Alternative Mechanism for O2 Formation in Natural Photosynthesis via Nucleophilic Oxo-Oxo Coupling

Development of artificial photosystems based on designed proteins for mechanistic insights into photosynthesis

This review aims to provide an overview of the progress in protein-based artificial photosystem design and their potential to uncover the underlying principles governing light-harvesting in photosynthesis. While significant advances have been made in this area, a gap persists in reviewing these advances. This review provides a perspective of the field, pinpointing knowledge gaps and unresolved challenges that warrant further inquiry. In particular, it delves into the key considerations when designing photosystems based on the chromophore and protein scaffold characteristics, presents the established strategies for artificial photosystems engineering with their advantages and disadvantages, and underscores the recent breakthroughs in understanding the molecular mechanisms governing light-harvesting, charge separation, and the role of the protein motions in the chromophore's excited state relaxation. By disseminating this knowledge, this article provides a foundational resource for defining the field of bio-hybrid photosystems and aims to inspire the continued exploration of artificial photosystems using protein design.

Synthetic Mn3Ce2O5-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The photosynthetic oxygen-evolving center (OEC) is a unique Mn4CaO5-cluster that catalyses water splitting into electrons, protons, and dioxygen. Precisely structural and functional mimicking of the OEC is a long-standing challenge and pressingly needed for understanding the structure-function relationship and catalytic mechanism of O-O bond formation. Herein we report two simple and robust artificial Mn3Ce2O5-complexes that display a remarkable structural similarity to the OEC in regarding of the ten-atom core (five metal ions and five oxygen bridges) and the alkyl carboxylate peripheral ligands. This Mn3Ce2O5-cluster can catalyse the water-splitting reaction on the surface of ITO electrode. These results clearly show that cerium can structurally and functionally replace both calcium and manganese in the cluster. Mass spectroscopic measurements demonstrate that the oxide bridges in the cluster are exchangeable and can be rapidly replaced by the isotopic oxygen of H2 18O in acetonitrile solution, which supports that the oxide bridge(s) may serve as the active site for the formation of O-O bond during the water-splitting reaction. These results would contribute to our understanding of the structure-reactivity relationship of both natural and artificial clusters and shed new light on the development of efficient water-splitting catalysts in artificial photosynthesis.

Mechanistic elucidation of O2 production from tBuOOH in water using the Mn(II) catalyst [Mn2(mcbpen)2(H2O)2]2+: a DFT study

This study employs density functional theory at the SMD/B3LYP-D3/6-311+G(2d,p),def2-TZVPP//SMD/B3LYP-D3/6-31G(d),SDD level of theory to explore the mechanistic details of O2 generation from tBuOOH, using H218O as the solvent, in the presence of the Mn(II) catalyst [Mn2(mcbpen)2(H2O)2]2+. Since this chemistry was reported to occur through the reaction of Mn(III)(μ-O)Mn(IV)-O˙ with water, we first revaluated this proposal and found that it occurs with an activation barrier greater than 36 kcal mol-1, ruling out the functioning of such a dimer as the active catalyst. Experimental evidence has shown that the oxidation of [Mn2(mcbpen)2(H2O)2]2+ by tBuOOH in H218O produces the Mn(IV) species [Mn(18O)(mcbpen)]+. Our investigations revealed a plausible mechanism for this observation in which [Mn (18O)(mcbpen)]+ acts as the active catalyst, generating the tert-butyl peroxyl radical (tBuOO˙) through its reaction with tBuOOH. In this proposed mechanism, the O-O bond is formed through the interaction of tBuOO˙ with another [Mn(18O)(mcbpen)]+, finally leading to the formation of the 16O18O product. Our findings underscore the pivotal role of [Mn(18O)(mcbpen)]+ in both generating the active species tBuOO˙ and consuming it to produce 16O18O. With activation barriers as low as about 9 kcal mol-1, these elementary steps highlight the feasibility of our proposed mechanism. Moreover, this mechanism elucidates why, experimentally, one of the oxygen atoms in the released O2 comes from water, while the other originates from tBuOOH. This research broadens our understanding of high oxidation state manganese chemistry, setting the stage for the development of more efficient Mn-based catalysts, aimed at improving processes in both renewable energy and synthetic chemistry.

Fresh Impetus in the Chemistry of Calcium Peroxides

Redox-inactive metal ions are essential in modulating the reactivity of various oxygen-containing metal complexes and metalloenzymes, including photosystem II (PSII). The heart of this unique membrane-protein complex comprises the Mn4CaO5 cluster, in which the Ca2+ ion acts as a critical cofactor in the splitting of water in PSII. However, there is still a lack of studies involving Ca-based reactive oxygen species (ROS) systems, and the exact nature of the interaction between the Ca2+ center and ROS in PSII still generates intense debate. Here, harnessing a novel Ca-TEMPO complex supported by the β-diketiminate ligand to control the activation of O2, we report the isolation and structural characterization of hitherto elusive Ca peroxides, a homometallic Ca hydroperoxide and a heterometallic Ca/K peroxide. Our studies indicate that the presence of K+ cations is a key factor controlling the outcome of the oxygenation reaction of the model Ca-TEMPO complex. Combining experimental observations with computational investigations, we also propose a mechanistic rationalization for the reaction outcomes. The designed approach demonstrates metal-TEMPO complexes as a versatile platform for O2 activation and advances the understanding of Ca/ROS systems.

Closing Kok's cycle of nature's water oxidation catalysis

The Mn4CaO5(6) cluster in photosystem II catalyzes water splitting through the Si state cycle (i = 0-4). Molecular O2 is formed and the natural catalyst is reset during the final S3 → (S4) → S0 transition. Only recently experimental breakthroughs have emerged for this transition but without explicit information on the S0-state reconstitution, thus the progression after O2 release remains elusive. In this report, our molecular dynamics simulations combined with density functional calculations suggest a likely missing link for closing the cycle, i.e., restoring the first catalytic state. Specifically, the formation of closed-cubane intermediates with all hexa-coordinate Mn is observed, which would undergo proton release, water dissociation, and ligand transfer to produce the open-cubane structure of the S0 state. Thereby, we theoretically identify the previously unknown structural isomerism in the S0 state that acts as the origin of the proposed structural flexibility prevailing in the cycle, which may be functionally important for nature's water oxidation catalysis.

The Effect of Removal of External Proteins PsbO, PsbP and PsbQ on Flash-Induced Molecular Oxygen Evolution and Its Biphasicity in Tobacco PSII

The oxygen evolution within photosystem II (PSII) is one of the most enigmatic processes occurring in nature. It is suggested that external proteins surrounding the oxygen-evolving complex (OEC) not only stabilize it and provide an appropriate ionic environment but also create water channels, which could be involved in triggering the ingress of water and the removal of O2 and protons outside the system. To investigate the influence of these proteins on the rate of oxygen release and the efficiency of OEC function, we developed a measurement protocol for the direct measurement of the kinetics of oxygen release from PSII using a Joliot-type electrode. PSII-enriched tobacco thylakoids were used in the experiments. The results revealed the existence of slow and fast modes of oxygen evolution. This observation is model-independent and requires no specific assumptions about the initial distribution of the OEC states. The gradual removal of exogenous proteins resulted in a slowdown of the rapid phase (~ms) of O2 release and its gradual disappearance while the slow phase (~tens of ms) accelerated. The role of external proteins in regulating the biphasicity and efficiency of oxygen release is discussed based on observed phenomena and current knowledge.

Structure Function Studies of Photosystem II Using X-Ray Free Electron Lasers

The structure and mechanism of the water-oxidation chemistry that occurs in photosystem II have been subjects of great interest. The advent of X-ray free electron lasers allowed the determination of structures of the stable intermediate states and of steps in the transitions between these intermediate states, bringing a new perspective to this field. The room-temperature structures collected as the photosynthetic water oxidation reaction proceeds in real time have provided important novel insights into the structural changes and the mechanism of the water oxidation reaction. The time-resolved measurements have also given us a view of how this reaction-which involves multielectron, multiproton processes-is facilitated by the interaction of the ligands and the protein residues in the oxygen-evolving complex. These structures have also provided a picture of the dynamics occurring in the channels within photosystem II that are involved in the transport of the substrate water to the catalytic center and protons to the bulk.

Mechanisms of Mn(V)-Oxo to Mn(IV)-Oxyl Conversion: From Closed-Cubane Photosystem II to Mn(V) Catalysts and the Role of the Entering Ligands

The low activation barrier for O-O coupling in the closed-cubane Oxygen-Evolving Centre (OEC) of Photosystem II (PSII) requires water coordination with the Mn4 'dangler' ion in the Mn(V)-oxo fragment. This coordination transforms the Mn(V)-oxo complex into a more reactive Mn4(IV)-oxyl species, enhancing O-O coupling. This study explains the mechanism behind the coordination and indicates that in the most stable form of the OEC, the Mn4 fragment adopts a trigonal bipyramidal geometry but needs to transition to a square pyramidal form to be activated for O-O coupling. This transition stabilizes the Mn4 dxy orbital, enabling electron transfer from the oxo ligand to the dxy orbital, converting the oxo ligand into an oxyl species. The role of the water is to coordinate with the square pyramidal structure, reducing the energy gap between the oxo and oxyl forms, thereby lowering the activation energy for O-O coupling. This mechanism applies not only to the OEC system but also to other Mn(V)-based catalysts. For other catalysts, ligands such as OH- stabilize the Mn(IV)-oxyl species better than water, improving catalyst activation for reactions like C-H bond activation. This study is the first to explain the Mn(V)-oxo to Mn(IV)-oxyl conversion, providing a new foundation for Mn-based catalyst design.

Water Ligands Regulate the Redox Leveling Mechanism of the Oxygen-Evolving Complex of the Photosystem II

Understanding how water ligands regulate the conformational changes and functionality of the oxygen-evolving complex (OEC) in photosystem II (PSII) throughout the catalytic cycle of oxygen evolution remains a highly intriguing and unresolved challenge. In this study, we investigate the effect of water insertion (WI) on the redox state of the OEC by using the molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) hybrid methods. We find that water binding significantly reduces the free energy change for proton-coupled electron transfer (PCET) from Mn to YZ•, underscoring the important regulatory role of water binding, which is essential for enabling the OEC redox-leveling mechanism along the catalytic cycle. We propose a water binding mechanism in which WI is thermodynamically favored by the closed-cubane form of the OEC, with water delivery mediated by Ca2+ ligand exchange. Isomerization from the closed- to open-cubane conformation at three post-WI states highlights the importance of the location of the MnIII center in the OEC and the orientation of its Jahn-Teller axis to conformational changes of the OEC, which might be critical for the formation of the O-O bond. These findings reveal a complex interplay between conformational changes in the OEC and the ligand environment during the activation of the OEC by YZ•. Analogous regulatory effects due to water ligand binding are expected to be important for a wide range of catalysts activated by redox state transitions in aqueous environments.

A Reappraisal of the S2 State of Nature's Water Oxidizing Complex in Its Low and High Spin Forms

Density functional theory calculated 14N hyperfine couplings are obtained for the Mn1 ligated π-N of residue His332 of the photosystem 2 water oxidizing complex. An open cubane, O4H, model closely matches the experimental coupling obtained for the high spin S = 5/2 form of the S2 state, supporting an open cubane structure for this state. We also investigate the unusual geometric features for the S2 state obtained by X-ray free electron laser structure determinations and rationalize it as an equilibrium occurring at room temperature between W1/O4 deprotonated and protonated forms of the open cubane structure.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

Rapid oxygen isotopic exchange between bicarbonate and water during photosynthesis

Whether rapid oxygen isotopic exchange between bicarbonate and water occurs in photosynthesis is the key to determine the source of oxygen by classic 18O-labeled photosynthetic oxygen evolution experiments. Here we show that both Microcystis aeruginosa and Chlamydomonas reinhardtii utilize a significant proportion (>16%) of added bicarbonate as a carbon source for photosynthesis. However, oxygen isotopic signal in added bicarbonate cannot be traced in the oxygen in organic matter synthesized by these photosynthetic organisms. This contradicts the current photosynthesis theory, which states that photosynthetic oxygen evolution comes only from water, and oxygen in photosynthetic organic matter comes only from carbon dioxide. We conclude that the photosynthetic organisms undergo rapid exchange of oxygen isotope between bicarbonate and water during photosynthesis. At the same time, this study also provides isotopic evidence for a new mechanism that half of the oxygen in photosynthetic oxygen evolution comes from bicarbonate photolysis and half comes from water photolysis, which provides a new explanation for the bicarbonate effect, and suggests that the Kok-Joliot cycle of photosynthetic oxygen evolution, must be modified to include a molecule of bicarbonate in addition to one molecule of water which in turn must be incorporated into the cycle instead of two water molecules. Furthermore, this study provides a theoretical basis for constructing a newer artificial photosynthetic reactor coupling light reactions with the dark reactions.

Assignment of the slowly exchanging substrate water of nature's water-splitting cofactor

Identifying the two substrate water sites of nature's water-splitting cofactor (Mn4CaO5 cluster) provides important information toward resolving the mechanism of O-O bond formation in Photosystem II (PSII). To this end, we have performed parallel substrate water exchange experiments in the S1 state of native Ca-PSII and biosynthetically substituted Sr-PSII employing Time-Resolved Membrane Inlet Mass Spectrometry (TR-MIMS) and a Time-Resolved 17O-Electron-electron Double resonance detected NMR (TR-17O-EDNMR) approach. TR-MIMS resolves the kinetics for incorporation of the oxygen-isotope label into the substrate sites after addition of H218O to the medium, while the magnetic resonance technique allows, in principle, the characterization of all exchangeable oxygen ligands of the Mn4CaO5 cofactor after mixing with H217O. This unique combination shows i) that the central oxygen bridge (O5) of Ca-PSII core complexes isolated from Thermosynechococcus vestitus has, within experimental conditions, the same rate of exchange as the slowly exchanging substrate water (WS) in the TR-MIMS experiments and ii) that the exchange rates of O5 and WS are both enhanced by Ca2+→Sr2+ substitution in a similar manner. In the context of previous TR-MIMS results, this shows that only O5 fulfills all criteria for being WS. This strongly restricts options for the mechanism of water oxidation.

Evolution in the Design of Water Oxidation Catalysts with Transition-Metals: A Perspective on Biological, Molecular, Supramolecular, and Hybrid Approaches

Increased demand for a carbon-neutral sustainable energy scheme augmented by climatic threats motivates the design and exploration of novel approaches that reserve intermittent solar energy in the form of chemical bonds in molecules and materials. In this context, inspired by biological processes, artificial photosynthesis has garnered significant attention as a promising solution to convert solar power into chemical fuels from abundantly found H2O. Among the two redox half-reactions in artificial photosynthesis, the four-electron oxidation of water according to 2H2O → O2 + 4H+ + 4e- comprises the major bottleneck and is a severe impediment toward sustainable energy production. As such, devising new catalytic platforms, with traditional concepts of molecular, materials and biological catalysis and capable of integrating the functional architectures of the natural oxygen-evolving complex in photosystem II would certainly be a value-addition toward this objective. In this review, we discuss the progress in construction of ideal water oxidation catalysts (WOCs), starting with the ingenuity of the biological design with earth-abundant transition metal ions, which then diverges into molecular, supramolecular and hybrid approaches, blurring any existing chemical or conceptual boundaries. We focus on the geometric, electronic, and mechanistic understanding of state-of-the-art homogeneous transition-metal containing molecular WOCs and summarize the limiting factors such as choice of ligands and predominance of environmentally unrewarding and expensive noble-metals, necessity of high-valency on metal, thermodynamic instability of intermediates, and reversibility of reactions that create challenges in construction of robust and efficient water oxidation catalyst. We highlight how judicious heterogenization of atom-efficient molecular WOCs in supramolecular and hybrid approaches put forth promising avenues to alleviate the existing problems in molecular catalysis, albeit retaining their fascinating intrinsic reactivities. Taken together, our overview is expected to provide guiding principles on opportunities, challenges, and crucial factors for designing novel water oxidation catalysts based on a synergy between conventional and contemporary methodologies that will incite the expansion of the domain of artificial photosynthesis.

Pivotal Role of Geometry Regulation on O-O Bond Formation Mechanism of Bimetallic Water Oxidation Catalysts

In this study, we highlight the impact of catalyst geometry on the formation of O-O bonds in Cu2 and Fe2 catalysts. A series of Cu2 complexes with diverse linkers are designed as electrocatalysts for water oxidation. Interestingly, the catalytic performance of these Cu2 complexes is enhanced as their molecular skeletons become more rigid, which contrasts with the behavior observed in our previous investigation with Fe2 analogs. Moreover, mechanistic studies reveal that the reactivity of the bridging O atom results in distinct pathways for O-O bond formation in Cu2 and Fe2 catalysts. In Cu2 systems, the coupling takes place between a terminal CuIII -OH and a bridging μ-O⋅ radical. Whereas in Fe2 systems, it involves the coupling of two terminal Fe-oxo entities. Furthermore, an in-depth structure-activity analysis uncovers the spatial geometric prerequisites for the coupling of the terminal OH with the bridging μ-O⋅ radical, ultimately leading to the O-O bond formation. Overall, this study emphasizes the critical role of precisely adjusting the spatial geometry of catalysts to align with the O-O bonding pathway.

Enhancing catalysis studies with chat generative pre-trained transformer (ChatGPT): Conversation with ChatGPT

The progress made in natural language processing (NLP) and large language models (LLMs), such as generative pre-trained transformers, (GPT) has provided exciting opportunities for enhancing research across various fields. Within the realm of catalysis studies, GPT-driven models present valuable support in expediting the exploration and comprehension of catalytic processes. This research underscores the significance of ChatGPT in catalysis research, emphasizing its prowess as a valuable tool for furthering scientific inquiries. It suggests that for an outstanding oxygen evolution reaction (OER) catalyst as a case study, scientists can leverage ChatGPT to extract deeper insights and brainstorm innovative approaches to grasp the mechanism better and refine current systems.

Electrocatalytic water oxidation with manganese phosphates

As inspired by the Mn4CaO5 oxygen evolution center in nature, Mn-based electrocatalysts have received overwhelming attention for water oxidation. However, the understanding of the detailed reaction mechanism has been a long-standing problem. Herein, homologous KMnPO4 and KMnPO4•H2O with 4-coordinated and 6-coordinated Mn centers, respectively, are prepared. The two catalysts constitute an ideal platform to study the structure-performance correlation. The presence of Mn(III), Mn(IV), and Mn(V) intermediate species are identified during water oxidation. The Mn(V)=O species is demonstrated to be the substance for O-O bond formation. In KMnPO4•H2O, the Mn coordination structure did not change significantly during water oxidation. In KMnPO4, the Mn coordination structure changed from 4-coordinated [MnO4] to 5-coordinated [MnO5] motif, which displays a triangular biconical configuration. The structure flexibility of [MnO5] is thermodynamically favored in retaining Mn(III)-OH and generating Mn(V)=O. The Mn(V)=O species is at equilibrium with Mn(IV)=O, the concentration of which determines the intrinsic activity of water oxidation. This study provides a clear picture of water oxidation mechanism on Mn-based systems.

Comprehensive Evaluation of Models for Ammonia Binding to the Oxygen Evolving Complex of Photosystem II

The identity and insertion pathway of the substrate oxygen atoms that are coupled to dioxygen by the oxygen-evolving complex (OEC) remains a central question toward understanding Nature's water oxidation mechanism. In several studies, ammonia has been used as a small "water analogue" to elucidate the pathway of substrate access to the OEC and to aid in determining which of the oxygen ligands of the tetramanganese cluster are substrates for O-O bond formation. On the basis of structural and spectroscopic investigations, five first-sphere binding modes of ammonia have been suggested, involving either substitution of an existing H2O/OH-/O2- group or addition as an extra ligand to a metal ion of the Mn4CaO5 cluster. Some of these modes, specifically the ones involving substitution, have already been subject to spectroscopy-oriented quantum chemical investigations, whereas more recent suggestions that postulate the addition of ammonia have not been examined so far with quantum chemistry for their agreement with spectroscopic data. Herein, we use a common structural framework and theoretical methodology to evaluate structural models of the OEC that represent all proposed modes of first-sphere ammonia interaction with the OEC in its S2 state. Criteria include energetic, magnetic, kinetic, and spectroscopic properties compared against available experimental EPR, ENDOR, ESEEM, and EDNMR data. Our results show that models featuring ammonia replacing one of the two terminal water ligands on Mn4 align best with experimental data, while they definitively exclude substitution of a bridging μ-oxo ligand as well as incorporation of ammonia as a sixth ligand on Mn1 or Mn4.

Direct spectroscopic evidence for the high-spin state of dioxidomanganese(V)

The spin state of metal centers in many catalytic reactions has been demonstrated to be a rate limiting factor when high-valent metal centers such as manganese are involved. Although numerous manganese(V) complexes, including a few manganese(V) oxo complexes, have been identified, thus far only one of these, [MnVH3 buea(O)], has been directly confirmed to exist in a high spin state. Such a high-spin manganese(V) center may play a crucial role in the dioxygen formation process in the elusive S4 state of the Kok cycle in photosystem II. In this study, we provide direct experimental evidence, using X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS), of gas phase [OMnO]+ as the second known high-spin manganese(V) oxo complex. We conclusively assign the ground state as 3B1 (C2v). Additionally, we provide fingerprint spectra not only for [OMnV O]+, but also for the high-spin hydroxidooxidomanganese(IV) ion [OMnIV OH]+ in its 4A'' (Cs) ground state that is expected to exhibit similar XAS and XMCD spectral signatures to neutral dioxidomanganese(IV).

Mapping the Oxygens in the Oxygen-Evolving Complex of Photosystem II by Their Nucleophilicity Using Quantum Descriptors

The oxygen-evolving complex (OEC) of Photosystem II catalyzes the water-splitting reaction using solar energy. Thus, understanding the reaction mechanism will inspire the design of biomimetic artificial catalysts that convert solar energy to chemical energy. Conceptual Density Functional Theory (CDFT) focuses on understanding the reactivity of molecules and the atomic contribution to the overall nucleophilicity and electrophilicity of the molecule using quantum descriptors. However, this method has not been applied to the OEC before. Here, we use Fukui functions and the dual descriptor to provide quantitative measures of the nucleophilicity and electrophilicity of oxygens in the OEC for different models in different S states. Our results show that the μ-oxo bridges connected to terminal Mn4 are nucleophilic, and those in the cube formed by Mn1, Mn2, and Mn3 are mostly electrophilic. The dual descriptors of the bridging oxygens in the OEC showed a similar reactivity to that of bridging oxygens in Mn model compounds. However, the terminal water W1, which is bound to Mn4, showed very strong reactivity in some of the S3 models. Thus, our calculations support the model that proposes the formation of the O2 molecule through nucleophilic attack by a terminal water.

Is There a Different Mechanism for Water Oxidation in Higher Plants?

The leading mechanism for the formation of O2 in photosystem II (PSII) has, during the past decade, been established as the so-called oxyl-oxo mechanism. In that mechanism, O2 is formed from a binding between an oxygen radical (oxyl) and a bridging oxo group. For the case of higher plants, that mechanism has recently been criticized. Instead, a nucleophilic attack of an oxo group on a five-coordinated Mn(V)═O group forming O2 has been suggested in a so-called water-unbound (WU) mechanism. In the present study, the WU mechanism has been investigated. It is found that the WU mechanism is just a variant of a previously suggested mechanism but with a reactant and a transition state that have much higher energies. The addition of a water molecule on the empty site of the Mn(V)═O center is very exergonic and leads back to the previously suggested oxyl-oxo mechanism.