Water oxidation in photosystem II

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.

Controlling Redox Potential of a Manganese(III)-Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity

A series of mononuclear manganese(III)-hydroxo and -aqua complexes, [MnIII(TBDAP)(OH)2]+ (1), [MnIII(TBDAP)(OH)(OH2)]2+ (2) and [MnIII(TBDAP)(OH2)2]3+ (3), were prepared from a manganese(II) precursor and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials (E1/2) in the order of 1 (-0.15 V) < 2 (0.56 V) < 3 (1.11 V), while pKa values exhibited a reverse trend in the order of 3 (3.87) < 2 (11.84). Employing the Bordwell Equation, the O-H bond dissociation free energies (BDFE) of [MnII(TBDAP)(OH)(OH2)]+ and [MnII(TBDAP)(OH2)2]2+, related to the driving force of 1 and 2 in hydrogen atom transfer (HAT), were determined as 75.3 and 77.3 kcal mol-1, respectively. It was found that the thermodynamic driving force of 2 in HAT becomes greater than that of 1 as the redox potential of 2 increases through protonation from 1 to 2. Kinetic studies on electrophilic reactions using a variety of substrates revealed that 1 is only weakly reactive with O-H bonds, whereas 2 can activate aliphatic C-H bonds in addition to O-H bonds. The reaction rates increased by 1.4 × 104-fold for the O-H bonds by 2 over 1, which was explained by the difference in BDFE and the tunneling effect. Furthermore, 3, possessing the highest redox potential value, was found to undergo an aromatic C-H bond activation reaction under mild conditions. These results provide valuable insights into enhancing electrophilic reactivity by modulating the redox potential of manganese(III)-hydroxo and -aqua complexes through protonation.

A Vision for the Future of Multiscale Modeling

The rise of modern computer science enabled physical chemistry to make enormous progresses in understanding and harnessing natural and artificial phenomena. Nevertheless, despite the advances achieved over past decades, computational resources are still insufficient to thoroughly simulate extended systems from first principles. Indeed, countless biological, catalytic and photophysical processes require ab initio treatments to be properly described, but the breadth of length and time scales involved makes it practically unfeasible. A way to address these issues is to couple theories and algorithms working at different scales by dividing the system into domains treated at different levels of approximation, ranging from quantum mechanics to classical molecular dynamics, even including continuum electrodynamics. This approach is known as multiscale modeling and its use over the past 60 years has led to remarkable results. Considering the rapid advances in theory, algorithm design, and computing power, we believe multiscale modeling will massively grow into a dominant research methodology in the forthcoming years. Hereby we describe the main approaches developed within its realm, highlighting their achievements and current drawbacks, eventually proposing a plausible direction for future developments considering also the emergence of new computational techniques such as machine learning and quantum computing. We then discuss how advanced multiscale modeling methods could be exploited to address critical scientific challenges, focusing on the simulation of complex light-harvesting processes, such as natural photosynthesis. While doing so, we suggest a cutting-edge computational paradigm consisting in performing simultaneous multiscale calculations on a system allowing the various domains, treated with appropriate accuracy, to move and extend while they properly interact with each other. Although this vision is very ambitious, we believe the quick development of computer science will lead to both massive improvements and widespread use of these techniques, resulting in enormous progresses in physical chemistry and, eventually, in our society.

Elucidating the catalytic mechanisms of O2 generation by [Mn2(μ-O)2(terpy)2(OH2)2]3+ using DFT calculations: a focus on ClO- as oxidant

The experimentally reported Mn(IV)Mn(III) complex [Mn2(μ-O)2(terpy)2(OH2)2]3+ has been observed catalyzing O2 generation with oxidants like ClO- and HSO5-. Previous mechanistic studies primarily focused on O2 generation with HSO5-, concluding that Mn(IV)Mn(III) acts as a catalyst, generating a Mn(IV)Mn(IV)-oxyl species as a key intermediate responsible for O-O bond formation. This computational study employs DFT calculations to investigate whether the catalytic generation of O2 using ClO- follows the same mechanism previously identified with HSO5- as the oxidant, or if it proceeds through an alternate pathway. To this end, we explored multiple pathways using ClO- as the oxidant. Interestingly, our findings confirm that in the case of ClO- as the oxidant, similar to what was observed with HSO5-, the Mn(IV)Mn(IV)-oxyl species indeed plays a crucial role in driving the catalytic evolution of O2 with the potential formation of the binuclear complexes Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH during the reaction. These complexes are reactive in producing O2, with activation free energies of 15.9 and 14.3 kcal mol-1, respectively. However, our calculations revealed that the Mn(IV)Mn(IV)-oxyl complex is significantly more reactive in producing O2 than Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH, with a lower free energy barrier of 8.1 kcal mol-1. Consequently, even though Mn(IV)Mn(IV)-oxyl is predicted to be present in much lower concentrations than Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH, it emerges as the species acting as the active catalyst for catalytic O2 generation. This study enhances our knowledge of high oxidation state (+3 and +4) manganese chemistry, highlighting its key role in catalysis and paving the way for more efficient Mn-based catalysts with broad applications.

Proton Domino Reactions at an Imidazole Relay Control the Oxidation of a TyrZ-His190 Artificial Mimic of Photosystem II

A close mimic of P680 and the TyrosineZ-Histidine190 pair in photosystem II (PS II) has been synthesized using a ruthenium chromophore and imidazole-phenol ligands. The intramolecular oxidation of the ligands by the photoproduced Ru(III) species is characterized by a small driving force, very similar to PS II where the complexity of kinetics was attributed to the reversibility of electron transfer steps. Laser flash photolysis revealed biphasic kinetics for ligand oxidation. The fast phase (τ<50 ns) corresponds to partial oxidation of the imidazole-phenol ligand, proton transfer within the hydrogen bond, and formation of a neutral phenoxyl radical. The slow phase (5-9 μs) corresponds to full oxidation of the ligand which is kinetically controlled by deprotonation of the distant 1-nitrogen of the imidazolium. These results show that imidazole with its two protonatable sites plays a special role as a proton relay in a 'proton domino' reaction.

Current analysis of cations substitution in the oxygen-evolving complex of photosystem II

Water oxidation in photosystem II (PSII) is performed by the oxygen-evolving complex Mn4CaO5 which can be extracted from PSII and then reconstructed using exogenous cations Mn(II) and Ca2+. The binding efficiency of other cations to the Mn-binding sites in Mn-depleted PSII was investigated without any positive results. At the same time, a study of the Fe cations interaction with Mn-binding sites showed that it binds at a level comparable with the binding of Mn cations. Binding of Fe(II) cations first requires its light-dependent oxidation. In general, the interaction of Fe(II) with Mn-depleted PSII has a number of features similar to the two-quantum model of photoactivation of the complex with the release of oxygen. Interestingly, incubation of Ca-depleted PSII with Fe(II) cations under certain conditions is accompanied by the formation of a chimeric cluster Mn/Fe in the oxygen-evolving complex. PSII with the cluster 2Mn2Fe was found to be capable of water oxidation, but only to the H2O2 intermediate. However, the cluster 3Mn1Fe can oxidize water to O2 with an efficiency about 25% of the original in the absence of extrinsic proteins PsbQ and PsbP. In the presence of these proteins, the efficiency of O2 evolution can reach 80% of the original when adding exogenous Ca2+. In this review, we summarized information on the formation of chimeric Mn-Fe clusters in the oxygen-evolving complex. The data cited may be useful for detailing the mechanism of water oxidation.

On the simulation and interpretation of substrate-water exchange experiments in photosynthetic water oxidation

Water oxidation by photosystem II (PSII) sustains most life on Earth, but the molecular mechanism of this unique process remains controversial. The ongoing identification of the binding sites and modes of the two water-derived substrate oxygens ('substrate waters') in the various intermediates (Si states, i = 0, 1, 2, 3, 4) that the water-splitting tetra-manganese calcium penta-oxygen (Mn4CaO5) cluster attains during the reaction cycle provides central information towards resolving the unique chemistry of biological water oxidation. Mass spectrometric measurements of single- and double-labeled dioxygen species after various incubation times of PSII with H218O provide insight into the substrate binding modes and sites via determination of exchange rates. Such experiments have revealed that the two substrate waters exchange with different rates that vary independently with the Si state and are hence referred to as the fast (Wf) and the slow (WS) substrate waters. New insight for the molecular interpretation of these rates arises from our recent finding that in the S2 state, under special experimental conditions, two different rates of WS exchange are observed that appear to correlate with the high spin and low spin conformations of the Mn4CaO5 cluster. Here, we reexamine and unite various proposed methods for extracting and assigning rate constants from this recent data set. The analysis results in a molecular model for substrate-water binding and exchange that reconciles the expected non-exchangeability of the central oxo bridge O5 when located between two Mn(IV) ions with the experimental and theoretical assignment of O5 as WS in all S states. The analysis also excludes other published proposals for explaining the water exchange kinetics.

Occupancy Analysis of Water Molecules inside Channels within 25 Å Radius of the Oxygen-Evolving Center of Photosystem II in Molecular Dynamics Simulations

At room temperature and neutral pH, the oxygen-evolving center (OEC) of photosystem II (PSII) catalyzes water oxidation. During this process, oxygen is released from the OEC, while substrate waters are delivered to the OEC and protons are passed from the OEC to the lumen through water channels known as the narrow or the O4 channel, broad or the Cl1 channel, and large or the O1 channel. Protein residues lining the surfaces of these channels play a critical role in stabilizing the hydrogen-bonding networks that assist in the process. We carried out an occupancy analysis to better understand the structural and possible substrate water dynamics in full PSII monomer molecular dynamics (MD) trajectories in both the S1 and S2 states. We find that the equilibrated positions of water molecules derived from MD-derived electron density maps largely match the experimentally observed positions in crystallography. Furthermore, the occupancy reduction in MD simulations of some water molecules inside the single-filed narrow channel also correlates well with the crystallographic data during a structural transition when the S1 state of the OEC advances to the S2 state. The overall reduced occupancies of water molecules are the source of their "vacancy-hopping" dynamic nature inside these channels, unlike water molecules inside an ice lattice where all water molecules have a fixed unit occupancy. We propose on the basis of findings in our structural and molecular dynamics analysis that the water molecule occupying a pocket formed by D1-D61, D1-S169, and O4 of the OEC could be the last steppingstone to enter into the OEC and that the broad channel may be favored for proton transfer.

Assembly and Repair of Photosystem II in Chlamydomonas reinhardtii

Oxygenic photosynthetic organisms use Photosystem II (PSII) to oxidize water and reduce plastoquinone. Here, we review the mechanisms by which PSII is assembled and turned over in the model green alga Chlamydomonas reinhardtii. This species has been used to make key discoveries in PSII research due to its metabolic flexibility and amenability to genetic approaches. PSII subunits originate from both nuclear and chloroplastic gene products in Chlamydomonas. Nuclear-encoded PSII subunits are transported into the chloroplast and chloroplast-encoded PSII subunits are translated by a coordinated mechanism. Active PSII dimers are built from discrete reaction center complexes in a process facilitated by assembly factors. The phosphorylation of core subunits affects supercomplex formation and localization within the thylakoid network. Proteolysis primarily targets the D1 subunit, which when replaced, allows PSII to be reactivated and completes a repair cycle. While PSII has been extensively studied using Chlamydomonas as a model species, important questions remain about its assembly and repair which are presented here.

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.

Three-state mathematical model for the assessment of DCMU-treated photosystem II heterogeneity

Photosystem II (PSII) is one of the main pigment-protein complexes of photosynthesis which is highly sensitive to unfavorable environmental factors. The heterogeneity of PSII properties is essential for the resistance of autotrophic organisms to stress factors. Assessment of the PSII heterogeneity may be used in environmental monitoring for on-line detection of contamination of the environment. We propose an approach to assess PSII oxygen-evolving complex and light-harvesting antenna heterogeneity that is based on mathematical modeling of the shape of chlorophyll a fluorescence rise of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated samples. The hierarchy of characteristic times of the processes considered in the model makes it possible to reduce the model to a system of three ordinary differential equations. The analytic solution of the reduced three-state model is expressed as a sum of two exponential functions, and it exactly reproduces the solution of the complete system within the time range from microseconds to hundreds of milliseconds. The combination of several such models for reaction centers with different properties made it possible to use it as an instrument to study PSII heterogeneity. PSII heterogeneity was studied for Chlamydomonas at different intensities of actinic light, for Scenedesmus under short-term heating, and for Chlorella grown in nitrate-enriched and nitrate-depleted media.

Spin polarized electron dynamics enhance water splitting efficiency by yttrium iron garnet photoanodes: a new platform for spin selective photocatalysis

This work presents a spectroscopic and photocatalytic comparison of water splitting using yttrium iron garnet (Y3Fe5O12, YIG) and hematite (α-Fe2O3) photoanodes. Despite similar electronic structures, YIG significantly outperforms widely studied hematite, displaying more than an order of magnitude increase in photocurrent density. Probing the charge and spin dynamics by ultrafast, surface-sensitive XUV spectroscopy reveals that the enhanced performance arises from (1) reduced polaron formation in YIG compared to hematite and (2) an intrinsic spin polarization of catalytic photocurrents in YIG. Ultrafast XUV measurements show a reduction in the formation of surface electron polarons compared to hematite due to site-dependent electron-phonon coupling. This leads to spin polarized photocurrents in YIG where efficient charge separation occurs on the Td sub-lattice compared to fast trapping and electron/hole pair recombination on the Oh sub-lattice. These lattice-dependent dynamics result in a long-lived spin aligned hole population at the YIG surface, which is directly observed using XUV magnetic circular dichroism. Comparison of the Fe M2,3 and O L1-edges show that spin aligned holes are hybridized between O 2p and Fe 3d valence band states, and these holes are responsible for highly efficient, spin selective water oxidation by YIG. Together, these results point to YIG as a new platform for highly efficient, spin selective photocatalysis.

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.

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.

An Investigation of the Influence of Tyrosine Local Interactions on Electron Hopping in a Model Protein

Multi-step electron transfer reactions are important to the function of many cellular systems. The ways in which such systems have evolved to direct electrons along specific pathways are largely understood, but less so are the ways in which the reduction-oxidation potentials of individual redox sites are controlled. We prepared a series of three new artificial variants of Pseudomonas aeruginosa azurin where a tyrosine (Tyr109) is situated between the native Cu ion and a Ru(II) photosensitizer tethered to a histidine (His107). Arginine, glutamine, or methionine were introduced as position 122, which is near to Tyr109. We investigated the rate of CuI oxidation by a flash-quench generated Ru(III) oxidant over pH values from 5 to 9. While the identity of the residue at position 122 affects some of the physical properties of Tyr109, the rates of CuI oxidation are only weakly dependent on the identity of the residue at 122. The results highlight that more work is still needed to understand how non-covalent interactions of redox active groups are affected in redox proteins.

Predicting the FCI Energy of Large Systems to Chemical Accuracy from Restricted Active Space Density Matrix Renormalization Group Calculations

We theoretically derive and validate with large scale simulations a remarkably accurate power law scaling of errors for the restricted active space density matrix renormalization group (DMRG-RAS) method [J. Phys. Chem. A 126, 9709] in electronic structure calculations. This yields a new extrapolation method, DMRG-RAS-X, which reaches chemical accuracy for strongly correlated systems such as the chromium dimer, dicarbon up to a large cc-pVQZ basis and even a large chemical complex such as the FeMoco with significantly lower computational demands than those of previous methods. The method is free of empirical parameters, performed robustly and reliably in all examples we tested, and has the potential to become a vital alternative method for electronic structure calculations in quantum chemistry and more generally for the computation of strong correlations in nuclear and condensed matter physics.

Harvesting electrons and holes from photodriven symmetry-breaking charge separation within a perylenediimide photosynthetic model dimer

Understanding how to utilize symmetry-breaking charge separation (SB-CS) offers a path toward increasingly efficient light-harvesting technologies. This process plays a central role in the first step of photosynthesis, in which the dimeric "special pair" of the photosynthetic reaction center enters a coherent SB-CS state after photoexcitation. Previous research on SB-CS in both biological and synthetic chromophore dimers has focused on increasing the efficiency of light-driven processes. In a chromophore dimer undergoing SB-CS, the energy of the radical ion pair product is nearly isoenergetic with that of the lowest excited singlet (S1) state of the dimer. This means that very little energy is lost from the absorbed photon. In principle, the relatively high energy electron and hole generated by SB-CS within the chromophore dimer can each be transferred to adjacent charge acceptors to extend the lifetime of the electron-hole pair, which can increase the efficiency of solar energy conversion. To investigate this possibility, we have designed a bis-perylenediimide cyclophane (mPDI2) covalently linked to a secondary electron donor, peri-xanthenoxanthene (PXX) and a secondary electron acceptor, partially fluorinated naphthalenediimide (FNDI). Upon selective photoexcitation of mPDI2, transient absorption spectroscopy shows that mPDI2 undergoes SB-CS, followed by two secondary charge transfer reactions to generate a PXX•+-mPDI2-FNDI•- radical ion pair having a nearly 3 µs lifetime. This strategy has the potential to increase the efficiency of molecular systems for artificial photosynthesis and photovoltaics.

Tracking the first electron transfer step at the donor side of oxygen-evolving photosystem II by time-resolved infrared spectroscopy

In oxygen-evolving photosystem II (PSII), the multi-phasic electron transfer from a redox-active tyrosine residue (TyrZ) to a chlorophyll cation radical (P680+) precedes the water-oxidation chemistry of the S-state cycle of the Mn4Ca cluster. Here we investigate these early events, observable within about 10 ns to 10 ms after laser-flash excitation, by time-resolved single-frequency infrared (IR) spectroscopy in the spectral range of 1310-1890 cm-1 for oxygen-evolving PSII membrane particles from spinach. Comparing the IR difference spectra at 80 ns, 500 ns, and 10 µs allowed for the identification of quinone, P680 and TyrZ contributions. A broad electronic absorption band assignable P680+ was used to trace largely specifically the P680+ reduction kinetics. The experimental time resolution was taken into account in least-square fits of P680+ transients with a sum of four exponentials, revealing two nanosecond phases (30-46 ns and 690-1110 ns) and two microsecond phases (4.5-8.3 µs and 42 µs), which mostly exhibit a clear S-state dependence, in agreement with results obtained by other methods. Our investigation paves the road for further insight in the early events associated with TyrZ oxidation and their role in the preparing the PSII donor side for the subsequent water oxidation chemistry.

Assessment of genetic variation among wheat genotypes for drought tolerance utilizing microsatellite markers and morpho-physiological characteristics

Drought is a major abiotic stress that severely limits sustainable wheat (Triticum aestivum L.) productivity via morphological and physio-biochemical alterations of cellular processes. The complex nature and polygenic control of drought tolerance traits make breeding tolerant genotypes quite challenging. However, naturally occurring variabilities among wheat germplasm resources could potentially help combating drought. The present study was conducted to assess the drought tolerance of 18 Bangladeshi hexaploid wheat genotypes, focusing on the identification of potent sources of diversity by combining microsatellite markers, also known as single sequence repeat markers, and morpho-physiological characteristics that might help accelerating wheat crop improvement programs. Initially, the genotypes were evaluated using 25 microsatellite markers followed by an on-field evaluation of 7 morphological traits (plant height, spike number, spike length, grains per spike, 1000-grain weight, grain yield, biological yield) and 6 physiological traits (SPAD value, membrane stability index, leaf relative water content, proline content, canopy temperature depression, and leaf K+ ion content). The field-trial was conducted in a factorial fashion of 18 wheat genotypes and two water regimes (control and drought) following a split-plot randomized complete block design. Regardless of genotype, drought was significantly damaging for all the tested traits; however, substantial variability in drought stress tolerance was evident among the genotypes. Spike length, 1000-grain weight, SPAD value, leaf relative water content, canopy temperature depression, proline content, and potassium (K+) ion content were the most representative of drought-induced growth and yield impairments and also correlated well with the contrasting ability of genotypic tolerance. Microsatellite markers amplified 244 alleles exhibiting 79% genetic diversity. Out of 25 markers, 23 was highly polymorphic showing 77% average polymorphism. Morpho-physiological trait-based hierarchical clustering and microsatellite marker-based neighbor-jointing clustering both revealed three genotypic clusters with 71% co-linearity between them. In both cases, the genotypes Kanchan, BAW-1147, BINA Gom 1, BARI Gom 22, BARI Gom 26, and BARI Gom 33 were found to be comparatively more tolerant than the other tested genotypes, showing potential for cultivation in water-deficit environments. The findings of this study would contribute to the present understanding of drought tolerance in wheat and would provide a basis for future genotype selection for drought-tolerant wheat breeding programs.

Reactivities and Electronic Structures of μ-1,2-Peroxo and μ-1,2-Superoxo CoIIICoIII Complexes: Electrophilic Reactivity and O2 Release Induced by Oxidation

Peroxo complexes are key intermediates in water oxidation catalysis (WOC). Cobalt plays an important role in WOC, either as oxides CoOx or as {CoIII(μ-1,2-peroxo)CoIII} complexes, which are the oldest peroxo complexes known. The oxidation of {CoIII(μ-1,2-peroxo)CoIII} complexes had usually been described to form {CoIII(μ-1,2-superoxo)CoIII} complexes; however, recently the formation of {CoIV(μ-1,2-peroxo)CoIII} species were suggested. Using a bis(tetradentate) dinucleating ligand, we present here the synthesis and characterization of {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} and {CoIII(μ-OH)2CoIII} complexes. Oxidation of {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} at -40 °C in CH3CN provides the stable {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} species and activates electrophilic reactivity. Moreover, {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} catalyzes water oxidation, not molecularly but rather via CoOx films. While {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} can be reversibly deprotonated with DBU at -40 °C in CH3CN, {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} undergoes irreversible conversions upon reaction with bases to a new intermediate that is also the decay product of {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} in aqueous solution at pH > 2. Based on a combination of experimental methods, the new intermediate is proposed to have a {CoII(μ-OH)CoIII} core formed by the release of O2 from {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} confirmed by a 100% yield of O2 upon photocatalytic oxidation of {CoIII(μ-1,2-peroxo)(μ-OH)CoIII}. This release of O2 by oxidation of a peroxo intermediate corresponds to the last step in molecular WOC.

Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [PD1PD2]. PHOTOSYNTHESIS RESEARCH 2023:10.1007/s11120-023-01049-3. [PMID: 37751034 DOI: 10.1007/s11120-023-01049-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Flash-induced absorption changes in the Soret region arising from the [PD1PD2]+ state, the chlorophyll cation radical formed upon light excitation of Photosystem II (PSII), were measured in Mn-depleted PSII cores at pH 8.6. Under these conditions, TyrD is i) reduced before the first flash, and ii) oxidized before subsequent flashes. In wild-type PSII, when TyrD● is present, an additional signal in the [PD1PD2]+-minus-[PD1PD2] difference spectrum was observed when compared to the first flash when TyrD is not oxidized. The additional feature was "W-shaped" with troughs at 434 nm and 446 nm. This feature was absent when TyrD was reduced, but was present (i) when TyrD was physically absent (and replaced by phenylalanine) or (ii) when its H-bonding histidine (D2-His189) was physically absent (replaced by a Leucine). Thus, the simple difference spectrum without the double trough feature at 434 nm and 446 nm, seemed to require the native structural environment around the reduced TyrD and its H bonding partners to be present. We found no evidence of involvement of PD1, ChlD1, PheD1, PheD2, TyrZ, and the Cytb559 heme in the W-shaped difference spectrum. However, the use of a mutant of the PD2 axial His ligand, the D2-His197Ala, shows that the PD2 environment seems involved in the formation of "W-shaped" signal.

Electronic Structure of the Complete Series of Gas-Phase Manganese Acetylacetonates by X-ray Absorption Spectroscopy

Metal centers in transition metal-ligand complexes occur in a variety of oxidation states causing their redox activity and therefore making them relevant for applications in physics and chemistry. The electronic state of these complexes can be studied by X-ray absorption spectroscopy, which is, however, due to the complex spectral signature not always straightforward. Here, we study the electronic structure of gas-phase cationic manganese acetylacetonate complexes Mn(acac)1-3+ using X-ray absorption spectroscopy at the metal center and ligand constituents. The spectra are well reproduced by multiconfigurational wave function theory, time-dependent density functional theory as well as parameterized crystal field and charge transfer multiplet simulations. This enables us to get detailed insights into the electronic structure of ground-state Mn(acac)1-3+ and extract empirical parameters such as crystal field strength and exchange coupling from X-ray excitation at both the metal and ligand sites. By comparison to X-ray absorption spectra of neutral, solvated Mn(acac)2,3 complexes, we also show that the effect of coordination on the L3 excitation energy, routinely used to identify oxidation states, can contribute about 40-50% to the observed shift, which for the current study is 1.9 eV per oxidation state.

Quantum Computation of Hydrogen Bond Dynamics and Vibrational Spectra

Calculating observable properties of chemical systems is often classically intractable and widely viewed as a promising application of quantum information processing. Here, we introduce a new framework for solving generic quantum chemical dynamics problems using quantum logic. We experimentally demonstrate a proof-of-principle instance of our method using the QSCOUT ion-trap quantum computer, where we experimentally drive the ion-trap system to emulate the quantum wavepacket dynamics corresponding to the shared-proton within an anharmonic hydrogen bonded system. Following the experimental creation and propagation of the shared-proton wavepacket on the ion-trap, we extract measurement observables such as its time-dependent spatial projection and its characteristic vibrational frequencies to spectroscopic accuracy (3.3 cm-1 wavenumbers, corresponding to >99.9% fidelity). Our approach introduces a new paradigm for studying the chemical dynamics and vibrational spectra of molecules and opens the possibility to describe the behavior of complex molecular processes with unprecedented accuracy.

Molecular Structure Related to an S = 5/2 High-Spin S2 State Manganese Cluster of Photosystem II Investigated by Q-Band Pulse EPR Spectroscopy

The high-spin S₂ state of the photosynthetic oxygen-evolving cluster Mn₄CaO₅, corresponding to the g = 4.1 signal for X-band electron paramagnetic resonance (EPR), was investigated using Q-band pulsed EPR, which detected a main peak at g = 3.10 and satellite peaks at 5.25, 4.55, and 2.80. We evaluated the spin state as the zero-field splitting of D = 0.465 cm^-1 and E/D = 0.245 with S = 5/2. The temperature dependence of the T₁ relaxation time revealed that the excited-state energy was 28.7 cm^-1 higher than that of the S = 5/2 ground state. By comparing present quantum mechanical (QM) calculation models, a closed-cubane structure with the protonation state of two oxygens, W1 (= OH^-) and W2 (= H₂O), was the most probable structure for the S = 5/2 state. The three-pulse electron spin-echo envelope modulation (ESEEM) detected the nuclear signal, which was assigned to nitrogen as His332 ligated to the Mn1 ion. The obtained hyperfine constant for the nitrogen signal was significantly reduced from that in the S = 1/2 low-spin state. These results indicate that the S = 5/2 spin state arises from the closed-cubane structure.

Proximal Methionine Amino Acid Residue Affects the Properties of Redox-Active Tryptophan in an Artificial Model Protein

Redox-active amino acid residues are at the heart of biological electron-transfer reactions. They play important roles in natural protein functions and are implicated in disease states (e.g., oxidative-stress-associated disorders). Tryptophan (Trp) is one such redox-active amino acid residue, and it has long been known to serve a functional role in proteins. Broadly speaking, there is still much to learn about the local features that make some Trp redox active and others inactive. Herein, we describe a new protein model system where we investigate how a methionine (Met) residue proximal to a redox-active Trp affects its reactivity and spectroscopy. We use an artificial variant of azurin from Pseudomonas aeruginosa to produce these models. We employ a series of UV-visible spectroscopy, electrochemistry, electron paramagnetic resonance, and density functional theory experiments to demonstrate the effect that placing Met near Trp radicals has in the context of redox proteins. The introduction of Met proximal to Trp lowers its reduction potential by ca. 30 mV and causes clear shifts in the optical spectra of the corresponding radicals. While the effect may be small, it is significant enough to be a way for natural systems to tune Trp reactivity.

Mutation of a metal ligand stabilizes the high-spin form of the S2 state in the O2-producing Mn4CaO5 cluster of photosystem II

The residue D1-D170 bridges Mn4 with the Ca ion in the O₂-evolving Mn₄CaO₅ cluster of Photosystem II. Recently, the D1-D170E mutation was shown to substantially alter the S_n+1-minus-S_n FTIR difference spectra [Debus RJ (2021) Biochemistry 60:3841-3855]. The mutation was proposed to alter the equilibrium between different Jahn-Teller conformers of the S₁ state such that (i) a different S₁ state conformer is stabilized in D1-D170E than in wild-type and (ii) the S₁ to S₂ transition in D1-D170E produces a high-spin form of the S₂ state rather than the low-spin form that is produced in wild-type. In this study, we employed EPR spectroscopy to test if a high-spin form of the S₂ state is formed preferentially in D1-D170E PSII. Our data show that illumination of dark-adapted D1-D170E PSII core complexes does indeed produce a high-spin form of the S₂ state rather than the low-spin multiline form that is produced in wild-type. This observation provides further experimental support for a change in the equilibrium between S state conformers in both the S₁ and S₂ states in a site-directed mutant that retains substantial O₂ evolving activity.

High-efficiency oxygen evolution by photosystem II oxygen-evolving complex containing 3Mn per reaction center

Ca-depleted photosystem II membranes obtained by treatment with acidic buffer do not contain Ca²⁺ in the Mn₄CaO₅ cluster but contain all extrinsic proteins protecting this cluster (PSII(-Ca/low pH)). However, unlike native photosystem II, Mn cluster in PSII(-Ca/low pH) samples is available for small-sized reductants. Using this property, we investigated the substitution possibility of Mn cation(s) with Fe cation(s) to obtain a chimeric cluster in PSII(-Ca/low pH) samples containing extrinsic proteins. We found that Fe(II) cation replaces Mn cation at pH 6.5, however, PSII(-Ca/low pH) membranes with the 3Mn1Fe chimeric cluster in the oxygen-evolving complex evolve O₂ with high intensity in the presence of exogenous Ca²⁺. The O₂ evolution rate is about 80% of the same rate in PSII(-Ca/low pH) membranes.

Closing the green gap of photosystem I with synthetic fluorophores for enhanced photocurrent generation in photobiocathodes

One restriction for biohybrid photovoltaics is the limited conversion of green light by most natural photoactive components. The present study aims to fill the green gap of photosystem I (PSI) with covalently linked fluorophores, ATTO 590 and ATTO 532. Photobiocathodes are prepared by combining a 20 μm thick 3D indium tin oxide (ITO) structure with these constructs to enhance the photocurrent density compared to setups based on native PSI. To this end, two electron transfer mechanisms, with and without a mediator, are studied to evaluate differences in the behavior of the constructs. Wavelength-dependent measurements confirm the influence of the additional fluorophores on the photocurrent. The performance is significantly increased for all modifications compared to native PSI when cytochrome c is present as a redox-mediator. The photocurrent almost doubles from -32.5 to up to -60.9 μA cm^-2. For mediator-less photobiocathodes, interestingly, drastic differences appear between the constructs made with various dyes. While the turnover frequency (TOF) is doubled to 10 e^-/PSI/s for PSI-ATTO590 on the 3D ITO compared to the reference specimen, the photocurrents are slightly smaller since the PSI-ATTO590 coverage is low. In contrast, the PSI-ATTO532 construct performs exceptionally well. The TOF increases to 31 e^-/PSI/s, and a photocurrent of -47.0 μA cm^-2 is obtained. This current is a factor of 6 better than the reference made with native PSI in direct electron transfer mode and sets a new record for mediator-free photobioelectrodes combining 3D electrode structures and light-converting biocomponents.

Bio-Inspired Bimetallic Cooperativity Through a Hydrogen Bonding Spacer in CO2 Reduction

At the core of carbon monoxide dehydrogenase (CODH) active site two metal ions together with hydrogen bonding scheme from amino acids orchestrate the interconversion between CO₂ and CO. We have designed a molecular catalyst implementing a bimetallic iron complex with an embarked second coordination sphere with multi-point hydrogen-bonding interactions. We found that, when immobilized on carbon paper electrode, the dinuclear catalyst enhances up to four fold the heterogeneous CO₂ reduction to CO in water with an improved selectivity and stability compared to the mononuclear analogue. Interestingly, quasi-identical catalytic performances are obtained when one of the two iron centers was replaced by a redox inactive Zn metal, questioning the cooperative action of the two metals. Snapshots of X-ray structures indicate that the two metalloporphyrin units tethered by a urea group is a good compromise between rigidity and flexibility to accommodate CO₂ capture, activation, and reduction.

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

O2 formation in photosystem II (PSII) is a vital event on Earth, but the exact mechanism remains unclear. The presently prevailing theoretical model is "radical coupling" (RC) involving a Mn(IV)-oxyl unit in an "open-cubane" Mn4CaO6 cluster, which is supported experimentally by the S3 state of cyanobacterial PSII featuring an additional Mn-bound oxygenic ligand. However, it was recently proposed that the major structural form of the S3 state of higher plants lacks this extra ligand, and that the resulting S4 state would feature instead a penta-coordinate dangler Mn(V)=oxo, covalently linked to a "closed-cubane" Mn3CaO4 cluster. For this proposal, we explore here a large number of possible pathways of O-O bond formation and demonstrate that the "nucleophilic oxo-oxo coupling" (NOOC) between Mn(V)=oxo and μ3-oxo is the only eligible mechanism in such a system. The reaction is facilitated by a specific conformation of the cluster and concomitant water binding, which is delayed compared to the RC mechanism. An energetically feasible process is described starting from the valid S4 state through the sequential formation of peroxide and superoxide, followed by O2 release and a second water insertion. The newly found mechanism is consistent with available experimental thermodynamic and kinetic data and thus a viable alternative pathway for O2 formation in natural photosynthesis, in particular for higher plants.

Influence of the antiseptic octenidine on spectral characteristics and energy migration processes in photosystem II core complexes

Herein, the effect of cationic antiseptics (chlorhexidine, picloxidine, miramistin, octenidine) on the initial processes of the delivery of light energy and its efficient use by the reaction centers in intact spinach photosystem II core complexes has been investigated. The characteristic effects-an increase in the fluorescence yield of light-harvesting pigments and a slowdown in the rate of energy migration in bacterial photosynthetic chromatophores has been recently demonstrated mainly in the presence of octenidine (Strakhovskaya et al., in Photosynth Res 147:197-209, 2021; Knox et al., in Photosynth Res, https://doi.org/10.1007/s11120-022-00909-8 , 2022). In this study, we also observed that in the presence of octenidine, the fluorescence intensity of photosystem II core complexes increases by 5-10 times, and the rate of energy migration from antennae to the reaction centers decreases by 3 times. In addition, with an increase in the concentration of this antiseptic, a new effect related to a shift of the spectrum, absorption and fluorescence to the short-wavelength region has been found. Similar effects were observed when detergent Triton X-100 was added to photosystem II samples. We concluded that the antiseptic primarily affects the structure of the internal light-harvesting antenna (CP43 and CP47), through which the excitation energy is delivered to the reaction center. As a result of such an impact, the chlorophyll molecules in this structure are destabilized and their optical and functional characteristics change.

Water oxidation in oxygenic photosynthesis studied by magnetic resonance techniques

The understanding of light-induced biological water oxidation in oxygenic photosynthesis is of great importance both for biology and (bio)technological applications. The chemically difficult multistep reaction takes place at a unique protein-bound tetra-manganese/calcium cluster in photosystem II whose structure has been elucidated by X-ray crystallography (Umena et al. Nature 2011, 473, 55). The cluster moves through several intermediate states in the catalytic cycle. A detailed understanding of these intermediates requires information about the spatial and electronic structure of the Mn₄ Ca complex; the latter is only available from spectroscopic techniques. Here, the important role of Electron Paramagnetic Resonance (EPR) and related double resonance techniques (ENDOR, EDNMR), complemented by quantum chemical calculations, is described. This has led to the elucidation of the cluster's redox and protonation states, the valence and spin states of the manganese ions and the interactions between them, and contributed substantially to the understanding of the role of the protein surrounding, as well as the binding and processing of the substrate water molecules, the O-O bond formation and dioxygen release. Based on these data, models for the water oxidation cycle are developed.

Characterization of the Rate-Limiting Steps in the Dark-To-Light Transitions of Closed Photosystem II: Temperature Dependence and Invariance of Waiting Times during Multiple Light Reactions

Rate-limiting steps in the dark-to-light transition of Photosystem II (PSII) were discovered by measuring the variable chlorophyll-a fluorescence transients elicited by single-turnover saturating flashes (STSFs). It was shown that in diuron-treated samples: (i) the first STSF, despite fully reducing the QA quinone acceptor molecule, generated only an F1(<Fm) fluorescence level; (ii) to produce the maximum (Fm) level, additional excitations were required, which, however, (iii) were effective only with sufficiently long Δτ waiting times between consecutive STSFs. Detailed studies revealed the gradual formation of the light-adapted charge-separated state, PSIIL. The data presented here substantiate this assignment: (i) the Δτ1/2 half-increment rise (or half-waiting) times of the diuron-treated isolated PSII core complexes (CCs) of Thermostichus vulcanus and spinach thylakoid membranes displayed similar temperature dependences between 5 and −80 °C, with substantially increased values at low temperatures; (ii) the Δτ1/2 values in PSII CC were essentially invariant on the Fk−to-Fk+1 (k = 1−4) increments both at 5 and at −80 °C, indicating the involvement of the same physical mechanism during the light-adaptation process of PSIIL. These data are in harmony with the earlier proposed role of dielectric relaxation processes in the formation of the light-adapted charge-separated state and in the variable chlorophyll-a fluorescence of PSII.

Structure-function correlations and system dynamics in oxygenic photosynthesis: classical perspectives and murburn precepts

HIGHLIGHTS

Contemporary beliefs on oxygenic photosynthesis are critiqued.Murburn model is suggested as an alternative explanation.In the new model, diffusible reactive species are the main protagonists.All pigments are deemed photo-redox active in the new stochastic mechanism.NADPH synthesis occurs via simple electron transfers, not via elaborate ETC.Oxygenesis is delocalized and not just centered at Mn-Complex.Energetics of murburn proposal for photophosphorylation is provided.The proposal ushers in a paradigm shift in photosynthesis research.

Structure of a dimeric photosystem II complex from a cyanobacterium acclimated to far-red light

Photosystem II (PSII) is the water-splitting enzyme central to oxygenic photosynthesis. To drive water oxidation, light is harvested by accessory pigments, mostly chlorophyll (Chl) a molecules, which absorb visible light (400-700 nm). Some cyanobacteria facultatively acclimate to shaded environments by altering their photosynthetic machinery to additionally absorb far-red light (FRL, 700-800 nm), a process termed far-red light photoacclimation or FaRLiP. During far-red light photoacclimation, FRL-PSII is assembled with FRL-specific isoforms of the subunits PsbA, PsbB, PsbC, PsbD, and PsbH, and some Chl-binding sites contain Chls d or f instead of the usual Chl a. The structure of an apo-FRL-PSII monomer lacking the FRL-specific PsbH subunit has previously been determined, but visualization of the dimeric complex has remained elusive. Here, we report the cryo-EM structure of a dimeric FRL-PSII complex. The site assignments for Chls d and f are consistent with those assigned in the previous apo-FRL-PSII monomeric structure. All sites that bind Chl d or Chl f at high occupancy exhibit a FRL-specific interaction of the formyl moiety of the Chl d or Chl f with the protein environment, which in some cases involves a phenylalanine sidechain. The structure retains the FRL-specific PsbH2 subunit, which appears to alter the energetic landscape of FRL-PSII, redirecting energy transfer from the phycobiliprotein complex to a Chl f molecule bound by PsbB2 that acts as a bridge for energy transfer to the electron transfer chain. Collectively, these observations extend our previous understanding of the structure-function relationship that allows PSII to function using lower energy FRL.

Elucidating the role of adsorption during artificial photosynthesis: H2O and CO2 adsorption isotherms over TiO2 reveal thermal effects under UV illumination

Adsorption measurements of CO₂ and H₂O over TiO₂ surfaces in dark and under illumination were carried out to reveal the ensuing bottlenecks of the initial steps of the artificial photosynthesis reaction. When the adsorption isotherms of both CO₂ and H₂O were measured under illumination, the results were comparable to isotherms measured at higher temperatures in dark. This evidence is interpreted as the presence of hot spots, due to charge carrier recombination reactions. Differential heat of adsorption measurements revealed that H₂O adsorption on TiO₂ is stronger, and with a higher coverage than that of CO₂. Dissociation of water is an energetically uphill reaction, and the local hot spots due to charge carrier recombination in indirect bandgap semiconductors can enhance the reaction probability. At higher temperatures, higher reaction probabilities are expected and estimated by a thermodynamic analysis for water splitting reaction. The potential role of these hot spots during natural and artificial photosynthetic reactions is discussed.

Water oxidation reaction in the presence of a dinuclear Mn(II)-semicarbohydrazone coordination compound

Water splitting, producing of oxygen, and hydrogen molecules, is an essential reaction for clean energy resources and is one of the challenging reactions for artificial photosynthesis. The Mn₄Ca cluster in photosystem II (PS-II) is responsible for water oxidation in natural photosynthesis. Due to this, water oxidation reaction by Mn coordination compounds is vital for mimicking the active core of the oxygen-evolving complex in PS-II. Here, a new dinuclear Mn(II)-semicarbohydrazone coordination compound, [Mn(HL)(µ-N₃)Cl]₂ (1), was synthesized and characterized by various methods. The structure of compound 1 was determined by single crystal X-ray analysis, which revealed the Mn(II) ions have distorted octahedral geometry as (MnN₄OCl). This geometry is created by coordinating of oxygen and two nitrogen donor atoms from semicarbohydrazone ligand, two nitrogen atoms from azide bridges, and chloride anion. Compound 1 was used as a catalyst for electrochemical water oxidation, and the surface of the electrode after the reaction was investigated by scanning electron microscopy, energy dispersive spectrometry, and powder X-ray diffraction analyses. Linear sweep voltammetry (LSV) experiments revealed that the electrode containing 1 shows high activity for chemical water oxidation with an electrochemical overpotential as low as 377 mV. Although our findings showed that the carbon paste electrode in the presence of 1 is an efficient electrode for water oxidation, it could not withstand water oxidation catalysis under bulk electrolysis and finally converted to Mn oxide nanoparticles which were active for water oxidation along with compound 1.

Investigation of electrocatalytic activity of a new mononuclear Mn(II) complex for water oxidation in alkaline media

Water splitting is a promising way to alleviate the energy crisis. In nature, water oxidation is done by a tetranuclear manganese cluster in photosystem II. Therefore, the study of water oxidation by Mn complexes is attractive in water splitting systems. In this report, a new mononuclear Mn(II) complex, MnL₂ (HL = (E)-3-hydroxy-N'-(pyridin-2-ylmethylene)-2-naphthohydrazide) was prepared and characterized by spectroscopic techniques and single-crystal X-ray diffraction. Crystallographic analysis indicated that the geometry around the Mn(II) ion is distorted octahedral. The MnN₄O₂ coordination moiety is achieved by bounding of oxygen and two nitrogen donor atoms of two hydrazone ligands. The synthesized complex was also investigated for electrochemical water oxidation using electrochemical techniques, scanning electron microscopy, energy dispersive spectrometry, and PXRD analysis. Linear sweep voltammetry experiment showed that the modified carbon paste electrode by the complex displays high activity for water oxidation reaction with an overpotential of 565 mV at a current density of 10 mA cm^-2 and Tafel slope of 105 mV dec^-1 in an alkaline solution. It was found that the complex structure finally changes during the reaction and converts to Mn oxide nanoparticles which act as active catalytic species and oxidize the water.

Combined Impact of Excess Zinc and Cadmium on Elemental Uptake, Leaf Anatomy and Pigments, Antioxidant Capacity, and Function of Photosynthetic Apparatus in Clary Sage (Salvia sclarea L.)

Clary sage (Salvia sclarea L.) is a medicinal plant that has the potential to be used for phytoextraction of zinc (Zn) and cadmium (Cd) from contaminated soils by accumulating these metals in its tissues. Additionally, it has been found to be more tolerant to excess Zn than to Cd stress alone; however, the interactive effects of the combined treatment with Zn and Cd on this medicinal herb, and the protective strategies of Zn to alleviate Cd toxicity have not yet been established in detail. In this study, clary sage plants grown hydroponically were simultaneously exposed to Zn (900 µM) and Cd (100 μM) for 8 days to obtain more detailed information about the plant responses and the role of excess Zn in mitigating Cd toxicity symptoms. The leaf anatomy, photosynthetic pigments, total phenolic and anthocyanin contents, antioxidant capacity (by DPPH and FRAP analyses), and the uptake and distribution of essential elements were investigated. The results showed that co-exposure to Zn and Cd leads to an increased leaf content of Fe and Mg compared to the control, and to increased leaf Ca, Mn, and Cu contents compared to plants treated with Cd only. This is most likely involved in the defense mechanisms of excess Zn against Cd toxicity to protect the chlorophyll content and the functions of both photosystems and the oxygen-evolving complex. The data also revealed that the leaves of clary sage plants subjected to the combined treatment have an increased antioxidant capacity attributed to the higher content of polyphenolic compounds. Furthermore, light microscopy indicated more alterations in the leaf morphology after Cd-only treatment than after the combined treatment. The present study shows that excess Zn could mitigate Cd toxicity in clary sage plants.

Peroxide-Selective Reduction of O2 at Redox-Inactive Rare-Earth(III) Triflates Generates an Ambiphilic Peroxide

Metal peroxides are key species involved in a range of critical biological and synthetic processes. Rare-earth (group III and the lanthanides; Sc, Y, La-Lu) peroxides have been implicated as reactive intermediates in catalysis; however, reactivity studies of isolated, structurally characterized rare-earth peroxides have been limited. Herein, we report the peroxide-selective (93-99% O₂^2-) reduction of dioxygen (O₂) at redox-inactive rare-earth triflates in methanol using a mild metallocene reductant, decamethylferrocene (Fc*). The first molecular praseodymium peroxide ([Pr^III₂(O₂^2-)(18C6)₂(EG)₂][OTf]₄; 18C6 = 18-crown-6, EG = ethylene glycol, ^-OTf = ^-O₃SCF₃; 2-Pr) was isolated and characterized by single-crystal X-ray diffraction, Raman spectroscopy, and NMR spectroscopy. 2-Pr displays high thermal stability (120 °C, 50 mTorr), is protonated by mild organic acids [pK_a1(MeOH) = 5.09 ± 0.23], and engages in electrophilic (e.g., oxygen atom transfer) and nucleophilic (e.g., phosphate-ester cleavage) reactivity. Our mechanistic studies reveal that the rate of oxygen reduction is dictated by metal-ion accessibility, rather than Lewis acidity, and suggest new opportunities for differentiated reactivity of redox-inactive metal ions by leveraging weak metal-ligand binding events preceding electron transfer.

Binding and functions of the two chloride ions in the oxygen-evolving center of photosystem II

Light-driven water oxidation in photosynthesis occurs at the oxygen-evolving center (OEC) of photosystem II (PSII). Chloride ions (Cl^-) are essential for oxygen evolution by PSII, and two Cl^- ions have been found to specifically bind near the Mn₄CaO₅ cluster in the OEC. The retention of these Cl^- ions within the OEC is critically supported by some of the membrane-extrinsic subunits of PSII. The functions of these two Cl^- ions and the mechanisms of their retention both remain to be fully elucidated. However, intensive studies performed recently have advanced our understanding of the functions of these Cl^- ions, and PSII structures from various species have been reported, aiding the interpretation of previous findings regarding Cl^- retention by extrinsic subunits. In this review, we summarize the findings to date on the roles of the two Cl^- ions bound within the OEC. Additionally, together with a short summary of the functions of PSII membrane-extrinsic subunits, we discuss the mechanisms of Cl^- retention by these extrinsic subunits.

Mimicking the Oxygen-Evolving Center in Photosynthesis

The oxygen-evolving center (OEC) in photosystem II (PSII) of oxygenic photosynthetic organisms is a unique heterometallic-oxide Mn₄CaO₅-cluster that catalyzes water splitting into electrons, protons, and molecular oxygen through a five-state cycle (S_n, n = 0 ~ 4). It serves as the blueprint for the developing of the man-made water-splitting catalysts to generate solar fuel in artificial photosynthesis. Understanding the structure-function relationship of this natural catalyst is a great challenge and a long-standing issue, which is severely restricted by the lack of a precise chemical model for this heterometallic-oxide cluster. However, it is a great challenge for chemists to precisely mimic the OEC in a laboratory. Recently, significant advances have been achieved and a series of artificial Mn₄XO₄-clusters (X = Ca/Y/Gd) have been reported, which closely mimic both the geometric structure and the electronic structure, as well as the redox property of the OEC. These new advances provide a structurally well-defined molecular platform to study the structure-function relationship of the OEC and shed new light on the design of efficient catalysts for the water-splitting reaction in artificial photosynthesis.

Reversible Structural Isomerization of Nature's Water Oxidation Catalyst Prior to O-O Bond Formation

Photosynthetic water oxidation is catalyzed by a manganese–calcium oxide cluster, which experiences five “S-states” during a light-driven reaction cycle. The unique “distorted chair”-like geometry of the Mn₄CaO₅₍₆₎ cluster shows structural flexibility that has been frequently proposed to involve “open” and “closed”-cubane forms from the S₁ to S₃ states. The isomers are interconvertible in the S₁ and S₂ states, while in the S₃ state, the open-cubane structure is observed to dominate inThermosynechococcus elongatus (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the S₃⁺Y_z state to the S₃ⁿY_z^• → S₄⁺Y_z step, and report for the first time that the reversible isomerism, which is suppressed in the S₃⁺Y_z state, is fully recovered in the ensuing S₃ⁿY_z^• state due to the proton release from a manganese-bound water ligand. The altered coordination strength of the manganese–ligand facilitates formation of the closed-cubane form, in a dynamic equilibrium with the open-cubane form. This tautomerism immediately preceding dioxygen formation may constitute the rate limiting step for O₂ formation, and exert a significant influence on the water oxidation mechanism in photosystem II.

Water Oxidation by Pentapyridyl Base Metal Complexes? A Case Study

The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O₂ release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]⁺ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)₃]²⁺/S₂O₈^2– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the M^III/M^II oxidation, but none was able to yield a detectable amount of a M^IV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl^– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond M^III. On that basis, design features for next-generation molecular WOCs are suggested.

We scrutinize the water oxidation activity for pentapyridyl metal complexes [M^II(Py5R)Cl]⁺ (M = Mn, Fe, Co, Ni; R = OH, OMe). Analysis of their stability, redox, and water-binding properties shows that the complexes are not able to reach high-valent intermediate states and do not catalyze water oxidation in their molecular form.

Probing the proton release by Photosystem II in the S1 to S2 high-spin transition

The stoichiometry and kinetics of the proton release were investigated during each transition of the S-state cycle in Photosystem II (PSII) from Thermosynechococcus elongatus containing either a Mn₄CaO₅ (PSII/Ca) or a Mn₄SrO₅ (PSII/Sr) cluster. The measurements were done at pH 6.0 and pH 7.0 knowing that, in PSII/Ca at pH 6.0 and pH 7.0 and in PSII/Sr at pH 6.0, the flash-induced S₂-state is in a low-spin configuration (S₂^LS) whereas in PSII/Sr at pH 7.0, the S₂-state is in a high-spin configuration (S₂^HS) in half of the centers. Two measurements were done; the time-resolved flash dependent i) absorption of either bromocresol purple at pH 6.0 or neutral red at pH 7.0 and ii) electrochromism in the Soret band of P_D1 at 440 nm. The fittings of the oscillations with a period of four indicate that one proton is released in the S₁ to S₂^HS transition in PSII/Sr at pH 7.0. It has previously been suggested that the proton released in the S₂^LS to S₃ transition would be released in a S₂^LSTyr_Z^• → S₂^HSTyr_Z^• transition before the electron transfer from the cluster to Tyr_Z^• occurs. The release of a proton in the S₁Tyr_Z^• → S₂^HSTyr_Z transition would logically imply that this proton release is missing in the S₂^HSTyr_Z^• to S₃Tyr_Z transition. Instead, the proton release in the S₁ to S₂^HS transition in PSII/Sr at pH 7.0 was mainly done at the expense of the proton release in the S₃ to S₀ and S₀ to S₁ transitions. However, at pH 7.0, the electrochromism of P_D1 seems larger in PSII/Sr when compared to PSII/Ca in the S₃ state. This points to the complex link between proton movements in and immediately around the Mn₄ cluster and the mechanism leading to the release of protons into the bulk.

Location of two Mn2+ affinity sites in photosystem II detected by pulsed electron-electron double resonance

In this study, we identified two Mn²⁺ sites in apo-Photosystem II (PSII) using the pulsed electron-electron double resonance (PELDOR). A Mn²⁺ ion was bound to apo-PSII on the deactivation of the oxygen-evolving complex. The electron-electron magnetic dipole interaction of the Mn²⁺ to Y_D^· was estimated to be 2.4 MHz. The site was assigned at the position between His332 and Glu189 in the D1 polypeptide, which is close to the Mn1 site in mature PS II. Using recent structures observed under electron microscopes (EM), the location of the Mn²⁺ site on photoactivation was reevaluated. The position between Asp170 and Glu189 in the D1 polypeptide is a good candidate for the initial high-affinity site for photoactivation. Based on a comparison with the PELDOR results, the two EM structures were evaluated.