Infrared spectroscopy of the nitrogenase MoFe protein under electrochemical control: potential-triggered CO binding

Light-driven, bias-free nitrogenase-based bioelectrochemical cell for ammonia generation

Nitrogen fixation is a key process that sustains life on Earth. Nitrogenase is the sole enzyme capable of fixing nitrogen under ambient conditions. Extensive research efforts have been dedicated to elucidating the enzyme mechanism and its artificial activation through high applied voltage, photochemistry, or strong reducing agents. Harnessing light irradiation to minimize the required external bias can lower the process's high energy investment. Herein, we present the development of photo-bioelectrochemical cells (PBECs) utilizing BiVO4/CoP or CdS/NiO photoanodes for nitrogenase activation toward N2 fixation. The constructed PBEC based on BiVO4/CoP photoanode requires minimal external bias (200 mV) and suppresses O2 generation that allows efficient activation of the nitrogenase enzyme, using glucose as an electron donor. In a second developed PBEC configuration, CdS/NiO photoanode was used, enabling bias-free activation of the nitrogenase-based cathode to produce 100 μM of ammonia at a faradaic efficiency (FE) of 12%. The ammonia production was determined by a commonly used fluorescence probe and further validated using 1H-NMR spectroscopy. The presented PBECs lay the foundation for biotic-abiotic systems to directly activate enzymes toward value-added chemicals by light-driven reactions.

The Fe Protein Cycle Associated with Nitrogenase Catalysis Requires the Hydrolysis of Two ATP for Each Single Electron Transfer Event

A central feature of the current understanding of dinitrogen (N₂) reduction by the enzyme nitrogenase is the proposed coupling of the hydrolysis of two ATP, forming two ADP and two Pi, to the transfer of one electron from the Fe protein component to the MoFe protein component, where substrates are reduced. A redox-active [4Fe-4S] cluster associated with the Fe protein is the agent of electron delivery, and it is well known to have a capacity to cycle between a one-electron-reduced [4Fe-4S]¹⁺ state and an oxidized [4Fe-4S]²⁺ state. Recently, however, it has been shown that certain reducing agents can be used to further reduce the Fe protein [4Fe-4S] cluster to a super-reduced, all-ferrous [4Fe-4S]⁰ state that can be either diamagnetic (S = 0) or paramagnetic (S = 4). It has been proposed that the super-reduced state might fundamentally alter the existing model for nitrogenase energy utilization by the transfer of two electrons per Fe protein cycle linked to hydrolysis of only two ATP molecules. Here, we measure the number of ATP consumed for each electron transfer under steady-state catalysis while the Fe protein cluster is in the [4Fe-4S]¹⁺ state and when it is in the [4Fe-4S]⁰ state. Both oxidation states of the Fe protein are found to operate by hydrolyzing two ATP for each single-electron transfer event. Thus, regardless of its initial redox state, the Fe protein transfers only one electron at a time to the MoFe protein in a process that requires the hydrolysis of two ATP.

Biomimetic mineralization based on self-assembling peptides

Biomimetic science has attracted great interest in the fields of chemistry, biology, materials science, and energy. Biomimetic mineralization is the process of synthesizing inorganic minerals under the control of organic molecules or biomolecules under mild conditions. Peptides are the motifs that constitute proteins, and can self-assemble into various hierarchical structures and show a high affinity for inorganic substances. Therefore, peptides can be used as building blocks for the synthesis of functional biomimetic materials. With the participation of peptides, the morphology, size, and composition of mineralized materials can be controlled precisely. Peptides not only provide well-defined templates for the nucleation and growth of inorganic nanomaterials but also have the potential to confer inorganic nanomaterials with high catalytic efficiency, selectivity, and biotherapeutic functions. In this review, we systematically summarize research progress in the formation mechanism, nanostructural manipulation, and applications of peptide-templated mineralized materials. These can further inspire researchers to design structurally complex and functionalized biomimetic materials with great promising applications.

Enzymatic and Microbial Electrochemistry: Approaches and Methods

The coupling of enzymes and/or intact bacteria with electrodes has been vastly investigated due to the wide range of existing applications. These span from biomedical and biosensing to energy production purposes and bioelectrosynthesis, whether for theoretical research or pure applied industrial processes. Both enzymes and bacteria offer a potential biotechnological alternative to noble/rare metal-dependent catalytic processes. However, when developing these biohybrid electrochemical systems, it is of the utmost importance to investigate how the approaches utilized to couple biocatalysts and electrodes influence the resulting bioelectrocatalytic response. Accordingly, this tutorial review starts by recalling some basic principles and applications of bioelectrochemistry, presenting the electrode and/or biocatalyst modifications that facilitate the interaction between the biotic and abiotic components of bioelectrochemical systems. Focus is then directed toward the methods used to evaluate the effectiveness of enzyme/bacteria-electrode interaction and the insights that they provide. The basic concepts of electrochemical methods widely employed in enzymatic and microbial electrochemistry, such as amperometry and voltammetry, are initially presented to later focus on various complementary methods such as spectroelectrochemistry, fluorescence spectroscopy and microscopy, and surface analytical/characterization techniques such as quartz crystal microbalance and atomic force microscopy. The tutorial review is thus aimed at students and graduate students approaching the field of enzymatic and microbial electrochemistry, while also providing a critical and up-to-date reference for senior researchers working in the field.

The crystalline state as a dynamic system: IR microspectroscopy under electrochemical control for a [NiFe] hydrogenase

Controlled formation of catalytically-relevant states within crystals of complex metalloenzymes represents a significant challenge to structure-function studies. Here we show how electrochemical control over single crystals of [NiFe] hydrogenase 1 (Hyd1) from Escherichia coli makes it possible to navigate through the full array of active site states previously observed in solution. Electrochemical control is combined with synchrotron infrared microspectroscopy, which enables us to measure high signal-to-noise IR spectra in situ from a small area of crystal. The output reports on active site speciation via the vibrational stretching band positions of the endogenous CO and CN- ligands at the hydrogenase active site. Variation of pH further demonstrates how equilibria between catalytically-relevant protonation states can be deliberately perturbed in the crystals, generating a map of electrochemical potential and pH conditions which lead to enrichment of specific states. Comparison of in crystallo redox titrations with measurements in solution or of electrode-immobilised Hyd1 confirms the integrity of the proton transfer and redox environment around the active site of the enzyme in crystals. Slowed proton-transfer equilibria in the hydrogenase in crystallo reveals transitions which are only usually observable by ultrafast methods in solution. This study therefore demonstrates the possibilities of electrochemical control over single metalloenzyme crystals in stabilising specific states for further study, and extends mechanistic understanding of proton transfer during the [NiFe] hydrogenase catalytic cycle.

The Conversion of Carbon Monoxide and Carbon Dioxide by Nitrogenases

Nitrogenases are the only known family of enzymes that catalyze the reduction of molecular nitrogen (N₂) to ammonia (NH₃). The N₂ reduction drives biological nitrogen fixation and the global nitrogen cycle. Besides the conversion of N₂, nitrogenases catalyze a whole range of other reductions, including the reduction of the small gaseous substrates carbon monoxide (CO) and carbon dioxide (CO₂) to hydrocarbons. However, it remains an open question whether these ‘side reactivities’ play a role under environmental conditions. Nonetheless, these reactivities and particularly the formation of hydrocarbons have spurred the interest in nitrogenases for biotechnological applications. There are three different isozymes of nitrogenase: the molybdenum and the alternative vanadium and iron‐only nitrogenase. The isozymes differ in their metal content, structure, and substrate‐dependent activity, despite their homology. This minireview focuses on the conversion of CO and CO₂ to methane and higher hydrocarbons and aims to specify the differences in activity between the three nitrogenase isozymes.

Electrochemical control of [FeFe]-hydrogenase single crystals reveals complex redox populations at the catalytic site

Elucidating the distribution of intermediates at the active site of redox metalloenzymes is vital to understanding their highly efficient catalysis. Here we demonstrate that it is possible to generate, and detect, the key catalytic redox states of an [FeFe]-hydrogenase in a protein crystal. Individual crystals of the prototypical [FeFe]-hydrogenase I from Clostridium pasteurianum (CpI) are maintained under electrochemical control, allowing for precise tuning of the redox potential, while the crystal is simultaneously probed via Fourier Transform Infrared (FTIR) microspectroscopy. The high signal/noise spectra reveal potential-dependent variation in the distribution of redox states at the active site (H-cluster) according to state-specific vibrational bands from the endogeneous CO and CN- ligands. CpI crystals are shown to populate the same H-cluster states as those detected in solution, including the oxidised species Hox, the reduced species Hred/HredH+, the super-reduced HsredH+ and the hydride species Hhyd. The high sensitivity and precise redox control offered by this approach also facilitates the detection and characterisation of low abundance species that only accumulate within a narrow window of conditions, revealing new redox intermediates.

The Spectroscopy of Nitrogenases

Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.

Natural and Engineered Electron Transfer of Nitrogenase

As the only enzyme currently known to reduce dinitrogen (N2) to ammonia (NH3), nitrogenase is of significant interest for bio-inspired catalyst design and for new biotechnologies aiming to produce NH3 from N2. In order to reduce N2, nitrogenase must also hydrolyze at least 16 equivalents of adenosine triphosphate (MgATP), representing the consumption of a significant quantity of energy available to biological systems. Here, we review natural and engineered electron transfer pathways to nitrogenase, including strategies to redirect or redistribute electron flow in vivo towards NH3 production. Further, we also review strategies to artificially reduce nitrogenase in vitro, where MgATP hydrolysis is necessary for turnover, in addition to strategies that are capable of bypassing the requirement of MgATP hydrolysis to achieve MgATP-independent N2 reduction.

Reduction of Substrates by Nitrogenases

Nitrogenase is the enzyme that catalyzes biological N₂ reduction to NH₃. This enzyme achieves an impressive rate enhancement over the uncatalyzed reaction. Given the high demand for N₂ fixation to support food and chemical production and the heavy reliance of the industrial Haber-Bosch nitrogen fixation reaction on fossil fuels, there is a strong need to elucidate how nitrogenase achieves this difficult reaction under benign conditions as a means of informing the design of next generation synthetic catalysts. This Review summarizes recent progress in addressing how nitrogenase catalyzes the reduction of an array of substrates. New insights into the mechanism of N₂ and proton reduction are first considered. This is followed by a summary of recent gains in understanding the reduction of a number of other nitrogenous compounds not considered to be physiological substrates. Progress in understanding the reduction of a wide range of C-based substrates, including CO and CO₂, is also discussed, and remaining challenges in understanding nitrogenase substrate reduction are considered.

Spectroelectrochemistry, the future of visualizing electrode processes by hyphenating electrochemistry with spectroscopic techniques

Reviewing the future of electrochemistry combined with infrared, Raman, and nuclear magnetic resonance spectroscopy as well as mass spectrometry.

Nitrogenase Bioelectrochemistry for Synthesis Applications

The fixation of atmospheric dinitrogen to ammonia by industrial technologies (such as the Haber Bosch process) has revolutionized humankind. In contrast to industrial technologies, a single enzyme is known for its ability to reduce or "fix" dinitrogen: nitrogenase. Nitrogenase is a complex oxidoreductase enzymatic system that includes a catalytic protein (where dinitrogen is reduced) and an electron-transferring reductase protein (termed the Fe protein) that delivers the electrons necessary for dinitrogen fixation. The catalytic protein most commonly contains a FeMo cofactor (called the MoFe protein), but it can also contain a VFe or FeFe cofactor. Besides their ability to fix dinitrogen to ammonia, these nitrogenases can also reduce substrates such as carbon dioxide to formate. Interestingly, the VFE nitrogenase can also form carbon-carbon bonds. The vast majority of research surrounding nitrogenase employs the Fe protein to transfer electrons, which is also associated with the rate-limiting step of nitrogenase catalysis and also requires the hydrolysis of adenosine triphosphate. Thus, there is significant interest in artificially transferring electrons to the catalytic nitrogenase proteins. In this Account, we review nitrogenase electrocatalysis whereby electrons are delivered to nitrogenase from electrodes. We first describe the use of an electron mediator (cobaltocene) to transfer electrons from electrodes to the MoFe protein. The reduction of protons to molecular hydrogen was realized, in addition to azide and nitrite reduction to ammonia. Bypassing the rate-limiting step within the Fe protein, we also describe how this approach was used to interrogate the rate-limiting step of the MoFe protein: metal-hydride protonolysis at the FeMo-co. This Account next reviews the use of cobaltocene to mediate electron transfer to the VFe protein, where the reduction of carbon dioxide and the formation of carbon-carbon bonds (yielding the formation of ethene and propene) was realized. This approach also found success in mediating electron transfer to the FeFe catalytic protein, which exhibited improved carbon dioxide reduction in comparison to the MoFe protein. In the final example of mediated electron transfer to the catalytic protein, this Account also reviews recent work where the coupling of infrared spectroscopy with electrochemistry enabled the potential-dependent binding of carbon monoxide to the FeMo-co to be studied. As an alternative to mediated electron transfer, recent work that has sought to transfer electrons to the catalytic proteins in the absence of electron mediators (by direct electron transfer) is also reviewed. This approach has subsequently enabled a thermodynamic landscape to be proposed for the cofactors of the catalytic proteins. Finally, this Account also describes nitrogenase electrocatalysis whereby electrons are first transferred from an electrode to the Fe protein, before being transferred to the MoFe protein alongside the hydrolysis of adenosine triphosphate. In this way, increased quantities of ammonia can be electrocatalytically produced from dinitrogen fixation. We discuss how this has led to the further upgrade of electrocatalytically produced ammonia, in combination with additional enzymes (diaphorase, alanine dehydrogenase, and transaminase), to selective production of chiral amine intermediates for pharmaceuticals. This Account concludes by discussing current and future research challenges in the field of electrocatalytic nitrogen fixation by nitrogenase.

Spectroscopic Description of the E1 State of Mo Nitrogenase Based on Mo and Fe X-ray Absorption and Mössbauer Studies

Mo nitrogenase (N2ase) utilizes a two-component protein system, the catalytic MoFe and its electron-transfer partner FeP, to reduce atmospheric dinitrogen (N₂) to ammonia (NH₃). The FeMo cofactor contained in the MoFe protein serves as the catalytic center for this reaction and has long inspired model chemistry oriented toward activating N₂. This field of chemistry has relied heavily on the detailed characterization of how Mo N2ase accomplishes this feat. Understanding the reaction mechanism of Mo N2ase itself has presented one of the most challenging problems in bioinorganic chemistry because of the ephemeral nature of its catalytic intermediates, which are difficult, if not impossible, to singly isolate. This is further exacerbated by the near necessity of FeP to reduce native MoFe, rendering most traditional means of selective reduction inept. We have now investigated the first fundamental intermediate of the MoFe catalytic cycle, E₁, as prepared both by low-flux turnover and radiolytic cryoreduction, using a combination of Mo Kα high-energy-resolution fluorescence detection and Fe K-edge partial-fluorescence-yield X-ray absorption spectroscopy techniques. The results demonstrate that the formation of this state is the result of an Fe-centered reduction and that Mo remains redox-innocent. Furthermore, using Fe X-ray absorption and ⁵⁷Fe Mössbauer spectroscopies, we correlate a previously reported unique species formed under cryoreducing conditions to the natively formed E₁ state through annealing, demonstrating the viability of cryoreduction in studying the catalytic intermediates of MoFe.

Beyond fossil fuel-driven nitrogen transformations

Nitrogen is fundamental to all of life and many industrial processes. The interchange of nitrogen oxidation states in the industrial production of ammonia, nitric acid, and other commodity chemicals is largely powered by fossil fuels. A key goal of contemporary research in the field of nitrogen chemistry is to minimize the use of fossil fuels by developing more efficient heterogeneous, homogeneous, photo-, and electrocatalytic processes or by adapting the enzymatic processes underlying the natural nitrogen cycle. These approaches, as well as the challenges involved, are discussed in this Review.

A Thermodynamic Model for Redox-Dependent Binding of Carbon Monoxide at Site-Differentiated, High Spin Iron Clusters

Binding of N2 and CO by the FeMo-cofactor of nitrogenase depends on the redox level of the cluster, but the extent to which pure redox chemistry perturbs the affinity of high spin iron clusters for π-acids is not well understood. Here, we report a series of site-differentiated iron clusters that reversibly bind CO in redox states FeII4 through FeIIFeIII3. One electron redox events result in small changes in the affinity for (at most ∼400-fold) and activation of CO (at most 28 cm-1 for νCO). The small influence of redox chemistry on the affinity of these high spin, valence-localized clusters for CO is in stark contrast to the large enhancements (105-1022 fold) in π-acid affinity reported for monometallic and low spin, bimetallic iron complexes, where redox chemistry occurs exclusively at the ligand binding site. While electron-loading at metal centers remote from the substrate binding site has minimal influence on the CO binding energetics (∼1 kcal·mol-1), it provides a conduit for CO binding at an FeIII center. Indeed, internal electron transfer from these remote sites accommodates binding of CO at an FeIII, with a small energetic penalty arising from redox reorganization (∼2.6 kcal·mol-1). The ease with which these clusters redistribute electrons in response to ligand binding highlights a potential pathway for coordination of N2 and CO by FeMoco, which may occur on an oxidized edge of the cofactor.

Comparison of hydroxycarboxylato imidazole molybdenum(iv) complexes and nitrogenase protein structures: indirect evidence for the protonation of homocitrato FeMo-cofactors

Glycolato and lactato imidazole molybdenum(iv) complexes are used for structural comparison with FeMo-cofactors of MoFe-protein structures.

Nitrogen Reduction to Ammonia on a Biomimetic Mononuclear Iron Centre: Insights into the Nitrogenase Enzyme

Nitrogenases catalyse nitrogen fixation to ammonia on a multinuclear Fe‐Mo centre, but their mechanism and particularly the order of proton and electron transfer processes that happen during the catalytic cycle is still unresolved. Recently, a unique biomimetic mononuclear iron model was developed using tris(phosphine)borate (TPB) ligands that was shown to convert N₂ into NH₃. Herein, we present a computational study on the [(TPB)FeN₂]⁻ complex and describe its conversion into ammonia through the addition of electrons and protons. In particular, we tested the consecutive proton transfer on only the distal nitrogen atom or alternated protonation of the distal/proximal nitrogen. It is found that the lowest energy pathway is consecutive addition of three protons to the same site, which forms ammonia and an iron‐nitrido complex. In addition, the proton transfer step of complexes with the metal in various oxidation and spin states were tested and show that the pK_a values of biomimetic mononuclear nitrogenase intermediates vary little with iron oxidation states. As such, the model gives several possible NH₃ formation pathways depending on the order of electron/proton transfer, and all should be physically accessible in the natural system. These results may have implications for enzymatic nitrogenases and give insight into the catalytic properties of mononuclear iron centres.

Protein Film Infrared Electrochemistry Demonstrated for Study of H2 Oxidation by a [NiFe] Hydrogenase

Understanding the chemistry of redox proteins demands methods that provide precise control over redox centers within the protein. The technique of protein film electrochemistry, in which a protein is immobilized on an electrode surface such that the electrode replaces physiological electron donors or acceptors, has provided functional insight into the redox reactions of a range of different proteins. Full chemical understanding requires electrochemical control to be combined with other techniques that can add additional structural and mechanistic insight. Here we demonstrate a technique, protein film infrared electrochemistry, which combines protein film electrochemistry with infrared spectroscopic sampling of redox proteins. The technique uses a multiple-reflection attenuated total reflectance geometry to probe a redox protein immobilized on a high surface area carbon black electrode. Incorporation of this electrode into a flow cell allows solution pH or solute concentrations to be changed during measurements. This is particularly powerful in addressing redox enzymes, where rapid catalytic turnover can be sustained and controlled at the electrode allowing spectroscopic observation of long-lived intermediate species in the catalytic mechanism. We demonstrate the technique with experiments on E. coli hydrogenase 1 under turnover (H2 oxidation) and non-turnover conditions.

Bioelectrochemical Haber-Bosch Process: An Ammonia-Producing H2 /N2 Fuel Cell

Nitrogenases are the only enzymes known to reduce molecular nitrogen (N₂ ) to ammonia (NH₃ ). By using methyl viologen (N,N'-dimethyl-4,4'-bipyridinium) to shuttle electrons to nitrogenase, N₂ reduction to NH₃ can be mediated at an electrode surface. The coupling of this nitrogenase cathode with a bioanode that utilizes the enzyme hydrogenase to oxidize molecular hydrogen (H₂ ) results in an enzymatic fuel cell (EFC) that is able to produce NH₃ from H₂ and N₂ while simultaneously producing an electrical current. To demonstrate this, a charge of 60 mC was passed across H₂  /N₂ EFCs, which resulted in the formation of 286 nmol NH₃  mg^-1 MoFe protein, corresponding to a Faradaic efficiency of 26.4 %.