Mn4CaO5 Cluster In Photosystem Research Articles

Independent Mutation of Two Bridging Carboxylate Ligands Stabilizes Alternate Conformers of the Photosynthetic O2-Evolving Mn4CaO5 Cluster in Photosystem II.

The O2-evolving Mn4CaO5 cluster in photosystem II is ligated by six carboxylate residues. One of these is D170 of the D1 subunit. This carboxylate bridges between one Mn ion (Mn4) and the Ca ion. A second carboxylate ligand is D342 of the D1 subunit. This carboxylate bridges between two Mn ions (Mn1 and Mn2). D170 and D342 are located on opposite sides of the Mn4CaO5 cluster. Recently, it was shown that the D170E mutation perturbs both the intricate networks of H-bonds that surround the Mn4CaO5 cluster and the equilibrium between different conformers of the cluster in two of its lower oxidation states, S1 and S2, while still supporting O2 evolution at approximately 50% the rate of the wild type. In this study, we show that the D342E mutation produces much the same alterations to the cluster's FTIR and EPR spectra as D170E, while still supporting O2 evolution at approximately 20% the rate of the wild type. Furthermore, the double mutation, D170E + D342E, behaves similarly to the two single mutations. We conclude that D342E alters the equilibrium between different conformers of the cluster in its S1 and S2 states in the same manner as D170E and perturbs the H-bond networks in a similar fashion. This is the second identification of a Mn4CaO5 metal ligand whose mutation influences the equilibrium between the different conformers of the S1 and S2 states without eliminating O2 evolution. This finding has implications for our understanding of the mechanism of O2 formation in terms of catalytically active/inactive conformations of the Mn4CaO5 cluster in its lower oxidation states.

Substitution of Ca2+ and changes in the H-bond network near the oxygen-evolving complex of photosystem II.

Ca2+, which provides binding sites for ligand water molecules W3 and W4 in the Mn4CaO5 cluster, is a prerequisite for O2 evolution in photosystem II (PSII). We report structural changes in the H-bond network and the catalytic cluster itself upon the replacement of Ca2+ with other alkaline earth metals, using a quantum mechanical/molecular mechanical approach. The small radius of Mg2+ makes W3 donate an H-bond to D1-Glu189 in Mg2+-PSII. If an additional water molecule binds at the large surface of Ba2+, it donates H-bonds to D1-Glu189 and the ligand water molecule at the dangling Mn, altering the H-bond network. The potential energy profiles of the H-bond between D1-Tyr161 (TyrZ) and D1-His190 and the interconversion between the open- and closed-cubane S2 conformations remain substantially unaltered upon the replacement of Ca2+. Remarkably, the O5⋯Ca2+ distance is shortest among all O5⋯metal distances irrespective of the radius being larger than that of Mg2+. Furthermore, Ca2+ is the only alkaline earth metal that equalizes the O5⋯metal and O2⋯metal distances and facilitates the formation of the symmetric cubane structure.

Role of D1-Glu65 in Proton Transfer during Photosynthetic Water Oxidation in Photosystem II.

Photosynthetic water oxidation takes place at the Mn4CaO5 cluster in photosystem II (PSII) through a light-driven cycle of five intermediates called S states (S0-S4). Although the PSII structures have shown the presence of several channels around the Mn4CaO5 cluster leading to the lumen, the pathways for proton release in the individual S-state transitions remain unidentified. Here, we studied the involvement of the so-called Cl channel in proton transfer during water oxidation by examining the effect of the mutation of D1-Glu65, a key residue in this channel, to Ala using Fourier transform infrared difference and time-resolved infrared spectroscopies together with thermoluminescence and delayed luminescence measurements. It was shown that the structure and the redox property of the catalytic site were little affected by the D1-Glu65Ala mutation. In the S2 → S3 transition, the efficiency was still high and the transition rate was only moderately retarded in the D1-Glu65Ala mutant. In contrast, the S3 → S0 transition was significantly inhibited by this mutation. These results suggest that proton transfer in the S2 → S3 transition occurs through multiple pathways including the Cl channel, whereas this channel likely serves as a single pathway for proton exit in the S3 → S0 transition.

Formation of the Metastable MnIII Water Oxidation Intermediate in Birnessite is Controlled by a Dissolution-Deposition Process Involving Labile MnII.

Birnessite, the closest naturally occurring analog of the Mn4 CaO5 cluster of photosystem II, is an important model compound in the development of bio-inspired electrocatalysts for the water oxidation reaction. The present work reports the formation mechanism of the key MnIII intermediate realized through the study of the effects of several electrolyte anions and cations on the catalytic efficiency of birnessite. In situ spectroelectrochemical measurements show that the activity is controlled by a dynamic dissolution-oxidation process, wherein MnIII is formed through the oxidation of labile uncomplexed MnII that reversibly shuttles between the birnessite and the electrolyte in a manner similar to the photoactivation in photosystem II. The role of electrolyte cations of different ionic radii and hydration strengths is to control the interlayer spacing, whereas electrolyte anions control the extent of deprotonation of complexed MnII in the lattice. Both in turn govern the shuttling efficiency of uncomplexed MnII and its subsequent electro-oxidation to MnIII .

Effects of x-ray free-electron laser pulse intensity on the Mn K β 1,3 x-ray emission spectrum in photosystem II-A case study for metalloprotein crystals and solutions.

In the last ten years, x-ray free-electron lasers (XFELs) have been successfully employed to characterize metalloproteins at room temperature using various techniques including x-ray diffraction, scattering, and spectroscopy. The approach has been to outrun the radiation damage by using femtosecond (fs) x-ray pulses. An example of an important and damage sensitive active metal center is the Mn4CaO5 cluster in photosystem II (PS II), the catalytic site of photosynthetic water oxidation. The combination of serial femtosecond x-ray crystallography and Kβ x-ray emission spectroscopy (XES) has proven to be a powerful multimodal approach for simultaneously probing the overall protein structure and the electronic state of the Mn4CaO5 cluster throughout the catalytic (Kok) cycle. As the observed spectral changes in the Mn4CaO5 cluster are very subtle, it is critical to consider the potential effects of the intense XFEL pulses on the Kβ XES signal. We report here a systematic study of the effects of XFEL peak power, beam focus, and dose on the Mn Kβ1,3 XES spectra in PS II over a wide range of pulse parameters collected over seven different experimental runs using both microcrystal and solution PS II samples. Our findings show that for beam intensities ranging from ∼5 × 1015 to 5 × 1017 W/cm2 at a pulse length of ∼35 fs, the spectral effects are small compared to those observed between S-states in the Kok cycle. Our results provide a benchmark for other XFEL-based XES studies on metalloproteins, confirming the viability of this approach.

PKa of the ligand water molecules in the oxygen-evolving Mn4CaO5 cluster in photosystem II

Release of the protons from the substrate water molecules is prerequisite for O2 evolution in photosystem II (PSII). Proton-releasing water molecules with low pKa values at the catalytic moiety can be the substrate water molecules. In some studies, one of the ligand water molecules, W2, is regarded as OH−. However, the PSII crystal structure shows neither proton acceptor nor proton-transfer pathway for W2, which is not consistent with the assumption of W2 = OH−. Here we report the pKa values of the four ligand water molecules, W1 and W2 at Mn4 and W3 and W4 at Ca2+, of the Mn4CaO5 cluster. pKa(W1) ≈ pKa(W2) << pKa(W3) ≈ pKa(W4) in the Mn4CaO5 cluster in water. However, pKa(W1) ≈ pKa(D1-Asp61) << pKa(W2) in the PSII protein environment. These results suggest that in PSII, deprotonation of W2 is energetically disfavored as far as W1 exists.

Probing the role of arginine 323 of the D1 protein in photosystem II function.

The Mn4 CaO5 cluster of photosystem II (PSII) advances sequentially through five oxidation states (S0 to S4 ). Under the enzyme cycle, two water molecules are oxidized, O2 is generated and four protons are released into the lumen. Umena et al. (2011) have proposed that, with other charged amino acids, the R323 residue of the D1 protein could contribute to regulate a proton egress pathway from the Mn4 CaO5 cluster and TyrZ via a proton channel identified from the 3D structure. To test this suggestion, a PsbA3/R323E site-directed mutant has been constructed and the properties of its PSII have been compared to those of the PsbA3-PSII by using EPR spectroscopy, polarography, thermoluminescence and time-resolved UV-visible absorption spectroscopy. Neither the oscillations with a period four nor the kinetics and S-state-dependent stoichiometry of the proton release were affected. However, several differences have been found: (1) the P680 + decay in the hundreds of ns time domain was much slower in the mutant, (2) the S2 QA - /DCMU and S3 QA - /DCMU radiative charge recombination occurred at higher temperatures and (3) the S0 TyrZ • , S1 TyrZ • , S2 TyrZ • split EPR signals induced at 4.2 K by visible light from the S0 TyrZ , S1 TyrZ , S2 TyrZ , respectively, and the (S2 TyrZ • )' induced by NIR illumination at 4.2 K of the S3 TyrZ state differed. It is proposed that the R323 residue of the D1 protein interacts with TyrZ likely via the H-bond network previously proposed to be a proton channel. Therefore, rather than participating in the egress of protons to the lumen, this channel could be involved in the relaxations of the H-bonds around TyrZ by interacting with the bulk, thus tuning the driving force required for TyrZ oxidation.

Role of the O4 Channel in Photosynthetic Water Oxidation as Revealed by Fourier Transform Infrared Difference and Time-Resolved Infrared Analysis of the D1-S169A Mutant.

Photosynthetic water oxidation takes place at the Mn4CaO5 cluster in photosystem II. Although the atomic structures of its intermediates called S states have recently been reported, the catalytic mechanism of water oxidation has not been well understood. Here, to investigate the involvement of the O4 site of the Mn4CaO5 cluster and a water channel from O4 in the water oxidation reaction, we examined the effects of D1-S169A mutation, which perturbs the interaction of a water molecule hydrogen-bonded with O4, by thermoluminescence (TL), Fourier transform infrared (FTIR) difference, and time-resolved infrared (TRIR) measurements. The observed upshifts of TL peaks and some changes in FTIR spectra upon S169A mutation revealed the perturbations of the redox potential of the Mn4CaO5 cluster and the interactions of the surrounding hydrogen bond network. In contrast, FTIR oscillation patterns and TRIR traces showed only minor effects of the mutation on the efficiencies and kinetics of individual S-state transitions. It was thus concluded that the O4 site plays a role in retaining the redox potential and the structure of the hydrogen bond network, whereas it is unlikely to be directly involved in the catalytic reaction of substrate water except for proton transfer through the O4 water chain.

Assessment of the manganese cluster’s oxidation state via photoactivation of photosystem II microcrystals

Knowledge of the manganese oxidation states of the oxygen-evolving Mn4CaO5 cluster in photosystem II (PSII) is crucial toward understanding the mechanism of biological water oxidation. There is a 4 decade long debate on this topic that historically originates from the observation of a multiline electron paramagnetic resonance (EPR) signal with effective total spin of S = 1/2 in the singly oxidized S2 state of this cluster. This signal implies an overall oxidation state of either Mn(III)3Mn(IV) or Mn(III)Mn(IV)3 for the S2 state. These 2 competing assignments are commonly known as "low oxidation (LO)" and "high oxidation (HO)" models of the Mn4CaO5 cluster. Recent advanced EPR and Mn K-edge X-ray spectroscopy studies converge upon the HO model. However, doubts about these assignments have been voiced, fueled especially by studies counting the number of flash-driven electron removals required for the assembly of an active Mn4CaO5 cluster starting from Mn(II) and Mn-free PSII. This process, known as photoactivation, appeared to support the LO model since the first oxygen is reported to evolve already after 7 flashes. In this study, we improved the quantum yield and sensitivity of the photoactivation experiment by employing PSII microcrystals that retained all protein subunits after complete manganese removal and by oxygen detection via a custom built thin-layer cell connected to a membrane inlet mass spectrometer. We demonstrate that 9 flashes by a nanosecond laser are required for the production of the first oxygen, which proves that the HO model provides the correct description of the Mn4CaO5 cluster's oxidation states.

Fourier transform infrared and mass spectrometry analyses of a site-directed mutant of D1-Asp170 as a ligand to the water-oxidizing Mn4CaO5 cluster in photosystem II

The Mn4CaO5 cluster, the catalytic center of water oxidation in photosystem II (PSII), is coordinated by six carboxylate and one imidazole ligands. The roles of these ligands in the water oxidation mechanism remain largely unknown. In this study, we constructed a D1-D170H mutant, in which the Asp ligand bridging Mn and Ca ions was replaced with His, in the cyanobacterium Synechocystis sp. PCC 6803, and analyzed isolated PSII core complexes using Fourier transform infrared (FTIR) difference spectroscopy and mass spectrometry (MS). The S2-minus-S1 FTIR difference spectrum of the PSII complexes of the D1-D170H mutant showed features virtually identical to those of the wild-type PSII. MS analysis further showed that ~70% of D1 proteins from the PSII complexes of D1-D170H possessed the wild-type amino acid sequence, although only the mutated sequence was detected in genomic DNA in the same batch of cells for PSII preparations. In contrast, a D1-S169A mutant as a control showed a modified FTIR spectrum and only a mutated D1 protein. It is thus concluded that the FTIR spectrum of the D1-D170H mutant actually reflects that of wild-type PSII, whereas the Mn4CaO5 cluster is not formed in PSII with D1-D170H mutation. Although the mechanism of production of the wild-type D1 protein in the D1-D170H mutant is unknown at present, a caution is necessary in the analysis of site-directed mutants of crucial residues in the D1 protein, and mutation has to be confirmed not only at the DNA level but also at the amino acid level.

Proton Release Process during the S2-to-S3 Transition of Photosynthetic Water Oxidation As Revealed by the pH Dependence of Kinetics Monitored by Time-Resolved Infrared Spectroscopy.

Photosynthetic water oxidation takes place at the Mn4CaO5 cluster in photosystem II via a light-driven cycle of intermediates called S states (S0-S4). Clarifying how electron and proton transfer reactions are coupled with each other in the S2 → S3 transition, which occurs just before O-O bond formation, is crucial for understanding the water oxidation mechanism. Here, we investigated the pH dependence of the kinetics of the S2 → S3 transition using time-resolved infrared (TRIR) spectroscopy to identify the proton release phase in this transition. TRIR measurements of YD-less PSII core complexes from the D2-Y160F mutant of Thermosynechococcus elongatus showed that the last phase in this transition (τ ∼ 350 μs at pH 6) was strongly dependent on pH, and its time constant at pH 5 was larger than that at pH 8 by a factor of >3. In contrast, the earlier phase with a time constant of ∼100 μs was virtually independent of pH. These results strongly support the view that proton release is a rate-limiting step of the proton-coupled electron transfer in the last phase of the S2 → S3 transition. This proton release enables electron transfer by removing an excessive positive charge from the catalytic center and hence decreasing its redox potential.

Mechanism of protonation of the over-reduced Mn4CaO5 cluster in photosystem II

Based on characterization by X-ray absorption spectroscopy, it has been proposed that the Mn4CaO5 cluster in the crystal structure of the water-oxidizing enzyme, photosystem II (PSII), may represent an over-reduced form arising from reduction by the X-ray beam. Using a quantum mechanical/molecular mechanical approach, and assuming that all of the μ-oxo bridges are deprotonated in S1, we analyzed the reduction process of the Mn4CaO5 cluster. In the crystal structure, the O atom (O5), which is linked with three Mn atoms and one Ca atom, has no H-bond. When reduced to S–2, unexpectedly, a water molecule at Ca2+ (W3) reoriented itself, formed a H-bond with O5, and released a proton to O5, resulting in formation of OH− at both W3 and O5. Once generated, the OH− group at O5 was stable, because the W3…O5 H-bond had already disappeared. A weak binding of H2O at Ca2+ led W3 to reorient and serve as a proton donor to O5 upon over-reduction.

Temperature dependence of the high-spin S2 to S3 transition in Photosystem II: Mechanistic consequences

The Mn4CaO5-cluster in Photosystem II advances through five oxidation states, S0 to S4, before water is oxidized and O2 is generated. The S2-state exhibits either a low-spin, S = 1/2 (S2LS), or a high-spin state, S = 5/2 (S2HS). Increasing the pH favors the S2HS configuration and mimics the formation of TyrZ in the S2LS-state at lower pH values (Boussac et al. Biochim. Biophys. Acta 1859 (2018) 342). Here, the temperature dependence of the S2HS to S3 transition was studied by EPR spectroscopy at pH 8.6. The present data strengthened the involvement of S2HS as a transient state in the S2LSTyrZ → S2HSTyrZ → S3TyrZ transition. Depending on the temperature, the S2HS progresses to S3 states exhibiting different EPR properties. One S3-state with a S = 3 signal, supposed to have a structure with the water molecule normally inserted in S2 to S3 transition, can be formed at temperatures as low as 77 K. This suggests that this water molecule is already bound in the S2HS state at pH 8.6. The nature of the EPR invisible S3 state, formed down to 4.2 K from a S2HS state, and that of the EPR detectable S3 state formed down to 77 K are discussed. It is proposed that in the S2LS to S3 transition, at pH < 8.6, the proton release (Sugiura et al. Biochim. Biophys. Acta 1859 (2018) 1259), the S2LS to S2HS conversion and the binding of the water molecule are all triggered by the formation of TyrZ.

Spin, Valence, and Structural Isomerism in the S3 State of the Oxygen-Evolving Complex of Photosystem II as a Manifestation of Multimetallic Cooperativity.

Photosynthetic water oxidation is catalyzed by a Mn4CaO5-cluster in photosystem II through an S-state cycle. Understanding the roles of heterogeneity in each S-state, as identified recently by the EPR spectroscopy, is very important to gain a complete description of the catalytic mechanism. We performed herein hybrid DFT calculations within the broken-symmetry formalism and associated analyses of Heisenberg spin models to study the electronic and spin structures of various isomeric structural motifs (hydroxo-oxo, oxyl-oxo, peroxo, and superoxo species) in the S3 state. Our extensive study reveals several factors that affect the spin ground state: (1) (formal) Mn oxidation state; (2) metal-ligand covalency; (3) coordination geometry; and (4) structural change of the Mn cluster induced by alternations in Mn···Mn distances. Some combination of these effects could selectively stabilize/destabilize some spin states. We found that the high spin state ( Stotal = 6) of the oxyl-oxo species can be causative for catalytic function, which manifests through mixing of the metal-ligand character in magnetic orbitals at relatively short O5···O6 distances (<2.0 Å) and long MnA···O5 distances (>2.0 Å). These results will serve as a basis to conceptually identify and rationalize the physicochemical synergisms that can be evoked by the unique "distorted chair" topology of the cluster through cooperative Jahn-Teller effects on multimetallic centers.

Initial Mn2+ binding site in photoassembly of the water-oxidizing Mn4CaO5 cluster in photosystem II as studied by quantum mechanics/molecular mechanics calculations

The initial Mn2+ binding site in photoassembly of the Mn4CaO5 cluster, the catalytic center of water oxidation in photosystem II, was studied using quantum mechanics/molecular mechanics calculations. Among the five metal sites (Mn1–Mn4 and Ca) of the Mn4CaO5 cluster, the Mn1 site involving D1-H332 was most stable for Mn2+ binding, while the Mn2 site showed a slightly higher energy comparable to the thermal fluctuation energy at room temperature. It is thus suggested that Mn2+ binding at the Mn1 site, likely in equilibrium with that at the Mn2 site, is the initial step of photoassembly of the Mn4CaO5 cluster.

Mechanism of Proton-Coupled Electron Transfer in the S0-to-S1 Transition of Photosynthetic Water Oxidation As Revealed by Time-Resolved Infrared Spectroscopy.

Photosynthetic water oxidation takes place at the Mn4CaO5 cluster in photosystem II through a light-driven cycle of intermediates called S states (S0-S4). To unravel the mechanism of water oxidation, it is essential to understand the coupling of electron- and proton-transfer reactions during the S-state transitions. Here, we monitored the reaction process in the S0 → S1 transition using time-resolved infrared (TRIR) spectroscopy. The TRIR signals of the pure contribution of the S0 → S1 transition was obtained by measurement upon a flash after dark adaptation following three flashes. The S0 → S1 traces at the vibrational frequencies of carboxylate groups and hydrogen bond networks around the Mn4CaO5 cluster showed a single phase with a time constant of ∼45 μs. A relatively small H/D kinetic isotope effect of ∼1.2 together with the absence of a slower phase suggests that proton release is coupled with electron transfer, which is a rate-limiting step. The high rate of proton-coupled electron transfer, which is even higher than pure electron transfer in the S1 → S2 transition, is consistent with the previous theoretical prediction that a hydroxo bridge of the Mn4CaO5 cluster gives rise to barrierless deprotonation upon S1 formation through a strongly hydrogen-bonded water molecule.

Mechanism of Methanol Inhibition of Photosynthetic Water Oxidation As Studied by Fourier Transform Infrared Difference and Time-Resolved Infrared Spectroscopies.

Photosynthetic water oxidation is performed at the Mn4CaO5 cluster in photosystem II. In this study, we investigated the effect of methanol, an analogue of water, on the water oxidation reaction and its interaction site using Fourier transform infrared (FTIR) difference and time-resolved infrared (TRIR) spectroscopies. Flash-induced FTIR difference measurement of the S-state cycle showed that methanol decreases mainly the efficiency of the S3 → S0 transition. TRIR measurement further showed that methanol slowed the rates of the S2 → S3 and S3 → S0 transitions. FTIR difference spectra upon the S1 → S2 transition exhibited prominent methanol-induced changes in the amide I and II bands of the main chains, whereas little change was observed in the bands of carboxylate groups, histidine side chains, and a water network in the vicinity of the Mn4CaO5 cluster. Similar tendencies were also observed with ethanol and 2-propanol. The C-O stretching vibration of methanol was further identified in the S2-minus-S1 spectrum using 18O-labeled methanol. These results indicate that methanol and small alcohols are bound near the Mn4CaO5 cluster but with no direct interaction. They probably replace a water molecule in a water channel around the Mn4CaO5 cluster, possibly interacting with a main chain amide. It is thus suggested that this replacement of water with methanol or a small alcohol inhibits water/proton transfer during the S2 → S3 and S3 → S0 transitions, which in turn provides experimental support for the view that these two transitions involve the water uptake and proton release processes.

Impact of D1-V185 on the Water Molecules That Facilitate O2 Formation by the Catalytic Mn4CaO5 Cluster in Photosystem II

The oxidations of the O2-evolving Mn4CaO5 cluster in Photosystem II are coupled to the release of protons to the thylakoid lumen via one or more proton egress pathways. These pathways are comprised of extensive networks of hydrogen-bonded water molecules and amino acid side chains. The hydrophobic residue, D1-V185, is adjacent to numerous water molecules in one of these pathways. The D1-V185N mutation dramatically slows O-O bond formation. This impairment has been attributed to a disruption of the hydrogen-bonded water molecules that are crucial for proton egress or whose rearrangement is required for catalysis. In this study, Fourier transform infrared spectroscopy was employed to characterize the impact of the D1-V185N mutation on the carboxylate groups and water molecules that form a network of hydrogen bonds in this putative proton egress pathway. By analyzing carboxylate stretching modes, carbonyl stretching modes of hydrogen-bonded carboxylic acids, O-H stretching modes of hydrogen-bonded water molecules, and D-O-D bending modes, we obtain evidence that the D1-V185N mutation perturbs the extensive network of hydrogen bonds that extends from YZ to D1-D61 to a greater extent than any mutation yet examined but does not alter the water molecules that interact directly with D1-D61. The mutation also alters the environments of the carboxylate groups whose p Ka values change in response to the S1 to S2 and S2 to S3 transitions. Finally, the mutation alters the environment of the water molecule whose bending mode vanishes during the S2 to S3 transition, consistent with assigning the Ca2+-bound W3 as the water molecule that deprotonates and joins oxo bridge O5 during the S2 to S3 transition, possibly as the second substrate water molecule for O2 formation.

Characterization of fluoride inhibition in photosystem II lacking extrinsic PsbP and PsbQ subunits

Photosynthetic oxygen evolution occurs through the oxidation of water at a catalytic Mn4CaO5 cluster in photosystem II and is promoted by chloride, which binds at two sites near the Mn4CaO5 cluster. Fluoride is a competitive inhibitor of chloride activation, but study of its effects is complicated by the possibility that it may form an insoluble CaF2 complex. In this study, the effects of fluoride were studied using PSII lacking the PsbP and PsbQ subunits, which help to regulate the requirements for the inorganic cofactors Ca2+ and Cl−. In this preparation, which allows easy exchange of ions, it was found that F− does not directly remove Ca2+ even when catalytic turnovers take place, suggesting that fluoride is not able to access the inner coordination sphere of Ca2+. By monitoring the loss in O2 evolution activity, the dissociation constant of F− was estimated to be about 1 mM in intact PSII, consistent with previous studies, and about 77 mM in PSII lacking the extrinsic subunits. The significantly higher value for PSII lacking PsbP and PsbQ is consistent with results for other ions. The effects of F− on electron transfer to Tyr Z was also studied and found to show similar trends in PSII with and without the two extrinsic subunits, but with a more pronounced effect in PSII lacking the extrinsic subunits. These results indicate that in PSII lacking PsbP and PsbQ, fluoride does not directly interact with or remove Ca2+ and inhibits O2 evolution in a manner comparable to PSII with the extrinsic subunits intact.

An octanuclear Cu(ii) cluster with a bio-inspired Cu4O4 cubic fragment for efficient photocatalytic water oxidation.

An octanuclear Cu(ii) cluster [Cu8(dpk·OH)8(OAc)4](ClO4)4 (dpk·OH = the monoanion of the hydrated, gem-diol form of di-2-pyridyl ketone) that contains two Cu4O4 cubic fragments similar to the Mn4CaO5 cluster in photosystem II (PSII) was found to be an efficient catalyst for photocatalytic water oxidation. In the [Ru(bpy)3]2+/Na2S2O8 system, it afforded an optimal oxygen yield, turnover number (TON) and turnover frequency (TOF) of 35.6%, 178 and 3.6 s-1, respectively, which are the highest values amongst all of the Cu-based photocatalytic water oxidation catalysts (WOCs).