Mechanism of nitrogen fixation by nitrogenase: the next stage

Engineering an Ag2Se@PANi core-shell nanozymes - Klebsiella pasteurii hybrid system with enhanced ammonia synthesis

Microbially mediated nitrogen fixation offers a sustainable and eco-friendly alternative to the energy-intensive Haber-Bosch reaction for ammonia production. However, the efficient conversion of atmospheric nitrogen into ammonia via microorganism remains a notable challenge. In this work, we successfully enhanced ammonia production in Klebsiella pasteurii Sb-24 by integrating polyaniline (PANi) coated Ag2Se core-shell nanozymes through a self-assembly process and revealed the underlying mechanisms. The results of this investigation revealed that Ag2Se@PANi nanozymes expedited the logarithmic growth phase of K. pasteurii Sb-24 cells, enhanced nitrogen fixation activity, and effectively reduced reactive oxygen species levels in Sb-24 cells, with more pronounced enhancements observed at 38 °C compared to 28 °C. Upon optimization, the biohybrid system achieved a maximum NH4+ production of 0.48 ± 0.05 μg·mL-1 at 38 °C, surpassing the output of pure bacterium by 333 %. Further exploration into interfacial electron transfer mechanisms uncovered that Ag2Se@PANi nanozymes efficiently facilitated electron transfer into K. pasteurii Sb-24 cells at 38 °C. These findings were corroborated by the upregulation of proteins involved in nitrogen fixation (i.e. Mo-Fe protein) and electron transfer as identified in the proteomic analysis. This study presents an effective strategy to enhancing ammonia production using PANi-based thermoelectric composites and lays a solid foundation for future research into the biomanufacturing using hybrid systems.

Proton-Modulated Nickel Hydride Electrocatalysis for the Hydrogenation of Unsaturated Bonds and Olefin Isomerization

Transition-metal hydrides stand as indispensable intermediates in both energy conversion and organic synthesis. Their electrochemical generation represents a compelling sustainable approach, enabling precise control over the reactivity and expanding the scope of electrocatalytic hydrogenation and isomerization. However, a major challenge in Ni-catalyzed electrochemical hydrogenation is the competing hydrogen evolution reaction (HER), which has led to various innovative strategies aimed at circumventing Ni-H formation. Here, we pursued an alternative approach by designing a bifunctional ligand with a pendant amine moiety to promote Ni-H formation. This design enabled selective (semi)hydrogenation of a diverse range of substrates, including terminal and internal alkynes, alkenes, and aldehydes, achieving an unprecedented substrate scope. Remarkably, we also demonstrated tunable positional selectivity for olefin isomerization by employing different types of proton sources. Our hydrogenation and isomerization method also exhibits excellent functional group tolerance, streamlining access to pharmaceuticals and their derivatives. Computational studies revealed the crucial, noninnocent role of the proton source in modulating metal hydride selectivity, either through hydrogen bonding, direct protonation of the pendant amine, or facilitation of protodemetalation.

Theoretical Study of Imide Formation in Nitrogen Fixation Catalyzed by Molybdenum Complex Bearing PCP-Type Pincer Ligand with Metallocenes

Homogeneous catalysts using a mononuclear molybdenum nitride (Mo≡N) complex bearing PCP-type pincer ligands allow nitrogen fixation under very mild conditions. The catalytic cycle involves three hydrogenation processes yielding an Mo-ammine complex [MoI(NH3)(PCP)] from the Mo-nitride complex [MoI(N)(PCP)]. We primarily focused on the first hydrogenation step, forming an Mo-imide complex [MoI(NH)(PCP)] since previous experimental and theoretical studies suggest that imide formation is the rate-limiting step in the catalytic cycle. The choice of protonating agent and reductant strongly influences the catalytic reactivity in imide formation. In this computational quantum chemical study, 2,4,6-collidinium (ColH+) was employed as the protonation agent, while metallocenes Cp2MII and decamethylmetallocenes Cp*2MII (M = V, Cr, Mn, Fe, Co, and Ni) were employed as reductants. The reaction of ColH+ with the metallocenes yields protonated metallocenes, where a cyclopentadienyl ring of the metallocenes is protonated. Protonated Cp*2CrII and Cp*2CoII are potential proton-coupled electron transfer (PCET) mediators to facilitate the imide formation of [MoI(N)(PCP)] with low activation free energies. The concerted reaction mechanism was compared with the stepwise reaction, where ColH+ directly protonates [MoI(N)(PCP)], followed by reduction with the decamethylmetallocenes. Furthermore, we analyzed how proton transfer and electron transfer are concerted in the reaction of the PCET mediators with [MoI(N)(PCP)] by tracing electronic states along the reaction coordinates.

Terminal hydride formation in [FeFe] hydrogenase: understanding the role of the dithiolate bridgehead

[FeFe]-hydrogenases are highly-active hydrogen-conversion biocatalysts using Earth-abundant metals in their active-site. Understanding their mechanism may enable design of catalysts for renewable energy storage. Here, observation of the crucial Fe-hydride-containing (Hhyd) intermediate in a PDT-variant of [FeFe]-hydrogenase reveals deeper insight into the role of the dithiolate bridgehead in the catalytic mechanism.

Substrate-Dependent Hydridic and Radical Reactivity of Triiron Hydride Clusters

The reactivity of iron clusters with one of more μ-hydrides and in the weak field pertains to catalysis on surfaces and biological metal cluster cofactors. As a model, then, a reactivity survey of the weak-field ligated iron hydride clusters Fe3H3L (1) and (FeCO)2Fe(μ3-H)L (2) (where L3- is a tris(β-diketiminate)cyclophanate) with Brønsted acids, organochlorides, acetyl chloride, boron trihalides, and titanium electrophiles is reported. Complex 1 reacts with Brønsted acids H2O and [Et3NH][Cl] to afford Fe3(OH)3L (3) and Fe3H2ClL (4), respectively, consistent with hydridic reactivity. Clusters 1 and 2 react readily with organochlorides, such as CCl4, CHCl3, and CH2Cl2, with identified intermediates supporting a radical pathway. Complex 1 reacts with trityl chloride (2 equiv) to selectively afford Fe3HCl2L (5) with reductive elimination of dihydrogen observed. Mixed-valent complex 2 reacts with AcCl to afford (FeCO)Fe2HClL (9). The scope of reactivity displayed implicates possible pathways accessible to larger clusters in biology or on metal surfaces.

In-tandem Electrochemical Reduction of Nitrate to Ammonia on Ultrathin-Sheet-Assembled Iron-Nickel Alloy Nanoflowers

The development of alternative routes for ammonia (NH3) synthesis with high Faradaic efficiency (FE) is crucial for energy conservation and to achieve zero carbon emissions. Electrocatalytic nitrate (NO3 -) reduction to NH3 (e-NO3RRA) is a promising alternative to the energy-intensive, fossil-fuel-driven Haber-Bosch process. The implementation of this innovative NH3 synthesis technique requires an efficient electrocatalyst and in-depth mechanistic understanding of e-NO3RRA. In this study, we developed an ultrathin sheet (μm) iron-nickel nanoflower alloy through electrodeposition and used it for e-NO3RRA under alkaline conditions. The prepared Fe-Ni alloy exhibited an FE of 97.28±1.36 % at -238 mVRHE and an NH3 yield rate up to 3999.1±242.59 μg h-1 cm-2. Experimental electrolysis, in situ Raman spectroscopy, and density functional theory calculations showed that the adsorption and reduction of NO3 - to NO2 - occurred on the Fe surface, whereas subsequent hydrogenation of NO2 - to NH3 occurred preferentially on the Ni surface. The catalysts exhibited comparable FE for at least 10 cycles, with a long-term stability of 216 h. Electron paramagnetic resonance results confirmed that adsorbed hydrogen was consumed during e-NO3RRA. This work introduces a sustainable, robust, and efficient Fe-Ni alloy electrocatalyst, offering an environmentally friendly approach for synthesizing NH3 from NO3 --contaminated water.

Metalloenzyme-Inspired Cluster Fabrication within Mesoporous Channels Featuring Optimized Catalytic Microenvironments for Efficient Neutral pH H2O2 Electrosynthesis

In nature, some metalloenzymes facilitate highly efficient catalytic transformations of small molecules, primarily attributed to the effective coupling between their metal cluster active sites and the surrounding microenvironment. Inspired by this, a thermotropic redispersion strategy to incorporate bismuth nanoclusters (Bi NCs) into mesoporous channels, mimicking metalloenzyme-like catalysis to enhance the two-electron oxygen reduction reaction (2e- ORR) for efficient neutral pH H2O2 electrosynthesis, is developed. This model electrocatalyst exhibits exceptional 2e- ORR performance with >95% H2O2 selectivity across 0.2-0.6 V vs RHE in neutral electrolyte. Notably, the system produces up to 7.2 wt% neutral H2O2 solution at an industrially relevant current density of ≈320 mA cm-2, with 90% Faradaic efficiency for H2O2 over 120 h in a flow cell, demonstrating significant practical potential. Mechanistic insights reveal that the introduction of Bi NCs enhances the adsorption of the *OOH intermediate, facilitating a highly active 2e- ORR process. Moreover, the mesoporous channels of the carbon support create a favorable catalytic microenvironment for O2 aeration and local alkalinity, further boosting H2O2 productivity. This catalyst design mimics metalloenzymes by optimal integration of the active site with the surrounding microenvironment, offering valuable insights for the rational design of nature-inspired small-molecule catalysts.

Exploring N2 activation using novel Lewis acid/base pairs: computational insight into frustrated Lewis pair reactivity

The activation of dinitrogen (N2) is a crucial step in synthesizing nitrogen-based compounds and remains a significant challenge due to its strong triple bond. Currently, industrial N2 conversion relies on the Haber-Bosch process, a highly energy-intensive method that utilizes transition metal-based catalysts. Frustrated Lewis pairs (FLPs) have emerged as a promising alternative for N2 activation without the need for transition metals. In this work, we employ density functional theory (DFT) to investigate the activation of N2 by transition metal-free Lewis acids (LAs) and bases (LBs). Our study demonstrates that LAs play a crucial role in capturing N2 and determining the thermodynamics of activation, while LBs play a complementary role by reducing the bond order of the N2 molecule, thereby promoting activation. The efficiency of N2 capture is directly linked to the electroaccepting characteristics of the LAs. A principal component analysis (PCA) reveals that the key factors influencing the electroaccepting power of LAs are the degree of pyramidalization and orbital occupation at the acidic site, as well as the local electrophilicity index. The LA-N2 interaction is found to be electrostatic with partially covalent character. Among the 21 LAs analyzed, triptycene-based systems exhibit the highest stability in forming LA-N2 complexes, highlighting their potential as effective N2-capturing agents. However, the N2 triple bond remains largely intact, necessitating the involvement of LBs in LA-N2-LB complexes for full activation, in a "push-pull" mechanism. Six LBs are analyzed in complexes with the most promising LAs. Bonding analysis indicates that the LB-N2 interaction can be regarded as a covalent bond, which may explain the main role of the LB in the reduction of the N2 bond order. Furthermore, the bond activation is significantly enhanced by increasing the nucleophilicity of the LB. Among all the LA-LB pair combinations, only three exhibit the defining characteristics of frustrated Lewis pairs (FLPs), with moderate interaction energies and substantial LA-LB distances. Our findings suggest that FLPs composed of triptycene-based LAs and tris-tert-butylphosphine represent the most promising candidates for N2 activation.

Dual-Shelled CeO2 Hollow Spheres Decorated with MXene Quantum Dots for Efficient Electrocatalytic Nitrogen Oxidation

Electrocatalytic nitrogen oxidation (NOR) provides a promising alternative strategy for synthesizing nitric acid from widespread N2, which overcomes the disadvantages of Haber-Bosch-Ostwald process. However, the NOR process suffers from the limitation of high N≡N bonding energy, sluggish kinetics, and low efficiency. It is prerequisite to develop more efficient NOR electrocatalysts. Herein, dual-shelled CeO2 hollow spheres (D-CeO2) are synthesized and modified with Ti3C2 MXene quantum dots (MQDs) for NOR, which exhibited a NO3 - yield rate of 71.25 µg h-1 mgcat -1 and Faradic Efficiency (FE) of 31.80% at 1.7 V versus RHE. The unique quantum size effect and abundant edge active sites lead to more effective capture of nitrogen. Moreover, the dual-shelled hollow structure will gather intermediate products in the interlayer of the core-shell to facilitate N2 fixation. The in situ Fourier transform infrared (FTIR) spectroscopy confirmed the formation of *NO and NO3 - species during the NOR, and the kinetics and possible pathways of NOR are calculated by density functional theory (DFT). In addition, a Zn-N2 reaction device is assembled with D-CeO2/MQDs as anode and Zn plate as cathode, obtaining an extremely high NO3 - yield rate of 104.57 µg h-1 mgcat -1 at 1 mA cm-2.

Favoring the Originally Unfavored Oxygen for Enhancing Nitrogen-to-Nitrate Electroconversion

Current nitrate production involves a two-step thermochemical process that is energy-intensive and generates substantial CO2 emissions. Sustainable NO3- production via the nitrogen electrooxidation reaction powered by renewable electricity is highly desirable, but the Faradaic efficiency (FE) at high production rates is unsatisfactory due to competition from the oxygen evolution reaction (OER). In this study, we propose reengineering the catalyst's microstructure-to-macroenvironment interface by particularly utilizing the previously considered unfavored oxygen from the OER. We demonstrate that the re-engineered interface facilitates a record-breaking FE of 35.52% under 8 atm air, with an impressive increase in FE (41.56%) observed during a continuous electrochemical process lasting for 60 h due to the in situ formation of the O2-rich macro-interface environment. The revelation is anticipated to furnish groundbreaking perspectives for the reaction systems design in electrochemical nitrate production and other electrocatalytic fields.

Massively Parallel Tensor Network State Algorithms on Hybrid CPU-GPU Based Architectures

The interplay of quantum and classical simulation and the delicate divide between them is in the focus of massively parallelized tensor network state (TNS) algorithms designed for high performance computing (HPC). In this contribution, we present novel algorithmic solutions together with implementation details to extend current limits of TNS algorithms on HPC infrastructure building on state-of-the-art hardware and software technologies. Benchmark results obtained via large-scale density matrix renormalization group (DMRG) simulations on single node multiGPU NVIDIA A100 system are presented for selected strongly correlated molecular systems addressing problems on Hilbert space dimensions up to 4.17 × 1035.

A Spectroscopic Criterion for Identifying the Degree of Ground-Level Near-Degeneracy Derived from Effective Hamiltonian Analyses of Three-Coordinate Iron Complexes

The fascinating magnetic and catalytic properties of coordinatively unsaturated 3d metal complexes are a manifestation of their electronic structures, in particular their nearly doubly or triply degenerate orbital ground levels. Here, we propose a criterion to determine the degree of degeneracy of this class of complexes based on their experimentally accessible magnetic anisotropy (parametrized by the electron spin g- and zero-field splitting (ZFS)-tensors). The criterion is derived from a comprehensive spectroscopic and theoretical study in the trigonal planar iron(0) complex, [(IMes)Fe(dvtms)] (IMes = 1,3-di(2',4',6'-trimethylphenyl)imidazol-2-ylidene, dvtms = divinyltetramethyldisiloxane, 1). Accurate ZFS-values (D = +33.54 cm-1, E/D = 0.09) and g-values (g ∥ = 1.96, g ⊥ = 2.45) of the triplet (S = 1) ground level of complex 1 were determined by complementary THz-EPR spectroscopy and SQUID magnetometry. In-depth effective Hamiltonian (EH) analyses coupled to wave-function-based ab initio calculations show that 1 features a ground level with three energetically close-lying orbital states with a "two-above-one" energy pattern. The observed magnetic anisotropy results from mixing of the two excited electronic states with the ground state by spin-orbit coupling (SOC). EH investigations on 1 and related complexes allowed us to generalize this finding and establish the anisotropy of the g - and ZFS-tensors as spectroscopic markers for assigning two- or three-fold orbital near-degeneracy.

In vivo synthesis of semiconductor nanoparticles in Azotobacter vinelandii for light-driven ammonia production

Ammonia (NH3) is an important commodity chemical used as an agricultural fertilizer and hydrogen-storage material. There has recently been much interest in developing an environmentally benign process for NH3 synthesis. Here, we report enhanced production of ammonia from diazotrophs under light irradiation using hybrid composites of inorganic nanoparticles (NPs) and bacterial cells. The primary focus of this study lies in the intracellular biosynthesis of semiconductor NPs within Azotobacter vinelandii, a diazotroph, when bacterial cells are cultured in a medium containing precursor molecules. For example, enzymes in bacterial cells, such as cysteine desulfurase, convert cysteine (Cys) into precursors for cadmium sulfide (CdS) synthesis when supplied with CdCl2. Photoexcited charge carriers in the biosynthesized NPs are transferred to nitrogen fixation enzymes, e.g., nitrogenase, facilitating the production of ammonium ions. Notably, the intracellular biosynthesis approach minimizes cell toxicity compared to extracellular synthesis due to the diminished generation of reactive oxygen species. The biohybrid system based on the in vivo approach results in a fivefold increase in ammonia production (0.45 mg gDCW-1 h-1) compared to the case of diazotroph cells only (0.09 mg gDCW-1 h-1).

Defect Engineering of Metal-Based Atomically Thin Materials for Catalyzing Small-Molecule Conversion Reactions

Recently, metal-based atomically thin materials (M-ATMs) have experienced rapid development due to their large specific surface areas, abundant electrochemically accessible sites, attractive surface chemistry, and strong in-plane chemical bonds. These characteristics make them highly desirable for energy-related conversion reactions. However, the insufficient active sites and slow reaction kinetics leading to unsatisfactory electrocatalytic performance limited their commercial application. To address these issues, defect engineering of M-ATMs has emerged to increase the active sites, modify the electronic structure, and enhance the catalytic reactivity and stability. This review provides a comprehensive summary of defect engineering strategies for M-ATM nanostructures, including vacancy creation, heteroatom doping, amorphous phase/grain boundary generation, and heterointerface construction. Introducing recent advancements in the application of M-ATMs in electrochemical small molecule conversion reactions (e.g., hydrogen, oxygen, carbon dioxide, nitrogen, and sulfur), which can contribute to a circular economy by recycling molecules like H2, O2, CO2, N2, and S. Furthermore, a crucial link between the reconstruction of atomic-level structure and catalytic activity via analyzing the dynamic evolution of M-ATMs during the reaction process is established. The review also outlines the challenges and prospects associated with M-ATM-based catalysts to inspire further research efforts in developing high-performance M-ATMs.

Recent advances in graphitic carbon nitride-based nanocomposites for energy storage and conversion applications

Graphitic carbon nitride (g-C3N4) has gained significant attention as a promising nonmetallic semiconductor photocatalyst due to its photochemical stability, favorable electronic properties, and efficient light absorption. Nevertheless, its practical applications are hindered by limitations such as low specific surface area, rapid recombination of photogenerated charge carriers, poor electrical conductivity, and restricted photo-response ranges. This review explores recent advancements in the synthesis, modification and application of g-C3N4and its nanocomposites with a focus on addressing these challenges. Key strategies for enhancing g-C3N4include various synthesis methods (solvothermal, microwave-assisted, sol-gel, and vapor deposition), doping, defect engineering, heterojunction formation, and surface modifications. Their potential in energy storage and conversion applications, including photocatalytic hydrogen production, carbon dioxide reduction, nitrogen fixation, and electrochemical energy storage are also highlighted. Overall, the review underscores the importance of structural and morphological modifications in improving the photoelectrochemical performance of g-C3N4-based nanocomposites, providing insights for future development and optimization.

Side-On Bound Beryllium Dinitrogen Complex: A Precursor for Complete Conversion of Dinitrogen to Ammonia Mediated by N-Heterocyclic Carbene

The complete conversion of dinitrogen to ammonia mediated by a side-on N2-bound carbene-beryllium complex, [NHC-Be(η2-N2)] has been studied considering both the symmetric and unsymmetric pathways. N-heterocyclic carbenes complexed with Be(η2-N2) moieties were considered substrates in our study. We found that two mechanistic pathways were possible for the reduction of dinitrogen to form ammonia. Our calculations revealed that the symmetric pathway is more favorable compared to the unsymmetric one. The interconversion of the complex from the symmetric product to the unsymmetric one involves a large activation energy barrier for the proton transfer pathway. Both of these pathways were associated with high exergonicity, and the N-N bond is observed to be elongated, which indicates that the NHC-Be(η2-N2) complex is a promising candidate for dinitrogen activation and subsequent reduction, resulting in the formation of ammonia. The bonding scenario of the NHC-Be(η2-N2) complex can be explained well by the famous Dewar-Chatt-Duncanson (DCD) model. Our calculations reveal that the symmetric pathway is found to be more suitable due to more negative values of change in Gibbs free energy. Solvent phase calculations have identified the viability of the NHC-Be(η2-N2) complex, indicating that the complex is sustainable in low-polar organic solvents, such as toluene and diethyl ether.

Construction of N-E bonds via Lewis acid-promoted functionalization of chromium-dinitrogen complexes

Direct conversion of dinitrogen (N2) into N-containing compounds beyond ammonia under ambient conditions remains a longstanding challenge. Herein, we present a Lewis acid-promoted strategy for diverse nitrogen-element bonds formation from N2 using chromium dinitrogen complex [Cp*(IiPr2Me2)Cr(N2)2]K (1). With the help of Lewis acids AlMe3 and BF3, we successfully trap a series of fleeting diazenido intermediates and synthesize value-added compounds containing N-B, N-Ge, and N-P bonds with 3 d metals, offering a method for isolating unstable intermediates. Furthermore, the formation of N-C bonds is realized under more accessible conditions that avoid undesired side reactions. DFT calculations reveal that Lewis acids enhance the participation of dinitrogen units in the frontier orbitals, thereby promoting electrophilic functionalization. Moreover, Lewis acid replacement and a base-induced end-on to side-on switch of [NNMe] unit in [(Cp*(IiPr2Me2)CrNN(BEt3)(Me)] (8) are achieved.

Investigating the Molybdenum Nitrogenase Mechanistic Cycle Using Spectroelectrochemistry

Molybdenum nitrogenase plays a crucial role in the biological nitrogen cycle by catalyzing the reduction of dinitrogen (N2) to ammonia (NH3) under ambient conditions. However, the underlying mechanisms of nitrogenase catalysis, including electron and proton transfer dynamics, remain only partially understood. In this study, we covalently attached molybdenum nitrogenase (MoFe) to gold electrodes and utilized surface-enhanced infrared absorption spectroscopy (SEIRA) coupled with electrochemistry techniques to investigate its catalytic mechanism. Our biohybrid system enabled electron transfer via a mild mediator, likely mimicking the natural electron flow through the P-cluster to FeMoco, the enzyme's active site. For the first time, we experimentally observed both terminal and bridging S-H stretching frequencies, resulting from the protonation of bridging sulfides in FeMoco during turnover conditions providing direct evidence of their role in catalysis. These experimental observations are further supported by QM/MM calculations. Additionally, we investigated CO inhibition, demonstrating both CO binding and unbinding dynamics under electrochemical conditions. These insights not only advance our understanding of the mechanistic cycle of molybdenum nitrogenase but also establish a foundation for studying alternative nitrogenases, including vanadium and iron nitrogenases.

Biomimetic [MFe3S4]3+ Cubanes (M = V/Mo) as Catalysts for a Fischer-Tropsch-like Hydrocarbon Synthesis─A Computational Study

Nitrogenase is the enzyme primarily responsible for reducing atmospheric nitrogen to ammonia. There are three general forms of nitrogenase based on the metal ion present in the cofactor binding site, namely, molybdenum-dependent nitrogenases with the iron-molybdenum cofactor (FeMoco), the vanadium-dependent nitrogenases with FeVco, and the iron-only nitrogenases. It has been shown that the vanadium-dependent nitrogenases tend to have a lesser efficacy in reducing dinitrogen but a higher efficacy in binding and reducing carbon monoxide. In biomimetic chemistry, [MFe3S4] (M = Mo/V) cubanes have been synthesized, studied, and shown to be promising mimics of some of the geometric and electronic properties of the nitrogenase cofactors. In this work, a density functional theory (DFT) study is presented on Fischer-Tropsch catalysis by these cubane complexes by studying CO binding and reduction to hydrocarbons. Our work implies that molybdenum has stronger binding interactions with the iron-sulfur framework of the cubane, which results in easier reduction of substrates like N2H4. However, this inhibits the binding and activation of CO, and hence, the molybdenum-containing complexes are less suitable for Fischer-Tropsch catalysis than vanadium-containing complexes.

Nitrification Mechanisms for the P460 Enzymes

The oxidation of hydroxylamine was studied by quantum chemical modeling. Hydroxylamine is the product of ammonia oxidation in ammonia monooxygenase. That mechanism has been studied recently by quantum chemical modeling as here. Only two enzymes can oxidize hydroxylamine, hydroxylamine oxidase and cytochrome-P460. Both employ the unusual P460-heme cofactor. In hydroxylamine oxidase, there is a covalently linked tyrosine, while in cytochrome-P460, there is a covalently linked lysine. The calculations give explanations for the experimental findings that NO is the final product in hydroxylamine oxidase, while N2O is the final product in cytochrome-P460. The effect of the covalent attachments has been investigated, and reasons for their presence have been given. The methodology used, which was proven to give very good agreement with experiments for several redox enzymes, again leads to excellent agreement with experimental findings.

The Development, Essence and Perspective of Nitrogen Reduction to Ammonia

Ammonia plays a pivotal role in agriculture and meanwhile holds promising potential as an energy vector for the hydrogen economy, where the nitrogen reduction to ammonia is a critical pathway for achieving sustainable development. Over the past hundred years, ammonia synthesis has undergone several breakthrough developments from Haber-Bosch process to photo/electro-catalysis and Li-mediated strategy, but still faces the challenges of low yield rate, selectivity and efficiency. Therefore, there is a pressing demand to develop efficient and green ammonia synthesis from nitrogen. This review summarizes the development of the nitrogen reduction to ammonia, highlighting six milestones during the whole journey. From the development direction, this work finds and extracts the essence of ammonia synthesis, that is the reaction pathways are affected by the energy barrier of reaction intermediates, which can be altered by proton sources, auxiliaries and catalysts. Then this work discusses the detailed overview of the significant development of proton source, auxiliaries and catalysts. Finally, based on the essence, the possible opportunities of ammonia synthesis from nitrogen reduction are presented, including the design of new ammonia synthesis pathways and efficient catalysts. The deep insight of nitrogen reduction to ammonia will provide a design guidance for efficient ammonia synthesis.

Structural basis for the conformational protection of nitrogenase from O2

The low reduction potentials required for the reduction of dinitrogen (N2) render metal-based nitrogen-fixation catalysts vulnerable to irreversible damage by dioxygen (O2)1-3. Such O2 sensitivity represents a major conundrum for the enzyme nitrogenase, as a large fraction of nitrogen-fixing organisms are either obligate aerobes or closely associated with O2-respiring organisms to support the high energy demand of catalytic N2 reduction4. To counter O2 damage to nitrogenase, diazotrophs use O2 scavengers, exploit compartmentalization or maintain high respiration rates to minimize intracellular O2 concentrations4-8. A last line of damage control is provided by the 'conformational protection' mechanism9, in which a [2Fe:2S] ferredoxin-family protein termed FeSII (ref. 10) is activated under O2 stress to form an O2-resistant complex with the nitrogenase component proteins11,12. Despite previous insights, the molecular basis for the conformational O2 protection of nitrogenase and the mechanism of FeSII activation are not understood. Here we report the structural characterization of the Azotobacter vinelandii FeSII-nitrogenase complex by cryo-electron microscopy. Our studies reveal a core complex consisting of two molybdenum-iron proteins (MoFePs), two iron proteins (FePs) and one FeSII homodimer, which polymerize into extended filaments. In this three-protein complex, FeSII mediates an extensive network of interactions with MoFeP and FeP to position their iron-sulphur clusters in catalytically inactive but O2-protected states. The architecture of the FeSII-nitrogenase complex is confirmed by solution studies, which further indicate that the activation of FeSII involves an oxidation-induced conformational change.

Reactivity of Polynuclear Niobium Oxynitride Cluster Anions Nb4N5-xOx - (x=0-5) toward N2

The activation of N₂ under mild conditions remains a significant challenge in chemistry. Understanding how the composition of ligands modulates the reactivity of metal centers is pivotal for the rational design of efficient catalysts for nitrogen fixation. Herein, the reactions between polynuclear niobium oxynitride anions Nb4N5-xOx - (x=0-5) and N2 were investigated by employing mass spectrometry, photoelectron imaging spectroscopy, and theoretical calculations. The rate constants of Nb4N5-xOx -/N2 gradually decrease for x=0 to x=4, and then increase again for x=5. The sharp increase of the rate constants of Nb4O5 -/N2 corresponds to a decrease in the electron detachment energy of the Nb4O5 - cluster in the photoelectron spectroscopic experiments. Theoretical calculations suggest that the low-coordinated Nb-Nb sites in Nb4N5-xOx - (x=0-5) behaves as the active centers to bind N2 in the side-on/end-on manner. Mechanistic analysis reveals that reducing the N/O ratio leads to higher electron densities on the active Nb-Nb centers and decreased positive charge on the metal atoms, which hinders the approach of N2 to the clusters. This finding discloses fundamental insights into the impact of N/O ratio in fine-tuning the reactivity of metal centers toward N2 adsorption in related catalytic processes.

Deciphering Molecular Mechanisms and Diversity of Plant Holobiont Bacteria: Microhabitats, Community Ecology, and Nutrient Acquisition

While gaining increasing attention, plant-microbiome-environment interactions remain insufficiently understood, with many aspects still underexplored. This article explores bacterial biodiversity across plant compartments, including underexplored niches such as seeds and flowers. Furthermore, this study provides a systematic dataset on the taxonomic structure of the anthosphere microbiome, one of the most underexplored plant niches. This review examines ecological processes driving microbial community assembly and interactions, along with the discussion on mechanisms and diversity aspects of processes concerning the acquisition of nitrogen, phosphorus, potassium, and iron-elements essential in both molecular and ecological contexts. These insights are crucial for advancing molecular biology, microbial ecology, environmental studies, biogeochemistry, and applied studies. Moreover, the authors present the compilation of molecular markers for discussed processes, which will find application in (phylo)genetics, various (meta)omic approaches, strain screening, and monitoring. Such a review can be a valuable source of information for specialists in the fields concerned and for applied researchers, contributing to developments in sustainable agriculture, environmental protection, and conservation biology.

Sulfide release and rebinding in the mechanism for nitrogenase

Nitrogenases are the only enzymes that activate the strong triple bond in N2. The mechanism for the activation has been very difficult to determine in spite of decades of work. In previous modeling studies it has been suggested that the mechanism for nitrogen activation starts out by four pre-activation steps (A0-A4) before catalysis. That suggestion led to excellent agreement with experimental Elecrtron Paramagnetic Resonance (EPR) observations in the step where N2 becomes protonated (E4). An important part of the pre-activation is that a sulfide is released. In the present paper, the details of the pre-activation are modeled, including the release of the sulfide. Several possible transition states for the release have been obtained. An A4(E0) state is reached which is very similar to the E4 state. For completeness, the steps going back from A4(E0) to A0 after catalysis are also modeled, including the insertion of a sulfide.

Spatiotemporal Variation in Dissolved, Bioavailable, and Particulate Elements and the Abundance of Harmful Algae in Grand Lake

Harmful algal blooms (HABs) are often linked to the increased loading of limiting nutrients such as nitrogen and phosphorus. Little is known about the relevance of other biogenic elements, the supplies of which are spatiotemporally heterogeneous, on HABs. We measured the dissolved, bioavailable, and particulate concentrations of 26 elements at four locations draining different catchments of a large reservoir during three seasons, in addition to the total abundance of phytoplankton and % of cyanobacteria. Finally, we manipulated a key element (Fe) in microcosms to test its effect on the community. Phytoplankton abundance and community structure varied spatiotemporally, with minimal variation in N/P. The variation in environmental supplies of several other elements was correlated with phytoplankton abundance, as well as up to 3 orders of magnitude differences in cyanobacterial yield. Bioassays manipulating Fe impacted total phytoplankton as well as the abundance of cyanobacteria, with Fe-chelated treatments resulting in a significant decline in phytoplankton as well as cyanobacterial yield. In summary, we found substantial heterogeneity in elemental supplies that are relevant to the phytoplankton community. Exploring the relevance of the entire system of elements in the context of HABs may be more rewarding than studies emphasizing a subset of elements.

Unravelling the potential of low-valent tunable vanadium complexes in the nitrogen reduction reaction (NRR)

Density functional theory calculations have been carried out to investigate the potential of several hitherto unknown low-valent tripodal vanadium complexes towards conversion of dinitrogen to ammonia as a function of different equatorial (PiPr2 and SiPr) and bridgehead groups (B, C and Si). All the newly proposed vanadium complexes were probed towards understanding their efficiency in some of the key steps involved in the dinitrogen fixation process. They were found to be successful in preventing the release of hydrazine during the nitrogen reduction reaction. We have performed a comprehensive mechanistic study by considering all the possible pathways (distal, alternate and hybrid) to understand the efficiency of some of the proposed catalysts towards the dinitrogen reduction process. The exergonic reaction free energies obtained for some of the key steps and the presence of thermally surmountable barrier heights involved in the catalytic cycle indicate that these complexes may be considered as suitable platforms for the functionalization of dinitrogen.

Molecular investigation of harmful cyanobacteria reveals hidden risks and niche partitioning in Kenyan Lakes

Despite the global expansion of cyanobacterial harmful algal blooms (cHABs), research is biased to temperate systems within the global north, such as the Laurentian Great Lakes. This lack of diversity represents a significant gap in the field and jeopardizes the health of those who reside along at-risk watersheds in the global south. The African Great Lake, Lake Victoria, is understudied despite serving as the second largest lake by surface area and demonstrating year-round cHABs. Here, we address this knowledge gap by performing a molecular survey of cHAB communities in three anthropogenically and ecologically important freshwater systems of Victoria's Kenyan watershed: Winam Gulf (Lake Victoria), Lake Simbi and Lake Naivasha. We identified a bloom of non-toxic Dolichospermum and toxic Microcystis in the Winam Gulf, with data suggesting sulfur limitation shapes competition dynamics between these two bloom-formers. Though we did not detect a bloom in Naivasha, it contained the largest diversity of cHAB genera amongst the three lakes. In turn, our results indicated methane metabolism may allow non-toxic picoplankton to outcompete cHAB genera, while suggesting Synechococcus spp. serves as a methane source and sink in this system. Lake Simbi exhibited a non-toxic Limnospira bloom at the time of sampling with very low abundances of cHAB genera present. Subsequently, these results were employed to design a cHAB screening and risk assessment framework for local stakeholders. Cumulatively, this work serves to increase cHAB research efforts on the international scale while serving as an impetus for cHAB monitoring on the local scale.

Towards planetary boundary sustainability of food processing wastewater, by resource recovery & emission reduction: A process system engineering perspective

Meeting the needs of a growing population calls for a change from linear production systems that exacerbate the depletion of finite natural resources and the emission of environmental pollutants. These linear production systems have resulted in the human-driven perturbation of the Earth's natural biogeochemical cycles and the transgression of environmentally safe operating limits. One solution that can help alleviate the environmental issues associated both with resource stress and harmful emissions is resource recovery from waste. In this review, we address the recovery of resources from food and beverage processing wastewater (FPWW), which offers a synergistic solution to some of the environmental issues with traditional food production. Research on resource recovery from FPWW typically focuses on technologies to recover specific resources without considering integrative process systems to recover multiple resources while simultaneously satisfying regulations on final effluent quality. Process Systems Engineering (PSE) offers methodologies able to address this holistic process design problem, including modelling the trade-offs between competing objectives. Optimisation of FPWW treatment and resource recovery has significant scope to reduce the environmental impacts of food production systems. There is significant potential to recover carbon, nitrogen, and phosphorus resources while respecting effluent quality limits, even when the significant uncertainties inherent to wastewater systems are considered. This review article gives an overview of the environmental challenges we face, discussed within the framework of the planetary boundary, and highlights the impacts caused by the agri-food sector. This paper also presents a comprehensive review of the characteristics of FPWW and available technologies to recover carbon and nutrient resources from wastewater streams with a particular focus on bioprocesses. PSE research and modelling advances are discussed in this review. Based on this discussion, we conclude the article with future research directions.

Nitrogen fixation by methanogenic Archaea, literature review and DNA database-based analysis; significance in face of climate change

Archaea represents a significant population of up to 10% in soil microbial communities. The role of Archaea in soil is often overlooked mainly due to its unculturability. Among the three domains of life biological nitrogen fixation (BNF) is mainly a trait of Eubacteria and some Archaea. Archaea mediated processes like BNF may become even more important in the face of global Climate change. Although there are reports on nitrogen fixation by Archaea, to best of our knowledge there is no comprehensive report on BNF by Archaea under environmental stresses typical to climate change. Here we report a survey of literature and DNA database to study N2-fixation among Archaea. A total of 37 Archaea belonging to Methanogens of the phylum Euryarchaeota within the class Methanococcus, Methanomicrobia Methanobacteria, and Methanotrophic ANME2 lineages either contain genes for BNF or are known to fix atmospheric N2. Archaea were found to have their nif genes arranged as clusters of 6-8 genes in a single operon. The genes code for commonly found Mo-nitrogenase while in some archaea the genes for alternative metal nitrogenases like vnf were also found. The nifHDK gene similarity matrices show that Archaea shared the highest similarity with the nifHDK gene of anaerobic Clostridium beijerinckii. Although there are various theories about the origin of N2-fixation in Archaea, the most acceptable is the origin of N2-fixation first in bacteria and its subsequent transfer to Archaea. Since Archaea can survive under extreme environmental conditions their role in BNF should be studied especially in soil under environmental stress.

Barriers impeding research data sharing on chronic disease prevention among the older adults in low-and middle-income countries: a systematic review

Introduction

Chronic diseases, including cardiovascular disease, diabetes, cancer, and chronic respiratory diseases, are a growing public health concern in low-and middle-income countries (LMICs) among the older population. The current review aimed to identify the main barriers that impede researchers from sharing research data on the prevention of chronic diseases in older adults living in LMICs). The review included both older women and men from these countries.

Methods

Studies were selected from 11 databases, including Web of Science, Scopus, PubMed, Taylor and Francis, Biomedical Central, BioOne, CINAHL, EBSCOHost, ScienceDirect, Wiley Online, and Google Scholar, were then transferred to CADIMA, an online tool for screening purposes, and a total of 1,305,316 studies were identified through a robust search strategy. CADIMA also ensured the quality of all studies in this review. The sampling techniques were performed by selecting and screening studies per this review's eligibility criteria. Ultimately, 13 studies were found to meet these criteria. A PRISMA flow chart was used to map out the number of studies that were identified, included, and excluded.

Results

Five main barriers were consistently highlighted, including a lack of necessary resources (9, 69%), dealing with complex and sensitive research data (2,15%), lack of policies, procedures, guidelines (5,38%), medical big data processing and integration (2,15%), and inadequate ethical considerations, legal compliance, and privacy protection (6,46%). Discussion: By shedding light on these obstacles, researchers can develop strategies to overcome the identified barriers and address areas requiring further investigation. The registration details of this review can be found under PROSPERO 2023 CRD42023437385, underscoring the importance of this review in advancing our collective understanding of chronic disease prevention among older adults worldwide.

Systematic review registration

PROSPERO, identifier CRD42023437385, available at: https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42023437385.

Light-driven nitrogen fixation routes for green ammonia production

The global goal for decarbonization of the energy sector and the chemical industry could become a reality by a massive increase in renewable-based technologies. For this clean energy transition, the versatile green ammonia may play a key role in the future as a fossil-free fertilizer, long-term energy storage medium, chemical feedstock, and clean burning fuel for transportation and decentralized power generation. The high energy-intensive industrial ammonia production has triggered researchers to look for a step change in new synthetic approaches powered by renewable energies. This review provides a comprehensive comparison of light-mediated N2 fixation technologies for green ammonia production, including photocatalytic, photoelectrocatalytic, PV-electrocatalytic and photothermocatalytic routes. Since these approaches are still at laboratory scale, we examine the most recent developments and discuss the open challenges for future improvements. Last, we offer a technoeconomic comparison of current and emerging ammonia production technologies, highlighting costs, barriers, recommendations, and potential opportunities for the real development of the next generation of green ammonia solutions.

Cross-Coupling of Mo- and V-Nitrogenases Permits Protein-Mediated Protection from Oxygen Deactivation

Nitrogenases catalyze dinitrogen (N2) fixation to ammonia (NH3). While these enzymes are highly sensitive to deactivation by molecular oxygen (O2) they can be produced by obligate aerobes for diazotrophy, necessitating a mechanism by which nitrogenase can be protected from deactivation. In the bacterium Azotobacter vinelandii, one mode of such protection involves an O2-responsive ferredoxin-type protein ("Shethna protein II", or "FeSII") which is thought to bind with Mo-dependent nitrogenase's two component proteins (NifH and NifDK) to form a catalytically stalled yet O2-tolerant tripartite protein complex. This protection mechanism has been reported for Mo-nitrogenase, however, in vitro assays with V-nitrogenase suggest that this mechanism is not universal to the three known nitrogenase isoforms. Here we report that the reductase of the V-nitrogenase (VnfH) can engage in this FeSII-mediated protection mechanism when cross-coupled with Mo-nitrogenase NifDK. Interestingly, the cross-coupling of the Mo-nitrogenase reductase NifH with the V-nitrogenase VnfDGK protein does not yield such protection.

A Metal-Sulfur-Carbon Catalyst Mimicking the Two-Component Architecture of Nitrogenase

The production of ammonia (NH3) from nitrogen sources involves competitive adsorption of different intermediates and multiple electron and proton transfers, presenting grand challenges in catalyst design. In nature nitrogenases reduce dinitrogen to NH3 using two component proteins, in which electrons and protons are delivered from Fe protein to the active site in MoFe protein for transfer to the bound N2. We draw inspiration from this structural enzymology, and design a two-component metal-sulfur-carbon (M-S-C) catalyst composed of sulfur-doped carbon-supported ruthenium (Ru) single atoms (SAs) and nanoparticles (NPs) for the electrochemical reduction of nitrate (NO3 -) to NH3. The catalyst demonstrates a remarkable NH3 yield rate of ~37 mg L-1 h-1 and a Faradaic efficiency of ~97 % for over 200 hours, outperforming those consisting solely of SAs or NPs, and even surpassing most reported electrocatalysts. Our experimental and theoretical investigations reveal the critical role of Ru SAs with the coordination of S in promoting the formation of the HONO intermediate and the subsequent reduction reaction over the NP-surface nearby. Such process results in a more energetically accessible pathway for NO3 - reduction on Ru NPs co-existing with SAs. This study proves a better understanding of how M-S-Cs act as a synthetic nitrogenase mimic during ammonia synthesis, and contributes to the future mechanism-based catalyst design.

Mechanism of Dinitrogen Photoactivation by P2PPhFe Complexes: Thermodynamic and Kinetic Computational Studies

The P2PPhFe(N2)(H)2 catalyst showed a significant ammonia yield under light irradiation. However, under thermal conditions, the hydrogen evolution reaction (HER) is favored over the nitrogen reduction reaction (N2RR), making P2PPhFe(N2)(H)2 an ideal system for studying the competition between both reactions. In this study, we used a series of computational tools to elucidate the photochemical reaction mechanism for the N2RR and thermal pathways leading to the HER with this catalyst. We calculated the energy profile for each transformation and estimated the rate constants for each step. Our results, which are consistent with experimental observations, indicate that photoinduced H2 elimination from P2PPhFe(N2)(H)2 promotes the formation of P2PPhFe(N2)2, which is on-path for N2RR. However, this elimination process is kinetically hindered due to high-energy barriers. Furthermore, our calculations reveal enhanced dinitrogen activation upon the conversion of P2PPhFe(N2)(H)2 to P2PPhFe(N2)2.

Isocyanide Substituent Influences Reductive Elimination versus Migratory Insertion in Reaction with an [Fe2(μ-H)2]2+ Complex

Iron hydrides are proposed reactive intermediates for N2 and CO conversion in industrial and biological processes. Here, we report a reactivity study of a low-coordinate di(μ-hydrido)diiron(II) complex, Fe2(μ-H)2L, where L2- is a bis(β-diketiminate) cyclophane, with isocyanides, which have electronic structures related to N2 and CO. The reaction outcome is influenced by the isocyanide substituent, with 2,6-xylyl isocyanide leading to H2 loss, to form a bis(μ-1,1-isocyanide)diiron(I) complex, whereas all of the other tested isocyanides insert into the Fe-H bond to give (μ-1,2-iminoformyl) complexes. Steric bulk of the isocyanide substituent determines the extent of insertion (i.e., into one or both Fe-H-Fe units) with tert-butyl isocyanide reacting to yield the mono-(μ-1,2-iminoformyl)diiron(II) complex, exclusively, and isopropyl- and methyl isocyanides affording the bis(μ-1,2-iminoformyl)diiron(II) products. Treatment of Fe2(μ-1,2-CHNtBu)(μ-H)L with 2,6-xylyl isocyanide (or XylNC) yields Fe2(μ-XylNC)2L and tert-butylaldimine as one of the organic products.

Photoelectrocatalytic-Microbial Biohybrid for Nitrogen Reduction

Nitrogen (N2) conversion to ammonia (NH3) in a mild condition is a big chemical challenge. The whole-cell diazotrophs based biological NH3 synthesis is one of the most promising strategies. Herein, the first attempt of photoelectrochemical-microbial (PEC-MB) biohybrid is contributed for artificial N2 fixation, where Azotobacter vinelandii (A. vinelandii) is interfaced directly with polydopamine encapsulated nickel oxide (NiO) nanosheets (NiO@PDA). By virtue of excellent bio-adhesive activity, high conductivity, and good biocompatibility of PDA layer, abundant A. vinelandii are effectively adsorbed on NiO@PDA to form NiO@PDA/A. vinelandii biohybrid, and the rationally designed biohybrid achieved a record-high NH3 production yield of 1.85 µmol h-1/108 cells (4.14 µmol h-1 cm-2). In addition, this biohybrid can operate both under illumination with a PEC model or in dark with an electrocatalytic (EC) model to implement long-term and successional NH3 synthesis. The enhancement mechanism of NH3 synthesis in NiO@PDA/A. vinelandii biohybrid can be ascribed to the increase of nicotinamide adenine dinucleotide-hydrogen (NADH) and adenosine 5-triphosphate (ATP) concentrations and over expression of nitrogen-fixing genes of nifH, nifD and nifK in nitrogenase. This innovative PEC-MB biohybrid strategy sheds light on the fundamental mechanism and establishes proof of concept of biotic-abiotic photosynthetic systems for sustainable chemical production.

Quantum algorithms for scientific computing

Quantum computing promises to provide the next step up in computational power for diverse application areas. In this review, we examine the science behind the quantum hype, and the breakthroughs required to achieve true quantum advantage in real world applications. Areas that are likely to have the greatest impact on high performance computing (HPC) include simulation of quantum systems, optimization, and machine learning. We draw our examples from electronic structure calculations and computational fluid dynamics which account for a large fraction of current scientific and engineering use of HPC. Potential challenges include encoding and decoding classical data for quantum devices, and mismatched clock speeds between classical and quantum processors. Even a modest quantum enhancement to current classical techniques would have far-reaching impacts in areas such as weather forecasting, aerospace engineering, and the design of 'green' materials for sustainable development. This requires significant effort from the computational science, engineering and quantum computing communities working together.

Tensor Network State Algorithms on AI Accelerators

We introduce novel algorithmic solutions for hybrid CPU-multiGPU tensor network state algorithms utilizing non-Abelian symmetries building on AI-motivated state-of-the-art hardware and software technologies. The presented numerical simulations on the FeMo cofactor, which plays a crucial role in converting atmospheric nitrogen to ammonia, are far beyond the scope of traditional approaches. Our large-scale SU(2) spin adapted density matrix renormalization group calculations up to bond dimension D = 216 on complete active space (CAS) size of 18 electrons in 18 orbitals [CAS(18, 18)] demonstrate that the current limit of exact solution, i.e. full-CI limit, can be achieved in fraction of time. Furthermore, benchmarks up to CAS(113, 76) demonstrate the utilization of NVIDIA's highly specialized AI accelerators via NVIDIA Tensor Cores, leading to performance around 115 TFLOPS on a single node supplied with eight NVIDIA A100 devices. As a consequence of reaching 71% of the full capacity of the hardware, the cubic scaling of computational time with bond dimension can be reduced to a linear form for a broad range of D values; thus, breaking the current computational limits of small CAS spaces in ab initio quantum chemistry and material science is becoming a reality. In comparison to strict U(1) implementations with matching accuracy, our solution has an estimated effective performance of 300-500 TFLOPS, which emphasizes the mutual need for both algorithmic and technological developments to push current frontiers on classical computation.

Nitrogen-Fixing Gamma Proteobacteria Azotobacter vinelandii-A Blueprint for Nitrogen-Fixing Plants?

The availability of fixed nitrogen limits overall agricultural crop production worldwide. The so-called modern "green revolution" catalyzed by the widespread application of nitrogenous fertilizer has propelled global population growth. It has led to imbalances in global biogeochemical nitrogen cycling, resulting in a "nitrogen problem" that is growing at a similar trajectory to the "carbon problem". As a result of the increasing imbalances in nitrogen cycling and additional environmental problems such as soil acidification, there is renewed and increasing interest in increasing the contributions of biological nitrogen fixation to reduce the inputs of nitrogenous fertilizers in agriculture. Interestingly, biological nitrogen fixation, or life's ability to convert atmospheric dinitrogen to ammonia, is restricted to microbial life and not associated with any known eukaryotes. It is not clear why plants never evolved the ability to fix nitrogen and rather form associations with nitrogen-fixing microorganisms. Perhaps it is because of the large energy demand of the process, the oxygen sensitivity of the enzymatic apparatus, or simply failure to encounter the appropriate selective pressure. Whatever the reason, it is clear that this ability of crop plants, especially cereals, would transform modern agriculture once again. Successfully engineering plants will require creating an oxygen-free niche that can supply ample energy in a tightly regulated manner to minimize energy waste and ensure the ammonia produced is assimilated. Nitrogen-fixing aerobic bacteria can perhaps provide a blueprint for engineering nitrogen-fixing plants. This short review discusses the key features of robust nitrogen fixation in the model nitrogen-fixing aerobe, gamma proteobacteria Azotobacter vinelandii, in the context of the basic requirements for engineering nitrogen-fixing plants.

Atomic-level design of biomimetic iron-sulfur clusters for biocatalysis

Designing biomimetic materials with high activity and customized biological functions by mimicking the central structure of biomolecules has become an important avenue for the development of medical materials. As an essential electron carrier, the iron-sulfur (Fe-S) clusters have the advantages of simple structure and high electron transport capacity. To rationally design and accurately construct functional materials, it is crucial to clarify the electronic structure and conformational relationships of Fe-S clusters. However, due to the complex catalytic mechanism and synthetic process in vitro, it is hard to reveal the structure-activity relationship of Fe-S clusters accurately. This review introduces the main structural types of Fe-S clusters and their catalytic mechanisms first. Then, several typical structural design strategies of biomimetic Fe-S clusters are systematically introduced. Furthermore, the development of Fe-S clusters in the biocatalytic field is enumerated, including tumor treatment, antibacterial, virus inhibition and plant photoprotection. Finally, the problems and development directions of Fe-S clusters are summarized. This review aims to guide people to accurately understand and regulate the electronic structure of Fe-S at the atomic level, which is of great significance for designing biomimetic materials with specific functions and expanding their applications in biocatalysis.

Review of cathodic electroactive bacteria: Species, properties, applications and electron transfer mechanisms

Cathodic electroactive bacteria (C-EAB) which are capable of accepting electrons from solid electrodes provide fresh avenues for pollutant removal, biosensor design, and electrosynthesis. This review systematically summarized the burgeoning applications of the C-EAB over the past decade, including 1) removal of nitrate, aromatic derivatives, and metal ions; 2) biosensing based on biocathode; 3) electrosynthesis of CH4, H2, organic carbon, NH3, and protein. In addition, the mechanisms of electron transfer by the C-EAB are also classified and summarized. Extracellular electron transfer and interspecies electron transfer have been introduced, and the electron transport mechanism of typical C-EAB, such as Shewanella oneidensis MR-1, has been combed in detail. By bringing to light this cutting-edge area of the C-EAB, this review aims to stimulate more interest and research on not only exploring great potential applications of these electron-accepting bacteria, but also developing steady and scalable processes harnessing biocathodes.

Final E5 to E8 Steps in the Nitrogenase Mechanism for Nitrogen Fixation

Nitrogenase converts nitrogen in the air to ammonia. It is often regarded as the second most important enzyme in nature after photosystem II. The mechanism for how nitrogenase is able to perform the difficult task of cleaving the strong bond in N2 is debated. It is known that for every electron that is donated to N2, two ATP are hydrolyzed. In the experimentally suggested mechanism, the activation occurs after four reductions of the ground state, but there is no suggestion for how the enzyme uses the hydrolysis energy to perform catalysis. In the theoretical mechanism, it is suggested that hydrolysis is used to reduce the electron donor. In previous papers, the steps leading to the activation of N2 in the so-called E4 state has been investigated, using both the experimental and theoretical mechanism, showing that only the theoretical one leads to agreement with EPR observations for E4. In the present paper, the four steps following E4, leading to the release of two ammonia molecules, are described using the same methodology as used in the previous studies.

Bioinspired recognition in metal-organic frameworks enabling precise sieving separation of fluorinated propylene and propane mixtures

The separation of fluorinated propane/propylene mixtures remains a major challenge in the electronics industry. Inspired by biological ion channels with negatively charged inner walls that allow selective transport of cations, we presented a series of formic acid-based metal-organic frameworks (MFA) featuring biomimetic multi-hydrogen confined cavities. These MFA materials, especially the cobalt formate (CoFA), exhibit specific recognition of hexafluoropropylene (C3F6) while facilitating size exclusion of perfluoropropane (C3F8). The dual-functional adsorbent offers multiple binding sites to realize intelligent selective recognition of C3F6, as supported by theoretical calculations and in situ spectroscopic experiments. Mixed-gas breakthrough experiments validate the capability of CoFA to produce high-purity (>5 N) C3F8 in a single step. Importantly, the stability and cost-effective scalable synthesis of CoFA underscore its extraordinary potential for industrial C3F6/C3F8 separations. This bioinspired molecular recognition approach opens new avenues for the efficient purification of fluorinated electronic specialty gases.

Enzymatic CO2 reduction catalyzed by natural and artificial Metalloenzymes

The continuously increasing level of atmospheric CO2 in the atmosphere has led to global warming. Converting CO2 into other carbon compounds could mitigate its atmospheric levels and produce valuable products, as CO2 also serves as a plentiful and inexpensive carbon feedstock. However, the inert nature of CO2 poses a major challenge for its reduction. To meet the challenge, nature has evolved metalloenzymes using transition metal ions like Fe, Ni, Mo, and W, as well as electron-transfer partners for their functions. Mimicking these enzymes, artificial metalloenzymes (ArMs) have been designed using alternative protein scaffolds and various metallocofactors like Ni, Co, Re, Rh, and FeS clusters. Both the catalytic efficiency and the scope of CO2-reduction product of these ArMs have been improved over the past decade. This review first focuses on the natural metalloenzymes that directly reduce CO2 by discussing their structures and active sites, as well as the proposed reaction mechanisms. It then introduces the common strategies for electrochemical, photochemical, or photoelectrochemical utilization of these native enzymes for CO2 reduction and highlights the most recent advancements from the past five years. We also summarize principles of protein design for bio-inspired ArMs, comparing them with native enzymatic systems and outlining challenges and opportunities in enzymatic CO2 reduction.

Enhanced catalytic performance through a single-atom preparation approach: a review on ruthenium-based catalysts

The outstanding catalytic properties of single-atom catalysts (SACs) stem from the maximum atom utilization and unique quantum size effects, leading to ever-increasing research interest in SACs in recent years. Ru-based SACs, which have shown excellent catalytic activity and selectivity, have been brought to the frontier of the research field due to their lower cost compared with other noble catalysts. The synthetic approaches for preparing Ru SACs are rather diverse in the open literature, covering a wide range of applications. In this review paper, we attempt to disclose the synthetic approaches for Ru-based SACs developed in the most recent years, such as defect engineering, coordination design, ion exchange, the dipping method, and electrochemical deposition etc., and discuss their representative applications in both electrochemical and organic reaction fields, with typical application examples given of: Li-CO2 batteries, N2 reduction, water splitting and oxidation of benzyl alcohols. The mechanisms behind their enhanced catalytic performance are discussed and their structure-property relationships are revealed in this review. Finally, future prospects and remaining unsolved issues with Ru SACs are also discussed so that a roadmap for the further development of Ru SACs is established.

57Fe nuclear resonance vibrational spectroscopic studies of tetranuclear iron clusters bearing terminal iron(iii)-oxido/hydroxido moieties

57Fe nuclear resonance vibrational spectroscopy (NRVS) has been applied to study a series of tetranuclear iron ([Fe4]) clusters based on a multidentate ligand platform (L3-) anchored by a 1,3,5-triarylbenzene linker and pyrazolate or (tertbutylamino)pyrazolate ligand (PzNH t Bu-). These clusters bear a terminal Fe(iii)-O/OH moiety at the apical position and three additional iron centers forming the basal positions. The three basal irons are connected with the apical iron center via a μ4-oxido ligand. Detailed vibrational analysis via density functional theory calculations revealed that strong NRVS spectral features below 400 cm-1 can be used as an oxidation state marker for the overall [Fe4] cluster core. The terminal Fe(iii)-O/OH stretching frequencies, which were observed in the range of 500-700 cm-1, can be strongly modulated (energy shifts of 20-40 cm-1 were observed) upon redox events at the three remote basal iron centers of the [Fe4] cluster without the change of the terminal Fe(iii) oxidation state and its coordination environment. Therefore, the current study provides a quantitative vibrational analysis of how the remote iron centers within the same iron cluster exert exquisite control of the chemical reactivities and thermodynamic properties of the specific iron site that is responsible for small molecule activation.

Water-catalyzed iron-molybdenum carbyne formation in bimetallic acetylene transformation

Transition metal carbyne complexes are of fundamental importance in carbon-carbon bond formation, alkyne metathesis, and alkyne coupling reactions. Most reported iron carbyne complexes are stabilized by incorporating heteroatoms. Here we show the synthesis of bioinspired FeMo heterobimetallic carbyne complexes by the conversion of C2H2 through a diverse series of intermediates. Key reactions discovered include the reduction of a μ-η2:η2-C2H2 ligand with a hydride to produce a vinyl ligand (μ-η1:η2-CH = CH2), tautomerization of the vinyl ligand to a carbyne (μ-CCH3), and protonation of either the vinyl or the carbyne compound to form a hydrido carbyne heterobimetallic complex. Mechanistic studies unveil the pivotal role of H2O as a proton shuttle, facilitating the proton transfer that converts the vinyl group to a bridging carbyne.

Seven-Coordinate Group 6 Metal Hydrides Obtained by H2 Activation at B(C6F5)3 Adducts of N2 Complexes: Frustrated Lewis Pair-Type Reactivity of The B-N Linkage

The adducts 2M,R of general formula trans-[(L)M{R2P(CH2)2PR2}2{N2B(C6F5)3}] (L=ø or N2, M=Mo or W, R=Et or Ph), formed from Lewis acid-base pairing of B(C6F5)3 to a dinitrogen ligand of zero-valent group 6 bis(phosphine) complexes trans-[M{R2P(CH2)2PR2}2(N2)2] are shown to react with dihydrogen to afford hepta-coordinated bis(hydride) complexes [M(H)2{R2P(CH2)2PR2}{N2B(C6F5)3}] 3M,R which feature the rare ability to activate both dinitrogen and dihydrogen at a single metal center, except in the case where M=Mo and R=Ph for which fast precipitation of insoluble [Mo(H)4(dppe)2] (dppe=1,2-bis(diphenylphosphino)ethane) occurs. The frustrated Lewis pair (FLP)-related reactivity of the B-N linkage in compounds 3W,R was explored and led to distal N functionalization without involvement of the hydride ligands. It is shown in one example that the resulting bis(hydride) diazenido compounds may also be obtained through a sequence involving first FLP-type N-functionalization followed by oxidative addition of H2. Those oily compounds were found to have limited stability in solution or in their isolated states. Finally, treatment of 3W,Et with the Lewis base N,N-dimethylaminopyridine (DMAP) affords the simple but unknown bis(hydride)-dinitrogen species [W(H)2(depe)2(N2)] 11Et (depe=1,2-bis(diethylphosphino)ethane) which direct, selective formation from trans-[W(N2)2(depe)2] is not possible.

Improvement of photocatalytic ammonia production of cobalt ferrite nanoparticles utilizing microporous ZSM-5 type ferrisilicate zeolite

The development of decarbonized synthesis approaches is a critical step in the fabrication of ammonia, an indispensable chemical and a potential carbon-neutral energy carrier. In this regard, the photocatalytic production technology has gained ample attention as a sustainable alternative to energy-intensive and environmentally detrimental Haber-Bosch process. Here, we present cobalt ferrite nanoparticles supported on microporous ZSM-5 type ferrisilicate zeolite as a desirable novel photocatalyst for the ammonia generation. The zeolite introduced as a microporous support increasing the catalytically active sites. A straightforward one-pot sol-gel method was used to synthesize cobalt ferrite (CoFe2O4) and CoFe2O4/ferrisilicate (CF/FS) nanocomposites with various weight percentages (10, 25 and 50%) of CoFe2O4. The photocatalytic performances of the samples in the production of ammonia were investigated under visible light irradiation. The highest rate of NH4+ production (484.74 µmol L-1 h-1) was achieved using the CF50%/FS photocatalyst. The distribution of < 50 nm-sized CoFe2O4 nanoparticles on the surface of the zeolite, as demonstrated by TEM images, and extensive BET surface areas are presented as convincing evidences for the improved photocatalytic activity paticularly in CF50%/FS photocatalyst.