Electroenzymatic C–C Bond Formation from CO2

Integration of biocompatible hydrogen evolution catalyst developed from metal-mix solutions with microbial electrosynthesis

Microbial conversion of CO2 to multi-carbon compounds such as acetate and butyrate is a promising valorisation technique. For those reactions, the electrochemical supply of hydrogen to the biocatalyst is a viable approach. Earlier we have shown that trace metals from microbial growth media spontaneously form in situ electro-catalysts for hydrogen evolution. Here, we show biocompatibility with the successful integration of such metal mix-based HER catalyst for immediate start-up of microbial acetogenesis (CO2 to acetate). Also, n-butyrate formation started fast (after twenty days). Hydrogen was always produced in excess, although productivity decreased over the 36 to 50 days, possibly due to metal leaching from the cathode. The HER catalyst boosted microbial productivity in a two-step microbial community bioprocess: acetogenesis by a BRH-c20a strain and acetate elongation to n-butyrate by Clostridium sensu stricto 12 (related) species. These findings provide new routes to integrate electro-catalysts and micro-organisms showing respectively bio and electrochemical compatibility.

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.

Advanced Electroanalysis for Electrosynthesis

Electrosynthesis is a popular, environmentally friendly substitute for conventional organic methods. It involves using charge transfer to stimulate chemical reactions through the application of a potential or current between two electrodes. In addition to electrode materials and the type of reactor employed, the strategies for controlling potential and current have an impact on the yields, product distribution, and reaction mechanism. In this Review, recent advances related to electroanalysis applied in electrosynthesis were discussed. The first part of this study acts as a guide that emphasizes the foundations of electrosynthesis. These essentials include instrumentation, electrode selection, cell design, and electrosynthesis methodologies. Then, advances in electroanalytical techniques applied in organic, enzymatic, and microbial electrosynthesis are illustrated with specific cases studied in recent literature. To conclude, a discussion of future possibilities that intend to advance the academic and industrial areas is presented.

Insights into Enzyme Reactions with Redox Cofactors in Biological Conversion of CO2

Carbon dioxide (CO2) is the most abundant component of greenhouse gases (GHGs) and directly creates environmental issues such as global warming and climate change. Carbon capture and storage have been proposed mainly to solve the problem of increasing CO2 concentration in the atmosphere; however, more emphasis has recently been placed on its use. Among the many methods of using CO2, one of the key environmentally friendly technologies involves biologically converting CO2 into other organic substances such as biofuels, chemicals, and biomass via various metabolic pathways. Although an efficient biocatalyst for industrial applications has not yet been developed, biological CO2 conversion is the needed direction. To this end, this review briefly summarizes seven known natural CO2 fixation pathways according to carbon number and describes recent studies in which natural CO2 assimilation systems have been applied to heterogeneous in vivo and in vitro systems. In addition, studies on the production of methanol through the reduction of CO2 are introduced. The importance of redox cofactors, which are often overlooked in the CO2 assimilation reaction by enzymes, is presented; methods for their recycling are proposed. Although more research is needed, biological CO2 conversion will play an important role in reducing GHG emissions and producing useful substances in terms of resource cycling.

Bioelectrocatalytic Synthesis: Concepts and Applications

Bioelectrocatalytic synthesis is the conversion of electrical energy into value-added products using biocatalysts. These methods merge the specificity and selectivity of biocatalysis and energy-related electrocatalysis to address challenges in the sustainable synthesis of pharmaceuticals, commodity chemicals, fuels, feedstocks and fertilizers. However, the specialized experimental setups and domain knowledge for bioelectrocatalysis pose a significant barrier to adoption. This review introduces key concepts of bioelectrosynthetic systems. We provide a tutorial on the methods of biocatalyst utilization, the setup of bioelectrosynthetic cells, and the analytical methods for assessing bioelectrocatalysts. Key applications of bioelectrosynthesis in ammonia production and small-molecule synthesis are outlined for both enzymatic and microbial systems. This review serves as a necessary introduction and resource for the non-specialist interested in bioelectrosynthetic research.

Light-driven Transformation of Carbon Monoxide into Hydrocarbons using CdS@ZnS : VFe Protein Biohybrids

Enzymatic Fisher-Tropsch (FT) process catalyzed by vanadium (V)-nitrogenase can convert carbon monoxide (CO) to longer-chain hydrocarbons (>C2) under ambient conditions, although this process requires high-cost reducing agent(s) and/or the ATP-dependent reductase as electron and energy sources. Using visible light-activated CdS@ZnS (CZS) core-shell quantum dots (QDs) as alternative reducing equivalent for the catalytic component (VFe protein) of V-nitrogenase, we first report a CZS : VFe biohybrid system that enables effective photo-enzymatic C-C coupling reactions, hydrogenating CO into hydrocarbon fuels (up to C4) that can be hardly achieved with conventional inorganic photocatalysts. Surface ligand engineering optimizes molecular and opto-electronic coupling between QDs and the VFe protein, realizing high efficiency (internal quantum yield >56 %), ATP-independent, photon-to-fuel production, achieving an electron turnover number of >900, that is 72 % compared to the natural ATP-coupled transformation of CO into hydrocarbons by V-nitrogenase. The selectivity of products can be controlled by irradiation conditions, with higher photon flux favoring (longer-chain) hydrocarbon generation. The CZS : VFe biohybrids not only can find applications in industrial CO removal for high-value-added chemical production by using the cheap, renewable solar energy, but also will inspire related research interests in understanding the molecular and electronic processes in photo-biocatalytic systems.

Enzymatic Fischer-Tropsch-Type Reactions

The Fischer-Tropsch (FT) process converts a mixture of CO and H2 into liquid hydrocarbons as a major component of the gas-to-liquid technology for the production of synthetic fuels. Contrary to the energy-demanding chemical FT process, the enzymatic FT-type reactions catalyzed by nitrogenase enzymes, their metalloclusters, and synthetic mimics utilize H+ and e- as the reducing equivalents to reduce CO, CO2, and CN- into hydrocarbons under ambient conditions. The C1 chemistry exemplified by these FT-type reactions is underscored by the structural and electronic properties of the nitrogenase-associated metallocenters, and recent studies have pointed to the potential relevance of this reactivity to nitrogenase mechanism, prebiotic chemistry, and biotechnological applications. This review will provide an overview of the features of nitrogenase enzymes and associated metalloclusters, followed by a detailed discussion of the activities of various nitrogenase-derived FT systems and plausible mechanisms of the enzymatic FT reactions, highlighting the versatility of this unique reactivity while providing perspectives onto its mechanistic, evolutionary, and biotechnological implications.

Industrial bioelectrochemistry for waste valorization: State of the art and challenges

Bioelectrochemistry has gained importance in recent years for some of its applications on waste valorization, such as wastewater treatment and carbon dioxide conversion, among others. The aim of this review is to provide an updated overview of the applications of bioelectrochemical systems (BESs) for waste valorization in the industry, identifying current limitations and future perspectives of this technology. BESs are classified according to biorefinery concepts into three different categories: (i) waste to power, (ii) waste to fuel and (iii) waste to chemicals. The main issues related to the scalability of bioelectrochemical systems are discussed, such as electrode construction, the addition of redox mediators and the design parameters of the cells. Among the existing BESs, microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) stand out as the more advanced technologies in terms of implementation and R&D investment. However, there has been little transfer of such achievements to enzymatic electrochemical systems. It is necessary that enzymatic systems learn from the knowledge reached with MFC and MEC to accelerate their development to achieve competitiveness in the short term.

Bioelectrocatalysis for CO2 reduction: recent advances and challenges to develop a sustainable system for CO2 utilization

Activation and turning CO₂ into value added products is a promising orientation to address environmental issues caused by CO₂ emission. Currently, electrocatalysis has a potent well-established role for CO₂ reduction with fast electron transfer rate; but it is challenged by the poor selectivity and low faradic efficiency. On the other side, biocatalysis, including enzymes and microbes, has been also employed for CO₂ conversion to target C_n products with remarkably high selectivity; however, low solubility of CO₂ in the liquid reaction phase seriously affects the catalytic efficiency. Therefore, a new synergistic role in bioelectrocatalysis for CO₂ reduction is emerging thanks to its outstanding selectivity, high faradic efficiency, and desirable valuable C_n products under mild condition that are surveyed in this review. Herein, we comprehensively discuss the results already obtained for the integration craft of enzymatic-electrocatalysis and microbial-electrocatalysis technologies. In addition, the intrinsic nature of the combination is highly dependent on the electron transfer. Thus, both direct electron transfer and mediated electron transfer routes are modeled and concluded. We also explore the biocompatibility and synergistic effects of electrode materials, which emerge in combination with tuned enzymes and microbes to improve catalytic performance. The system by integrating solar energy driven photo-electrochemical technics with bio-catalysis is further discussed. We finally highlight the significant findings and perspectives that have provided strong foundations for the remarkable development of green and sustainable bioelectrocatalysis for CO₂ reduction, and that offer a blueprint for C_n valuable products originate from CO₂ under efficient and mild conditions.

Ionization energies of metallocenes: a coupled cluster study of cobaltocene

Open-shell transition metal chemistry presents challenges to contemporary electronic structure methods, based on either density functional or wavefunction theory. While CCSD(T) is the well-trusted gold standard for maingroup thermochemistry, the accuracy and robustness of the method is less clear for open-shell transition metal chemistry, requiring benchmarking of CCSD(T)-based protocols against either higher-level theory or experiment. Ionization energies (IEs) of metallocenes provide an interesting test case with metallocenes being common redox reagents as well as playing roles as redox mediators and cocatalysts in redox catalysis. Using highly accurate ZEKE-MATI experimental measurements of gas phase adiabatic (5.3275 ± 0.0006 eV) and vertical (5.4424 ± 0.0006 eV) ionization energies of cobaltocene, we systematically assessed the accuracy of the local coupled-cluster method DLPNO-CCSD(T) with respect to geometry, reference determinant, basis set size and extrapolation schemes, PNO cut-off and extrapolation, local triples approximation, relativistic effects and core-valence correlation. We show that PNO errors are controllable via the recently introduced PNO extrapolation schemes and that the expensive iterative triples (T₁) contribution can be made more manageable by calculating it as a smaller-basis/smaller PNO-cutoff correction. The reference determinant turns out to be a critical aspect in these calculations with the HF determinant resulting in large DLPNO-CCSD(T) errors, likely due to the qualitatively flawed molecular orbital spectrum. The BP86 functional on the other hand was found to provide reference orbitals giving small DLPNO-CCSD(T) errors, likely due to more realistic orbitals as suggested by the more consistent MO spectrum compared to HF. A protocol including complete basis set extrapolations with correlation-consistent basis sets, complete PNO space extrapolations, iterative triples- and core-valence correlation corrections was found to give errors of -0.07 eV and -0.03 eV for adiabatic- and vertical-IE of cobaltocene, respectively, giving close to chemical accuracy for both properties. A computationally efficient DLPNO-CCSD(T) protocol was devised and tested against adiabatic ionization energies of 6 different metallocenes (V, Cr, Mn, Fe, Co, Ni). For the other metallocenes, the iterative triples (T₁) and PNO extrapolation contributions turn out to be even more important. The results give errors close to the experimental uncertainty, similar to recent auxiliary-field quantum Monte Carlo results. The quality of the reference determinant orbitals is identified as the main source of uncertainty in CCSD(T) calculations of metallocenes.

Conversion of Interfacial Chemical Bonds for Inducing Efficient Photoelectrocatalytic Water Splitting

Sp-C-hybridized alkyne bonds present the natural advantages of interacting with metal atoms and have the ability to generate a large number of new catalytic active sites on the surface and the interfaces, thus greatly promoting the efficient progress of various light/electrochemical reactions. In this work, we have successfully fabricated a novel type of interfacial structure containing sp-C-Mo/O bonds and mixed Mo valence states with outstanding catalytic activity and stability for photoelectrocatalytic (PEC) overall water splitting in a wide pH range (0-14), due to the presence of sp-carbon-rich graphdiyne. For example, in alkaline conditions (pH = 14), the overpotentials of oxygen and hydrogen evolution reactions at 10 mA cm-2 are 165 and 8 mV. When being used as an electrolyzer, the cell voltage of this catalyst is only 1.40 V to achieve 10 mA cm-2. The high PEC activity of graphdiyne@molybdenum oxide originates from the conversion of chemical bonds at the sp-C hybrid interface and the coexistence of multivalent states of molybdenum, triggering a large number of catalytic active sites, greatly promoting charge transfer and lowering water dissociation energy.

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.

Fast CO2 hydration kinetics impair heterogeneous but improve enzymatic CO2 reduction catalysis

The performance of heterogeneous catalysts for electrocatalytic CO₂ reduction (CO₂R) suffers from unwanted side reactions and kinetic inefficiencies at the required large overpotential. However, immobilised CO₂R enzymes — such as formate dehydrogenase — can operate with high turnover and selectivity at a minimal overpotential and are therefore ‘ideal’ model catalysts. Here, through the co-immobilisation of carbonic anhydrase, we study the effect of CO₂ hydration on the local environment and performance of a range of disparate CO₂R systems from enzymatic (formate dehydrogenase) to heterogeneous systems. We show that the co-immobilisation of carbonic anhydrase increases the kinetics of CO₂ hydration at the electrode. This benefits enzymatic CO₂ reduction — despite the decrease in CO₂ concentration — due to a reduction in local pH change, whereas it is detrimental to heterogeneous catalysis (on Au), because the system is unable to suppress the H₂ evolution side reaction. Understanding the role of CO₂ hydration kinetics within the local environment on the performance of electrocatalyst systems provides important insights for the development of next generation synthetic CO₂R catalysts.

Bioelectrocatalytic Cofactor Regeneration Coupled to CO2 Fixation in a Redox-Active Hydrogel for Stereoselective C-C Bond Formation

The sustainable capture and conversion of carbon dioxide (CO2 ) is key to achieving a circular carbon economy. Bioelectrocatalysis, which aims at using renewable energies to power the highly specific, direct transformation of CO2 into value added products, holds promise to achieve this goal. However, the functional integration of CO2 -fixing enzymes onto electrode materials for the electrosynthesis of stereochemically complex molecules remains to be demonstrated. Here, we show the electricity-driven regio- and stereoselective incorporation of CO2 into crotonyl-CoA by an NADPH-dependent enzymatic reductive carboxylation. Co-immobilization of a ferredoxin NADP+ reductase and crotonyl-CoA carboxylase/reductase within a 2,2'-viologen-modified hydrogel enabled iterative NADPH recycling and stereoselective formation of (2S)-ethylmalonyl-CoA, a prospective intermediate towards multi-carbon products from CO2 , with 92±6 % faradaic efficiency and at a rate of 1.6±0.4 μmol cm-2  h-1 . This approach paves the way for realizing even more complex bioelectrocatalyic cascades in the future.

Nickel Nanoparticles Immobilized over Mesoporous SBA-15 for Efficient Carbonylative Coupling Reactions Utilizing CO2: A Spotlight

Ecofriendly routes for the synthesis of carbamates and carbonylative coupling products such as benzyl formate derivatives are very demanding for both academia and industries. Foreseeing a sustainable green future, we systematically analyzed the synthesis history of both these chemicals, mentioning their pros and cons. As a step towards green chemistry, here we have optimized the reaction conditions for the synthesis of various benzyl formates from corresponding benzyl halides and carbamates from substituted anilines and alkyl halides catalyzed by Ni(0) nanoparticles (NPs) immobilized over amine-functionalized ordered mesoporous SBA-15 material in the presence of CO₂ as C1 source. This spotlight on applications is aimed to provide a clear outlook to date regarding the gradual progress in the synthesis of both these aforementioned chemicals and finally addresses further efforts for overcoming the current challenges.

Taming Electron Transfers: From Breaking Bonds to Creating Molecules

The electron is the ultimate redox reagent to build and reshape molecular structures. Understanding and controlling the parameters underlying dissociative electron transfer (DET) reactivity and its coupling with proton transfer is crucial for combining selectivity, kinetics and energy efficiency in molecular chemistry. Reactivity understanding and mechanistic elements in DET processes are traced back and key examples of current research efforts are presented, demonstrating a large variety of applications. The involvement of DET pathways indeed encompasses a broad range of processes such as photoredox catalysis, CO₂ reduction and alcohol oxidation. Interplay between these experimental examples and fundamental mechanistic study provides a powerful path to the understanding of driving force-rate relationships, which is crucial for the development of future generations of energy efficient catalytic schemes in redox organic chemistry.

Integrating abiotic chemical catalysis and enzymatic catalysis in living cells

We review hybrid systems of abiotic catalysis and enzymatic catalysis, which function in living cells. This research direction will stimulate multidisciplinary fields, including complex molecule synthesis, energy production, and life science.

Unleashing the Power of Biocatalysts

Biocatalysis is a continuously expanding subfield in chemical biology. Herein, I describe two categories of biocatalysts, the LEGO-brick-like and game-console-like type, both of which can streamline the synthetic routes to therapeutics. A multi-disciplinary approach to expand the biocatalytic toolkit will open up opportunities to develop new therapeutics.

Enzymatic Bioreactors: An Electrochemical Perspective

Biocatalysts provide a number of advantages such as high selectivity, the ability to operate under mild reaction conditions and availability from renewable resources that are of interest in the development of bioreactors for applications in the pharmaceutical and other sectors. The use of oxidoreductases in biocatalytic reactors is primarily focused on the use of NAD(P)-dependent enzymes, with the recycling of the cofactor occurring via an additional enzymatic system. The use of electrochemically based systems has been limited. This review focuses on the development of electrochemically based biocatalytic reactors. The mechanisms of mediated and direct electron transfer together with methods of immobilising enzymes are briefly reviewed. The use of electrochemically based batch and flow reactors is reviewed in detail with a focus on recent developments in the use of high surface area electrodes, enzyme engineering and enzyme cascades. A future perspective on electrochemically based bioreactors is presented.

Insights into Nitrogenase Bioelectrocatalysis for Green Ammonia Production

There is a growing interest in using ammonia as a liquid carrier of hydrogen for energy applications. Currently, ammonia is produced industrially by the Haber-Bosch process, which requires high temperature and high pressure. In contrast, bacteria have naturally evolved an enzyme known as nitrogenase, that is capable of producing ammonia and hydrogen at ambient temperature and pressure. Therefore, nitrogenases are attractive as a potentially more efficient means to produce ammonia via harnessing the unique properties of this enzyme. In recent years, exciting progress has been made in bioelectrocatalysis using nitrogenases to produce ammonia. Here, the prospects for developing biological ammonia production are outlined, key advances in bioelectrocatalysis by nitrogenases are highlighted, and possible solutions to the obstacles faced in realising this goal are discussed.

A molecular mediator for reductive concerted proton-electron transfers via electrocatalysis

Electrocatalytic approaches to the activation of unsaturated substrates via reductive concerted proton-electron transfer (CPET) must overcome competing, often kinetically dominant hydrogen evolution. We introduce the design of a molecular mediator for electrochemically triggered reductive CPET through the synthetic integration of a Brønsted acid and a redox mediator. Cathodic reduction at the cobaltocenium redox mediator substantially weakens the homolytic nitrogen-hydrogen bond strength of a Brønsted acidic anilinium tethered to one of the cyclopentadienyl rings. The electrochemically generated molecular mediator is demonstrated to transform a model substrate, acetophenone, to its corresponding neutral α-radical via a rate-determining CPET.

Reactivity, Mechanism, and Assembly of the Alternative Nitrogenases

Biological nitrogen fixation is catalyzed by the enzyme nitrogenase, which facilitates the cleavage of the relatively inert triple bond of N2. Nitrogenase is most commonly associated with the molybdenum-iron cofactor called FeMoco or the M-cluster, and it has been the subject of extensive structural and spectroscopic characterization over the past 60 years. In the late 1980s and early 1990s, two "alternative nitrogenase" systems were discovered, isolated, and found to incorporate V or Fe in place of Mo. These systems are regulated by separate gene clusters; however, there is a high degree of structural and functional similarity between each nitrogenase. Limited studies with the V- and Fe-nitrogenases initially demonstrated that these enzymes were analogously active as the Mo-nitrogenase, but more recent investigations have found capabilities that are unique to the alternative systems. In this review, we will discuss the reactivity, biosynthetic, and mechanistic proposals for the alternative nitrogenases as well as their electronic and structural properties in comparison to the well-characterized Mo-dependent system. Studies over the past 10 years have been particularly fruitful, though key aspects about V- and Fe-nitrogenases remain unexplored.

Gold nanoclusters cause selective light-driven biochemical catalysis in living nano-biohybrid organisms

Living nano-biohybrid organisms or nanorgs combine the specificity and well-designed surface chemistry of an enzyme catalyst site, with the strong light absorption and efficient charge injection (for biocatalytic reaction) from inorganic materials. Previous efforts in harvesting sunlight for renewable and sustainable photochemical conversion of inexpensive feedstocks to biochemicals using nanorgs focused on the design of semiconductor nanoparticles or quantum dots (QDs). However, metal nanoparticles and nanoclusters (NCs), such as gold (Au), offer strong light absorption properties and biocompatibility for potential application in living nanorgs. Here we show that optimized, sub-1 nanometer Au NCs-nanorgs can carry out selective biochemical catalysis with high turnover number (10⁸ mol mol^-1 of cells) and turnover frequency (>2 × 10⁷ h^-1). While the differences of size, light absorption, and electrochemical properties between these NCs (with 18, 22, and 25 atoms) are small, large differences in their light-activated properties dictate that 22 atom Au NCs are best suited for forming living nanorgs to drive photocatalytic ammonia production from air. Based on our experiments, these Au₂₂ NC-nanorgs demonstrate 29.3% quantum efficiency of converting absorbed photons to the desired chemical, and 12.9% efficiency of photon-to-fuel conversion based on energy input-output. Further, by comparing the light-driven ammonia production yield between strains producing Mo-Fe nitrogenase with and without histidine tags, we demonstrate that preferential coupling of Au NCs to the nitrogenase through Au-histidine interactions is crucial for effective electron transfer and subsequent product generation. Together, these results provide the design rules for forming Au NCs-nanorgs and can have important implications for carrying out light-driven biochemical catalysis for renewable solar fuel generation.

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.

Purple bacteria photo-bioelectrochemistry: enthralling challenges and opportunities

Perspective of research directions exploring purple bacteria photo-bioelectrochemistry: from harvesting photoexcited electrons to bioelectrochemical systems development.

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.

Sequential Co-immobilization of Enzymes in Metal-Organic Frameworks for Efficient Biocatalytic Conversion of Adsorbed CO2 to Formate

The main challenges in multienzymatic cascade reactions for CO₂ reduction are the low CO₂ solubility in water, the adjustment of substrate channeling, and the regeneration of co-factor. In this study, metal-organic frameworks (MOFs) were prepared as adsorbents for the storage of CO₂ and at the same time as solid supports for the sequential co-immobilization of multienzymes via a layer-by-layer self-assembly approach. Amine-functionalized MIL-101(Cr) was synthesized for the adsorption of CO₂. Using amine-MIL-101(Cr) as the core, two HKUST-1 layers were then fabricated for the immobilization of three enzymes chosen for the reduction of CO₂ to formate. Carbonic anhydrase was encapsulated in the inner HKUST-1 layer and hydrated the released CO₂ to HCO3-. Bicarbonate ions then migrated directly to the outer HKUST-1 shell containing formate dehydrogenase and were converted to formate. Glutamate dehydrogenase on the outer MOF layer achieved the regeneration of co-factor. Compared with free enzymes in solution using the bubbled CO₂ as substrate, the immobilized enzymes using stored CO₂ as substrate exhibited 13.1-times higher of formate production due to the enhanced substrate concentration. The sequential immobilization of enzymes also facilitated the channeling of substrate and eventually enabled higher catalytic efficiency with a co-factor-based formate yield of 179.8%. The immobilized enzymes showed good operational stability and reusability with a cofactor cumulative formate yield of 1077.7% after 10 cycles of reusing.

Strategies for Bioelectrochemical CO2 Reduction

Atmospheric CO₂ is a cheap and abundant source of carbon for synthetic applications. However, the stability of CO₂ makes its conversion to other carbon compounds difficult and has prompted the extensive development of CO₂ reduction catalysts. Bioelectrocatalysts are generally more selective, highly efficient, can operate under mild conditions, and use electricity as the sole reducing agent. Improving the communication between an electrode and a bioelectrocatalyst remains a significant area of development. Through the examples of CO₂ reduction catalyzed by electroactive enzymes and whole cells, recent advancements in this area are compared and contrasted.

Expansion of Redox Chemistry in Designer Metalloenzymes

Many artificial enzymes that catalyze redox reactions have important energy, environmental, and medical applications. Native metalloenzymes use a set of redox-active amino acids and cofactors as redox centers, with a potential range between -700 and +800 mV versus standard hydrogen electrode (SHE, all reduction potentials are versus SHE). The redox potentials and the orientation of redox centers in native metalloproteins are optimal for their redox chemistry. However, the limited number and potential range of native redox centers challenge the design and optimization of novel redox chemistry in metalloenzymes. Artificial metalloenzymes use non-native redox centers and could go far beyond the natural range of redox potentials for novel redox chemistry. In addition to designing protein monomers, strategies for increasing the electron transfer rate in self-assembled protein complexes and protein-electrode or -nanomaterial interfaces will be discussed. Redox reactions in proteins occur on redox active amino acid residues (Tyr, Trp, Met, Cys, etc.) and cofactors (iron sulfur clusters, flavin, heme, etc.). The redox potential of these redox centers cover a ∼1.5 V range and is optimized for their specific functions. Despite recent progress, tuning the redox potential for amino acid residues or cofactors remains challenging. Many redox-active unnatural amino acids (UAAs) can be incorporated into protein via genetic codon expansion. Their redox potentials extend the range of physiologically relevant potentials. Indeed, installing new redox cofactors with fined-tuned redox potentials is essential for designing novel redox enzymes. By combining UAA and redox cofactor incorporation, we harnessed light energy to reduce CO₂ in a fluorescent protein, mimicking photosynthetic apparatus in nature. Manipulating the position and reduction potential of redox centers inside proteins is important for optimizing the electron transfer rate and the activity of artificial enzymes. Learning from the native electron transfer complex, protein-protein interactions can be enhanced by increasing the electrostatic interaction between proteins. An artificial oxidase showed close to native enzyme activity with optimized interaction with electron transfer partner and increased electron transfer efficiency. In addition to the de novo design of protein-protein interaction, protein self-assembly methods using scaffolds, such as proliferating cell nuclear antigen, to efficiently anchor enzymes and their redox partners. The self-assembly process enhances electron transfer efficiency and enzyme activity by bringing redox centers into close proximity of each other. In addition to protein self-assembly, protein-electrode or protein-nanomaterial self-assembly can also promote efficient electron transfer from inorganic materials to enzyme active sites. Such hybrid systems combine the efficiency of enzyme reactions and the robustness of electrodes or nanomaterials, often with advantageous catalytic activities. By combining these strategies, we can not only mimic some of nature's most fascinating reactions, such as photosynthesis and aerobic respiration, but also transcend nature toward environmental, energy, and health applications.

Interfacing nature's catalytic machinery with synthetic materials for semi-artificial photosynthesis

Semi-artificial photosynthetic systems aim to overcome the limitations of natural and artificial photosynthesis while providing an opportunity to investigate their respective functionality. The progress and studies of these hybrid systems is the focus of this forward-looking perspective. In this Review, we discuss how enzymes have been interfaced with synthetic materials and employed for semi-artificial fuel production. In parallel, we examine how more complex living cellular systems can be recruited for in vivo fuel and chemical production in an approach where inorganic nanostructures are hybridized with photosynthetic and non-photosynthetic microorganisms. Side-by-side comparisons reveal strengths and limitations of enzyme- and microorganism-based hybrid systems, and how lessons extracted from studying enzyme hybrids can be applied to investigations of microorganism-hybrid devices. We conclude by putting semi-artificial photosynthesis in the context of its own ambitions and discuss how it can help address the grand challenges facing artificial systems for the efficient generation of solar fuels and chemicals.