Biological Energy Conversion Research Articles

Fabrication of bio-abiotic hybrid living hydrogel for bifunctional electrochemical conversion

Design and intelligent use renewable natural bioenergy is an important challenge. Electric microorganism-based materials are being serve as an important part of bioenergy devices for energy release and collection, calling for suitable skeleton materials to anchor live microbes. Herein we verified the feasibility of constructing bio-abiotic hybrid living materials based on the combination of gelatin, Li-ions and exoelectrogenic bacteria Shewanella oneidensis manganese-reducing-1 (MR-1). The gelatin-based mesh contains abundant pores, allowing microbes to dock and small molecules to diffuse. The hybrid materials hold plentiful electronegative groups, which effectively anchor Li-ions and facilitate their transition. Moreover, the electrochemical characteristics of the materials can be modulated through changing the ratios of gelatin, bacteria and Li-ions. Based on the gelatin-Li-ion-microorganism hybrid materials, a bifunctional device was fabricated, which could play dual roles alternatively, generation of electricity as a microbial fuel cell and energy storage as a pseudocapacitor. The capacitance and the maximum voltage output of the device reaches 68 F·g-1 and 0.67 V, respectively. This system is a new platform and fresh start to fabricate bio-abiotic living materials for microbial electron storage and transfer. We expect the setup will extend to other living systems and devices for synthetic biological energy conversion.

The Winding Road from Origin to Emergence (of Life).

Humanity's strive to understand why and how life appeared on planet Earth dates back to prehistoric times. At the beginning of the 19th century, empirical biology started to tackle this question yielding both Charles Darwin's Theory of Evolution and the paradigm that the crucial trigger putting life on its tracks was the appearance of organic molecules. In parallel to these developments in the biological sciences, physics and physical chemistry saw the fundamental laws of thermodynamics being unraveled. Towards the end of the 19th century and during the first half of the 20th century, the tensions between thermodynamics and the "organic-molecules-paradigm" became increasingly difficult to ignore, culminating in Erwin Schrödinger's 1944 formulation of a thermodynamics-compliant vision of life and, consequently, the prerequisites for its appearance. We will first review the major milestones over the last 200 years in the biological and the physical sciences, relevant to making sense of life and its origins and then discuss the more recent reappraisal of the relative importance of metal ions vs. organic molecules in performing the essential processes of a living cell. Based on this reassessment and the modern understanding of biological free energy conversion (aka bioenergetics), we consider that scenarios wherein life emerges from an abiotic chemiosmotic process are both thermodynamics-compliant and the most parsimonious proposed so far.

QM/MM Free Energy Calculations of Long-Range Biological Protonation Dynamics by Adaptive and Focused Sampling.

Water-mediated proton transfer reactions are central for catalytic processes in a wide range of biochemical systems, ranging from biological energy conversion to chemical transformations in the metabolism. Yet, the accurate computational treatment of such complex biochemical reactions is highly challenging and requires the application of multiscale methods, in particular hybrid quantum/classical (QM/MM) approaches combined with free energy simulations. Here, we combine the unique exploration power of new advanced sampling methods with density functional theory (DFT)-based QM/MM free energy methods for multiscale simulations of long-range protonation dynamics in biological systems. In this regard, we show that combining multiple walkers/well-tempered metadynamics with an extended system adaptive biasing force method (MWE) provides a powerful approach for exploration of water-mediated proton transfer reactions in complex biochemical systems. We compare and combine the MWE method also with QM/MM umbrella sampling and explore the sampling of the free energy landscape with both geometric (linear combination of proton transfer distances) and physical (center of excess charge) reaction coordinates and show how these affect the convergence of the potential of mean force (PMF) and the activation free energy. We find that the QM/MM-MWE method can efficiently explore both direct and water-mediated proton transfer pathways together with forward and reverse hole transfer mechanisms in the highly complex proton channel of respiratory Complex I, while the QM/MM-US approach shows a systematic convergence of selected long-range proton transfer pathways. In this regard, we show that the PMF along multiple proton transfer pathways is recovered by combining the strengths of both approaches in a QM/MM-MWE/focused US (FUS) scheme and reveals new mechanistic insight into the proton transfer principles of Complex I. Our findings provide a promising basis for the quantitative multiscale simulations of long-range proton transfer reactions in biological systems.

Soft alchemy: a comprehensive guide to chemical reactions for pneumatic soft actuation

Soft robotics has emerged as a transformative field, leveraging bio-inspired novel actuation mechanisms to enable more adaptable, compliant, and sophisticated robotic systems. However, the portability of soft pneumatic actuators is typically constrained by the tethering to bulky power sources. This review offers a thorough analysis of autonomous power alternatives facilitated by chemical reactions for gas generation and absorption, a concept analogous to biological energy conversion processes. These bio-inspired strategies propel soft pneumatic actuators towards new horizons of autonomy and portability, essential for real-world applications. This comprehensive review explores the critical intersection of gas evolution reactions (GERs) and gas consumption reactions (GCRs) as a power source for pneumatic actuation in soft robotics. We here emphasize the importance and impact of bio-inspired design, control, efficiency, safety, and sustainability within soft robotics to not only mimic biological motions but to enhance them. This review explores the fundamentals of both pneumatic and chemically powered actuation, highlighting the need for careful consideration of reaction kinetics. Additionally, this work highlights key aspects of smart materials that draw from biological structures and response mechanisms, along with state-of-the-art techniques for precise pressure modulation. Finally, we chart prospective development pathways and provide a future outlook for bio-inspired soft robotics, emphasizing the transformative impact of integrating chemical actuation methods. This exploration underlines the quest for further autonomy in soft robotic systems and points towards the future opportunities in this exciting and fast-developing field.

Electron-Transport-Chain-Mediated Selective Growth of Gold Nanocrystals in the Intermembrane Space of Live Microbial Cells.

Hybridization of microbial cells with inorganic nanoparticles that could dramatically improve cellular functions such as electron transfer has been realized by the random attachment or stochastic entry of the nanoparticles. Clearly, the selective growth of inorganic nanoparticles on target functional organelles is ideal for such hybridization. Here, we report the selective growth of gold nanocrystals in the intermembrane space (IMS) of Escherichia coli by exploiting the electron transport chain (ETC). We systematically show that gold ions are permeated through porins in the outer membrane of E. coli and further reduced to gold nanocrystals by the ETC in live E. coli. We directly observe that the resulting gold nanocrystals exist only in the IMS by transmission electron microscopy measurements of cross-sectioned E. coli. Molecular dynamics simulations suggest that once gold ions are reduced to small nuclei by the ETC, the nuclei can be stably physisorbed onto ETC complexes, further supporting the ETC-mediated growth. Finally, we show that the ATP synthesis of E. coli where gold nanocrystals are formed in the IMS is up to 9 times higher than that of E. coli alone. We believe that our work can significantly contribute to not only improving microbial metabolic functions for biological energy conversion but also restoring physiological dysfunctions of microbial cells for biomedicine.

Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and added base.

The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(p-tbudhbpy)Cl (1) was used as a control complex and Mn(nPrdhbpy)Cl (2) contains a pendent -OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N'-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H2O2 and 2 is quantitative for H2O2, with O2 binding to the reduced Mn(ii) center being the rate-determining step. Upon addition of the conjugate base, N,N'-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H2O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O-O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H2O2 at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate.

Cu site differentiation in tetracopper(i) sulfide clusters enables biomimetic N2O reduction.

Copper clusters feature prominently in both metalloenzymes and synthetic nanoclusters that mediate catalytic redox transformations of gaseous small molecules. Such reactions are critical to biological energy conversion and are expected to be crucial parts of renewable energy economies. However, the precise roles of individual metal atoms within clusters are difficult to elucidate, particularly for cluster systems that are dynamic under operating conditions. Here, we present a metal site-specific analysis of synthetic Cu4(μ4-S) clusters that mimic the Cu Z active site of the nitrous oxide reductase enzyme. Leveraging the ability to obtain structural snapshots of both inactive and active forms of the synthetic model system, we analyzed both states using resonant X-ray diffraction anomalous fine structure (DAFS), a technique that enables X-ray absorption profiles of individual metal sites within a cluster to be extracted independently. Using DAFS, we found that a change in cluster geometry between the inactive and active states is correlated to Cu site differentiation that is presumably required for efficient activation of N2O gas. More precisely, we hypothesize that the Cu δ+⋯Cu δ- pairs produced upon site differentiation are poised for N2O activation, as supported by computational modeling. These results provide an unprecedented level of detail on the roles of individual metal sites within the synthetic cluster system and how those roles interplay with cluster geometry to impact the reactivity function. We expect this fundamental knowledge to inform understanding of metal clusters in settings ranging from (bio)molecular to nanocluster to extended solid systems involved in energy conversion.

Hydrogen Reduction Reduction (HRR)

The year 2023 marks the bicentennial of the discovery of heterogeneous H2 catalysis. Nowadays, the hydrogen oxidation reaction (HOR), which converts H2 to protons (H+), is one of the most important transformations in energy research. Meanwhile, hydrogenation reactions, which transfer net H-atoms (H·) to organic substrates from H2, play pivotal roles in various chemical industries. In comparison, it is surprising to see that a heterogenous catalytic reaction that directly converts H2 to hydrides (H–) still remains elusive. Hydride transfer is a critical elementary reaction step that spans biological catalysis, organic synthesis and energy conversion. Conventionally, hydride transfer reactions are performed using (bio)molecular hydride reagents under homogeneous conditions. Here we report a conceptually distinct heterogeneous hydride transfer reaction via the net electrocatalytic hydrogen reduction reaction (HRR), which reduces H2 to hydrides. The reaction proceeds by H2 dissociative adsorption on a metal electrode to form surface M−H species, which are then negatively polarized to drive hydride transfer to molecular hydride acceptors with up to 91% Faradaic efficiency. The hydride transfer reactivity of surface M−H species is highly tunable, and, depending on the electrode potential, the thermodynamic hydricity of Pt−H on the same Pt electrode can continuously span a range of >40 kcal/mol.In addition to this electrochemical transformation, we further developed an alternative strategy to achieve heterogeneous hydride transfer on dispersed metal catalysts under non-electrochemical conditions. In the presence of a base, deprotonation of surface M-H builds up electron density at the metal surface and thereby causes spontaneous polarization which can then drive hydride transfer from metal nanoparticles. Based on this, HRR can be performed under standard hydrogenation conditions with the addition of a base using dispersed metal catalysts, and the surface hydricity is now controlled by the solution basicity. We further demonstrate that this non-electrochemical HRR method can be applied to NADH regeneration which is closely related to industrial biocatalysis. The discovery of HRR reveals the fundamental hydride transfer reactivity of surface M-H species as well as its polarization-dependent nature. Both the electrochemical and non-electrochemical HRR methods establish sustainable strategies for accessing reactive hydrides directly from H2. Figure 1

Innovative Design and Development of Biological Fuel Cell-Based Energy Conversion System

Innovative Design and Development of Biological Fuel Cell-Based Energy Conversion System

2D materials towards energy conversion processes in nanofluidics.

Hierarchically assembled 2D material membranes are extremely promising platforms for energy conversion processes in nanofluidics. In this perspective, we discuss recent advances in the production of smart 2D material membranes that come close to mimicking biological energy conversion processes and how these efforts translate into the design of water purification systems, artificial photosynthesis, and solar energy conversion devices. As we depict here, 2D material membranes synergistically modulate the intrinsic active sites (nanopores), electron transport, mass transfer, and mechanical and chemical stability aiming at cost-effective and highly efficient smart membranes.

Site-Differentiated Iron-Sulfur Cluster Ligation Affects Flavin-Based Electron Bifurcation Activity.

Electron bifurcation is an elegant mechanism of biological energy conversion that effectively couples three different physiologically relevant substrates. As such, enzymes that perform this function often play critical roles in modulating cellular redox metabolism. One such enzyme is NADH-dependent reduced-ferredoxin: NADP+ oxidoreductase (NfnSL), which couples the thermodynamically favorable reduction of NAD+ to drive the unfavorable reduction of ferredoxin from NADPH. The interaction of NfnSL with its substrates is constrained to strict stoichiometric conditions, which ensures minimal energy losses from non-productive intramolecular electron transfer reactions. However, the determinants for this are not well understood. One curious feature of NfnSL is that both initial acceptors of bifurcated electrons are unique iron–sulfur (FeS) clusters containing one non-cysteinyl ligand each. The biochemical impact and mechanistic roles of site-differentiated FeS ligands are enigmatic, despite their incidence in many redox active enzymes. Herein, we describe the biochemical study of wild-type NfnSL and a variant in which one of the site-differentiated ligands has been replaced with a cysteine. Results of dye-based steady-state kinetics experiments, substrate-binding measurements, biochemical activity assays, and assessments of electron distribution across the enzyme indicate that this site-differentiated ligand in NfnSL plays a role in maintaining fidelity of the coordinated reactions performed by the two electron transfer pathways. Given the commonality of these cofactors, our findings have broad implications beyond electron bifurcation and mechanistic biochemistry and may inform on means of modulating the redox balance of the cell for targeted metabolic engineering approaches.

Ion Hydration under Nanoscale Confinement: Dimensionality and Scale Effects

How ions are hydrated in nanoconfined spaces is crucial for understanding many natural phenomena and practical applications, such as biological functionalities and energy conversion devices. In real systems, nanoconfinement shows structural diversity, but the influence of dimensionality and scale on ion hydration remains considerably unrevealed. Here, we study ion hydration under various confinements by systematic molecular dynamics simulations. In a given dimension, the structure and dynamics of water molecules in the first hydration shell are altered to a degree inversely correlated with the confinement scale, as long as there is no central bulk-like region. Further comparison of ion hydration among different dimensional systems shows that this scale effect becomes more pronounced in systems with lower dimensionality, due to a more significant water layering effect and lower probability for ions to stay away from confining surfaces. These findings provide a qualitatively new understanding of ion transport in biological channels and are instrumental for the design of functional nanofluidic devices.

Molecular Principles of Redox-Coupled Protonation Dynamics in Photosystem II.

Photosystem II (PSII) catalyzes light-driven water oxidization, releasing O2 into the atmosphere and transferring the electrons for the synthesis of biomass. However, despite decades of structural and functional studies, the water oxidation mechanism of PSII has remained puzzling and a major challenge for modern chemical research. Here, we show that PSII catalyzes redox-triggered proton transfer between its oxygen-evolving Mn4O5Ca cluster and a nearby cluster of conserved buried ion-pairs, which are connected to the bulk solvent via a proton pathway. By using multi-scale quantum and classical simulations, we find that oxidation of a redox-active Tyrz (Tyr161) lowers the reaction barrier for the water-mediated proton transfer from a Ca2+-bound water molecule (W3) to Asp61 via conformational changes in a nearby ion-pair (Asp61/Lys317). Deprotonation of this W3 substrate water triggers its migration toward Mn1 to a position identified in recent X-ray free-electron laser (XFEL) experiments [Ibrahim et al. Proc. Natl. Acad. Sci. USA 2020, 117, 12,624–12,635]. Further oxidation of the Mn4O5Ca cluster lowers the proton transfer barrier through the water ligand sphere of the Mn4O5Ca cluster to Asp61 via a similar ion-pair dissociation process, while the resulting Mn-bound oxo/oxyl species leads to O2 formation by a radical coupling mechanism. The proposed redox-coupled protonation mechanism shows a striking resemblance to functional motifs in other enzymes involved in biological energy conversion, with an interplay between hydration changes, ion-pair dynamics, and electric fields that modulate the catalytic barriers.

Resolving Chemical Dynamics in Biological Energy Conversion: Long-Range Proton-Coupled Electron Transfer in Respiratory Complex I.

ConspectusBiological energy conversion is catalyzed by membrane-bound proteins that transduce chemical or light energy into energy forms that power endergonic processes in the cell. At a molecular level, these catalytic processes involve elementary electron-, proton-, charge-, and energy-transfer reactions that take place in the intricate molecular machineries of cell respiration and photosynthesis. Recent developments in structural biology, particularly cryo-electron microscopy (cryoEM), have resolved the molecular architecture of several energy transducing proteins, but detailed mechanistic principles of their charge transfer reactions still remain poorly understood and a major challenge for modern biochemical research. To this end, multiscale molecular simulations provide a powerful approach to probe mechanistic principles on a broad range of time scales (femtoseconds to milliseconds) and spatial resolutions (101–106 atoms), although technical challenges also require balancing between the computational accuracy, cost, and approximations introduced within the model. Here we discuss how the combination of atomistic (aMD) and hybrid quantum/classical molecular dynamics (QM/MM MD) simulations with free energy (FE) sampling methods can be used to probe mechanistic principles of enzymes responsible for biological energy conversion. We present mechanistic explorations of long-range proton-coupled electron transfer (PCET) dynamics in the highly intricate respiratory chain enzyme Complex I, which functions as a redox-driven proton pump in bacterial and mitochondrial respiratory chains by catalyzing a 300 Å fully reversible PCET process. This process is initiated by a hydride (H–) transfer between NADH and FMN, followed by long-range (>100 Å) electron transfer along a wire of 8 FeS centers leading to a quinone biding site. The reduction of the quinone to quinol initiates dissociation of the latter to a second membrane-bound binding site, and triggers proton pumping across the membrane domain of complex I, in subunits up to 200 Å away from the active site. Our simulations across different size and time scales suggest that transient charge transfer reactions lead to changes in the internal hydration state of key regions, local electric fields, and the conformation of conserved ion pairs, which in turn modulate the dynamics of functional steps along the reaction cycle. Similar functional principles, which operate on much shorter length scales, are also found in some unrelated proteins, suggesting that enzymes may employ conserved principles in the catalysis of biological energy transduction processes.

Ship building design of general cargo vessel using comparator ship method in port route of Jakarta-Makassar-Manokwari

Marine-based industrial development includes several sectors including Sea Transportation Services, Ferry Services, Capture Fisheries, Offshore Oil & Gas, Marine Biological Resources, Marine Tourism, and Energy Conversion, which in general both management and operations require supporting facilities, namely ships. with certain types that can serve these interests. During the current economic recovery period, there need to be concrete and more innovative steps that must be pursued by all parties, both government and private, in order to reduce the time needed to improve the state’s economic condition. Hence, the role of marine potential is very vital to be further developed in the future. The optimization results of the comparison ship obtained the main dimensions of the design vessel, namely: Lbp = 81.19 m, B = 15.00 m, T = 5.50 m, H = 7.00 m, Cb = 0.75, Cm = 0.99, Cw = 0.84 Cpv = 0.89, Cph = 0.76, Fb = 1.50 m, Fn = 0.22, Displacement = 5280.91 tons, Volume = 5131.583 m3, GRT = 3910 tons, Vs = 12 knots, Lwl = 83.22 m. This comparator ship method makes it easy for ship designers with accurate results.

Current Progress of Interfacing Organic Semiconducting Materials with Bacteria.

Microbial bioelectronics require interfacing microorganisms with electrodes. The resulting abiotic/biotic platforms provide the basis of a range of technologies, including energy conversion and diagnostic assays. Organic semiconductors (OSCs) provide a unique strategy to modulate the interfaces between microbial systems and external electrodes, thereby improving the performance of these incipient technologies. In this review, we explore recent progress in the field on how OSCs, and related materials capable of charge transport, are being used within the context of microbial systems, and more specifically bacteria. We begin by examining the electrochemical communication modes in bacteria and the biological basis for charge transport. Different types of synthetic organic materials that have been designed and synthesized for interfacing and interrogating bacteria are discussed next, followed by the most commonly used characterization techniques for evaluating transport in microbial, synthetic, and hybrid systems. A range of applications is subsequently examined, including biological sensors and energy conversion systems. The review concludes by summarizing what has been accomplished so far and suggests future design approaches for OSC bioelectronics materials and technologies that hybridize characteristic properties of microbial and OSC systems.

Energy transduction by reversible electron bifurcation

Electron bifurcation is a biological energy conversion process that oxidizes a two-electron donor at medium potential, coupled to the reduction of a high- and a low-potential acceptor species. This process is often fully reversible (can occur close to ΔG≈0), allowing the creation of strong reductants with minimal free energy cost, using compounds with higher reduction potentials. For many years, the internal workings of electron bifurcating enzymes were poorly understood, especially regarding how short-circuit reactions are prevented. Recently, a conserved energy landscape was proposed to naturally insulate against short-circuit reactions, enabling efficient and reversible electron bifurcation (the reversible EB scheme). We review the physical principles that underpin the EB scheme, describe how the reversible EB scheme is distinct from previous views, and outline questions that remain open.

Transition metal-mediated O-O bond formation and activation in chemistry and biology.

Oxygen evolution and reduction reactions are fundamental processes in biological energy conversion schemes, which represent an attractive method for artificial energy conversion for a world still largely depending on fossil fuels. A range of metalloenzymes achieve these challenging tasks in biology by activating water and dioxygen using cheap and abundant transition metals, such as iron, copper, and manganese. High-valent metal-oxo/oxyl, metal-superoxo, and/or metal-(hydro)peroxo species are common reactive intermediates that are found in the O-O bond formation and activation reactions. The transient nature of the metal-oxygen intermediates has, however, prevented their isolation and characterization in most cases. As a consequence, unambiguous mechanistic assignments in the O-O bond formation and cleavage processes in biological and chemical entries remain elusive, especially for the intermediates and mechanisms involved in the O-O bond formation reactions. This viewpoint article aims at summarizing the information obtained to date in enzymatic and biomimetic systems that fuels the debate regarding the nature of the active oxidants and the mechanistic uncertainties associated with the transition metal-mediated O-O bond formation and cleavage reactions.

First plant cell atlas workshop report.

The societal challenges posed by a growing human population and climate change necessitate technical advances in plant science. Plant research makes vital contributions to society by advancing technologies that improve agricultural food production, biological energy capture and conversion, and human health. However, the plant biology community lacks a comprehensive understanding of molecular machinery, including their locations within cells, distributions and variations among different cell types, and real‐time dynamics. Fortunately, rapid advances in molecular methods, imaging, proteomics, and metabolomics made in the last decade afford unprecedented opportunities to develop a molecular‐level map of plant cells with high temporal and spatial resolution. The Plant Cell Atlas (PCA) initiative aims to generate a resource that will provide fresh insight into poorly understood aspects of plant cell structure and organization and enable the discovery of new cellular compartments and features. The PCA will be a community resource (www.plantcellatlas.org/) that describes the state of various plant cell types and integrates high‐resolution spatio‐temporal information of nucleic acids, proteins, and metabolites within plant cells. This first PCA initiative workshop convened scientists passionate about developing a comprehensive PCA to brainstorm about the state of the field, recent advances, the development of tools, and the future directions of this initiative. The workshop featured invited talks to share initial data, along with broader ideas for the PCA. Additionally, breakout sessions were organized around topics including the conceptual goals of the PCA, technical challenges, and community wants and needs. These activities connected scientists with diverse expertise and sparked important discussions about how to leverage and extend leading‐edge technologies and develop new techniques. A major outcome of the workshop was that the community wishes to redefine concepts of plant cell types and tissues quantitatively. A long‐term goal is to delineate all molecules within the cell at high spatio‐temporal resolution, obtain information about interacting molecular networks, and identify the contribution of these networks to development of the organism as a whole. As a first step, we wish to create comprehensive cellular and subcellular biomolecular maps of transcripts, proteins, and metabolites, track the dynamic interactions of these molecules intra‐ and intercellularly, discern complete states and transitions of specialized cell types, and integrate these disparate data points to generate testable models of cellular function. Ultimately, the PCA initiative will have a substantial positive impact by empowering a broad, diverse group of scientists to forge exciting paths in the field of plant science, facilitating connections with interested stakeholders beyond the scientific community, and enabling new agricultural technologies for a sustainable future.

Direct observation of transient species generated from protonation and deprotonation of the lowest triplet of p-nitrophenylphenol

Photo-induced proton coupled electron transfer (PCET) is essential in the biological, photosynthesis, catalysis and solar energy conversion processes. Recently, p-nitrophenylphenol (HO-Bp-NO2) has been used as a model compound to study the photo-induced PCET mechanism by using ultrafast spectroscopy. In transient absorption spectra both singlet and triplet states were observed to exhibit PCET behavior upon laser excitation of HO-Bp-NO2. When we focused on the PCET in the triplet state, a new sharp band attracted us. This band was recorded upon excitation of HO-Bp-NO2 in aprotic polar solvents, and has not been observed for p-nitrobiphenyl which is without hydroxyl substitution. In order to find out what the new band represents, acidic solutions were used as an additional proton donor considering the acidity of HO-Bp-NO2. With the help of results in strong (~10−1 mol/L) and weak (~10−4 mol/L) acidic solutions, the new band is identified as open shell singlet O-Bp-NO2H, which is generated through protonation of nitro O in 3HO-Bp-NO2 followed by deprotonation of hydroxyl. Kinetics analysis indicates that the formation of radical O-Bp-NO2 competes with O-Bp-NO2H in the way of concerted electron-proton transfer and/or proton followed electron transfers and is responsible for the low yield of O-Bp-NO2H. The results in the present work will make it clear how the 3HO-Bp-NO2 deactivates in aprotic polar solvents and provide a solid benchmark for the deeply studying the PCET mechanism in triplets of analogous aromatic nitro compounds.