Water-oxidizing Complex Research Articles

Photoconsumption of molecular oxygen on both donor and acceptor sides of photosystem II in Mn-depleted subchloroplast membrane fragments

Oxygen consumption in Mn-depleted photosystem II (PSII) preparations under continuous and pulsed illumination is investigated. It is shown that removal of manganese from the water-oxidizing complex (WOC) by high pH treatment leads to a 6-fold increase in the rate of O 2 photoconsumption. The use of exogenous electron acceptors and donors to PSII shows that in Mn-depleted PSII preparations along with the well-known effect of O 2 photoreduction on the acceptor side of PSII, there is light-induced O 2 consumption on the donor side of PSII (nearly 30% and 70%, respectively). It is suggested that the light-induced O 2 uptake on the donor side of PSII is related to interaction of O 2 with radicals produced by photooxidation of organic molecules. The study of flash-induced O 2 uptake finds that removal of Mn from the WOC leads to O 2 photoconsumption with maximum in the first flash, and its yield is comparable with the yield of O 2 evolution on the third flash measured in the PSII samples before Mn removal. The flash-induced O 2 uptake is drastically (by a factor of 1.8) activated by catalytic concentration (5–10 μM, corresponding to 2–4 Mn per RC) of Mn 2+, while at higher concentrations (> 100 μM) Mn 2+ inhibits the O 2 photoconsumption (like other electron donors: ferrocyanide and diphenylcarbazide). Inhibitory pre-illumination of the Mn-depleted PSII preparations (resulting in the loss of electron donation from Mn 2+) leads to both suppression of flash-induced O 2 uptake and disappearance of the Mn-induced activation of the O 2 photoconsumption. We assume that the light-induced O 2 uptake in Mn-depleted PSII preparations may reflect not only the negative processes leading to photoinhibition but also possible participation of O 2 or its reactive forms in the formation of the inorganic core of the WOC.

Bicarbonate Coordinates to Mn3+ during Photo-Assembly of the Catalytic Mn4Ca Core of Photosynthetic Water Oxidation: EPR Characterization

Assembly of the catalytic cluster, Mn4CaOxCly, comprising the water-oxidizing complex (WOC) of photosystem II (PSII), occurs during biogenesis in the presence of the apo-WOC-PSII complex, Mn2+, Ca2+ and Cl− cofactors under weak illumination. The in vitro assembly process known as photo-activation involves several intermediates that have been resolved in previous kinetic studies. (Bi)carbonate has been shown to stimulate the rate of formation and yield of the first stable light-induced Mn3+ assembly intermediate (IM1) from Mn2+ bound to the high-affinity assembly site in apo-WOC-PSII. 13C electron spin echo envelope modulation has previously revealed that (bi)carbonate is a ligand to this Mn2+. Herein, we use parallel-mode electron paramagnetic resonance (EPR) spectroscopy to characterize the Mn3+ photoproduct, which exists as a ternary complex with carbonate at the high-affinity assembly site (in the absence of Ca2+) formulated as [CO3-Mn3+-apo-WOC-PSII]. The EPR-derived spectral parameters of IM1 (the g value, 55Mn hyperfine coupling constant (AZ) and the ligand-field splitting parameters D/E) are independent of solution pH, in marked contrast to their strong pH dependence in the absence of bicarbonate. (Bi)carbonate coordination “chemically isolates” the IM1 from external pH changes, much like that caused by Ca2+ coordination, revealing similar roles in photo-assembly. The cumulative results reveal that (bi)carbonate and Ca2+ coordination control the ligand field strength and symmetry around the initial high-affinity Mn3+, consistent with the possible formation of a μ2-oxide bridge in IM1, [Mn3+(O2−)Ca2+]. These events greatly improve the quantum yield of subsequent steps in photo-assembly.

Development of Bioinspired Mn4O4−Cubane Water Oxidation Catalysts: Lessons from Photosynthesis

Hydrogen is the most promising fuel of the future owing to its carbon-free, high-energy content and potential to be efficiently converted into either electrical or thermal energy. The greatest technical barrier to accessing this renewable resource remains the inability to create inexpensive catalysts for the solar-driven oxidation of water. To date, the most efficient system that uses solar energy to oxidize water is the photosystem II water-oxidizing complex (PSII-WOC), which is found within naturally occurring photosynthetic organisms. The catalytic core of this enzyme is a CaMn(4)O(x) cluster, which is present in all known species of oxygenic phototrophs and has been conserved since the emergence of this type of photosynthesis about 2.5 billion years ago. The key features that facilitate the catalytic success of the PSII-WOC offer important lessons for the design of abiological water oxidation catalysts. In this Account, we examine the chemical principles that may govern the PSII-WOC by comparing the water oxidation capabilities of structurally related synthetic manganese-oxo complexes, particularly those with a cubical Mn(4)O(4) core ("cubanes"). We summarize this research, from the self-assembly of the first such clusters, through the elucidation of their mechanism of photoinduced rearrangement to release O(2), to recent advances highlighting their capability to catalyze sustained light-activated electrolysis of water. The [Mn(4)O(4)](6+) cubane core assembles spontaneously in solution from monomeric precursors or from [Mn(2)O(2)](3+) core complexes in the presence of metrically appropriate bidentate chelates, for example, diarylphosphinates (ligands of Ph(2)PO(2)(-) and 4-phenyl-substituted derivatives), which bridge pairs of Mn atoms on each cube face (Mn(4)O(4)L(6)). The [Mn(4)O(4)](6+) core is enlarged relative to the [Mn(2)O(2)](3+) core, resulting in considerably weaker Mn-O bonds. Cubanes are ferocious oxidizing agents, stronger than analogous complexes with the [Mn(2)O(2)](3+) core, as demonstrated both by the range of substrates they dehydrogenate or oxygenate (unactivated alkanes, for example) and the 25% larger O-H bond enthalpy of the resulting mu(3)-OH bridge. The cubane core topology is structurally suited to releasing O(2), and it does so in high yield upon removal of one phosphinate by photoexcitation in the gas phase or thermal excitation in the solid state. This is quite unlike other Mn-oxo complexes and can be attributed to the elongated Mn-O bond lengths and low-energy transition state to the mu-peroxo precursor. The photoproduct, [Mn(4)O(2)L(5)](+), an intact nonplanar butterfly core complex, is poised for oxidative regeneration of the cubane core upon binding of two water molecules and coupling to an anode. Catalytic evolution of O(2) and protons from water exceeding 1000 turnovers can be readily achieved by suspending the oxidized cubane, [Mn(4)O(4)L(6)](+), into a proton-conducting membrane (Nafion) preadsorbed onto a conducting electrode and electroxidizing the photoreduced butterfly complexes by the application of an external bias. Catalytic water oxidation can be achieved using sunlight as the only source of energy by replacing the external electrical bias with redox coupling to a photoanode incorporating a Ru(bipyridyl) dye.

Solar Fuels via Artificial Photosynthesis

Because sunlight is diffuse and intermittent, substantial use of solar energy to meet humanity's needs will probably require energy storage in dense, transportable media via chemical bonds. Practical, cost effective technologies for conversion of sunlight directly into useful fuels do not currently exist, and will require new basic science. Photosynthesis provides a blueprint for solar energy storage in fuels. Indeed, all of the fossil-fuel-based energy consumed today derives from sunlight harvested by photosynthetic organisms. Artificial photosynthesis research applies the fundamental scientific principles of the natural process to the design of solar energy conversion systems. These constructs use different materials, and researchers tune them to produce energy efficiently and in forms useful to humans. Fuel production via natural or artificial photosynthesis requires three main components. First, antenna/reaction center complexes absorb sunlight and convert the excitation energy to electrochemical energy (redox equivalents). Then, a water oxidation complex uses this redox potential to catalyze conversion of water to hydrogen ions, electrons stored as reducing equivalents, and oxygen. A second catalytic system uses the reducing equivalents to make fuels such as carbohydrates, lipids, or hydrogen gas. In this Account, we review a few general approaches to artificial photosynthetic fuel production that may be useful for eventually overcoming the energy problem. A variety of research groups have prepared artificial reaction center molecules. These systems contain a chromophore, such as a porphyrin, covalently linked to one or more electron acceptors, such as fullerenes or quinones, and secondary electron donors. Following the excitation of the chromophore, photoinduced electron transfer generates a primary charge-separated state. Electron transfer chains spatially separate the redox equivalents and reduce electronic coupling, slowing recombination of the charge-separated state to the point that catalysts can use the stored energy for fuel production. Antenna systems, employing a variety of chromophores that absorb light throughout the visible spectrum, have been coupled to artificial reaction centers and have incorporated control and photoprotective processes borrowed from photosynthesis. Thus far, researchers have not discovered practical solar-driven catalysts for water oxidation and fuel production that are robust and use earth-abundant elements, but they have developed artificial systems that use sunlight to produce fuel in the laboratory. For example, artificial reaction centers, where electrons are injected from a dye molecule into the conduction band of nanoparticulate titanium dioxide on a transparent electrode, coupled to catalysts, such as platinum or hydrogenase enzymes, can produce hydrogen gas. Oxidizing equivalents from such reaction centers can be coupled to iridium oxide nanoparticles, which can oxidize water. This system uses sunlight to split water to oxygen and hydrogen fuel, but efficiencies are low and an external electrical potential is required. Although attempts at artificial photosynthesis fall short of the efficiencies necessary for practical application, they illustrate that solar fuel production inspired by natural photosynthesis is achievable in the laboratory. More research will be needed to identify the most promising artificial photosynthetic systems and realize their potential.

Biomimetic and Microbial Approaches to Solar Fuel Generation

Photosynthesis is performed by a multitude of organisms, but in nearly all cases, it is variations on a common theme: absorption of light followed by energy transfer to a reaction center where charge separation takes place. This initial form of chemical energy is stabilized by the biosynthesis of carbohydrates. To produce these energy-rich products, a substrate is needed that feeds in reductive equivalents. When photosynthetic microorganisms learned to use water as a substrate some 2 billion years ago, a fundamental barrier against unlimited use of solar energy was overcome. The possibility of solar energy use has inspired researchers to construct artificial photosynthetic systems that show analogy to parts of the intricate molecular machinery of photosynthesis. Recent years have seen a reorientation of efforts toward creating integrated light-to-fuel systems that can use solar energy for direct synthesis of energy-rich compounds, so-called solar fuels. Sustainable production of solar fuels is a long awaited development that promises extensive solar energy use combined with long-term storage. The stoichiometry of water splitting into molecular oxygen, protons, and electrons is deceptively simple; achieving it by chemical catalysis has proven remarkably difficult. The reaction center Photosystem II couples light-induced charge separation to an efficient molecular water-splitting catalyst, a Mn(4)Ca complex, and is thus an important template for biomimetic chemistry. In our aims to design biomimetic manganese complexes for light-driven water oxidation, we link photosensitizers and charge-separation motifs to potential catalysts in supramolecular assemblies. In photosynthesis, production of carbohydrates demands the delivery of multiple reducing equivalents to CO(2). In contrast, the two-electron reduction of protons to molecular hydrogen is much less demanding. Virtually all microorganisms have enzymes called hydrogenases that convert protons to hydrogen, many of them with good catalytic efficiency. The catalytic sites of hydrogenases are now the center of attention of biomimetic efforts, providing prospects for catalytic hydrogen production with inexpensive metals. Thus, we might complete the water-to-fuel conversion: light + 2H(2)O --> 2H(2) + O(2). This reaction formula is to some extent already elegantly fulfilled by cyanobacteria and green algae, water-splitting photosynthetic microorganisms that under certain conditions also can produce hydrogen. An alternative route to hydrogen from solar energy is therefore to engineer these organisms to produce hydrogen more efficiently. This Account describes our original approach to combine research in these two fields: mimicking structural and functional principles of both Photosystem II and hydrogenases by synthetic chemistry and engineering cyanobacteria to become better hydrogen producers and ultimately developing new routes toward synthetic biology.

Ordered Mesoporous SBA-15 Type Graphitic Carbon Nitride: A Semiconductor Host Structure for Photocatalytic Hydrogen Evolution with Visible Light

Ordered mesoporous carbon nitride shows improved photoactivity and is promising as a host semiconductor scaffold for the design of hybrid visible-light-sensitive catalysts, as it enables the co-assembly of chromophoric antenna molecules, water-reduction catalysts (here exemplified by Pt), and/or water-oxidation complexes into the host matrix, thus generating a new type of biomimetic photocatalyst system for water splitting chemistry but also for selective organic synthesis.

Oxygen detection in biological systems

This article presents a brief description of analytical tools for monitoring evolution and consumption of molecular dioxygen in biological organisms. Based on its nature as a gas and its physical and chemical properties of the ground state ³Σ(g)O₂; different approaches have been developed for quantitative determinations: (i) manometry, (ii) formation of titratable sediments, (iii) solid state electrodes, (iv) EPR oximetry, (v) luminescence quenching, (vi) biological sensoring, (vii) mass spectrometry and (viii) amperometry. Among these methods mass spectrometry and amperometry are of special relevance for studies on the mechanisms of photosynthetic dioxygen evolution. Mass spectrometry is described in the article of Beckman et al. in this special issue. Therefore, the major part of this contribution focuses on amperometric methods that are currently widely used. Two different types of electrodes are described: (i) Clark-type electrode and (ii) Joliot-type electrode. The complementary advantages of both systems are outlined. A more detailed description comprises the potential of the Joliot-type electrode for mechanistic studies on the reactivity of the different redox states of the water oxidizing complex (WOC).

Highly Active and Robust Cp* Iridium Complexes for Catalytic Water Oxidation

A series of Cp*Ir catalysts are the most active known by over an order of magnitude for water oxidation with Ce(IV). DFT calculations support a Cp*Ir=O complex as an active species.

ChemInform Abstract: EPR Investigations of Synthetic Manganese Complexes as Biomimics of the Water Oxidation Complex in Photosystem II

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The psbO mutant of Chlamydomonas reinhardtii is capable of assembling stable, photochemically active reaction center of photosystem II

The functional status of photosystem II (PSII) complex in the dark-grown PsbO-deficient mutant of green alga Chlamydomonas reinhardtii was studied. It was found that ΔpsbO mutant cells of C. reinhardtii grown under heterotrophic conditions (dark + acetate) were capable of assembling stable, photochemically-competent reaction centers of PSII (as confirmed by immunological analysis of D1 protein level, pigments content and photoinduced changes of PSII chlorophyll fluorescence yield), while O2-evolution activity was not revealed. The ratio Fv/Fm for the dark-grown ΔpsbO mutant C. reinhardtii was 0.37 and that for the dark-grown wild type cells was 0.56. Analysis of chlorophyll fluorescence induction curve indicated that the absence of oxygen-evolving activity could be due to some defects in the organization of the PSII catalytic manganese cluster. Decrease of the rate of the electron donation from water-oxidizing complex to the PSII reaction center as well as the appearance of an additional transient fluorescence peak during the dark relaxation of Fv testify to the damages to the PSII donor side. The data obtained suggest that the dark-grown PsbO-deficient cells of C. reinhardtii are able to form stable, photochemically active PSII reaction center, unable to oxidize water due to probable defects in the assembly of the manganese cluster.

Photosynthetic Oxygen Evolution Is Not Reversed at High Oxygen Pressures: Mechanistic Consequences for the Water-Oxidizing Complex

We investigated the effects of elevated O(2) pressure on the production of O(2) by photosynthetic organisms in several species of plants, algae, and a cyanobacterium. Using a noninvasive fluorometry technique to monitor sequential turnover of the photosystem II (PSII) reaction center as a function of O(2) pressures, we showed that none of the reactions of water oxidation are affected by elevated O(2) pressures up to 50-fold greater than atmospheric conditions. Thus, the terminal step of O(2) release from the water oxidation complex (S(4) --> S(0) + O(2) + nH(+)) is not reversible in whole cells, leaves, or isolated thylakoid membranes containing PSII, in contrast to reports using detergent-extracted PSII complexes. This implies that there is no thermodynamically accessible intermediate that can be populated by preventing or reversing the O(2) release step with O(2) at atmospheric pressure. To assess the sensitivity of PSII charge recombination to O(2) pressure, we quantitatively modeled the consequences of two putative perturbations to the catalytic cycle of water oxidation within the framework of the Kok model. On the basis of the breadth of oxygenic phototrophs examined in this study, we conclude that O(2) accumulation in cells or the atmosphere does not suppress photosynthetic productivity through the reversal of water oxidation in contemporary phototrophs and would have been unlikely to influence the evolution of oxygenic photosynthesis.

Electrochemical investigation of Mn4O4-cubane water-oxidizing clusters

High valence states in manganese clusters are a key feature of the function of one of the most important catalysts found in nature, the water-oxidizing complex of photosystem II. We describe a detailed electrochemical investigation of two bio-inspired manganese-oxo complexes, [Mn(4)O(4)L(6)] (L = diphenylphosphinate (1) and bis(p-methoxyphenyl)phosphinate (2)), in solution, attached to an electrode surface and suspended within a Nafion film. These complexes contain a cubic [Mn(4)O(4)](6+) core stabilized by phosphinate ligands. They have previously been shown to be active and durable photocatalysts for the oxidation of water to dioxygen. A comparison of catalytic photocurrent generated by films deposited by two methods of electrode immobilization reveals that doping of the catalyst in Nafion results in higher photocurrent than was observed for a solid layer of cubane on an electrode surface. In dichloromethane solution, and under conditions of cyclic voltammetry, the one-electron oxidation processes 1/1(+) and 2/2(+) were found to be reversible and quasi-reversible, respectively. Some decomposition of 1(+) and 2(+) was detected on the longer timescale of bulk electrolysis. Both compounds also undergo a two-electron, chemically irreversible reduction in dichloromethane, with a mechanism that is dependent on scan rate and influenced by the presence of a proton donor. When immersed in aqueous electrolyte, the reduction process exhibits a limited level of chemical reversibility. These data provide insights into the catalytic operation of these molecules during photo-assisted electrolysis of water and highlight the importance of the strongly electron-donating ligand environment about the manganese ions in the ability of the cubanes to photocatalyze water oxidation at low overpotentials.

Photosystem II: The machinery of photosynthetic water splitting

This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair [Formula: see text] formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with P680+* as driving force and tyrosine Y(Z) as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special Q(B) binding site with Q(A)-* acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: [Formula: see text] where (1)(RC-PC)* denotes the excited singlet state (1)P680* of the reaction centre pigment complex. The structure of the catalytic Mn(4)O(X)Ca cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S(3), are discussed. The unravelling of the mode of O-O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S(3) and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the Q(B) site for PQ reduction to PQH(2) and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.

Function of plastoquinone in heat stress reactions of plants

The effect of high temperature treatment (40 °C, 3 h, illumination at 100 μmol m − 2 s − 1 ) on the photosynthetic electron flow in barley seedlings of different age was investigated. Thermoinduced inhibition of the liner electron flow due to partial impairment of the water oxidizing complex (WOC) and the increase in the extent of Q A − reoxidation by Tyr z ox in thylakoids isolated from 4-day-old leaves was shown by measurements of oxygen evolution using benzoquinone or potassium ferricyanide as electron acceptors, as well as by following Q A − reoxidation kinetics in the absence and presence of exogenous electron acceptors, DCBQ and DMBQ. Using HPLC analysis, an increase in the oxidation of the photoactive plastoquinone pool in young leaves under heating was shown. In older, 11-day-old leaves, heat treatment limited both photosynthetic electron flow and oxygen evolution. The same effects of heat shock on oxygen evolution caused an inhibition of electron flow on the donor side of PSII only. However, a rise in the proportion of PSII with Q A − reoxidized through recombination with the S 2/S 3 state of the WOC was observed. The addition of exogenous electron acceptors (DCBQ and DMBQ) and a donor (DPC) showed that the thermoinduced decrease in the electron transport rate was caused by an impediment of electron flow from Q A − to acceptor pool. The decrease in size of the photoactive PQ-pool and a change in the proportions of oxidized and reduced PQ in older leaves under heat treatment were shown. It was suggested that a thermoinduced change of the redox state of the PQ-pool and a redistribution of plastoquinone molecules between photoactive and non-photoactive pools are the mechanisms which reflect and regulate the response of the photosynthetic apparatus under heat stress conditions.

Importance of a single disulfide bond for the PsbO protein of photosystem II: protein structure stability and soluble overexpression in Escherichia coli

PsbO protein is an important constituent of the water-oxidizing complex, located on the lumenal side of photosystem II. We report here the efficient expression of the spinach PsbO in E. coli where the solubility depends entirely on the formation of the disulfide bond. The PsbO protein purified from a pET32 system that includes thioredoxin fusion is properly folded and functionally active. Urea unfolding experiments imply that the reduction of the single disulfide bridge decreases stability of the protein. Analysis of inter-residue contact density through the PsbO molecule shows that Cys51 is located in a cluster with high contact density. Reduction of the Cys28-Cys51 bond is proposed to perturb the packing interactions in this cluster and destabilize the protein as a whole. Taken together, our results give evidence that PsbO exists in solution as a compact highly ordered structure, provided that the disulfide bridge is not reduced.

Visible Light-Induced Electron Transfer from Di-μ-oxo-Bridged Dinuclear Mn Complexes to Cr Centers in Silica Nanopores

The compound (bpy) 2Mn (III)(mu-O) 2Mn (IV)(bpy) 2, a structural model relevant for the photosynthetic water oxidation complex, was coupled to single Cr (VI) charge-transfer chromophores in the channels of the nanoporous oxide AlMCM-41. Mn K-edge EXAFS spectroscopy confirmed that the di-mu-oxo dinuclear Mn core of the complex is unaffected when loaded into the nanoscale pores. Observation of the 16-line EPR signal characteristic of Mn (III)(mu-O) 2Mn (IV) demonstrates that the majority of the loaded complexes retained their nascent oxidation state in the presence or absence of Cr (VI) centers. The FT-Raman spectrum upon visible light excitation of the Cr (VI)-O (II) --> Cr (V)-O (I) ligand-to-metal charge transfer reveals electron transfer from Mn (III)(mu-O) 2Mn (IV) (Mn-O stretch at 700 cm (-1)) to Cr (VI), resulting in the formation of Cr (V) and Mn (IV)(mu-O) 2Mn (IV) (Mn-O stretch at 645 cm (-1)). All initial and final states are directly observed by FT-Raman or EPR spectroscopy, and the assignments are corroborated by X-ray absorption spectroscopy measurements. The endoergic charge separation products (Delta E o = -0.6 V) remain after several minutes, which points to spatial separation of Cr (V) and Mn (IV)(mu-O) 2Mn (IV) as a consequence of hole (O (I)) hopping as a major contributing mechanism. This is the first observation of visible light-induced oxidation of a potential water oxidation complex by a metal charge-transfer pump in a nanoporous environment. These findings will allow for the assembly and photochemical characterization of well-defined transition metal molecular units, with the ultimate goal of performing endothermic, multielectron transformations that are coupled to visible light electron pumps in nanostructured scaffolds.

Reconstitution of the water-oxidizing complex in manganese-depleted photosystem II preparations using synthetic Mn complexes: a fluorine-19 NMR study of the reconstitution process

Reconstitution of Mn-depleted photosystem II (PSII) particles was examined with synthetic trinuclear Mn complexes of newly developed tripod ligands. Rates of the electron transfer and oxygen evolution were up to 74-86 and 52-56% of those measured in native PSII. These values are higher than those for the PSII reconstituted by MnCl(2). The role of the tripod ligands during the reconstitution process was examined by (19)F NMR. Due to the high NMR sensitivity of the (19)F nucleus and the low abundance of fluorine atoms in natural PSII, it was possible to selectively observe the fluorine atoms on the tripod ligand. It was shown that the tripod ligands were released from the Mn complex after the reconstitution. We propose that the primary step in the reconstitution process is the prebinding of the Mn complex to the hydrophobic part of the PSII particle.

Structural, Magnetic Coupling and Oxidation State Trends in Models of the CaMn4 Cluster in Photosystem II

Density functional theory calculations are reported on a set of isomeric structures I, II and III that share the structural formula [CaMn4C9H10N2O16]q+.(H2O)3 (q= -1, 0, 1, 2, 3). Species I has a skeletal structure, which has been previously identified as a close match to the ligated CaMn4 cluster in Photosystem II, as characterized in the most recent 3.0 angstroms crystal structure. Structures II and III are rearrangements of I, which largely retain that model's bridging ligand framework, but feature metal atom positions broadly consistent with, respectively, the earlier 3.5 and 3.2 angstroms crystal structures for the Photosystem II water-oxidising complex (WOC). Our study explores the influence of the cluster charge state (and hence S state) on several important properties of the model structures; including the relative energies of the three models, their interconversion, trends in the individual Mn oxidation states, preferred hydration sites and favoured modes of magnetic coupling between the manganese atoms. We find that, for several of the explored cluster charge states, modest differences in the bridging-ligand geometry exert a powerful influence over the individual manganese oxidation states, but throughout these states the robustness of the tetrahedron formed by the Ca and three of the Mn atoms remains a significant feature and contrasts with the positional flexibility of the fourth Mn atom. Although structure I is lowest in energy for most S states, the energy differences between structures for a given S state are not large. Overall, structure II provides a better match for the EXAFS derived metal-metal distance parameters for the earlier S states (S0 to S2), but not for S3 in which a significant structural change is observed experimentally. In this S state structure III provides a closer fit. The implications of these results, for the possible action of the WOC, are discussed.

Hydrogencarbonate is not a tightly bound constituent of the water-oxidizing complex in photosystem II

Since the end of the 1950s hydrogencarbonate (‘bicarbonate’) is discussed as a possible cofactor of photosynthetic water-splitting, and in a recent X-ray crystallography model of photosystem II (PSII) it was displayed as a ligand of the Mn 4O x Ca cluster. Employing membrane-inlet mass spectrometry (MIMS) and isotope labelling we confirm the release of less than one (≈ 0.3) HCO 3 − per PSII upon addition of formate. The same amount of HCO 3 − release is observed upon formate addition to Mn-depleted PSII samples. This suggests that formate does not replace HCO 3 − from the donor side, but only from the non-heme iron at the acceptor side of PSII. The absence of a firmly bound HCO 3 − is corroborated by showing that a reductive destruction of the Mn 4O x Ca cluster inside the MIMS cell by NH 2OH addition does not lead to any CO 2/HCO 3 − release. We note that even after an essentially complete HCO 3 −/CO 2 removal from the sample medium by extensive degassing in the MIMS cell the PSII samples retain ≥ 75% of their initial flash-induced O 2-evolving capacity. We therefore conclude that HCO 3 − has only ‘indirect’ effects on water-splitting in PSII, possibly by being part of a proton relay network and/or by participating in assembly and stabilization of the water-oxidizing complex.

Single-Molecule Magnetism Properties of the First Strontium-Manganese Cluster [SrMn14O11(OMe)3(O2CPh)18(MeCN)2

The preparation and properties of the first strontium-manganese molecular complex are described. The reaction of (NBu(n)4)[Mn4O2(O2CPh)9(H 2O)] (4Mn(III)) with Sr(ClO4)2 in MeCN/MeOH led to the isolation of [SrMn14O11(OMe)3(O2CPh)18(MeCN)2] ( 1; 13Mn(III), Mn(II)). The structure of 1 consists of two [Mn4O3(OMe)] cubane units attached to a central, near-planar, trinuclear [Mn3O4] unit, to which are also attached a Mn and a Sr above the plane and a [Mn2O(OMe)] rhomb below the plane. Peripheral ligation is provided by 18 bridging benzoate and two terminal MeCN groups. Variable-temperature and -field dc magnetization (M) data were collected in the 1.8-10 K and 0.1-4.0 T ranges and fit by matrix diagonalization methods to give S = 9/2, D = -0.50(5) cm(-1), and g = 1.88(10), where S is the ground-state spin and D is the axial zero-field splitting parameter. Magnetization versus dc field sweeps at various temperatures and scan rates exhibited hysteresis loops, confirming 1 to be a new single-molecule magnet. Because complex 1 is the initial molecular example of intimately associated Mn and Sr atoms, Sr EXAFS studies have been performed for the first time on a synthetic Sr-containing molecule. This has also allowed comparisons with the EXAFS data on the Sr-substituted water oxidizing complex (WOC) of Photosystem II (PS II), which contains a SrMn4 complex.