Ligation of D1-His332 and D1-Asp170 to the manganese cluster of photosystem II from Synechocystis assessed by multifrequency pulse EPR spectroscopy

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

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

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

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

Insights into the protonation state and spin structure for the g = 2 multiline electron paramagnetic resonance signal of the oxygen-evolving complex

In photosystem II (PSII), one-electron oxidation of the most stable oxidation state of the Mn4CaO5 cluster (S1) leads to formation of two distinct states, the open-cubane S2 conformation [Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV)] with low spin and the closed-cubane S2 conformation [Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III)] with high spin. In electron paramagnetic resonance (EPR) spectroscopy, the open-cubane S2 conformation exhibits a g = 2 multiline signal. However, its protonation state remains unclear. Here, we investigated the protonation state of the open-cubane S2 conformation by calculating exchange couplings in the presence of the PSII protein environment and simulating the pulsed electron-electron double resonance (PELDOR). When a ligand water molecule, which forms an H-bond with D1-Asp61 (W1), is deprotonated at dangling Mn4(IV), the first-exited energy (34 cm-1) in manifold spin excited states aligns with the observed value in temperature-dependent pulsed EPR analyses, and the PELDOR signal is best reproduced. Consequently, the g = 2 multiline signal observed in EPR corresponds to the open-cubane S2 conformation with the deprotonated W1 (OH-).

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

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

Development of serial X-ray fluorescence holography for radiation-sensitive protein crystals

X-ray fluorescence holography (XFH) is a powerful atomic resolution technique capable of directly imaging the local atomic structure around atoms of a target element within a material. Although it is theoretically possible to use XFH to study the local structures of metal clusters in large protein crystals, the experiment has proven difficult to perform, especially on radiation-sensitive proteins. Here, the development of serial X-ray fluorescence holography to allow the direct recording of hologram patterns before the onset of radiation damage is reported. By combining a 2D hybrid detector and the serial data collection used in serial protein crystallography, the X-ray fluorescence hologram can be directly recorded in a fraction of the measurement time needed for conventional XFH measurements. This approach was demonstrated by obtaining the Mn Kα hologram pattern from the protein crystal Photosystem II without any X-ray-induced reduction of the Mn clusters. Furthermore, a method to interpret the fluorescence patterns as real-space projections of the atoms surrounding the Mn emitters has been developed, where the surrounding atoms produce large dark dips along the emitter-scatterer bond directions. This new technique paves the way for future experiments on protein crystals that aim to clarify the local atomic structures of their functional metal clusters, and for other related XFH experiments such as valence-selective XFH or time-resolved XFH.

Water oxidation in oxygenic photosynthesis studied by magnetic resonance techniques

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

Pulsed Multifrequency Electron Paramagnetic Resonance Spectroscopy Reveals Key Branch Points for One- vs Two-Electron Reactivity in Mn/Fe Proteins

Traditionally, the ferritin-like superfamily of proteins was thought to exclusively use a diiron active site in catalyzing a diverse array of oxygen-dependent reactions. In recent years, novel redox-active cofactors featuring heterobimetallic Mn/Fe active sites have been discovered in both the radical-generating R2 subunit of class Ic (R2c) ribonucleotide reductases (RNRs) and the related R2-like ligand-binding oxidases (R2lox). However, the protein-specific factors that differentiate the radical reactivity of R2c from the C-H activation reactions of R2lox remain unknown. In this work, multifrequency pulsed electron paramagnetic resonance (EPR) spectroscopy and ligand hyperfine techniques in conjunction with broken-symmetry density functional theory calculations are used to characterize the molecular and electronic structures of two EPR-active intermediates trapped during aerobic assembly of the R2lox Mn/Fe cofactor. A Mn^III(μ-O)(μ-OH)Fe^III species is identified as the first EPR-active species and represents a common state between the two classes of redox-active Mn/Fe proteins. The species downstream from the Mn^III(μ-O)(μ-OH)Fe^III state exhibits unique EPR properties, including unprecedented spectral breadth and isotope-dependent g-tensors, which are attributed to a weakly coupled, hydrogen-bonded Mn^III(μ-OH)Fe^III species. This final intermediate precedes formation of the Mn^III/Fe^III resting state and is suggested to be relevant to understanding the endogenous reactivity of R2lox.

Binding of the substrate analog methanol in the oxygen-evolving complex of photosystem II in the D1-N87A genetic variant of cyanobacteria

The solar water-splitting protein complex, photosystem II (PSII), catalyzes one of the most energetically demanding reactions in nature by using light energy to drive a catalyst capable of oxidizing water. The water oxidation reaction is catalyzed at the Mn₄Ca-oxo cluster in the oxygen-evolving complex (OEC), which cycles through five light-driven S-state intermediates (S₀-S₄). A detailed mechanism of the reaction remains elusive as it requires knowledge of the delivery and binding of substrate water in the higher S-state intermediates. In this study, we use two-dimensional (2D) hyperfine sublevel correlation spectroscopy, in conjunction with quantum mechanics/molecular mechanics (QM/MM) and density functional theory (DFT), to probe the binding of the substrate analog, methanol, in the S₂ state of the D1-N87A variant of PSII from Synechocystis sp. PCC 6803. The results indicate that the size and specificity of the "narrow" channel is altered in D1-N87A PSII, allowing for the binding of deprotonated ¹³C-labeled methanol at the Mn4(IV) ion of the catalytic cluster in the S₂ state. This has important implications on the mechanistic models for water oxidation in PSII.

Alteration of the O2-Producing Mn4Ca Cluster in Photosystem II by the Mutation of a Metal Ligand

The O₂-evolving Mn₄Ca cluster in photosystem II (PSII) is arranged as a distorted Mn₃Ca cube that is linked to a fourth Mn ion (denoted as Mn4) by two oxo bridges. The Mn4 and Ca ions are bridged by residue D1-D170. This is also the only residue known to participate in the high-affinity Mn(II) site that participates in the light-driven assembly of the Mn₄Ca cluster. In this study, we use Fourier transform infrared difference spectroscopy to characterize the impact of the D1-D170E mutation. On the basis of analyses of carboxylate and carbonyl stretching modes and the O-H stretching modes of hydrogen-bonded water molecules, we show that this mutation alters the extensive network of hydrogen bonds that surrounds the Mn₄Ca cluster in the same manner as that of many other mutations. It also alters the equilibrium between conformers of the Mn₄Ca cluster in the dark-stable S₁ state so that a high-spin form of the S₂ state is produced during the S₁-to-S₂ transition instead of the low-spin form that gives rise to the S₂ state multiline electron paramagnetic resonance signal. The mutation may also change the coordination mode of the carboxylate group at position 170 to unidentate ligation of Mn4. This is the first mutation of a metal ligand in PSII that substantially impacts the spectroscopic signatures of the Mn₄Ca cluster without substantially eliminating O₂ evolution. The results have significant implications for our understanding of the roles of alternate active/inactive conformers of the Mn₄Ca cluster in the mechanism of O₂ formation.

Pulse EPR Spectroscopic Characterization of the S3 State of the Oxygen-Evolving Complex of Photosystem II Isolated from Synechocystis

The S₃ state is the last semi-stable state in the water splitting reaction that is catalyzed by the Mn₄O₅Ca cluster that makes up the oxygen-evolving complex (OEC) of photosystem II (PSII). Recent high-field/frequency (95 GHz) electron paramagnetic resonance (EPR) studies of PSII isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus have found broadened signals induced by chemical modification of the S₃ state. These signals are ascribed to an S₃ form that contains a five-coordinate Mn^IV center bridged to a cuboidal Mn^IV₃O₄Ca unit. High-resolution X-ray free-electron laser studies of the S₃ state have observed the OEC with all-octahedrally coordinated Mn^IV in what is described as an open cuboid-like cluster. No five-coordinate Mn^IV centers have been reported in these S₃ state structures. Here, we report high-field/frequency (130 GHz) pulse EPR of the S₃ state in Synechocystis sp. PCC 6803 PSII as isolated in the presence of glycerol. The S₃ state of PSII from Synechocystis exhibits multiple broadened forms (≈69% of the total signal) similar to those seen in the chemically modified S₃ centers from T. elongatus. Field-dependent ELDOR-detected nuclear magnetic resonance resolves two classes of ⁵⁵Mn nuclear spin transitions: one class with small hyperfine couplings (|A| ≈ 1-7 MHz) and another with larger hyperfine couplings (|A| ≈ 100 MHz). These results are consistent with an all-Mn^IV₄ open cubane structure of the S₃ state and suggest that the broadened S₃ signals arise from a perturbation of Mn4A and/or Mn3B, possibly induced by the presence of glycerol in the as-isolated Synechocystis PSII.

D1-S169A substitution of photosystem II reveals a novel S2-state structure

In photosystem II (PSII), photosynthetic water oxidation occurs at the O₂-evolving complex (OEC), a tetramanganese-calcium cluster that cycles through light-induced redox intermediates (S₀-S₄) to produce oxygen from two substrate water molecules. The OEC is surrounded by a hydrogen-bonded network of amino-acid residues that plays a crucial role in proton transfer and substrate water delivery. Previously, we found that D1-S169 was crucial for water oxidation and its mutation to alanine perturbed the hydrogen-bonding network. In this study, we demonstrate that the activation energy for the S₂ to S₁ transition of D1-S169A PSII is higher than wild-type PSII with a ~1.7-2.7× slower rate of charge recombination with Q_A^- relative to wild-type PSII. Arrhenius analysis of the decay kinetics shows an E_a of 5.87 ± 1.15 kcal mol^-1 for decay back to the S₁ state, compared to 0.80 ± 0.13 kcal mol^-1 for the wild-type S₂ state. In addition, we find that ammonia does not affect the S₂-state EPR signal, indicating that ammonia does not bind to the Mn cluster in D1-S169A PSII. Finally, a QM/MM analysis indicates that an additional water molecule binds to the Mn4 ion in place of an oxo ligand O5 in the S₂ state of D1-S169A PSII. The altered S₂ state of D1-S169A PSII provides insight into the S₂➔S₃ state transition.

Two-dimensional 67Zn HYSCORE spectroscopy reveals that a Zn-bacteriochlorophyll aP′ dimer is the primary donor (P840) in the type-1 reaction centers of Chloracidobacterium thermophilum

Using pulsed EPR spectroscopy and isotopic labeling we demonstrate that reaction centers of Chloracidobacterium thermophilum have an unusual primary donor that is a dimer of Zn-bacteriochlorophyll a_P′ molecules.

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.

Reconciling Structural and Spectroscopic Fingerprints of the Oxygen-Evolving Complex of Photosystem II: A Computational Study of the S2 State

The catalytic cycle of photosynthetic water oxidation occurs at the Mn₄CaO₅ oxygen-evolving complex (OEC) of photosystem II. Extensive spectroscopic data have been collected on the intermediates, especially the S₂ (Kok) state, although the proton and electron inventories (Mn oxidation states) are still uncertain. The "high oxidation" paradigm assigns S₂ Mn oxidation level (III, IV, IV, IV) or (IV, IV, IV, III), whereas a "low oxidation" paradigm posits two additional electrons. Here, we investigate the geometric (X-ray diffraction, extended X-ray absorption fine structure) and spectroscopic (electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR)) properties of the S₂ state using quantum chemical density functional theory calculations, focusing on the neglected low paradigm. Two interconvertible electronic spin configurations are predicted as ground states, producing multiline ( S = 1/2) and broad ( S = 5/2) EPR signals in the low paradigm oxidation state (III, IV, III, III) and with W2 as OH^- and O5 as OH^-. They have "open" ( S = 5/2) and "closed" ( S = 1/2) Mn₃CaO₄-cubane geometries. Other energetically accessible isomers with ground spin states 1/2, 7/2, 9/2, or 11/2 can be obtained through perturbations of hydrogen-bonding networks (e.g., H⁺ from His337 to O3 or W2), consistent with experimental observations. Conformers with the low oxidation state configuration (III, IV, IV, II) also become energetically accessible when the protonation states are O5 (OH^-), W2 (H₂O), and neutral His337. The configuration with (III, IV, III, III) agrees well with earlier low-temperature EPR and ENDOR interpretations, whereas the Mn^II-containing configuration agrees partially with recent ENDOR data. However, the low oxidation paradigm does not yield isotropic ligand hyperfine interactions in good agreement with observed values. We conclude that the low Mn oxidation state proposal for the OEC can closely fit most of the available structural and electronic data for S₂ at accessible energies.

Lowering water oxidation overpotentials using the ionisable imidazole of copper(2-(2'-pyridyl)imidazole)

Rapid and low overpotential oxidation of water to dioxygen remains a key hurdle for storage of solar energy. Here, we address this issue by demonstrating that deprotonation of 2-(2'-pyridyl)-imidazole (pimH)-ligated copper complexes promotes water oxidation at low overpotential and low catalyst loading. This improves upon other work on homogeneous copper-based water oxidation catalysts, which are highly active, but limited by high overpotentials. EPR and UV-vis spectroscopic evaluation of catalyst speciation shows that at pH ≥ 12 coordinated pimH is deprotonated and a bis(hydroxide) Cu²⁺ active catalyst forms. Rapid electrochemical water oxidation (35 s^-1, 0.85 V onset potential) was observed with 150 μM catalyst. These results demonstrate that catalytic water oxidation potentials can be shifted by hundreds of mV in homogeneous metal catalysts bearing an ionisable imidazole ligand.

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.

Proton Matrix ENDOR Studies on Ca2+-depleted and Sr2+-substituted Manganese Cluster in Photosystem II

Proton matrix ENDOR spectra were measured for Ca(2+)-depleted and Sr(2+)-substituted photosystem II (PSII) membrane samples from spinach and core complexes from Thermosynechococcus vulcanus in the S2 state. The ENDOR spectra obtained were similar for untreated PSII from T. vulcanus and spinach, as well as for Ca(2+)-containing and Sr(2+)-substituted PSII, indicating that the proton arrangements around the manganese cluster in cyanobacterial and higher plant PSII and Ca(2+)-containing and Sr(2+)-substituted PSII are similar in the S2 state, in agreement with the similarity of the crystal structure of both Ca(2+)-containing and Sr(2+)-substituted PSII in the S1 state. Nevertheless, slightly different hyperfine separations were found between Ca(2+)-containing and Sr(2+)-substituted PSII because of modifications of the water protons ligating to the Sr(2+) ion. Importantly, Ca(2+) depletion caused the loss of ENDOR signals with a 1.36-MHz separation because of the loss of the water proton W4 connecting Ca(2+) and YZ directly. With respect to the crystal structure and the functions of Ca(2+) in oxygen evolution, it was concluded that the roles of Ca(2+) and Sr(2+) involve the maintenance of the hydrogen bond network near the Ca(2+) site and electron transfer pathway to the manganese cluster.

Electronic structure of the oxygen evolving complex in photosystem II, as revealed by 55Mn Davies ENDOR studies at 2.5 K

We report the first (55)Mn pulsed ENDOR studies on the S2 state multiline spin ½ centre of the oxygen evolving complex (OEC) in Photosystem II (PS II), at temperatures below 4.2 K. These were performed on highly active samples of spinach PS II core complexes, developed previously in our laboratories for photosystem spectroscopic use, at temperatures down to 2.5 K. Under these conditions, relaxation effects which have previously hindered observation of most of the manganese ENDOR resonances from the OEC coupled Mn cluster are suppressed. (55)Mn ENDOR hyperfine couplings ranging from ∼50 to ∼680 MHz are now seen on the S2 state multiline EPR signal. These, together with complementary high resolution X-band CW EPR measurements and detailed simulations, reveal that at least two and probably three Mn hyperfine couplings with large anisotropy are seen, indicating that three Mn(III) ions are likely present in the functional S2 state of the enzyme. This suggests a low oxidation state paradigm for the OEC (mean Mn oxidation level 3.0 in the S1 state) and unexpected Mn exchange coupling in the S2 state, with two Mn ions nearly magnetically silent. Our results rationalize a number of previous ligand ESEEM/ENDOR studies and labelled water exchange experiments on the S2 state of the photosystem, in a common picture which is closely consistent with recent photo-assembly (Kolling et al., Biophys. J. 2012, 103, 313-322) and large scale computational studies on the OEC (Gatt et al., Angew. Chem., Int. Ed. 2012, 51, 12025-12028, Kurashige et al. Nat. Chem. 2013, 5, 660-666).

Generalized approximate spin projection calculations of effective exchange integrals of the CaMn4O5 cluster in the S1 and S3 states of the oxygen evolving complex of photosystem II

Full geometry optimizations followed by the vibrational analysis were performed for eight spin configurations of the CaMn4O4X(H2O)3Y (X = O, OH; Y = H2O, OH) cluster in the S1 and S3 states of the oxygen evolution complex (OEC) of photosystem II (PSII). The energy gaps among these configurations obtained by vertical, adiabatic and adiabatic plus zero-point-energy (ZPE) correction procedures have been used for computation of the effective exchange integrals (J) in the spin Hamiltonian model. The J values are calculated by the (1) analytical method and the (2) generalized approximate spin projection (AP) method that eliminates the spin contamination errors of UB3LYP solutions. Using J values derived from these methods, exact diagonalization of the spin Hamiltonian matrix was carried out, yielding excitation energies and spin densities of the ground and lower-excited states of the cluster. The obtained results for the right (R)- and left (L)-opened structures in the S1 and S3 states are found to be consistent with available optical and magnetic experimental results. Implications of the computational results are discussed in relation to (a) the necessity of the exact diagonalization for computations of reliable energy levels, (b) magneto-structural correlations in the CaMn4O5 cluster of the OEC of PSII, (c) structural symmetry breaking in the S1 and S3 states, and (d) the right- and left-handed scenarios for the O-O bond formation for water oxidation.

Metal ion oxidation state assignment based on coordinating ligand hyperfine interaction

In exchange-coupled mixed-valence spin systems, the magnitude and sign of the effective ligand hyperfine interaction (HFI) can be useful in determining the formal oxidation state of the coordinating metal ion, as well as provide information about the coordination geometry. This is due to the fact that the observed ligand HFI is a function of the projection factor (Clebsch-Gordon coefficient) that maps the site spin value S i of the local paramagnetic center onto the total spin of the exchange-coupled system, S T. Recently, this relationship has been successfully exploited in identifying the oxidation state of the Mn ion coordinated by the sole nitrogenous ligand to the oxygen-evolving complex in certain states of photosystem II. The origin and evolution of these efforts is described.

Pulse electron paramagnetic resonance studies of the interaction of methanol with the S2 state of the Mn4O5Ca cluster of photosystem II

The binding of the substrate analogue methanol to the catalytic Mn4CaO5 cluster of the water-oxidizing enzyme photosystem II is known to alter the electronic structure properties of the oxygen-evolving complex without retarding O2-evolution under steady-state illumination conditions. We report the binding mode of (13)C-labeled methanol determined using 9.4 GHz (X-band) hyperfine sublevel-correlation (HYSCORE) and 34 GHz (Q-band) electron spin-echo electron nuclear double resonance (ESE-ENDOR) spectroscopies. These results are compared to analogous experiments on a mixed-valence Mn(III)Mn(IV) complex (2-OH-3,5-Cl2-salpn)2Mn(III)Mn(IV) (salpn = N,N'-bis(3,5-dichlorosalicylidene)-1,3-diamino-2-hydroxypropane) in which methanol ligates to the Mn(III) ion ( Larson et al. (1992) J. Am. Chem. Soc. , 114 , 6263 ). In the mixed-valence Mn(III,IV) complex, the hyperfine coupling to the (13)C of the bound methanol (Aiso = 0.65 MHz, T = 1.25 MHz) is appreciably larger than that observed for (13)C methanol associated with the Mn4CaO5 cluster poised in the S2 state, where only a weak dipolar hyperfine interaction (Aiso = 0.05 MHz, T = 0.27 MHz) is observed. An evaluation of the (13)C hyperfine interaction using the X-ray structure coordinates of the Mn4CaO5 cluster indicates that methanol does not bind as a terminal ligand to any of the manganese ions in the oxygen-evolving complex. We favor methanol binding in place of a water ligand to the Ca(2+) in the Mn4CaO5 cluster or in place of one of the waters that form hydrogen bonds with the oxygen bridges of the cluster.

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.

Participation of glutamate-333 of the D1 polypeptide in the ligation of the Mn₄CaO₅ cluster in photosystem II

In the 1.9 Å structural model of photosystem II (PDB: 3ARC), the amino acid residue Glu333 of the D1 polypeptide coordinates to the oxygen-evolving Mn₄CaO₅ cluster. This residue appears to be highly significant in that it bridges the two Mn ions (Mn(B3) and the "dangling" Mn(A4)) that are also bridged by the oxygen atom O5. This oxygen atom has been proposed to be derived from one of two substrate water molecules and to become incorporated into the product dioxygen molecule during the final step in the catalytic cycle. In addition, the backbone nitrogen of D1-Glu333 interacts directly with a nearby Cl⁻ atom. To further explore the influence of this structurally unique residue on the properties of the Mn₄CaO₅ cluster, the D1-E333Q mutant of the cyanobacterium Synechocystis sp. PCC 6803 was characterized with a variety of biophysical and spectroscopic methods, including polarography, EPR, X-ray absorption, and FTIR difference spectroscopy. The kinetics of oxygen release in the mutant were essentially unchanged from those in wild-type. In addition, the oxygen flash yields exhibited normal period-four oscillations having normal S state parameters, although the yields were lower, indicative of the mutant's lower steady-state dioxygen evolution rate of approximately 30% compared to that of the wild-type. The S₁ state Mn-XANES and Mn-EXAFS and S₂ state multiline EPR signals of purified D1-E333Q PSII core complexes closely resembled those of wild-type, aside from having lower amplitudes. The S(n+1)-minus-S(n) FTIR difference spectra showed only minor alterations to the carbonyl, amide, and carboxylate stretching regions. However, the mutation eliminated a negative peak at 3663 cm⁻¹ in the weakly H-bonding O-H stretching region of the S₂-minus-S₁ FTIR difference spectrum and caused an approximately 9 cm⁻¹ downshift of the negative feature in this region of the S₁-minus-S₀ FTIR difference spectrum. We conclude that fully functional Mn₄CaO₅ clusters assemble in the presence of the D1-E333Q mutation but that the mutation decreases the yield of assembled clusters and alters the H-bonding properties of one or more water molecules or hydroxide groups that are located on or near the Mn₄CaO₅ cluster and that either deprotonate or form stronger hydrogen bonds during the S₀ to S₁ and S₁ to S₂ transitions.

High-field EPR on membrane proteins - crossing the gap to NMR

In this review on advanced EPR spectroscopy, which addresses both the EPR and NMR communities, considerable emphasis is put on delineating the complementarity of NMR and EPR concerning the measurement of molecular interactions in large biomolecules. From these interactions, detailed information can be revealed on structure and dynamics of macromolecules embedded in solution- or solid-state environments. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultrafast electronics for signal data handling and processing have pushed to new horizons the limits of EPR spectroscopy and its multifrequency extensions concerning the sensitivity of detection, the selectivity with respect to interactions, and the resolution in frequency and time domains. One of the most important advances has been the extension of EPR to high magnetic fields and microwave frequencies, very much in analogy to what happens in NMR. This is exemplified by referring to ongoing efforts for signal enhancement in both NMR and EPR double-resonance techniques by exploiting dynamic nuclear or electron spin polarization via unpaired electron spins and their electron-nuclear or electron-electron interactions. Signal and resolution enhancements are particularly spectacular for double-resonance techniques such as ENDOR and PELDOR at high magnetic fields. They provide greatly improved orientational selection for disordered samples that approaches single-crystal resolution at canonical g-tensor orientations - even for molecules with small g-anisotropies. Exchange of experience between the EPR and NMR communities allows for handling polarization and resolution improvement strategies in an optimal manner. Consequently, a dramatic improvement of EPR detection sensitivity could be achieved, even for short-lived paramagnetic reaction intermediates. Unique structural and dynamic information is thus revealed that can hardly be obtained by any other analytical techniques. Micromolar quantities of sample molecules have become sufficient to characterize stable and transient reaction intermediates of complex molecular systems - offering highly interesting applications for chemists, biochemists and molecular biologists. In three case studies, representative examples of advanced EPR spectroscopy are reviewed: (I) High-field PELDOR and ENDOR structure determination of cation-anion radical pairs in reaction centers from photosynthetic purple bacteria and cyanobacteria (Photosystem I); (II) High-field ENDOR and ELDOR-detected NMR spectroscopy on the oxygen-evolving complex of Photosystem II; and (III) High-field electron dipolar spectroscopy on nitroxide spin-labelled bacteriorhodopsin for structure-function studies. An extended conclusion with an outlook to further developments and applications is also presented.

Ammonia binding to the oxygen-evolving complex of photosystem II identifies the solvent-exchangeable oxygen bridge (μ-oxo) of the manganese tetramer

The assignment of the two substrate water sites of the tetra-manganese penta-oxygen calcium (Mn4O5Ca) cluster of photosystem II is essential for the elucidation of the mechanism of biological O-O bond formation and the subsequent design of bio-inspired water-splitting catalysts. We recently demonstrated using pulsed EPR spectroscopy that one of the five oxygen bridges (μ-oxo) exchanges unusually rapidly with bulk water and is thus a likely candidate for one of the substrates. Ammonia, a water analog, was previously shown to bind to the Mn4O5Ca cluster, potentially displacing a water/substrate ligand [Britt RD, et al. (1989) J Am Chem Soc 111(10):3522-3532]. Here we show by a combination of EPR and time-resolved membrane inlet mass spectrometry that the binding of ammonia perturbs the exchangeable μ-oxo bridge without drastically altering the binding/exchange kinetics of the two substrates. In combination with broken-symmetry density functional theory, our results show that (i) the exchangable μ-oxo bridge is O5 {using the labeling of the current crystal structure [Umena Y, et al. (2011) Nature 473(7345):55-60]}; (ii) ammonia displaces a water ligand to the outer manganese (MnA4-W1); and (iii) as W1 is trans to O5, ammonia binding elongates the MnA4-O5 bond, leading to the perturbation of the μ-oxo bridge resonance and to a small change in the water exchange rates. These experimental results support O-O bond formation between O5 and possibly an oxyl radical as proposed by Siegbahn and exclude W1 as the second substrate water.

Use of electron paramagnetic resonance to solve biochemical problems

Electron paramagnetic resonance (EPR) spectroscopy is a very powerful biophysical tool that can provide valuable structural and dynamic information about a wide variety of biological systems. The intent of this review is to provide a general overview for biochemists and biological researchers of the most commonly used EPR methods and how these techniques can be used to answer important biological questions. The topics discussed could easily fill one or more textbooks; thus, we present a brief background on several important biological EPR techniques and an overview of several interesting studies that have successfully used EPR to solve pertinent biological problems. The review consists of the following sections: an introduction to EPR techniques, spin-labeling methods, and studies of naturally occurring organic radicals and EPR active transition metal systems that are presented as a series of case studies in which EPR spectroscopy has been used to greatly further our understanding of several important biological systems.

Biological water oxidation

Photosystem II (PSII), a multisubunit pigment-protein supercomplex found in cyanobacteria, algae, and plants, catalyzes a unique reaction in nature: the light-driven oxidation of water. Remarkable recent advances in the structural analysis of PSII now give a detailed picture of the static supercomplex on the molecular level. These data provide a solid foundation for future functional studies, in particular the mechanism of water oxidation and oxygen release. The catalytic core of the PSII is a tetramanganese-calcium cluster (Mn₄O₅Ca), commonly referred to as the oxygen-evolving complex (OEC). The function of the OEC rests on its ability to cycle through five metastable states (Si, i = 0-4), transiently storing four oxidizing equivalents, and in so doing, facilitates the four electron water splitting reaction. While the latest crystallographic model of PSII gives an atomic picture of the OEC, the exact connectivity within the inorganic core and the S-state(s) that the X-ray model represents remain uncertain. In this Account, we describe our joint experimental and theoretical efforts to eliminate these ambiguities by combining the X-ray data with spectroscopic constraints and introducing computational modeling. We are developing quantum chemical methods to predict electron paramagnetic resonance (EPR) parameters for transition metal clusters, especially focusing on spin-projection approaches combined with density functional theory (DFT) calculations. We aim to resolve the geometric and electronic structures of all S-states, correlating their structural features with spectroscopic observations to elucidate reactivity. The sequence of manganese oxidations and concomitant charge compensation events via proton transfer allow us to rationalize the multielectron S-state cycle. EPR spectroscopy combined with theoretical calculations provides a unique window into the tetramangenese complex, in particular its protonation states and metal ligand sphere evolution, far beyond the scope of static techniques such as X-ray crystallography. This approach has led, for example, to a detailed understanding of the EPR signals in the S₂-state of the OEC in terms of two interconvertible, isoenergetic structures. These two structures differ in their valence distribution and spin multiplicity, which has important consequences for substrate binding and may explain its low barrier exchange with solvent water. New experimental techniques and innovative sample preparations are beginning to unravel the complex sequence of substrate uptake/inclusion, which is coupled to proton release. The introduction of specific site perturbations, such as replacing Ca²⁺ with Sr²⁺, provides discrete information about the ligand environment of the individual Mn ions. In this way, we have identified a potential open coordination site for one Mn center, which may serve as a substrate binding site in the higher S-states, such as S₃ and S₄. In addition, we can now monitor the binding of the substrate water in the lower S-states (S₁ and S₂) using new EPR-detected NMR spectroscopies. These studies provided the first evidence that one of the substrates is subsumed into the complex itself and forms an oxo-bridge between two Mn ions. This result places important new restrictions on the mechanism of O-O bond formation. These new insights from nature's water splitting catalyst provide important criteria for the rational design of bioinspired synthetic catalysts.

Electronic structure of S(2) state of the oxygen-evolving complex of photosystem II studied by PELDOR

Photosynthetic water splitting is catalyzed by a Mn(4)CaO(5) cluster in photosystem II, whose structure was recently determined at a resolution of 1.9Å [Umena, Y. et al. 2011, Nature, 473:55-60]. To determine the electronic structure of the Mn(4)CaO(5) cluster, pulsed electron-electron double resonance (PELDOR) measurements were performed for the tyrosine residue Y(D)() and S(2) state signals with non-oriented and oriented photosystem II (PS II) samples. Based on these measurements, the spin density distributions were calculated by comparing with the experimental results. The best fitting parameters were obtained with a model in which Mn1 has a large positive projection, Mn3 has a small positive projection, and Mn2 and Mn4 have negative projections (the numbering of Mni (i=1-4) is based on the crystal structure at a 1.9Å resolution), which yielded spin projections of 1.97, -1.20, 1.19 and -0.96 for Mn1-4 ions. The results show that the Mn1 ion, which is coordinated by H332, D342 and E189, has a valence of Mn(III) in the S(2) state. The sign of the exchange interactions J(13) is positive, and the other signs are negative.