Quantifying the ion selectivity of the Ca2+ site in photosystem II: evidence for direct involvement of Ca2+ in O2 formation

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.

Redox Processes Involving Oxygen: The Surprising Influence of Redox-Inactive Lewis Acids

Metalloenzymes with heteromultimetallic active sites perform chemical reactions that control several biogeochemical cycles. Transformations catalyzed by such enzymes include dioxygen generation and reduction, dinitrogen reduction, and carbon dioxide reduction-instrumental transformations for progress in the context of artificial photosynthesis and sustainable fertilizer production. While the roles of the respective metals are of interest in all these enzymatic transformations, they share a common factor in the transfer of one or multiple redox equivalents. In light of this feature, it is surprising to find that incorporation of redox-inactive metals into the active site of such an enzyme is critical to its function. To illustrate, the presence of a redox-inactive Ca2+ center is crucial in the Oxygen Evolving Complex, and yet particularly intriguing given that the transformation catalyzed by this cluster is a redox process involving four electrons. Therefore, the effects of redox inactive metals on redox processes-electron transfer, oxygen- and hydrogen-atom transfer, and O-O bond cleavage and formation reactions-mediated by transition metals have been studied extensively. Significant effects of redox inactive metals have been observed on these redox transformations; linear free energy correlations between Lewis acidity and the redox properties of synthetic model complexes are observed for several reactions. In this Perspective, these effects and their relevance to multielectron processes will be discussed.

Interplay of two low-barrier hydrogen bonds in long-distance proton-coupled electron transfer for water oxidation

D1-Tyr161 (TyrZ) forms a low-barrier H-bond with D1-His190 and functions as a redox-active group in photosystem II. When oxidized to the radical form (TyrZ-O•), it accepts an electron from the oxygen-evolving Mn4CaO5 cluster, facilitating an increase in the oxidation state (Sn; n = 0-3). In this study, we investigated the mechanism of how TyrZ-O• drives proton-coupled electron transfer during the S2 to S3 transition using a quantum mechanical/molecular mechanical approach. In response to TyrZ-O• formation and subsequent loss of the low-barrier H-bond, the ligand water molecule at the Ca2+ site (W4) reorients away from TyrZ and donates an H-bond to D1-Glu189 at Mn4 of Mn4CaO5 together with an adjacent water molecule. The H-bond donation to the Mn4CaO5 cluster triggers the release of the proton from the lowest pKa site (W1 at Mn4) along the W1…D1-Asp61 low-barrier H-bond, leading to protonation of D1-Asp61. The interplay of the two low-barrier H-bonds, involving the Ca2+ interface and forming the extended Grotthuss-like network [TyrZ…D1-His190]-[Mn4CaO5]-[W1…D1-Asp61], rather than the direct electrostatic interaction, is likely a basis of the apparent long-distance interaction (11.4 Å) between TyrZ-O• formation and D1-Asp61 protonation.

Structural and energetic insights into Mn-to-Fe substitution in the oxygen-evolving complex

Manganese (Mn) serves as the catalytic center for water splitting in photosystem II (PSII), despite the abundance of iron (Fe) on earth. As a first step toward why Mn and not Fe is employed by Nature in the water oxidation catalyst, we investigated the Fe₄CaO₅ cluster in the PSII protein environment using a quantum mechanical/molecular mechanical (QM/MM) approach, assuming an equivalence between Mn(III/IV) and Fe(II/III). Substituting Mn with Fe resulted in the protonation of μ-oxo bridges at sites O2 and O3 by Arg357 and D1-His337, respectively. While the Mn₄CaO₅ cluster exhibits distinct open- and closed-cubane S₂ conformations, the Fe₄CaO₅ cluster lacks this variability due to an equal spin distribution over sites Fe1 and Fe4. The absence of a low-barrier H-bond between a ligand water molecule (W1) and D1-Asp61 in the Fe₄CaO₅ cluster may underlie its incapability for ligand water deprotonation, highlighting the relevance of Mn in natural water splitting.

Effect of lanthanides on oxidation of Mn2+ cations via a high-affinity Mn-binding site in photosystem II membranes

Lanthanide cations (La³⁺ and Tb³⁺) bind to the Ca-binding site of the oxygen-evolving complex in Ca-depleted PSII membranes and irreversibly inhibit the oxygen evolution. Оn the other hand, EPR measurement of Mn²⁺ concentration in buffer revealed that lanthanide cations inhibit the light-dependent oxidation of Mn²⁺ cations via the high-affinity Mn-binding site in Mn-depleted PSII membranes, which suggests that they bind to and inhibit the high-affinity Mn-binding site of the oxygen-evolving complex. The inhibition is irreversible, bound Ln³⁺ cation could not be washed out from the sample. Calcium ion inhibits oxidation of Mn²⁺ (5 μM) at very high concentration (tens mM) and the inhibition is reversible. In this work we measured the reduction rate of exogenic electron acceptor 2,6-dichlorophenolindophenol during the oxidation of Mn²⁺ cations in the Ca-depleted PSII and in the Ca-depleted PSII treated with lanthanides after extraction of Mn cluster from these preparations. We found that irreversible binding of the lanthanide cation to the Ca-binding site in the Ca-depleted PSII membranes leads to a partial inhibition of the high-affinity Mn-binding site.

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.

From manganese oxidation to water oxidation: assembly and evolution of the water-splitting complex in photosystem II

The manganese cluster of photosystem II has been the focus of intense research aiming to understand the mechanism of H₂O-oxidation. Great effort has also been applied to investigating its oxidative photoassembly process, termed photoactivation that involves the light-driven incorporation of metal ions into the active Mn₄CaO₅ cluster. The knowledge gained on these topics has fundamental scientific significance, but may also provide the blueprints for the development of biomimetic devices capable of splitting water for solar energy applications. Accordingly, synthetic chemical approaches inspired by the native Mn cluster are actively being explored, for which the native catalyst is a useful benchmark. For both the natural and artificial catalysts, the assembly process of incorporating Mn ions into catalytically active Mn oxide complexes is an oxidative process. In both cases this process appears to share certain chemical features, such as producing an optimal fraction of open coordination sites on the metals to facilitate the binding of substrate water, as well as the involvement of alkali metals (e.g., Ca²⁺) to facilitate assembly and activate water-splitting catalysis. This review discusses the structure and formation of the metal cluster of the PSII H₂O-oxidizing complex in the context of what is known about the formation and chemical properties of different Mn oxides. Additionally, the evolutionary origin of the Mn₄CaO₅ is considered in light of hypotheses that soluble Mn²⁺ was an ancient source of reductant for some early photosynthetic reaction centers ('photomanganotrophy'), and recent evidence that PSII can form Mn oxides with structural resemblance to the geologically abundant birnessite class of minerals. A new functional role for Ca²⁺ to facilitate sustained Mn²⁺ oxidation during photomanganotrophy is proposed, which may explain proposed physiological intermediates during the likely evolutionary transition from anoxygenic to oxygenic photosynthesis.

Photosystem 2 and the oxygen evolving complex: a brief overview

These special issues of photosynthesis research present papers documenting progress in revealing the many aspects of photosystem 2, a unique, one-of-a-kind complex system that can reduce a plastoquinone to a plastoquinol on every second flash of light and oxidize 2 H₂O to an O₂ on every fourth flash. This overview is a brief personal assessment of the progress observed by the author over a four-decade research career, including a discussion of some remaining unsolved issues. It will come as no surprise to readers that there are remaining questions given the complexity of PS2, and the efforts that have been needed so far to uncover its secrets. In fact, most readers will have their own lists of outstanding questions.

Single-Source Deposition of Mixed-Metal Oxide Films Containing Zirconium and 3d Transition Metals for (Photo)electrocatalytic Water Oxidation

The fabrication of mixed-metal oxide films holds promise for the development of practical photoelectrochemical catalyst coatings but currently presents challenges in terms of homogeneity, cost, and scalability. We report a straightforward and versatile approach to produce catalytically active zirconium-based films for electrochemical and photoelectrochemical water oxidation. The mixed-metal oxide catalyst films are derived from novel single-source precursor oxide cage compounds containing Zr with first-row transition metals such as Co, Fe, and Cu. The Zr-based film doped with Co on fluorine-doped tin oxide (FTO)-coated glass exhibits the highest electrocatalytic O₂ evolution performance in an alkaline medium and an operational stability above 18 h. The deposition of this film onto a BiVO₄ photoanode significantly enhances its photoelectrochemical activity toward solar water oxidation, lowering the onset potential by 0.12-0.21 V vs reversible hydrogen electrode (RHE) and improving the maximum photocurrent density by ∼50% to 2.41 mA cm^-2 for the CoZr-coated BiVO₄ photoanodes compared to that for bare BiVO₄.

Ca2+ effects on Fe(II) interactions with Mn-binding sites in Mn-depleted oxygen-evolving complexes of photosystem II and on Fe replacement of Mn in Mn-containing, Ca-depleted complexes

Fe(II) cations bind with high efficiency and specificity at the high-affinity (HA), Mn-binding site (termed the "blocking effect" since Fe blocks further electron donation to the site) of the oxygen-evolving complex (OEC) in Mn-depleted, photosystem II (PSII) membrane fragments (Semin et al. in Biochemistry 41:5854, 2002). Furthermore, Fe(II) cations can substitute for 1 or 2Mn cations (pH dependent) in Ca-depleted PSII membranes (Semin et al. in Journal of Bioenergetics and Biomembranes 48:227, 2016; Semin et al. in Journal of Photochemistry and Photobiology B 178:192, 2018). In the current study, we examined the effect of Ca²⁺ cations on the interaction of Fe(II) ions with Mn-depleted [PSII(-Mn)] and Ca-depleted [PSII(-Ca)] photosystem II membranes. We found that Ca²⁺ cations (about 50 mM) inhibit the light-dependent oxidation of Fe(II) (5 µM) by about 25% in PSII(-Mn) membranes, whereas inhibition of the blocking process is greater at about 40%. Blocking of the HA site by Fe cations also decreases the rate of charge recombination between Q_A^- and Y_Z^•+ from t_1/2 = 30 ms to 46 ms. However, Ca²⁺ does not affect the rate during the blocking process. An Fe(II) cation (20 µM) replaces 1Mn cation in the Mn₄CaO₅ catalytic cluster of PSII(-Ca) membranes at pH 5.7 but 2 Mn cations at pH 6.5. In the presence of Ca²⁺ (10 mM) during the substitution process, Fe(II) is not able to extract Mn at pH 5.7 and extracts only 1Mn at pH 6.5 (instead of two without Ca²⁺). Measurements of fluorescence induction kinetics support these observations. Inhibition of Mn substitution with Fe(II) cations in the OEC only occurs with Ca²⁺ and Sr²⁺ cations, which are also able to restore oxygen evolution in PSII(-Ca) samples. Nonactive cations like La³⁺, Ni²⁺, Cd²⁺, and Mg²⁺ have no influence on the replacement of Mn with Fe. These results show that the location and/or ligand composition of one Mn cation in the Mn₄CaO₅ cluster is strongly affected by calcium depletion or rebinding and that bound calcium affects the redox potential of the extractable Mn4 cation in the OEC, making it resistant to reduction.

Pentanuclear clusters resembling the cubane-dangler connectivity in the native oxygen-evolving center of photosystem II

A series of pentametallic "cubane-plus-dangler" complexes have been target synthesized. Among them, the [Fe3Ni2] aggregate strongly resembled the native oxygen-evolving center by mimicking the "cubane-plus-dangler" skeleton, the aqua binding site, and the connectivity between the pendent ion and the parent cubane. Our synthetic strategy that uses tri-substituted methanol as the "cubane-generator" and carboxylate as the pendant ligand provides a feasible approach for accessing model compounds of biological catalyst systems.

Chlororespiration Serves as Photoprotection for the Photo-Inactivated Oxygen-Evolving Complex in Zostera marina, a Marine Angiosperm

As an alternative electron sink, chlororespiration, comprising the NAD(P)H dehydrogenase complex and plastid terminal plastoquinone oxidase, may play a significant role in sustaining the redox equilibrium between stroma and thylakoid membrane. This study identified a distinct role for chlororespiration in the marine angiosperm Zostera marina, whose oxygen-evolving complex (OEC) is prone to photo-inactivation as a result of its inherent susceptibility to excess irradiation. The strong connectivity between OEC peripheral proteins and key chlororespiratory enzymes, as demonstrated in the interaction network of differentially expressed genes, suggested that the recovery of photo-inactivated OEC was connected with chlororespiration. Chlorophyll fluorescence, transcriptome and Western blot data verified a new physiological role for chlororespiration to function as photoprotection and generate a proton gradient across the thylakoid membrane for the recovery of photo-inactivated OEC. Chlororespiration was only activated in darkness following excess irradiation exposure, which might be related to electron deficiency in the electron transport chain because of the continuous impairment of the OEC. The activation of chlororespiration in Z. marina was prone to proactivity, which was also supported by the further activation of the oxidative pentose-phosphate pathway synthesizing NADPH to meet the demand of chlororespiration during darkness. This phenomenon is distinct from the common assumption that chlororespiration is prone to consuming redundant reducing power during the short transition phase from light to dark.

Probing the Structure and Binding Mode of EDTA on the Surface of Mn3O4 Nanoparticles for Water Oxidation by Advanced Electron Paramagnetic Resonance Spectroscopy

Identification of the surface structure of nanoparticles is important for understanding the catalytic mechanism and improving the properties of the particles. Here, we provide a detailed description of the coordination modes of ethylenediaminetetraacetate (EDTA) on Mn₃O₄ nanoparticles at the atomic level, as obtained by advanced electron paramagnetic resonance (EPR) spectroscopy. Binding of EDTA to Mn₃O₄ leads to dramatic changes in the EPR spectrum, with a 5-fold increase in the axial zero-field splitting parameter of Mn(II). This indicates significant changes in the coordination environment of the Mn(II) site; hence, the binding of EDTA causes a profound change in the electronic structure of the manganese site. Furthermore, the electron spin echo envelope modulation results reveal that two ¹⁴N atoms of EDTA are directly coordinated to the Mn site and a water molecule is coordinated to the surface of the nanoparticles. An Fourier transform infrared spectroscopy study shows that the Ca(II) ion is coordinated to the carboxylic ligands via the pseudobridging mode. The EPR spectroscopic results provide an atomic picture of surface-modified Mn₃O₄ nanoparticles for the first time. These results can enhance our understanding of the rational design of catalysts, for example, for the water oxidation reaction.

Water oxidation in photosystem II

Biological water oxidation, performed by a single enzyme, photosystem II, is a central research topic not only in understanding the photosynthetic apparatus but also for the development of water splitting catalysts for technological applications. Great progress has been made in this endeavor following the report of a high-resolution X-ray crystallographic structure in 2011 resolving the cofactor site (Umena et al. in Nature 473:55-60, 2011), a tetra-manganese calcium complex. The electronic properties of the protein-bound water oxidizing Mn4OxCa complex are crucial to understand its catalytic activity. These properties include: its redox state(s) which are tuned by the protein matrix, the distribution of the manganese valence and spin states and the complex interactions that exist between the four manganese ions. In this short review we describe how magnetic resonance techniques, particularly EPR, complemented by quantum chemical calculations, have played an important role in understanding the electronic structure of the cofactor. Together with isotope labeling, these techniques have also been instrumental in deciphering the binding of the two substrate water molecules to the cluster. These results are briefly described in the context of the history of biological water oxidation with special emphasis on recent work using time resolved X-ray diffraction with free electron lasers. It is shown that these data are instrumental for developing a model of the biological water oxidation cycle.

Redox Tuning via Ligand-Induced Geometric Distortions at a YMn3O4 Cubane Model of the Biological Oxygen Evolving Complex

The function of proteins involved in electron transfer is dependent on cofactors attaining the necessary reduction potentials. We establish a mode of cluster redox tuning through steric pressure on a synthetic model related to Photosystem II. Resembling the cuboidal [CaMn₃O₄] subsite of the biological oxygen evolving complex (OEC), [Mn₄O₄] and [YMn₃O₄] complexes featuring ligands of different basicity and chelating properties were characterized by cyclic voltammetry. In the absence of ligand-induced distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. Contraction of Y-oxo/Y–Mn distances by 0.1/0.15 Å enforced by a chelating ligand results in an increase of cluster reduction potential even in the presence of strongly basic donors. Related protein-induced changes in Ca-oxo/Ca–Mn distances may have similar effects in tuning the redox potential of the OEC through entatic states and may explain the cation size dependence on the progression of the S-state cycle.

The redox properties of [YMn₃O₄] and [Mn₄O₄] complexes featuring bridging ligands of different basicity and chelating properties are reported. In the absence of ligand-induced geometric distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. A chelating ligand results in contractions of Y-oxo distances by ∼0.1 Å, which correlates with an increase of cluster reduction potential even in the presence of strongly basic donors.

Growth and biochemical changes in quail bush (Atriplex lentiformis (Torr.) S.Wats) under Cd stress

Halophytes have several advantages to be more effective in metal phytoextraction. Little is known about the Cd-phytoextraction potential of Atriplex lentiformis under different levels of Cd. Seven levels of Cd (0, 40, 80, 120, 160, 200, and 240 mg per kg of soil) were added to A. lentiformis plants grown on pots filled with 5 kg of sandy loam soil. A. lentiformis plants achieve different defense mechanisms to meet the high concentration of Cd in the soil and plant. These mechanisms include reducing the number and area of leaves, minimizing chlorophyll synthesis, and enhancing synthesizing of oxalic acid, phenols, and proline. The critical point of Cd was 9.35 and 183 mg kg^-1 for available soil Cd and leaves concentrations, respectively. The maximum level of Cd displayed a 66% decrease in the chlorophyll content of the leaves. On the other hand, the oxalic acid, phenols, and proline in the leaves were increased significantly by 129, 100, and 200% when Cd increased from 0 to 240 mg. The tested plant removed 3.6% of the total soil Cd under the low Cd concentration (40 mg) but under the high level of Cd (240 mg), it only removed a negligible amount of soil Cd (0.74%). The current study confirmed that A. lentiformis plants lost the ability to cleanup Cd from contaminated soil under the high levels of contamination due to the high reduction in the production of dry matter.

Calcium, conformational selection, and redox-active tyrosine YZ in the photosynthetic oxygen-evolving cluster

In Photosystem II (PSII), YZ (Tyr161D1) participates in radical transfer between the chlorophyll donor and the Mn₄CaO₅ cluster. Under flashing illumination, the metal cluster cycles among five S_n states, and oxygen is evolved from water. The essential YZ is transiently oxidized and reduced on each flash in a proton-coupled electron transfer (PCET) reaction. Calcium is required for function. Of reconstituted divalent ions, only strontium restores oxygen evolution. YZ is predicted to hydrogen bond to calcium-bound water and to His190D1 in PSII structures. Here, we report a vibrational spectroscopic study of YZ radical and singlet in the presence of the metal cluster. The S₂ state is trapped by illumination at 190 K; flash illumination then generates the S₂YZ radical. Using reaction-induced FTIR spectroscopy and divalent ion depletion/substitution, we identify calcium-sensitive tyrosyl radical and tyrosine singlet bands in the S₂ state. In calcium-containing PSII, two CO stretching bands are detected at 1,503 and 1,478 cm^-1 These bands are assigned to two different radical conformers in calcium-containing PSII. At pH 6.0, the 1,503-cm^-1 band shifts to 1,507 cm^-1 in strontium-containing PSII, and the band is reduced in intensity in calcium-depleted PSII. These effects are consistent with a hydrogen-bonding interaction between the calcium site and one conformer of radical YZ. Analysis of the amide I region indicates that calcium selects for a PCET reaction in a subset of the YZ conformers, which are trapped in the S₂ state. These results support the interpretation that YZ undergoes a redox-coupled conformational change, which is calcium dependent.

Creation of a 3Mn/1Fe cluster in the oxygen-evolving complex of photosystem II and investigation of its functional activity

Extraction of Mn cations from the oxygen-evolving complex (OEC) of Ca-depleted PSII membranes (PSII[-Ca,4Mn]) by reductants like hydroquinone (H₂Q) occurs with lower efficiency at acidic pH (2Mn/reaction center [RC] are extracted at pH5.7) than at neutral pH (3Mn/RC are extracted at pH6.5) [Semin et al. Photosynth. Res. 125 (2015) 95]. Fe(II) also extracts Mn cations from PSII(-Ca,4Mn), but only 2Mn/RC at pH6.5, forming a heteronuclear 2Mn/2Fe cluster [Semin and Seibert, J. Bioenerg. Biomembr. 48 (2016) 227]. Here we investigated the efficiency of Mn extraction by Fe(II) at acidic pH and found that Fe(II) cations can extract only 1Mn/RC from PSII(-Ca,4Mn) membranes at pH 5.7, forming a 3Mn/1Fe cluster. Also we found that the presence of Fe cations in a heteronuclear cluster (2Mn/2Fe) increases the resistance of the remaining Mn cations to H₂Q action, since H₂Q can extract Mn cations from homonuclear Mn clusters of PSII(-Ca,4Mn) and PSII(-Ca,2Mn) membranes but not from the heteronuclear cluster in PSII(-Ca,2Mn,2Fe) membranes. H₂Q also cannot extract Mn from PSII membranes obtained by incubation of PSII(-Ca,4Mn) membranes with Fe(II) cations at pH5.7, which suggests the formation of a heteronuclear 3Mn/1Fe cluster in the OEC. Functional activity of PSII with a 3Mn/1Fe cluster was investigated. PSII preparations with a 3Mn/1Fe cluster in the OEC are able to photoreduce the exogenous electron acceptor 2,6-dichlorophenolindophenol, possibly due to incomplete oxidation of water molecules as is the case with PSII(-Ca,2Mn,2Fe) samples. However, in the contrast to PSII(-Ca,2Mn,2Fe) samples PSII(-Ca,3Mn,1Fe) membranes can evolve O₂ at a low rate in the presence of exogenous Ca²⁺ (at about 27% of the rate of O₂ evolution in native PSII membranes). The explanation for this phenomenon (either water splitting and production of molecular O₂ by the 3Mn/1Fe cluster or apparent O₂ evolution due to minor contamination of PSII(3Mn,1Fe) samples with PSII(-Ca,4Mn) membranes) is discussed.

Design of porphyrin-based ligands for the assembly of [d-block metal : calcium] bimetallic centers

A secondary binding-site for alkaline-earth cations is introduced on a porphyrin platform to obtain competent bitopicN,O-ligands.

Quantum mechanics/molecular mechanics simulation of the ligand vibrations of the water-oxidizing Mn4CaO5 cluster in photosystem II

During photosynthesis, the light-driven oxidation of water performed by photosystem II (PSII) provides electrons necessary to fix CO2, in turn supporting life on Earth by liberating molecular oxygen. Recent high-resolution X-ray images of PSII show that the water-oxidizing center (WOC) is composed of an Mn4CaO5 cluster with six carboxylate, one imidazole, and four water ligands. FTIR difference spectroscopy has shown significant structural changes of the WOC during the S-state cycle of water oxidation, especially within carboxylate groups. However, the roles that these carboxylate groups play in water oxidation as well as how they should be properly assigned in spectra are unresolved. In this study, we performed a normal mode analysis of the WOC using the quantum mechanics/molecular mechanics (QM/MM) method to simulate FTIR difference spectra on the S1 to S2 transition in the carboxylate stretching region. By evaluating WOC models with different oxidation and protonation states, we determined that models of high-oxidation states, Mn(III)2Mn(IV)2, satisfactorily reproduced experimental spectra from intact and Ca-depleted PSII compared with low-oxidation models. It is further suggested that the carboxylate groups bridging Ca and Mn ions within this center tune the reactivity of water ligands bound to Ca by shifting charge via their π conjugation.

Selective Uptake of Alkaline Earth Metals by Cyanobacteria Forming Intracellular Carbonates

The uptakes of calcium (Ca), strontium (Sr), and barium (Ba) by two cyanobacterial strains, Cyanothece sp. PCC7425 and Gloeomargarita lithophora, both forming intracellular carbonates, were investigated in laboratory cultures. In the culture medium BG-11 amended with 250 μM Ca and 50 or 250 μM Sr and Ba, G. lithophora accumulated first Ba, then Sr, and finally Ca. Sr and Ba were completely accumulated by G. lithophora cells at rates between 0.02 and 0.10 fmol h^-1 cell^-1 and down to extracellular concentrations below the detection limits of inductively coupled plasma atomic emission spectroscopy. Accumulation of Sr and Ba did not affect the growth rate of the strain. This sequential accumulation occurred mostly intracellularly within polyphosphate and carbonate granules and resulted in the formation of core-shell structures in carbonates. In contrast, Cyanothece sp. PCC7425 showed neither a preferential accumulation of heavier alkaline earth metals nor core-shell structures in the carbonates. This indicated that fractionation between alkaline earth metals was not inherent to intracellularly calcifying cyanobacteria but was likely a genetically based trait of G. lithophora. Overall, the capability of G. lithophora to sequester preferentially Sr and Ba at high rates may be of considerable interest for designing new remediation strategies and better understanding the geochemical cycles of these elements.

Mechanism of inhibition and decoupling of oxygen evolution from electron transfer in photosystem II by fluoride, ammonia and acetate

Ca(2+) extraction from oxygen-evolving complex (OEC) of photosystem II (PSII) is accompanied by decoupling of oxygen evolution/electron transfer processes [Semin et al. Photosynth. Res. 98 (2008) 235] and appearance of a broad EPR signal at g=2 (split "S3" signal) what can imply the relationship between these effects. Split signal have been observed not only in Ca-depleted PSII but also in PSII membranes treated by fluoride anions, sodium acetate, and NH4Cl. Here we investigated the question: can such compounds induce the decoupling effect during treatment of PSII like Ca(2+) extraction does? We found that F(-), sodium acetate, and NH4Cl inhibit O2 evolution in PSII membranes more effectively than the reduction of artificial electron acceptor 2,6-dichlorophenolindophenol, i.e. the action of these compounds is accompanied by decoupling of these processes in OEC. Similarity of effects observed after Ca(2+) extraction and F(-), CH3COO(-) or NH4Cl treatments suggests that these compounds can inactivate function of Ca(2+). Such inactivation could originate from disturbance of the network of functionally active hydrogen bonds around OEC formed with participation of Ca(2+). This inhibition effect is observed in the region of low concentration of inhibitors. Increasing of inhibitor concentration is accompanied by appearance of other sites of inhibition.

Redox-coupled substrate water reorganization in the active site of Photosystem II-The role of calcium in substrate water delivery

Photosystem II (PSII) catalyzes light-driven water splitting in nature and is the key enzyme for energy input into the biosphere. Important details of its mechanism are not well understood. In order to understand the mechanism of water splitting, we perform here large-scale density functional theory (DFT) calculations on the active site of PSII in different oxidation, spin and ligand states. Prior to formation of the O-O bond, we find that all manganese atoms are oxidized to Mn(IV) in the S3 state, consistent with earlier studies. We find here, however, that the formation of the S3 state is coupled to the movement of a calcium-bound hydroxide (W3) from the Ca to a Mn (Mn1 or Mn4) in a process that is triggered by the formation of a tyrosyl radical (Tyr-161) and its protonated base, His-190. We find that subsequent oxidation and deprotonation of this hydroxide on Mn1 result in formation of an oxyl-radical that can exergonically couple with one of the oxo-bridges (O5), forming an O-O bond. When O(2) leaves the active site, a second Ca-bound water molecule reorients to bridge the gap between the manganese ions Mn1 and Mn4, forming a new oxo-bridge for the next reaction cycle. Our findings are consistent with experimental data, and suggest that the calcium ion may control substrate water access to the water oxidation sites.

Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures

All cyanobacteria, algae, and plants use a similar water-oxidizing catalyst for water oxidation. This catalyst is housed in Photosystem II, a membrane-protein complex that functions as a light-driven water oxidase in oxygenic photosynthesis. Water oxidation is also an important reaction in artificial photosynthesis because it has the potential to provide cheap electrons from water for hydrogen production or for the reduction of carbon dioxide on an industrial scale. The water-oxidizing complex of Photosystem II is a Mn-Ca cluster that oxidizes water with a low overpotential and high turnover frequency number of up to 25-90 molecules of O2 released per second. In this Review, we discuss the atomic structure of the Mn-Ca cluster of the Photosystem II water-oxidizing complex from the viewpoint that the underlying mechanism can be informative when designing artificial water-oxidizing catalysts. This is followed by consideration of functional Mn-based model complexes for water oxidation and the issue of Mn complexes decomposing to Mn oxide. We then provide a detailed assessment of the chemistry of Mn oxides by considering how their bulk and nanoscale properties contribute to their effectiveness as water-oxidizing catalysts.

Redox potential tuning by redox-inactive cations in nature's water oxidizing catalyst and synthetic analogues

Fundamental differences between synthetic manganese clusters and the biological water oxidizing catalyst are demonstrated in the modulation of their redox potential by redox-inactive cations.

Characterization of the Sr(2+)- and Cd(2+)-Substituted Oxygen-Evolving Complex of Photosystem II by Quantum Mechanics/Molecular Mechanics Calculations

The Mn4CaO5 cluster in the oxygen-evolving complex is the catalytic core of the Photosystem II (PSII) enzyme, responsible for the water splitting reaction in oxygenic photosynthesis. The role of the redox-inactive ion in the cluster has not yet been fully clarified, although several experimental data are available on Ca2+-depleted and Ca2+-substituted PSII complexes, indicating Sr2+-substituted PSII as the only modification that preserves oxygen evolution. In this work, we investigated the structural and electronic properties of the PSII catalytic core with Ca2+ replaced with Sr2+ and Cd2+ in the S2 state of the Kok−Joliot cycle by means of density functional theory and ab initio molecular dynamics based on a quantum mechanics/ molecular mechanics approach. Our calculations do not reveal significant differences between the substituted and wild-type systems in terms of geometries, thermodynamics, and kinetics of two previously identified intermediate states along the S2 to S3 transition, namely, the open cubane S2 A and closed cubane S2 B conformers. Conversely, our calculations show different pKa values for the water molecule bound to the three investigated heterocations. Specifically, for Cd-substituted PSII, the pKa value is 5.3 units smaller than the respective value in wild type Ca-PSII. On the basis of our results, we conclude that, assuming all the cations sharing the same binding site, the induced difference in the acidity of the binding pocket might influence the hydrogen bonding network and the redox levels to prevent the further evolution of the cycle toward the S3 state.

Correlation between pH dependence of O2 evolution and sensitivity of Mn cations in the oxygen-evolving complex to exogenous reductants

Effects of pH, Ca(2+), and Cl(-) ions on the extraction of Mn cations from oxygen-evolving complex (OEC) in Ca-depleted photosystem II (PSII(-Ca)) by exogenous reductants hydroquinone (H2Q) and H2O2 were studied. Two of 4 Mn cations are released by H2Q and H2O2 at pHs 5.7, 6.5, and 7.5, and their extraction does not depend on the presence of Ca(2+) and Cl(-) ions. One of Mn cations ("resistant" Mn cation) cannot be extracted by H2Q and H2O2 at any pH. Extraction of 4th Mn ion ("flexible" Mn cation) is sensitive to pH, Ca(2+), and Cl(-). This Mn cation is released by reductants at pH 6.5 but not at pHs 5.7 and 7.5. A pH dependence curve of the oxygen-evolving activity in PSII(-Ca) membranes (in the presence of exogenous Ca(2+)) has a bell-shaped form with the maximum at pH 6.5. Thus, the increase in the resistance of flexible Mn cation in OEC to the action of reductants at acidic and alkaline pHs coincides with the decrease in oxygen evolution activity at these pHs. Exogenous Ca(2+) protects the extraction of flexible Mn cation at pH 6.5. High concentration of Cl(-) anions (100 mM) shifts the pH optimum of oxygen evolution to alkaline region (around pH 7.5), while the pH of flexible Mn extraction is also shifted to alkaline pH. This result suggests that flexible Mn cation plays a key role in the water-splitting reaction. The obtained results also demonstrate that only one Mn cation in Mn4 cluster is under strong control of calcium. The change in the flexible Mn cation resistance to exogenous reductants in the presence of Ca(2+) suggests that Ca(2+) can control the redox potential of this cation.

Lessons from Nature: A Bio-Inspired Approach to Molecular Design

Metalloproteins contain actives sites with intricate structures that perform specific functions with high selectivity and efficiency. The complexity of these systems complicates the study of their function and the understanding of the properties that give rise to their reactivity. One approach that has contributed to the current level of understanding of their biological function is the study of synthetic constructs that mimic one or more aspects of the native metalloproteins. These systems allow individual contributions to the structure and function to be analyzed and also permit spectroscopic characterization of the metal cofactors without complications from the protein environment. This Current Topic is a review of synthetic constructs as probes for understanding the biological activation of small molecules. These topics are developed from the perspective of seminal molecular design breakthroughs from the past that provide the foundation for the systems used today.

Tuning the Redox Properties of a Nonheme Iron(III)-Peroxo Complex Binding Redox-Inactive Zinc Ions by Water Molecules

Redox-inactive metal ions play important roles in tuning chemical properties of metal-oxygen intermediates. Herein we report the effect of water molecules on the redox properties of a nonheme iron(III)-peroxo complex binding redox-inactive metal ions. The coordination of two water molecules to a Zn(2+) ion in (TMC)Fe(III) -(O2 )-Zn(CF3 SO3 )2 (1-Zn(2+) ) decreases the Lewis acidity of the Zn(2+) ion, resulting in the decrease of the one-electron oxidation and reduction potentials of 1-Zn(2+) . This further changes the reactivities of 1-Zn(2+) in oxidation and reduction reactions; no reaction occurred upon addition of an oxidant (e.g., cerium(IV) ammonium nitrate (CAN)) to 1-Zn(2+) , whereas 1-Zn(2+) coordinating two water molecules, (TMC)Fe(III) -(O2 )-Zn(CF3 SO3 )2 -(OH2 )2 [1-Zn(2+) -(OH2 )2 ], releases the O2 unit in the oxidation reaction. In the reduction reactions, 1-Zn(2+) was converted to its corresponding iron(IV)-oxo species upon addition of a reductant (e.g., a ferrocene derivative), whereas such a reaction occurred at a much slower rate in the case of 1-Zn(2+) -(OH2 )2 . The present results provide the first biomimetic example showing that water molecules at the active sites of metalloenzymes may participate in tuning the redox properties of metal-oxygen intermediates.

Removal of Ca(2+) from the Oxygen-Evolving Complex in Photosystem II Has Minimal Effect on the Mn4O5 Core Structure: A Polarized Mn X-ray Absorption Spectroscopy Study

Ca(2+)-depleted and Ca(2+)-reconstituted spinach photosystem II was studied using polarized X-ray absorption spectroscopy of oriented PS II preparations to investigate the structural and functional role of the Ca(2+) ion in the Mn4O5Ca cluster of the oxygen-evolving complex (OEC). Samples were prepared by low pH/citrate treatment as one-dimensionally ordered membrane layers and poised in the Ca(2+)-depleted S1 (S1') and S2 (S2') states, the S2'YZ(•) state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca(2+)-reconstituted S1 state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism. Polarized EXAFS data of all states of Ca(2+)-depleted PS II investigated show only minor changes in distances and orientations of the Mn-Mn vectors compared to the Ca(2+)-containing OEC, which may be attributed to some loss of rigidity of the core structure. Thus, removal of the Ca(2+) ion does not lead to fundamental distortion or rearrangement of the tetranuclear Mn cluster, which indicates that the Ca(2+) ion in the OEC is not critical for structural maintenance of the cluster, at least in the S1 and S2 states, but fulfills a crucial catalytic function in the mechanism of the water oxidation reaction. On the basis of this structural information, reasons for the inhibitory effect of Ca(2+) removal are discussed, attributing to the Ca(2+) ion a fundamental role in organizing the surrounding (substrate) water framework and in proton-coupled electron transfer to YZ(•) (D1-Tyr161).

Manganese–calcium clusters supported by calixarenes

An investigation inspired by the structure of the oxygen-evolving complex of photosystem II has resulted in the isolation and structural characterization of the first examples of Mn/Ca clusters supported by a calixarene ligand.

Principles of Natural Photosynthesis

Nature relies on a unique and intricate biochemical setup to achieve sunlight-driven water splitting. Combined experimental and computational efforts have produced significant insights into the structural and functional principles governing the operation of the water-oxidizing enzyme Photosystem II in general, and of the oxygen-evolving manganese-calcium cluster at its active site in particular. Here we review the most important aspects of biological water oxidation, emphasizing current knowledge on the organization of the enzyme, the geometric and electronic structure of the catalyst, and the role of calcium and chloride cofactors. The combination of recent experimental work on the identification of possible substrate sites with computational modeling have considerably limited the possible mechanistic pathways for the critical O-O bond formation step. Taken together, the key features and principles of natural photosynthesis may serve as inspiration for the design, development, and implementation of artificial systems.

Investigations of the effect of the non-manganese metal in heterometallic-oxido cluster models of the oxygen evolving complex of photosystem II: lanthanides as substitutes for calcium

We report the syntheses and electrochemical properties of nine new clusters ([LLnMn^IV₃O₄(OAc)₃(DMF)_n]⁺ (Ln = La³⁺, Ce³⁺, Nd³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Yb³⁺, and Lu³⁺, n = 2 or 3)) supported by a ligand (L^3–) based on a 1,3,5-triarylbenzene motif appended with alkoxide and pyridine donors. All complexes were obtained by metal substitution of Ca²⁺ with lanthanides upon treatment of previously reported LMn₃CaO₄(OAc)₃(THF) with Ln(OTf)₃. Structural characterization confirmed that the clusters contain the [LnMn₃O₄] cubane motif. The effect of the redox-inactive centers on the electronic properties of the Mn₃O₄ cores was investigated by cyclic voltammetry. A linear correlation between the redox potential of the cluster and the ionic radii or pKa of the lanthanide metal ion was observed. Chemical reduction of the LMn^IV₃GdO₄(OAc)₃(DMF)₂ cluster with decamethylferrocene, resulted in the formation of LGdMn^IV₂Mn^IIIO₄(OAc)₃(DMF)₂, a rare example of mixed-valence [MMn₃O₄] cubane. The lanthanide-coordinated ligands can be substituted with other donors, including water, the biological substrate.

Mixed-oxide cubane clusters of manganese and lanthanides, [LnMn^IV₃O₄], were synthesized as models of the oxygen evolving complex (OEC) of photosystem II. The redox potentials of these clusters show linear dependence with the lanthanide Lewis acidity, measured as radius of the ion or the pKa of the metal aquo complex. Rare examples of mixed-valent [GdMn^IV₂Mn^IIIO₄] cluster and water coordination to the lanthanide in the [DyMn^IV₃O₄] cubane provide models of the various states of the OEC.

Redox-inactive metal ions modulate the reactivity and oxygen release of mononuclear non-haem iron(III)-peroxo complexes

Redox-inactive metal ions that function as Lewis acids play pivotal roles in modulating the reactivity of oxygen-containing metal complexes and metalloenzymes, such as the oxygen-evolving complex in photosystem II and its small-molecule mimics. Here we report the synthesis and characterization of non-haem iron(III)-peroxo complexes that bind redox-inactive metal ions, (TMC)Fe(III)-(μ,η(2):η(2)-O2)-M(n+) (M(n+) = Sr(2+), Ca(2+), Zn(2+), Lu(3+), Y(3+) and Sc(3+); TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). We demonstrate that the Ca(2+) and Sr(2+) complexes showed similar electrochemical properties and reactivities in one-electron oxidation or reduction reactions. However, the properties and reactivities of complexes formed with stronger Lewis acidities were found to be markedly different. Complexes that contain Ca(2+) or Sr(2+) ions were oxidized by an electron acceptor to release O2, whereas the release of O2 did not occur for complexes that bind stronger Lewis acids. We discuss these results in the light of the functional role of the Ca(2+) ion in the oxidation of water to dioxygen by the oxygen-evolving complex.

A nano-sized manganese oxide in a protein matrix as a natural water-oxidizing site

The purpose of this review is to present recent advances in the structural and functional studies of water-oxidizing center of Photosystem II and its surrounding protein matrix in order to synthesize artificial catalysts for production of clean and efficient hydrogen fuel.

Synthetic cluster models of biological and heterogeneous manganese catalysts for O2 evolution

Artificial photosynthesis has emerged as an important strategy toward clean and renewable fuels. Catalytic oxidation of water to O2 remains a significant challenge in this context. A mechanistic understanding of currently known heterogeneous and biological catalysts at a molecular level is highly desirable for fundamental reasons as well as for the rational design of practical catalysts. This Award Article discusses recent efforts in synthesizing structural models of the oxygen-evolving complex of photosystem II. These structural motifs are also related to heterogeneous mixed-metal oxide catalysts. A stepwise synthetic methodology was developed toward achieving the structural complexity of the targeted active sites. A geometrically restricted multinucleating ligand, but with labile coordination modes, was employed for the synthesis of low-oxidation-state trimetallic species. These precursors were elaborated to site-differentiated tetrametallic complexes in high oxidation states. This methodology has allowed for structure-reactivity studies that have offered insight into the effects of different components of the clusters. Mechanistic aspects of oxygen-atom transfer and incorporation from water have been interrogated. Significantly, a large and systematic effect of redox-inactive metals on the redox properties of these clusters was discovered. With the pKa value of the redox-inactive metal-aqua complex as a measure of the Lewis acidity, structurally analogous clusters display a linear dependence between the reduction potential and acidity; each pKa unit shifts the potential by ca. 90 mV. Implications for the function of the biological and heterogeneous catalysts are discussed.

Hydrogen sulfide alleviates lead-induced photosynthetic and ultrastructural changes in oilseed rape

The role of hydrogen sulfide (H2S) in alleviating lead (Pb) induced stress in oilseed rape (Brassica napus L.) was studied under laboratory conditions. Plants were grown hydroponically in greenhouse conditions under three levels (0, 100, and 400 µM) of Pb and three levels (0, 100 and 200 µM) of H2S donor, sodium hydrosulfide (NaHS). Application of H2S significantly improved the plant growth, root morphology, chlorophyll contents and photosynthetic activity in leaves of B. napus under Pb stress. Moreover, exogenously applied H2S significantly lowered the Pb concentration in shoots and roots of plants under Pb stress. The microscopic examination indicated that application of exogenous H2S enabled a clean mesophyll cell having a well developed chloroplast with thylakoid membranes and starch grains. A number of modifications could be observed in root tip cell i.e. mature mitochondria, long endoplasmic reticulum and golgibodies under combined application of H2S and Pb. On the basis of these findings, it can be concluded that application of exogenous H2S has a protective role on plant growth, net photosynthesis rate and ultrastructural changes in B. napus plants under high Pb exposures.

The structure and activation of substrate water molecules in Sr2+-substituted photosystem II

An EPR spectroscopy study with direct evidence that the Ca²⁺ ion plays a structural role in maintaining the hydrogen-bond network in photosystem II.