Artificial Photosynthesis for Production of ATP, NAD(P)H, and Hydrogen Peroxide

Cell-Free Production and Regeneration of Cofactors

Cofactors, such as adenosine triphosphate, nicotinamide adenine dinucleotide, and coenzyme A, are involved in nearly 50% of enzymatic reactions and widely used in biocatalytic production of useful chemicals. Although commercial production of cofactors has been mostly dependent on extraction from microbial cells, this approach has a theoretical limitation to achieve a high-titer, high-yield production of cofactors owing to the tight regulation of cofactor biosynthesis in living cells. Besides the cofactor production, their regeneration is also a key challenge to enable continuous use of costly cofactors and improve the feasibility of enzymatic chemical manufacturing. Construction and implementation of enzyme cascades for cofactor biosynthesis and regeneration in a cell-free environment can be a promising approach to these challenges. In this chapter, we present the available tools for cell-free cofactor production and regeneration, the pros and cons, and how they can contribute to promote the industrial application of enzymes.

Artificial Cells: Past, Present and Future

Artificial cells are constructed to imitate natural cells and allow researchers to explore biological process and the origin of life. The construction methods for artificial cells, through both top-down or bottom-up approaches, have achieved great progress over the past decades. Here we present a comprehensive overview on the development of artificial cells and their properties and applications. Artificial cells are derived from lipids, polymers, lipid/polymer hybrids, natural cell membranes, colloidosome, metal-organic frameworks and coacervates. They can be endowed with various functions through the incorporation of proteins and genes on the cell surface or encapsulated inside of the cells. These modulations determine the properties of artificial cells, including producing energy, cell growth, morphology change, division, transmembrane transport, environmental response, motility and chemotaxis. Multiple applications of these artificial cells are discussed here with a focus on therapeutic applications. Artificial cells are used as carriers for materials and information exchange and have been shown to function as targeted delivery systems of personalized drugs. Additionally, artificial cells can function to substitute for cells with impaired function. Enzyme therapy and immunotherapy using artificial cells have been an intense focus of research. Finally, prospects of future development of cell-mimic properties and broader applications are highlighted.

Transforming Escherichia coli Proteomembranes into Artificial Chloroplasts Using Molecular Photocatalysis

During the light-dependent reaction of photosynthesis, green plants couple photoinduced cascades of redox reactions with transmembrane proton translocations to generate reducing equivalents and chemical energy in the form of NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate), respectively. We mimic these basic processes by combining molecular ruthenium polypyridine-based photocatalysts and inverted vesicles derived from Escherichia coli. Upon irradiation with visible light, the interplay of photocatalytic nicotinamide reduction and enzymatic membrane-located respiration leads to the simultaneous formation of two biologically active cofactors, NADH (nicotinamide adenine dinucleotide) and ATP, respectively. This inorganic-biologic hybrid system thus emulates the cofactor delivering function of an active chloroplast.

One-Pot Highly Efficient Synthesis of N-Enrich Graphene Quantum Dots as a Photocatalytic Platform for NAD+/NADP+ Reduction

An efficient photocatalytic regeneration of nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH) has been carried out by two-electron reduction and protonation of NAD⁺ /NADP⁺ , induced by photons in the visible light region. This functional artificial photosynthetic counterpart of the complete energy-trapping occurring in natural photosystem I (PS I) is achieved with nitrogen-enrich graphene quantum dot (N-EGQD) as the light-harvesting photocatalyst. In buffer aqueous solution, this compound photo catalytically recycles a rhodium hydride complex of the type [Cp*Rh(bpy)H]⁺ (Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine) which can mediate hydride transfer processes leading to nucleotide co-factor reduction. Very promising yields of 73.31%/78.45% of NADH/NADPH with the excellent thermal stability of N-EGQD photocatalyst is observed. Thus, in this work, an efficient light-harvesting photocatalyst is synthesized for the regeneration of nicotinamide cofactor that has pharmaceuticals application.

Augmenting photosynthesis through facile AIEgen-chloroplast conjugation and efficient solar energy utilization

Photosynthesis is regarded as the foundation for sustaining life on our planet. Light-harvesting is the initial step that activates the subsequent photochemical reactions. In the photosystems, chloroplast is the basic light-driven metabolic factory of higher plant cells. However, there is an incomplete match between the solar radiation spectrum and absorption profile of chloroplasts. It is hard for the photosynthetic pigments to fully utilize the sunlight energy. Here, we designed two new aggregation-induced emission (AIE) molecules with activated alkyl groups (TPE-PPO and TPA-TPO). Via a facile metal-free "Click" reaction, we realized the substantial manipulation of live chloroplasts with the AIE luminogens (AIEgens). Owing to the matched photophysical properties, the AIEgens could harvest harmful ultraviolet radiation (HUVR) and photosynthetically inefficient radiation (PIR), and further convert them into photosynthetically active radiation (PAR) for chloroplast absorption. As a result, the conjugated AIEgen-chloroplast exhibited better capability of water splitting and electron separation. It promoted the generation of adenosine triphosphate (ATP), which is an important product of photosynthesis. This work provides an effective strategy for improving plant photosynthesis.

Solar-to-hydrogen peroxide energy conversion on resorcinol-formaldehyde resin photocatalysts prepared by acid-catalysed polycondensation

The photocatalytic generation of hydrogen peroxide from water and dioxygen (H₂O + 1/2O₂ → H₂O₂, ΔG° = +117 kJ mol^-1) under sunlight is a promising strategy for the artificial photosynthesis of a liquid fuel. We had previously found that resorcinol-formaldehyde (RF) resin powders prepared by the base-catalysed high-temperature hydrothermal method act as semiconductor photocatalysts for H₂O₂ generation. Herein, we report that RF resins prepared by the acid-catalysed high-temperature hydrothermal method (~523 K) using common acids at pH < 4 exhibit enhanced photocatalytic activity. The base- and acid-catalysed methods both produce methylene- and methine-bridged resins consisting of π-conjugated and π-stacked benzenoid-quinoid donor-acceptor resorcinol units. The acidic conditions result in the resins with a lower bandgap (1.7 eV) and higher conductivity because the lower-degree of crosslinking creates a strongly π-stacked architecture. The irradiation of the RF-acid resins with simulated sunlight in water with atmospheric-pressure O₂ generates H₂O₂ at a solar-to-chemical conversion efficiency of 0.7%, which is the highest efficiency ever reported for powder catalysts used in artificial photosynthesis.

Photocatalytic Hydrogen Evolution from Plastoquinol Analogues as a Potential Functional Model of Photosystem I

The recent development of a functional model of photosystem II (PSII) has paved a new way to connect the PSII model with a functional model of photosystem I (PSI). However, PSI functional models have yet to be reported. We report herein the first potential functional model of PSI, in which plastoquinol (PQH₂) analogues were oxidized to plastoquinone (PQ) analogues, accompanied by hydrogen (H₂) evolution. Photoirradiation of a deaerated acetonitrile (MeCN) solution containing hydroquinone derivatives (X-QH₂) as a hydrogen source, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) as a photoredox catalyst, and a cobalt(III) complex, Co^III(dmgH)₂pyCl (dmgH = dimethylglyoximate monoanion; py = pyridine) as a redox catalyst resulted in the evolution of H₂ and formation of the corresponding p-benzoquinone derivatives (X-Q) quantitatively. The maximum quantum yield for photocatalytic H₂ evolution from tetrachlorohydroquinone (Cl₄QH₂) with Acr⁺-Mes and Co^III(dmgH)₂pyCl and H₂O in deaerated MeCN was determined to be 10%. Photocatalytic H₂ evolution is started by electron transfer (ET) from Cl₄QH₂ to the triplet ET state of Acr⁺-Mes to produce Cl₄QH₂^•+ and Acr^•-Mes with a rate constant of 7.2 × 10⁷ M^-1 s^-1, followed by ET from Acr^•-Mes to Co^III(dmgH)₂pyCl to produce [Co^II(dmgH)₂pyCl]^-, accompanied by the regeneration of Acr⁺-Mes. On the other hand, Cl₄QH₂^•+ is deprotonated to produce Cl₄QH^•, which transfers either a hydrogen-atom transfer or a proton-coupled electron transfer to [Co^II(dmgH)₂pyCl]^- to produce a cobalt(III) hydride complex, [Co^III(H)(dmgH)₂pyCl]^-, which reacts with H⁺ to evolve H₂, accompanied by the regeneration of Co^III(dmgH)₂pyCl. The formation of [Co^II(dmgH)₂pyCl]^- was detected by electron paramagnetic resonance measurements.

Green Catalysts: Applied and Synthetic Photosynthesis

The biological process of photosynthesis was critical in catalyzing the oxygenation of Earth’s atmosphere 2.5 billion years ago, changing the course of development of life on Earth. Recently, the fields of applied and synthetic photosynthesis have utilized the light-driven protein–pigment supercomplexes central to photosynthesis for the photocatalytic production of fuel and other various valuable products. The reaction center Photosystem I is of particular interest in applied photosynthesis due to its high stability post-purification, non-geopolitical limitation, and its ability to generate the greatest reducing power found in nature. These remarkable properties have been harnessed for the photocatalytic production of a number of valuable products in the applied photosynthesis research field. These primarily include photocurrents and molecular hydrogen as fuels. The use of artificial reaction centers to generate substrates and reducing equivalents to drive non-photoactive enzymes for valuable product generation has been a long-standing area of interest in the synthetic photosynthesis research field. In this review, we cover advances in these areas and further speculate synthetic and applied photosynthesis as photocatalysts for the generation of valuable products.

Enhancing the Light Coverage of Photosynthetic Bacteria to Augment Photosynthesis by Conjugated Polymer Nanoparticles

By coating photosynthetic bacteria of Rhodopseudomonas palustris with conjugated polymers nanoparticles modified with positively charged peptide TAT (CPNs-TAT), a bio-optical hybrid composite of R. palustris/CPNs-TAT has been constructed. R. palustris/CPNs-TAT augments the light coverage of R. palustris to broaden the R. palustris absorption due to excellent light-harvesting properties of CPNs-TAT, especially in the ultraviolet region. It leads to converting ultraviolet light to visible light that could be absorbed by R. palustris, allowing antenna systems around the reaction center (RC) of the photosynthetic membrane to absorb more photons, thus photons are excited and transferred to the RC where the electron-hole separation occurs. Therefore, R. palustris/CPNs-TAT improves adenosine triphosphate (ATP) synthesis by increasing proton gradient, resulting in the enhancement of the photosynthetic activity. This effort combines synthetic light-harvesting materials with photosynthetic bacteria without complicated genetic techniques to obtain the hybrid bio-optical systems for augmenting photosynthesis beyond natural photosynthetic bacteria.

Photocatalytic redox reactions with metalloporphyrins

Metalloporphyrinoids are utilized as efficient sensitizers and catalysts in photosynthesis and the reverse reaction that is respiration. Because metalloporphyrinoids show strong absorption in the visible region and redox active, metalloporphyrinoids are also suited as photoredox catalysts for photo-driven redox reactions using solar energy. In particular, metalloporphyrins are utilized as pivotal components to mimic the structure and function of the photosynthetic reaction center. Metalloporphyrins are used as photoredox catalysts for hydrogen evolution from electron and proton sources combining hydrogen evolution catalysts. Metalloporphyrins also act as thermal redox catalysts for photocatalytic reduction of CO2 with photoredox catalysts. Metalloporphyrins are also used as dual catalysts for a photoredox catalyst for oxygenation of substrates with H2O and a redox catalyst for O2 reduction when dioxygen is used as a two-electron oxidant and H2O as an oxygen source, both of which are the greenest reactants. Free base porphyrins can also be employed as promising photoredox catalysts for C–C bond formation reactions.

A Novel Photo-Driven Hydrogenation Reaction of an NAD+-Type Complex Toward Artificial Photosynthesis

The photocatalytic reduction of carbon dioxide (CO₂) to value-added chemicals is an attractive strategy to utilize CO₂ as a feedstock for storing renewable energy, such as solar energy, in chemical bonds. Inspired by the biological function of the nicotinamide adenine dinucleotide redox couple (NAD⁺/NADH), we have been developing transition-metal complexes containing NAD⁺/NADH-functionalized ligands to create electro- and/or photochemically renewable hydride donors for the conversion of CO₂ into value-added chemicals. Our previous findings have provided insights for the development of photocatalytic organic hydride reduction reactions for CO₂, however, further examples, as well as investigation, of these photo-driven NAD⁺/NADH-type hydrogenation and organic hydride transfer reactions are required not only to explore the mechanism in detail but also to develop a highly efficient catalyst for artificial photosynthesis. In this paper, we report the synthesis, characterization, and photo-induced NAD⁺/NADH conversion properties of a new ruthenium(II) complex, [Ru(bpy)₂(Me-pn)](PF₆)₂ (1), which contains a new NAD⁺-type ligand, Me-pn (2-methyl-6-(pyridin-2-yl)-1,5-naphthyridine). In addition, we have succeeded in the isolation of the corresponding two-electron reduced ruthenium(II) complex containing the NADH-type ligand Me-pnHH (2-methyl-6-(pyridin-2-yl)-1,4-dihydro-1,5-naphthyridine), i.e., [Ru(bpy)₂(Me-pnHH)](PF₆)₂ (1HH), by the photo-induced hydrogenation reaction of 1. Thus, in this study, a new photo-driven NAD⁺/NADH-type hydrogenation reaction for possible CO₂ reduction using the NAD⁺/NADH redox function has been constructed.

Catalytic recycling of NAD(P)H

A large number of industrially relevant enzymes depend upon dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH) cofactors, which are too expensive to be added in stoichiometric amounts. Existing NAD(P)H-recycling systems suffer from low activity, or the generation of side products. This review focuses on NAD(P)H cofactor regeneration catalyzed by transition metal complexes such as rhodium, ruthenium and iridium complexes using cheap reducing agents such as hydrogen (H₂) and ethanol, which have attracted increasing attention as sustainable energy carriers. The catalytic mechanisms for the regioselective reduction of NAD(P)⁺ are discussed with emphasis on identification of catalytically active intermediates such as transition metal hydride complexes. Applications of NAD(P)H-recycling systems to develop artificial photosynthesis are also discussed.

Pentanuclear iron catalysts for water oxidation: substituents provide two routes to control onset potentials

The development of robust and efficient molecular catalysts based on earth-abundant transition metals for water oxidation reactions is a challenging research target. Our group recently demonstrated the high activity and stability of a pentairon-based water oxidation electrocatalyst (M. Okamura, M. Kondo, R. Kuga, Y. Kurashige, T. Yanai, S. Hayami, V. K. K. Praneeth, M. Yoshida, K. Yoneda, S. Kawata and S. Masaoka, Nature, 2016, 530, 465-468). However, the development of strategies to decrease onset potentials for catalysis remains challenging. In this article, we report the construction of a series of pentanuclear iron complexes by introducing electron-donating (methyl) and electron-withdrawing (bromo) substituents on the ligand. Two newly synthesized complexes exhibited five reversible redox processes, similar to what is seen with the parent complex. These complexes can also serve as homogeneous catalysts for water oxidation reactions, and the faradaic efficiencies of the reactions were high. Additionally, the onset potentials of the newly developed complexes were lower than that of the parent complex. Mechanistic insights revealed that there are two methods for decreasing onset potentials: control of the redox potentials of the pentairon complex and control of the reaction mechanism.

The hallmarks of living systems: towards creating artificial cells

Despite the astonishing diversity and complexity of living systems, they all share five common hallmarks: compartmentalization, growth and division, information processing, energy transduction and adaptability. In this review, we give not only examples of how cells satisfy these requirements for life and the ways in which it is possible to emulate these characteristics in engineered platforms, but also the gaps that remain to be bridged. The bottom-up synthesis of life-like systems continues to be driven forward by the advent of new technologies, by the discovery of biological phenomena through their transplantation to experimentally simpler constructs and by providing insights into one of the oldest questions posed by mankind, the origin of life on Earth.