Hydrogen Evolution from Water under Aerobic Conditions Catalyzed by a Cobalt ATCUN Metallopeptide

Decrypting the hydrogen evolution in alkaline water with novel magnetoactive cobalt(II) complex-driven cobalt oxide electrocatalysts

Under the gravity of future socio-economic development, the viability of water electrolysis still hinges on the accessibility of stable earth-abundant electrocatalysts and net energy efficiency. This work emphasizes the design and synthesis of two newly developed cobalt(II) complexes, [Co(HL)2(NCS)2] (Comono) and [Co2(L)3(CH3OH)]ClO4 (Codi), with a (N,O)-donor ligand, HL (2-methoxy-6-(((2-methoxyphenyl)imino)methyl)phenol). The study delves into understanding their structural, morphological, magnetic, and charge transport characteristics. Moreover, the study explores the potential of these complexes in catalyzing hydrogen production through heterogeneous electrocatalysis. The X-ray crystal structure of Comono reveals the octahedral geometry of the Co(II) ion, adopting two HL units and two NCS- units. The Codi complex exhibits a doubly-phenoxo-O-bridged (μ1,1) dinuclear complex, forming a typical octahedral geometry for both the Co(II) centres in coupling with three units of L-. Temperature-dependent magnetic susceptibility measurements showed that all of the Co(II) ion in Comono shows a typical paramagnetic behaviour for high spin octahedral Co(II) ions while the Co(II) centres in Codi are coupled with doubly-phenoxo-bridges bearing weak ferromagnetic characteristics at low temperature. Electron transport properties of the Co(II) complex-mediated Schottky device address the superior carrier mobility (μ) for Codi (9.21 × 10-5) over Comono (2.02 × 10-5 m2 v-1 s-1) with respective transit times of 1.70 × 10-9 and 7.77 × 10-9 s. Additionally, electron impedance spectral analysis supports the lower electrical transport resistance of Codi relative to Comono. The heterogeneous electrocatalytic HER activity of Codi and Comono in 0.1 M KOH shows excellent electrocatalytic efficiency in terms of the various electrochemical parameters. Constant potential electrolysis, multi-cycle CVs, and post-HER analysis reveal the pre-catalytic nature of the complexes, which in turn delivers Co3O4 nanoparticles as the active catalysts for efficient hydrogen evolution.

Efficiency Conceptualization Model: A Theoretical Method for Predicting the Turnover of Catalysts

In recent times, the theoretical prediction of catalytic efficiency is of utmost urgency. With the advent of density functional theory (DFT), reliable computations can delineate a quantitative aspect of the study. To this state-of-the-art approach, valuable incorporation would be a tool that can acknowledge the efficiency of a catalyst. In the current work, we developed the efficiency conceptualization model (ECM) that utilizes the quantum mechanical tool to achieve efficiency in terms of turnover frequency (TOF). Twenty-six experimentally designed transition metal (TM) water oxidation catalysts were chosen under similar experimental conditions of temperature, pressure, and pH to execute the same. The computations conclude that the Fe-based [Fe(OTf)2(Me2Pytacn)] (MWOC-17) is a highly active catalyst and, therefore, can endure for more time in the catalytic cycle. Our results conclude that the Ir-based catalysts [Cp*Ir(κ2-N,O)X] with MWOC-23: X=Cl; and MWOC-24: X=NO3 report the highest computed turnover numbers (TONs),τ c o m p u t e d T O N 0 ${\tau _{computed\;TON}^0 }$ of 406 and 490 against the highest experimental TONs,τ e x p e r i m e n t a l T O N ${\tau _{experimental\;TON} }$ of 1200 and 2000 respectively, whereas the Co-based [Co(12-TMC)]2+ (MWOC-19) has the lowest TONs (τ c o m p u t e d T O N 0 ${\tau _{computed\;TON}^0 }$ =19, τexperimental TON=16) among the chosen catalysts and thereby successful in corroborating the previous experimental results.

Heptanuclear Mixed-Valence Co4IIICo3II Molecular Wheel─A Molecular Analogue of Layered Double Hydroxides with Single-Molecule Magnet Behavior and Electrocatalytic Activity for Hydrogen Evolution Reactions

We present a bifunctional heptanuclear cobalt(II)/cobalt(III) molecular complex formulated as [Co7(μ3-OH)4(H2L1)2(HL2)2](NO3)6·6H2O (1) (where H5L1 is 2,2'-(((1E,1'E)-((2-hydroxy-5-methyl-1,3-phenylene)bis(methanylylidene))bis(azanylylidene))bis(propane-1,3-diol)) and H2L2 is 2-amino-1,3-propanediol). Compound 1 has been characterized by single-crystal X-ray diffraction analysis along with other spectral and magnetic measurements. Structural analysis indicates that 1 contains a mixed-valence Co7 cluster where a central Co(II) ion is connected to six different Co centers (four CoIII and two CoII ions) by four μ3-OH groups, giving rise to a planar heptanuclear cluster that resembles a molecular fragment of a layered double hydroxide (LDH). Two triply deprotonated (H2L1)3- ligands form the outer side of the cluster while two singly deprotonated (HL2)- ligands are located at the top and bottom of the central heptanuclear core. Variable temperature magnetic measurements indicate the presence of weak ferromagnetic CoII···CoII interactions (J = 3.53(6) cm-1) within the linear trinuclear CoII cluster. AC susceptibility measurements show that 1 is a field-induced single-molecule magnet (SMM) with τ0 = 8.2(7) × 10-7 s and Ueff = 11.3(4) K. The electrocatalytic hydrogen evolution reaction (HER) activity of 1 in homogeneous phase shows an overpotential of 455 mV, with a Faradaic efficiency of 81% and a TOF of 8.97 × 104 μmol H2 h-1 mol-1.

Hydrogen Evolution of a Unique DNAzyme Composed of Cobalt-Protoporphyrin IX and G-Quadruplex DNA

Molecular hydrogen (H2 ) is a clean and renewable fuel that has garnered significant interest in the search for alternatives to fossil fuels. Here, we constructed an artificial DNAzyme composed of cobalt-protoporphyrin IX (CoPP) and G-quadruplex DNA, possessing a unique H2 Oint ligand between the CoPP and G-quartet planes. We show for the first time that CoPP-DNAzyme catalyzes photo-induced H2 production under anaerobic conditions with a turnover number (TON) of 1229 ± 51 over 12 h at pH 6.05 and 10 °C. Compared with free-CoPP, complexation with G-quadruplex DNA resulted in a 4.7-fold increase in H2 production activity. The TON of the CoPP-DNAzyme revealed an optimal acid-base equilibrium with a pKa value of 7.60 ± 0.05, apparently originating from the equilibrium between Co(III)-H- and Co(I) states. Our results demonstrate that the H2 Oint ligand can augment and modulate the intrinsic catalytic activity of H2 production catalysts. These systems pave the way to using DNAzymes for H2 evolution in the direct conversion of solar energy to H2 from water.

The effect of metalation on antimicrobial piscidins imbedded in normal and oxidized lipid bilayers

Metalation of the N-terminal Amino Terminal Cu(ii)- and Ni(ii)-binding (ATCUN) motif may enhance the antimicrobial properties of piscidins. Molecular dynamics simulations of free and nickelated piscidins 1 and 3 (P1 and P3) were performed in 3 : 1 POPC/POPG and 2.6 : 1 : 0.4 POPC/POPG/aldo-PC bilayers (POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine: POPG, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol; aldo-PC, 1-palmitoyl-2-(9'-oxo-nonanoyl)-sn-glycero-3-phosphocholine) bilayer models. Nickel(ii) binding decreases the conformation dynamics of the ATCUN motif and lowers the charge of the N-terminus to allow it to embed deeper in the bilayer without significantly changing the overall depth due to interactions of the charged half-helix of the peptide with the headgroups. Phe1⋯Ni2+ cation-π and Phe2-Phe1 CH-π interactions contribute to a small fraction of structures within the nickelated P1 simulations and may partially protect a bound metal from metal-centered chemical activity. The substitution of Phe2 for Ile2 in P3 sterically blocks conformations with cation-π interactions offering less protection to the metal. This difference between metalated P1 and P3 may indicate a mechanism by which peptide sequence can influence antimicrobial properties. Any loss of bilayer integrity due to chain reversal of the oxidized phospholipid chains of aldo-PC may be enhanced in the presence of metalated piscidins.

A Series of Ni Complexes Based on a Versatile ATCUN-Like Tripeptide Scaffold to Decipher Key Parameters for Superoxide Dismutase Activity

The cellular level of reactive oxygen species (ROS) has to be controlled to avoid some pathologies, especially those linked to oxidative stress. One strategy for designing antioxidants consists of modeling natural enzymes involved in ROS degradation. Among them, nickel superoxide dismutase (NiSOD) catalyzes the dismutation of the superoxide radical anion, O₂^•-, into O₂ and H₂O₂. We report here Ni complexes with tripeptides derived from the amino-terminal Cu^II- and Ni^II-binding (ATCUN) motif that mimics some structural features found in the active site of the NiSOD. A series of six mononuclear Ni^II complexes were investigated in water at physiological pH with different first coordination spheres, from compounds with a N3S to N2S2 set, and also complexes that are in equilibrium between the N-coordination (N3S) and S-coordination (N2S2). They were fully characterized by a combination of spectroscopic techniques, including ¹H NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations and their redox properties studied by cyclic voltammetry. They all display SOD-like activity, with a k_cat ranging between 0.5 and 2.0 × 10⁶ M^-1 s^-1. The complexes in which the two coordination modes are in equilibrium are the most efficient, suggesting a beneficial effect of a nearby proton relay.

A self-sustaining serpentinization mega-engine feeds the fougerite nanoengines implicated in the emergence of guided metabolism

The demonstration by Ivan Barnes et al. that the serpentinization of fresh Alpine-type ultramafic rocks results in the exhalation of hot alkaline fluids is foundational to the submarine alkaline vent theory (AVT) for life's emergence to its 'improbable' thermodynamic state. In AVT, such alkaline fluids ≤ 150°C, bearing H2 > CH4 > HS--generated and driven convectively by a serpentinizing exothermic mega-engine operating in the ultramafic crust-exhale into the iron-rich, CO2> > > NO3--bearing Hadean ocean to result in hydrothermal precipitate mounds comprising macromolecular ferroferric-carbonate oxyhydroxide and minor sulfide. As the nanocrystalline minerals fougerite/green rust and mackinawite (FeS), they compose the spontaneously precipitated inorganic membranes that keep the highly contrasting solutions apart, thereby maintaining redox and pH disequilibria. They do so in the form of fine chimneys and chemical gardens. The same disequilibria drive the reduction of CO2 to HCOO- or CO, and the oxidation of CH4 to a methyl group-the two products reacting to form acetate in a sequence antedating the 'energy-producing' acetyl coenzyme-A pathway. Fougerite is a 2D-layered mineral in which the hydrous interlayers themselves harbor 2D solutions, in effect constricted to ~ 1D by preferentially directed electron hopping/tunneling, and proton Gröthuss 'bucket-brigading' when subject to charge. As a redox-driven nanoengine or peristaltic pump, fougerite forces the ordered reduction of nitrate to ammonium, the amination of pyruvate and oxalate to alanine and glycine, and their condensation to short peptides. In turn, these peptides have the flexibility to sequester the founding inorganic iron oxyhydroxide, sulfide, and pyrophosphate clusters, to produce metal- and phosphate-dosed organic films and cells. As the feed to the hydrothermal mound fails, the only equivalent sustenance on offer to the first autotrophs is the still mildly serpentinizing upper crust beneath. While the conditions here are very much less bountiful, they do offer the similar feed and disequilibria the survivors are accustomed to. Sometime during this transition, a replicating non-ribosomal guidance system is discovered to provide the rules to take on the incrementally changing surroundings. The details of how these replicating apparatuses emerged are the hard problem, but by doing so the progenote archaea and bacteria could begin to colonize what would become the deep biosphere. Indeed, that the anaerobic nitrate-respiring methanotrophic archaea and the deep-branching Acetothermia presently comprise a portion of that microbiome occupying serpentinizing rocks offers circumstantial support for this notion. However, the inescapable, if jarring conclusion is drawn that, absent fougerite/green rust, there would be no structured channelway to life.

Impact of N-heteroaromatic N-termini in Cu(II) ATCUN metallopeptides on their biorelevant redox activity

Cu(II) complexes with ATCUN peptide ligands have been investigated for their ROS (reactive oxygen species) generation and oxidative DNA degradation abilities. The biological activity of most ATCUN complexes such as Cu-GGH (Gly-Gly-His) is, however, low. Tuning the redox chemistry by incorporation of N-heteroaromatics reinstates ROS production which leads to efficient DNA cleavage.

Photocatalytic Hydrogen Evolution by a De Novo Designed Metalloprotein that Undergoes Ni-Mediated Oligomerization Shift

De novo metalloprotein design involves the construction of proteins guided by specific repeat patterns of polar and apolar residues, which, upon self-assembly, provide a suitable environment to bind metals and produce artificial metalloenzymes. While a wide range of functionalities have been realized in de novo designed metalloproteins, the functional repertoire of such constructs towards alternative energy-relevant catalysis is currently limited. Here we show the application of de novo approach to design a functional H2 evolving protein. The design involved the assembly of an amphiphilic peptide featuring cysteines at tandem a/d sites of each helix. Intriguingly, upon NiII addition, the oligomers shift from a major trimeric assembly to a mix of dimers and trimers. The metalloprotein produced H2 photocatalytically with a bell-shape pH dependence, having a maximum activity at pH 5.5. Transient absorption spectroscopy is used to determine the timescales of electron transfer as a function of pH. Selective outer sphere mutations are made to probe how the local environment tunes activity. A preferential enhancement of activity is observed via steric modulation above the NiII site, towards the N-termini, compared to below the NiII site towards the C-termini.

Novel Dithiolene Nickel Complex Catalysts for Electrochemical Hydrogen Evolution Reaction for Hydrogen Production in Nonaqueous and Aqueous Solutions

Three molecular catalysts based on mononuclear nickel(II) complexes with square planar geometries, [BzPy]2[Ni(mnt)2] (1), [BzPy]2[Ni(i-mnt)2] (2), and [BzPy]2[Ni(tdas)2] (3) (BzPy = benzyl pyridinium) are synthesized by the reaction of NiCl2∙6H2O, [BzPy]Br, and Na2(mnt)/Na2(i-mnt)/Na2(tdas) (mnt = 1,2-dicyanoethylene-1,2-dithiolate for (1), i-mnt = 2,2-dicyanoethylene-1,1-dithiolate for (2), and tdas = 1,2,5-thiadiazole-3,4-dithiolate for (3)), respectively. The structures and compositions of these three catalysts are characterized by XRD, elemental analysis, FT-IR, and ESI-MS. The electrochemical properties and the corresponding catalytic activities of these three catalysts are studied by cyclic voltammetry. The controlled-potential electrolysis with gas chromatography analysis confirms the hydrogen production with a turnover frequency (TOF) of 116.89, 165.51, and 189.16 moles of H2 per mole of catalyst per hour at a potential of − 0.99 V (versus SHE) in acetonitrile solutions containing the catalysts, respectively. In a neutral buffer solution, these three molecular catalysts exhibit a TOF of 411.85, 488.76, and 555.06 mol of H2 per mole of catalyst per hour at a potential of − 0.49 V (versus SHE), respectively, indicating that Complex 3 constitutes the better active catalyst than Complexes 1 and 2. For fundamental understanding, a catalytic HER mechanism is also proposed.

Graphical abstract

Repurposing a peptide antibiotic as a catalyst: a multicopper–daptomycin complex as a cooperative O–O bond formation and activation catalyst

A peptide antibiotic, daptomycin, was repurposed to a multicopper catalyst presenting cooperative rate enhancement in O–O bond formation and activation reactions.

Hydrothermal synthesis of long-chain hydrocarbons up to C24 with NaHCO3-assisted stabilizing cobalt

Abiotic CO₂ reduction on transition metal minerals has been proposed to account for the synthesis of organic compounds in alkaline hydrothermal systems, but this reaction lacks experimental support, as only short-chain hydrocarbons (<C₅) have been synthesized in artificial simulation. This presents a question: What particular hydrothermal conditions favor long-chain hydrocarbon synthesis? Here, we demonstrate the hydrothermal bicarbonate reduction at ∼300 °C and 30 MPa into long-chain hydrocarbons using iron (Fe) and cobalt (Co) metals as catalysts. We found the Co⁰ promoter responsible for synthesizing long-chain hydrocarbons to be extraordinarily stable when coupled with Fe-OH formation. Under these hydrothermal conditions, the traditional water-induced deactivation of Co is inhibited by bicarbonate-assisted CoO_x reduction, leading to honeycomb-native Co nanosheets generated in situ as a new motif. The Fe-OH formation, confirmed by operando infrared spectroscopy, enhances CO adsorption on Co, thereby favoring further reduction to long-chain hydrocarbons (up to C₂₄). These results not only advance theories for an abiogenic origin for some petroleum accumulations and the hydrothermal hypothesis of the emergence of life but also introduce an approach for synthesizing long-chain hydrocarbons by nonnoble metal catalysts for artificial CO₂ utilization.

Biohybrid Systems for Improved Bioinspired, Energy-Relevant Catalysis

Biomimetic catalysts, ranging from small-molecule metal complexes to supramolecular assembles, possess many exciting properties that could address salient challenges in industrial-scale manufacturing. Inspired by natural enzymes, these biohybrid catalytic systems demonstrate superior characteristics, including high activity, enantioselectivity, and enhanced aqueous solubility, over their fully synthetic counterparts. However, instability and limitations in the prediction of structure-function relationships are major drawbacks that often prevent the application of biomimetic catalysts outside of the laboratory. Despite these obstacles, recent advances in synthetic enzyme models have improved our understanding of complicated biological enzymatic processes and enabled the production of catalysts with increased efficiency. This review outlines important developments and future prospects for the design and application of bioinspired and biohybrid systems at multiple length scales for important, biologically relevant, clean energy transformations.

A De Novo-Designed Artificial Metallopeptide Hydrogenase: Insights into Photochemical Processes and the Role of Protonated Cys

Hydrogenase enzymes produce H₂ gas, which can be a potential source of alternative energy. Inspired by the [NiFe] hydrogenases, we report the construction of a de novo-designed artificial hydrogenase (ArH). The ArH is a dimeric coiled coil where two cysteine (Cys) residues are introduced at tandem a/d positions of a heptad to create a tetrathiolato Ni binding site. Spectroscopic studies show that Ni binding significantly stabilizes the peptide producing electronic transitions characteristic of Ni-thiolate proteins. The ArH produces H₂ photocatalytically, demonstrating a bell-shaped pH-dependence on activity. Fluorescence lifetimes and transient absorption spectroscopic studies are undertaken to elucidate the nature of pH-dependence, and to monitor the reaction kinetics of the photochemical processes. pH titrations are employed to determine the role of protonated Cys on reactivity. Through combining these results, a fine balance is found between solution acidity and the electron transfer steps. This balance is critical to maximize the production of Ni^I -peptide and protonation of the Ni^II -H^- intermediate (Ni-R) by a Cys (pK_a ≈6.4) to produce H₂ .

Biochemical and artificial pathways for the reduction of carbon dioxide, nitrite and the competing proton reduction: effect of 2nd sphere interactions in catalysis

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment.

Increasing the rate of the hydrogen evolution reaction in neutral water with protic buffer electrolytes

Electrocatalytic generation of H2 is challenging in neutral pH water, where high catalytic currents for the hydrogen evolution reaction (HER) are particularly sensitive to the proton source and solution characteristics. A tris(hydroxymethyl)aminomethane (TRIS) solution at pH 7 with a [2Fe-2S]-metallopolymer electrocatalyst gave catalytic current densities around two orders of magnitude greater than either a more conventional sodium phosphate solution or a potassium chloride (KCl) electrolyte solution. For a planar polycrystalline Pt disk electrode, a TRIS solution at pH 7 increased the catalytic current densities for H2 generation by 50 mA/cm2 at current densities over 100 mA/cm2 compared to a sodium phosphate solution. As a special feature of this study, TRIS is acting not only as the primary source of protons and the buffer of the pH, but the protonated TRIS ([TRIS-H]+) is also the sole cation of the electrolyte. A species that is simultaneously the proton source, buffer, and sole electrolyte is termed a protic buffer electrolyte (PBE). The structure-activity relationships of the TRIS PBE that increase the HER rate of the metallopolymer and platinum catalysts are discussed. These results suggest that appropriately designed PBEs can improve HER rates of any homogeneous or heterogeneous electrocatalyst system. General guidelines for selecting a PBE to improve the catalytic current density of HER systems are offered.

Designed Metal-ATCUN Derivatives: Redox- and Non-redox-Based Applications Relevant for Chemistry, Biology, and Medicine

The designed "ATCUN" motif (amino-terminal copper and nickel binding site) is a replica of naturally occurring ATCUN site found in many proteins/peptides, and an attractive platform for multiple applications, which include nucleases, proteases, spectroscopic probes, imaging, and small molecule activation. ATCUN motifs are engineered at periphery by conjugation to recombinant proteins, peptides, fluorophores, or recognition domains through chemically or genetically, fulfilling the needs of various biological relevance and a wide range of practical usages. This chemistry has witnessed significant growth over the last few decades and several interesting ATCUN derivatives have been described. The redox role of the ATCUN moieties is also an important aspect to be considered. The redox potential of designed M-ATCUN derivatives is modulated by judicious choice of amino acid (including stereochemistry, charge, and position) that ultimately leads to the catalytic efficiency. In this context, a wide range of M-ATCUN derivatives have been designed purposefully for various redox- and non-redox-based applications, including spectroscopic probes, target-based catalytic metallodrugs, inhibition of amyloid-β toxicity, and telomere shortening, enzyme inactivation, biomolecules stitching or modification, next-generation antibiotic, and small molecule activation.

Biosynthetic Approaches towards the Design of Artificial Hydrogen-Evolution Catalysts

Hydrogen is a clean and sustainable form of fuel that can minimize our heavy dependence on fossil fuels as the primary energy source. The need of finding greener ways to generate H₂ gas has ignited interest in the research community to synthesize catalysts that can produce H₂ by the reduction of H⁺ . The natural H₂ producing enzymes hydrogenases have served as an inspiration to produce catalytic metal centers akin to these native enzymes. In this article we describe recent advances in the design of a unique class of artificial hydrogen evolving catalysts that combine the features of the active site metal(s) surrounded by a polypeptide component. The examples of these biosynthetic catalysts discussed here include i) assemblies of synthetic cofactors with native proteins; ii) peptide-appended synthetic complexes; iii) substitution of native cofactors with non-native cofactors; iv) metal substitution from rubredoxin; and v) a reengineered Cu storage protein into a Ni binding protein. Aspects of key design considerations in the construction of these artificial biocatalysts and insights gained into their chemical reactivity are discussed.

Scaffold-Based Functional Models of [Fe]-Hydrogenase (Hmd): Building the Bridge between Biological Structure and Molecular Function

The well-known dinuclear [FeFe] and [NiFe] hydrogenase enzymes are redox-based proton reduction and H₂ oxidation catalysts. In comparison, the structural and functional aspects of the mononuclear nonredox hydrogenase, known as [Fe]-hydrogenase or Hmd, have been less explored because of the relatively recent crystallographic elucidation of the enzyme active site. Additionally, the synthetic challenges posed by the highly substituted and asymmetric coordination environment of the iron guanylylpyridinol (FeGP) cofactor have hampered functional biomimetic modeling studies to a large extent. The active site contains an octahedral low-spin Fe(II) center with the following coordination motifs: a bidentate acyl-pyridone moiety (C,N) and cysteinyl-S in a facial arrangement; two cis carbonyl ligands; and a H₂O/H₂ binding site. In [Fe]-hydrogenase, heterolytic H₂ activation putatively by the pendant pyridone/pyridonate-O base serving as a proton acceptor. Following H₂ cleavage, an intermediate Fe-H species is thought to stereoselectively transfer a hydride to the substrate methenyl-H₄MPT⁺, thus forming methylene-H₄MPT. In the past decade, chemists, inspired by the elegant organometallic chemistry inherent to the FeGP cofactor, have synthesized a number of faithful structural models. However, functional systems are still relatively limited and often rely on abiological ligands or metal centers that obfuscate a direct correlation to nature's design.Our group has developed a bioinspired suite of synthetic analogues of Hmd to better understand the effects of structure on the stability and functionality of the Hmd active site, with a special emphasis on using a scaffold-based ligand design. This systematic approach has contributed to a deeper understanding of the unique ligand array of [Fe]-hydrogenase in nature and has ultimately resulted in the first functional synthetic models without the aid of abiological ligands. This Account reviews the reactivity of the functional anthracene-scaffolded synthetic models developed by our group in the context of current mechanistic understanding drawn from both protein crystallography and computational studies. Furthermore, we introduce a novel thermodynamic framework to place the reactivity of our model systems in context and provide an outlook on the future study of [Fe]-hydrogenase synthetic models through both a structural and functional lens.

Light-driven catalysis with engineered enzymes and biomimetic systems

Efforts to drive catalytic reactions with light, inspired by natural processes like photosynthesis, have a long history and have seen significant recent growth. Successfully engineering systems using biomolecular and bioinspired catalysts to carry out light-driven chemical reactions capitalizes on advantages offered from the fields of biocatalysis and photocatalysis. In particular, driving reactions under mild conditions and in water, in which enzymes are operative, using sunlight as a renewable energy source yield environmentally friendly systems. Furthermore, using enzymes and bioinspired systems can take advantage of the high efficiency and specificity of biocatalysts. There are many challenges to overcome to fully capitalize on the potential of light-driven biocatalysis. In this mini-review, we discuss examples of enzymes and engineered biomolecular catalysts that are activated via electron transfer from a photosensitizer in a photocatalytic system. We place an emphasis on selected forefront chemical reactions of high interest, including CH oxidation, proton reduction, water oxidation, CO₂ reduction, and N₂ reduction.

Artificial Metalloenzymes: Recent Developments and Innovations in Bioinorganic Catalysis

Cellular life is orchestrated by the biochemical components of cells that include nucleic acids, lipids, carbohydrates, proteins, and cofactors such as metabolites and metals, all of which coalesce and function synchronously within the cell. Metalloenzymes allow for such complex chemical processes, as they catalyze a myriad of biochemical reactions both efficiently and selectively, where the metal cofactor provides additional functionality to promote reactivity not readily achieved in their absence. While the past 60 years have yielded considerable insight on how enzymes catalyze these reactions, a need to engineer and develop artificial metalloenzymes has been driven not only by industrial and therapeutic needs, but also by innate human curiosity. The design of miniature enzymes, both rationally and through serendipity, using both organic and inorganic building blocks has been explored by many scientists over the years and significant progress has been made. Herein, recent developments over the past 5 years in areas that have not been recently reviewed are summarized, and prospects for future research in these areas are addressed.

Engineering a solid-state metalloprotein hydrogen evolution catalyst

Hydrogen has the potential to play an important role in decarbonising our energy systems. Crucial to achieving this is the ability to produce clean sources of hydrogen using renewable energy sources. Currently platinum is commonly used as a hydrogen evolution catalyst, however, the scarcity and expense of platinum is driving the need to develop non-platinum-based catalysts. Here we report a protein-based hydrogen evolution catalyst based on a recombinant silk protein from honeybees and a metal macrocycle, cobalt protoporphyrin (CoPPIX). We enhanced the hydrogen evolution activity three fold compared to the unmodified silk protein by varying the coordinating ligands to the metal centre. Finally, to demonstrate the use of our biological catalyst, we built a proton exchange membrane (PEM) water electrolysis cell using CoPPIX-silk as the hydrogen evolution catalyst that is able to produce hydrogen with a 98% Faradaic efficiency. This represents an exciting advance towards allowing protein-based catalysts to be used in electrolysis cells.

The oxygen reactivity of an artificial hydrogenase designed in a reengineered copper storage protein

The O₂ reactivity of an artificial biomolecular hydrogenase, the nickel binding protein (NBP) is investigated.

Protein three-dimensional structures at the origin of life

Proteins are relatively easy to synthesize, compared to nucleic acids and it is likely that there existed a stage prior to the RNA world which can be called the protein world. Some of the three-dimensional (3D) peptide structures in these proteins have, we argue, been conserved since then and may constitute the oldest biological relics in existence. We focus on 3D peptide motifs consisting of up to eight or so amino acid residues. The best known of these is the 'nest', a three- to seven-residue protein motif, which has the function of binding anionic atoms or groups of atoms. Ten per cent of amino acids in typical proteins belong to a nest, so it is a common motif. A five-residue nest is found as part of the well-known P-loop that is a recurring feature of many ATP or GTP-binding proteins and it has the function of binding the phosphate part of these ligands. A synthetic hexapeptide, ser-gly-ala-gly-lys-thr, designed to resemble the P-loop, has been shown to bind inorganic phosphate. Another type of nest binds iron-sulfur centres. A range of other simple motifs occur with various intriguing 3D structures; others bind cations or form channels that transport potassium ions; other peptides form catalytically active haem-like or sheet structures with certain transition metals. Amyloid peptides are also discussed. It now seems that the earliest polypeptides were far from being functionless stretches, and had many of the properties, both binding and catalytic, that might be expected to encourage and stabilize simple life forms in the hydrothermal vents of ocean depths.

Redesign of a Copper Storage Protein into an Artificial Hydrogenase

We report the construction of an artificial hydrogenase (ArH) by reengineering a Cu storage protein (Cspl) into a Ni-binding protein (NBP) employing rational metalloprotein design. The hypothesis driven design approach involved deleting existing Cu sites of Csp1 and identification of a target tetrathiolate Ni binding site within the protein scaffold followed by repacking the hydrophobic core. Guided by modeling, the NBP was expressed and purified in high purity. NBP is a well-folded and stable construct displaying native-like unfolding behavior. Spectroscopic and computational studies indicated that the NBP bound nickel in a distorted square planar geometry that validated the design. Ni(II)-NBP is active for photo-induced H₂ evolution following a reductive quenching mechanism. Ni(II)-NBP catalyzed H⁺ reduction to H₂ gas electrochemically as well. Analysis of the catalytic voltammograms established a proton-coupled electron transfer (PCET) mechanism. Electrolysis studies confirmed H₂ evolution with quantitative Faradaic yields. Our studies demonstrate an important scope of rational metalloprotein design that allows imparting functions into protein scaffolds that have natively not evolved to possess the same function of the target metalloprotein constructs.

Oxidative Cleavage of Cellobiose by Lytic Polysaccharide Monooxygenase (LPMO)-Inspired Copper Complexes

The potentially tridentate ligand bis[(1-methyl-2-benzimidazolyl)ethyl]amine (2BB) was employed to prepare copper complexes [(2BB)Cu^I]OTf and [(2BB)Cu^II(H₂O)₂](OTf)₂ as bioinspired models of lytic polysaccharide copper-dependent monooxygenase (LPMO) enzymes. Solid-state characterization of [(2BB)Cu^I]OTf revealed a Cu(I) center with a T-shaped coordination environment and metric parameters in the range of those observed in reduced LPMOs. Solution characterization of [(2BB)Cu^II(H₂O)₂](OTf)₂ indicates that [(2BB)Cu^II(H₂O)₂]²⁺ is the main species from pH 4 to 7.5; above pH 7.5, the hydroxo-bridged species [{(2BB)Cu^II(H₂O) _x }₂(μ-OH)₂]²⁺ is also present, on the basis of cyclic voltammetry and mass spectrometry. These observations imply that deprotonation of the central amine of Cu(II)-coordinated 2BB is precluded, and by extension, amine deprotonation in the histidine brace of LPMOs appears unlikely at neutral pH. The complexes [(2BB)Cu^I]OTf and [(2BB)Cu^II(H₂O)₂](OTf)₂ act as precursors for the oxidative degradation of cellobiose as a cellulose model substrate. Spectroscopic and reactivity studies indicate that a dicopper(II) side-on peroxide complex generated from [(2BB)Cu^I]OTf/O₂ or [(2BB)Cu^II(H₂O)₂](OTf)₂/H₂O₂/NEt₃ oxidizes cellobiose both in acetonitrile and aqueous phosphate buffer solutions, as evidenced from product analysis by high-performance liquid chromatography-mass spectrometry. The mixture of [(2BB)Cu^II(H₂O)₂](OTf)₂/H₂O₂/NEt₃ results in more extensive cellobiose degradation. Likewise, the use of both [(2BB)Cu^I]OTf and [(2BB)Cu^II(H₂O)₂](OTf)₂ with KO₂ afforded cellobiose oxidation products. In all cases, a common Cu(II) complex formulated as [(2BB)Cu^II(OH)(H₂O)]⁺ was detected by mass spectrometry as the final form of the complex.

Protein Substrates for Reaction Discovery: Site-Selective Modification with Boronic Acid Reagents

Chemical modification of natural proteins must navigate difficult selectivity questions in a complex polyfunctional aqueous environment, within a narrow window of acceptable conditions. Limits on solvent mixtures, pH, and temperature create challenges for most synthetic methods. While a protein's complex polyfunctional environment undoubtedly creates challenges for traditional reactions, we wondered if it also might create opportunities for pursuing new bioconjugation reactivity directly on protein substrates. This Account describes our efforts to date to discover and develop new and useful reactivity for protein modification by starting from an open-ended screen of potential transition-metal catalysts for boronic acid reactivity with a model protein substrate. By starting from a broad screen, we were hoping to take advantage of the very many potential reactive sites on even a small model protein. And perhaps more importantly, whole proteins as reaction screening substrates might exhibit uniquely reactive local environments, the results of a dense combination of functional groups that would be nearly impossible to mimic in a small-molecule context. This effort has resulted in the discovery of four new protein modification reactions with boronic acid reagents, including a remarkable modification of specific backbone N-H bonds. This histidine-directed Chan-Lam coupling, based on specific proximity of an imidazole and two amide groups, is one important example of powerful reactivity that depends on a combination of functional groups that proteins make possible. Other bioconjugation reactions uncovered include a three-component tyrosine metalation with rhodium(III), a nickel-catalyzed cysteine arylation, and an unusual ascorbate-mediated oxidative process for N-terminal modification. The remarkably broad scope of reactivity types encountered in this work is a testament to the breadth of boronic acid reactivity. It is also a demonstration of the diverse reactivities that are possible by the combined alteration of boronic acid structure and metal promoter. The discovery of specific backbone modification chemistry has been a broadly empowering reactivity. Pyroglutamate, a naturally occurring posttranslational modification, exhibits remarkably high reactivity in histidine-directed backbone modification, which allows us to treat pyroglutamate as a reactive bioorthogonal handle that is readily incorporated into proteins of interest by natural machinery. In another research direction, the development of a vinylogous photocleavage system has allowed us to view backbone modification as a photocaging modification which is released by exposure to light.

Cobalt Metallopeptide Electrocatalyst for the Selective Reduction of Nitrite to Ammonium

A cobalt-tripeptide complex (CoGGH) is developed as an electrocatalyst for the selective six-electron, eight-proton reduction of nitrite to ammonium in aqueous buffer near neutral pH. The onset potential for nitrite reduction occurs at -0.65 V vs Ag/AgCl (1 M KCl). Controlled potential electrolysis at -0.90 V generates ammonium with a faradaic efficiency of 90 ± 3% and a turnover number of 3550 ± 420 over 5.5 h. CoGGH also catalyzes the reduction of the proposed intermediates nitric oxide and hydroxylamine to ammonium. These results reveal that a simple metallopeptide is an active functional mimic of the complex enzymes cytochrome c nitrite reductase and siroheme-containing nitrite reductase.

Directing the reactivity of metal hydrides for selective CO2 reduction

A critical challenge in electrocatalytic CO₂ reduction to renewable fuels is product selectivity. Desirable products of CO₂ reduction require proton equivalents, but key catalytic intermediates can also be competent for direct proton reduction to H₂ Understanding how to manage divergent reaction pathways at these shared intermediates is essential to achieving high selectivity. Both proton reduction to hydrogen and CO₂ reduction to formate generally proceed through a metal hydride intermediate. We apply thermodynamic relationships that describe the reactivity of metal hydrides with H⁺ and CO₂ to generate a thermodynamic product diagram, which outlines the free energy of product formation as a function of proton activity and hydricity (∆G_H-), or hydride donor strength. The diagram outlines a region of metal hydricity and proton activity in which CO₂ reduction is favorable and H⁺ reduction is suppressed. We apply our diagram to inform our selection of [Pt(dmpe)₂](PF₆)₂ as a potential catalyst, because the corresponding hydride [HPt(dmpe)₂]⁺ has the correct hydricity to access the region where selective CO₂ reduction is possible. We validate our choice experimentally; [Pt(dmpe)₂](PF₆)₂ is a highly selective electrocatalyst for CO₂ reduction to formate (>90% Faradaic efficiency) at an overpotential of less than 100 mV in acetonitrile with no evidence of catalyst degradation after electrolysis. Our report of a selective catalyst for CO₂ reduction illustrates how our thermodynamic diagrams can guide selective and efficient catalyst discovery.

Hydrogen evolution from water catalyzed by cobalt-mimochrome VI*a, a synthetic mini-protein

The folding of a synthetic mini-hydrogenase is shown to enhance catalyst efficiency and longevity.

A synthetic enzyme is reported that electrocatalytically reduces protons to hydrogen (H₂) in water near neutral pH under aerobic conditions. Cobalt mimochrome VI*a (CoMC6*a) is a mini-protein with a cobalt deuteroporphyrin active site within a scaffold of two synthetic peptides covalently bound to the porphyrin. Comparison of the activity of CoMC6*a to that of cobalt microperoxidase-11 (CoMP11-Ac), a cobalt porphyrin catalyst with a single “proximal” peptide and no organized secondary structure, reveals that CoMC6*a has significantly enhanced longevity, yielding a turnover number exceeding 230 000, in comparison to 25 000 for CoMP11-Ac. Furthermore, comparison of cyclic voltammograms of CoMC6*a and CoMP11-Ac indicates that the trifluoroethanol-induced folding of CoMC6*a lowers the overpotential for catalytic H₂ evolution by up to 100 mV. These results demonstrate that even a minimal polypeptide matrix can enhance longevity and efficiency of a H₂-evolution catalyst.