7.60 × 10^-11-7.60 × 10^-8 M with a detection limit of 0.021 nM and was successfully applied for the determination of MP in corn samples.

NO Reduction to N2O Triggered by a Dinuclear Dinitrosyl Iron Complex via the Associated Pathways of Hyponitrite Formation and NO Disproportionation

In spite of the comprehensive study of the metal-mediated conversion of NO to N2O disclosing the conceivable processes/mechanism in biological and biomimetic studies, in this study, the synthesis cycles and mechanism of NO reduction to N2O triggered by the electronically localized dinuclear {Fe(NO)2}10-{Fe(NO)2}9 dinitrosyl iron complex (DNIC) [Fe(NO)2(μ-bdmap)Fe(NO)2(THF)] (1) (bdmap = 1,3- bis(dimethylamino)-2-propanolate) were investigated in detail. Reductive conversion of NO to N2O triggered by complex 1 in the presence of exogenous ·NO occurs via the simultaneous formation of hyponitrite-bound {[Fe2(NO)4(μ-bdmap)]2(κ4-N2O2)} (2) and [NO2]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO2)] (3) (NO disproportionation yielding N2O and complex 3). EPR/IR spectra, single-crystal X-ray diffraction, and the electrochemical study uncover the reversible redox transformation of {Fe(NO)2}9-{Fe(NO)2}9 [Fe2(NO)4(μ-bdmap)(μ-OC4H8)]+ (7) ↔ {Fe(NO)2}10-{Fe(NO)2}9 1 ↔ {Fe(NO)2}10-{Fe(NO)2}10 [Fe(NO)2(μ-bdmap)Fe(NO)2]- (6) and characterize the formation of complex 1. Also, the synthesis study and DFT computation feature the detailed mechanism of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 DNIC 1 reducing NO to N2O via the associated hyponitrite-formation and NO-disproportionation pathways. Presumably, the THF-bound {Fe(NO)2}9 unit of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 complex 1 served as an electron buffering reservoir for accommodating electron redistribution, and the {Fe(NO)2}10 unit of complex 1 acted as an electron-transfer channel to drive exogeneous ·NO coordination to yield proposed relay intermediate κ2-N,O-[NO]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO)] (A) for NO reduction to N2O.

A Nonheme Mononuclear {FeNO}7 Complex that Produces N2 O in the Absence of an Exogenous Reductant

A new nonheme iron(II) complex, Fe^II (Me₃ TACN)((OSi^Ph2 )₂ O) (1), is reported. Reaction of 1 with NO_(g) gives a stable mononitrosyl complex Fe(NO)(Me₃ TACN)((OSi^Ph2 )₂ O) (2), which was characterized by Mössbauer (δ=0.52 mm s^-1 , |ΔE_Q |=0.80 mm s^-1 ), EPR (S=3/2), resonance Raman (RR) and Fe K-edge X-ray absorption spectroscopies. The data show that 2 is an {FeNO}⁷ complex with an S=3/2 spin ground state. The RR spectrum (λ_exc =458 nm) of 2 combined with isotopic labeling (¹⁵ N, ¹⁸ O) reveals ν(N-O)=1680 cm^-1 , which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO)=1681 cm^-1 ). Complex 2 reacts rapidly with H₂ O in THF to produce the N-N coupled product N₂ O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N₂ O in the absence of an exogenous reductant.

Activation of O2 and NO in heme-copper oxidases - mechanistic insights from computational modelling

Heme-copper oxidases are transmembrane enzymes involved in aerobic and anaerobic respiration. The largest subgroup contains the cytochrome c oxidases (CcO), which reduce molecular oxygen to water. A significant part of the free energy released in this exergonic process is conserved as an electrochemical gradient across the membrane, via two processes, electrogenic chemistry and proton pumping. A deviant subgroup is the cytochrome c dependent NO reductases (cNOR), which reduce nitric oxide to nitrous oxide and water. This is also an exergonic reaction, but in this case none of the released free energy is conserved. Computational studies applying hybrid density functional theory to cluster models of the bimetallic active sites in the heme-copper oxidases are reviewed. To obtain a reliable description of the reaction mechanisms, energy profiles of the entire catalytic cycles, including the reduction steps have to be constructed. This requires a careful combination of computational results with certain experimental data. Computational studies have elucidated mechanistic details of the chemical parts of the reactions, involving cleavage and formation of covalent bonds, which have not been obtainable from pure experimental investigations. Important insights regarding the mechanisms of energy conservation have also been gained. The computational studies show that the reduction potentials of the active site cofactors in the CcOs are large enough to afford electrogenic chemistry and proton pumping, i.e. efficient energy conservation. These results solve a conflict between different types of experimental data. A mechanism for the proton pumping, involving a specific and crucial role for the active site tyrosine, conserved in all CcOs, is suggested. For the cNORs, the calculations show that the low reduction potentials of the active site cofactors are optimized for fast elimination of the toxic NO molecules. At the same time, the low reduction potentials lead to endergonic reduction steps with high barriers. To prevent even higher barriers, which would lead to a too slow reaction, when the electrochemical gradient across the membrane is present, the chemistry must occur in a non-electrogenic manner. This explains why there is no energy conservation in cNOR.

Spectroscopic Evidence of Hyponitrite Radical Intermediate in NO Disproportionation at a MOF-Supported Mononuclear Copper Site

Dianionic hyponitrite (N₂ O₂ ^2- ) is often proposed, based on model complexes, as the key intermediate in reductive coupling of nitric oxide to nitrous oxide at the bimetallic active sites of heme-copper oxidases and nitric oxide reductases. In this work, we examine the gas-solid reaction of nitric oxide with the metal-organic framework Cu^I -ZrTpmC* with a suite of in situ spectroscopies and density functional theory simulations, and identify an unusual chelating N₂ O₂ ^.- intermediate. These results highlight the advantage provided by site-isolation in metal-organic frameworks (MOFs) for studying important reaction intermediates, and provide a mechanistic scenario compatible with the proposed one-electron couple in these enzymes.

Molecular understanding of heteronuclear active sites in heme-copper oxidases, nitric oxide reductases, and sulfite reductases through biomimetic modelling

Heme-copper oxidases (HCO), nitric oxide reductases (NOR), and sulfite reductases (SiR) catalyze the multi-electron and multi-proton reductions of O2, NO, and SO32-, respectively. Each of these reactions is important to drive cellular energy production through respiratory metabolism and HCO, NOR, and SiR evolved to contain heteronuclear active sites containing heme/copper, heme/nonheme iron, and heme-[4Fe-4S] centers, respectively. The complexity of the structures and reactions of these native enzymes, along with their large sizes and/or membrane associations, make it challenging to fully understand the crucial structural features responsible for the catalytic properties of these active sites. In this review, we summarize progress that has been made to better understand these heteronuclear metalloenzymes at the molecular level though study of the native enzymes along with insights gained from biomimetic models comprising either small molecules or proteins. Further understanding the reaction selectivity of these enzymes is discussed through comparisons of their similar heteronuclear active sites, and we offer outlook for further investigations.

Not Limited to Iron: A Cobalt Heme-NO Model Facilitates N-N Coupling with External NO in the Presence of a Lewis Acid to Generate N2 O

Some bacterial heme proteins catalyze the coupling of two NO molecules to generate N2 O. We previously reported that a heme Fe-NO model engages in this N-N bond-forming reaction with NO. We now demonstrate that (OEP)CoII (NO) similarly reacts with 1 equiv of NO in the presence of the Lewis acids BX3 (X=F, C6 F5 ) to generate N2 O. DFT calculations support retention of the CoII oxidation state for the experimentally observed adduct (OEP)CoII (NO⋅BF3 ), the presumed hyponitrite intermediate (P.+ )CoII (ONNO⋅BF3 ), and the porphyrin π-radical cation by-product of this reaction, and that the π-radical cation formation likely occurs at the hyponitrite stage. In contrast, the Fe analogue undergoes a ferrous-to-ferric oxidation state conversion during this reaction. Our work shows that cobalt hemes are chemically competent to engage in the NO-to-N2 O conversion reaction.

Nitric Oxide Reductase Activity in Heme-Nonheme Binuclear Engineered Myoglobins through a One-Electron Reduction Cycle

Fe_BMbs are structural and functional models of native bacterial nitric oxide reductases (NORs) generated through engineering of myoglobin. These biosynthetic models replicate the heme-nonheme diiron site of NORs and allow substitutions of metal centers and heme cofactors. Here, we provide evidence for multiple NOR turnover in monoformyl-heme-containing Fe_BMb1 proteins loaded with Fe^II, Co^II, or Zn^II metal ions at the Fe_B site (Fe^II/Co^II/Zn^II-Fe_BMb1(MF-heme)). FTIR detection of the ν(NNO) band of N₂O at 2231 cm^-1 provides a direct quantitative measurement of the product in solution. A maximum number of turnover is observed with Fe^II-Fe_BMb1(MF-heme), but the NOR activity is retained when the Fe_B site is loaded with Zn^II. These data support the viability of a one-electron semireduced pathway for the reduction of NO at binuclear centers in reducing conditions.

Surface-Enhanced Infrared Absorption Spectroscopy of Bacterial Nitric Oxide Reductase under Electrochemical Control Using a Vibrational Probe of Carbon Monoxide

Nitric oxide reductases (NORs) reduce nitric oxide to nitrous oxide in the denitrification pathway of the global nitrogen cycle. NORs contain four iron cofactors and the NO reduction occurs at the heme b₃/nonheme Fe_B binuclear active site. The determination of reduction potentials of the iron cofactors will help us elucidate the enzymatic reaction mechanism. However, previous reports on these potentials remain controversial. Herein, we performed electrochemical and surface-enhanced infrared absorption (SEIRA) spectroscopic measurements of Pseudomonas aeruginosa NOR immobilized on gold electrodes. Cyclic voltammograms exhibited two reduction peaks at -0.11 and -0.44 V vs SHE, and a SEIRA spectrum using a vibrational probe of CO showed a characteristic band at 1972 cm^-1 at -0.4 V vs SHE, which was assigned to νCO of heme b₃-CO. Our results suggest that the reduction of heme b₃ initiates the enzymatic NO reduction.

Upon further analysis, neither cytochrome c554 from Nitrosomonas europaea nor its F156A variant display NO reductase activity, though both proteins bind nitric oxide reversibly

A re-investigation of the interaction with NO of the small tetraheme protein cytochrome c₅₅₄ (C₅₅₄) from Nitrosomonas europaea has shown that the 5-coordinate heme II of the two- or four-electron-reduced protein will nitrosylate reversibly. The process is first order in C₅₅₄, first order in NO, and second-order overall. The rate constant for NO binding to the heme is 3000 ± 140 M^-1s^-1, while that for dissociation is 0.034 ± 0.009 s^-1; the degree of protein reduction does not appear to significantly influence the nitrosylation rate. In contrast to a previous report (Upadhyay AK, et al. J Am Chem Soc 128:4330, 2006), this study found no evidence of C₅₅₄-catalyzed NO reduction, either with [Formula: see text] or with [Formula: see text] Some sub-stoichiometric oxidation of the lowest potential heme IV was detected when [Formula: see text] was exposed to an excess of NO, but this is believed to arise from partial intramolecular electron transfer that generates {Fe(NO)}⁸ at heme II. The vacant heme II coordination site of C₅₅₄ is crowded by three non-bonding hydrophobic amino acids. After replacing one of these (Phe156) with the smaller alanine, the nitrosylation rate for F156A^2- and F156A^4- was about 400× faster than for the wild type, though the rate of the reverse denitrosylation process was almost unchanged. Unlike in the wild-type C₅₅₄, the 6-coordinate low-spin hemes of F156A^4- oxidized over the course of several minutes after exposure to NO. Concomitant formation of N₂O could explain this heme oxidation, though alternative explanations are equally plausible given the available data.