The g = 2 multiline EPR signal of the S2 state of the photosynthetic oxygen-evolving complex originates from a ground spin state

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.

Tetranuclear [MnIIIMn3IVO4] Complexes as Spectroscopic Models of the S2 State of the Oxygen Evolving Complex in Photosystem II

Despite extensive biochemical, spectroscopic, and computational studies, the mechanism of biological water oxidation by the oxygen evolving complex (OEC) of Photosystem II remains a subject of significant debate. Mechanistic proposals are guided by the characterization of reaction intermediates such as the S₂ state, which features two characteristic EPR signals at g = 2 and g = 4.1. Two nearly isoenergetic structural isomers have been proposed as the source of these distinct signals, but relevant structure-electronic structure studies remain rare. Herein, we report the synthesis, crystal structure, electrochemistry, XAS, magnetic susceptibility, variable temperature CW-EPR, and pulse EPR data for a series of [Mn^IIIMn₃^IVO₄] cuboidal complexes as spectroscopic models of the S₂ state of the OEC. Resembling the oxidation state and EPR spectra of the S₂ state of the OEC, these model complexes show two EPR signals, a broad low field signal and a multiline signal, that are remarkably similar to the biological system. The effect of systematic changes in the nature of the bridging ligands on spectroscopy were studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Our model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes need to be carefully assessed.

Investigating the Secondary Structure of Membrane Peptides Utilizing Multiple 2H-Labeled Hydrophobic Amino Acids via Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy

An electron spin echo envelope modulation (ESEEM) approach was used to probe local secondary structures of membrane proteins and peptides. This ESEEM method detects dipolar couplings between ²H-labeled nuclei on the side chains of an amino acid (Leu or Val) and a strategically placed nitroxide spin-label in the proximity up to 8 Å. ESEEM spectra patterns for different samples correlate directly to the periodic structural feature of different secondary structures. Since this pattern can be affected by the side chain length and flexibility of the ²H-labeled amino acid used in the experiment, it is important to examine several different hydrophobic amino acids (d₃ Ala, d₈ Val, d₈ Phe) utilizing this ESEEM approach. In this work, a series of ESEEM data were collected on the AChR M2δ membrane peptide to build a reference for the future application of this approach for various biological systems. The results indicate that, despite the relative intensity and signal-to-noise level, all amino acids share a similar ESEEM modulation pattern for α-helical structures. Thus, all commercially available ²H-labeled hydrophobic amino acids can be utilized as probes for the further application of this ESEEM approach. Also, the ESEEM signal intensities increase as the side chain length gets longer or less rigid. In addition, longer side chain amino acids had a larger ²H ESEEM FT peak centered at the ²H Larmor frequency for the i ± 4 sample when compared to the corresponding i ± 3 sample. For shorter side chain amino acids, the ²H ESEEM FT peak intensity ratio between i ± 4 and i ± 3 was not well-defined.

Structural changes in the oxygen-evolving complex of photosystem II induced by the S1 to S2 transition: A combined XRD and QM/MM study

The S₁ → S₂ transition of the oxygen-evolving complex (OEC) of photosystem II does not involve the transfer of a proton to the lumen and occurs at cryogenic temperatures. Therefore, it is commonly thought to involve only Mn oxidation without any significant change in the structure of the OEC. Here, we analyze structural changes upon the S₁ → S₂ transition, as revealed by quantum mechanics/molecular mechanics methods and the isomorphous difference Fourier method applied to serial femtosecond X-ray diffraction data. We find that the main structural change in the OEC is in the position of the dangling Mn and its coordination environment.

EPR spectroscopy of the manganese cluster of photosystem II

Electron paramagnetic resonance (EPR) spectroscopy is a valuable tool for understanding the oxidation state and chemical environment of the Mn4Ca cluster of photosystem II. Since the discovery of the multiline signal from the S2 state, EPR spectroscopy has continued to reveal details about the catalytic center of oxygen evolution. At present EPR signals from nearly all of the S-states of the Mn4Ca cluster, as well as from modified and intermediate states, have been observed. This review article describes the various EPR signals obtained from the Mn4Ca cluster, including the metalloradical signals due to interaction of the cluster with a nearby organic radical.

Towards a spin coupling model for the Mn4 cluster in Photosystem II

The X-band EPR spectra of the IR sensitive untreated PSII and of MeOH- and NH(3)-treated PSII from spinach in the S(2)-state are simulated with collinear and rhombic g- and Mn-hyperfine tensors. The obtained principal values indicate a 1Mn(III)3Mn(IV) composition for the Mn(4) cluster. The four isotropic components of the Mn-hyperfine tensors are found in good agreement with the previously published values determined from EPR and (55)Mn-ENDOR data. Assuming intrinsic isotropic components of the Mn-hyperfine interactions identical to those of the Mn-catalase, spin density values are calculated. A Y-shape 4J-coupling scheme is explored to reproduce the spin densities for the untreated PSII. All the required criteria such as a S=1/2 ground state with a low lying excited spin state (30 cm(-1)) and an easy conversion to a S=5/2 system responsible for the g=4.1 EPR signal are shown to be satisfied with four antiferromagnetic interactions lying between -290 and -130 cm(-1).

EPR studies of the water oxidizing complex in the S1 and the higher S states: the manganese cluster and Y(Z) radical

The parallel polarization electron paramagnetic resonance (EPR) method has been applied to investigate manganese EPR signals of native S1 and S3 states of the water oxidizing complex (WOC) in photosystem (PS) II. The EPR signals in both states were assigned to thermally excited states with S=1, from which zero-field interaction parameters D and E were derived. Three kinds of signals, the doublet signal, the singlet-like signal and g=11-15 signal, were detected in Ca2+-depleted PS II. The g=11-15 signal was observed by parallel and perpendicular modes and assigned to a higher oxidation state beyond S2 in Ca2+-depleted PS II. The singlet-like signal was associated with the g=11-15 signal but not with the Y(Z) (the tyrosine residue 161 of the D1 polypeptide in PS II) radical. The doublet signal was associated with the Y(Z) radical as proved by pulsed electron nuclear double resonance (ENDOR) and ENDOR-induced EPR. The electron transfer mechanism relevant to the role of Y(Z) radical was discussed.

Comparative studies of the S0 and S2 multiline electron paramagnetic resonance signals from the manganese cluster in Photosystem II

Electron paramagnetic resonance (EPR) spectroscopy is one of the major techniques used to analyse the structure and function of the water oxidising complex (WOC) in Photosystem II. The discovery of an EPR signal from the S0 state has opened the way for new experiments, aiming to characterise the S0 state and elucidate the differences between the different S states. We present a review of the biochemical and biophysical characterisation of the S0 state multiline signal that has evolved since its discovery, and compare these results to previous and recent data from the S2 multiline signal. We also present some new data from the S2 state reached on the second turnover of the enzyme.

EPR and ENDOR studies of the water oxidizing complex of Photosystem II

A comparative study of X-band EPR and ENDOR of the S2 state of photosystem II membrane fragments and core complexes in the frozen state is presented. The S2 state was generated either by continuous illumination at T=200 K or by a single turn-over light flash at T=273 K yielding entirely the same S2 state EPR signals at 10 K. In membrane fragments and core complex preparations both the multiline and the g=4.1 signals were detected with comparable relative intensity. The absence of the 17 and 23 kDa proteins in the core complex preparation has no effect on the appearance of the EPR signals. (1)H-ENDOR experiments performed at two different field positions of the S2 state multiline signal of core complexes permitted the resolution of four hyperfine (hf) splittings. The hf coupling constants obtained are 4.0, 2.3, 1.1 and 0.6 MHz, in good agreement with results that were previously reported (Tang et al. (1993) J Am Chem Soc 115: 2382-2389). The intensities of all four line pairs belonging to these hf couplings are diminished in D2O. A novel model is presented and on the basis of the two largest hfc's distances between the manganese ions and the exchangeable protons are deduced. The interpretation of the ENDOR data indicates that these hf couplings might arise from water which is directly ligated to the manganese of the water oxidizing complex in redox state S2.

Simulation of the S2 state multiline electron paramagnetic resonance signal of photosystem II: a multifrequency approach

The S2 state electron paramagnetic resonance (EPR) multiline signal of Photosystem II has been simulated at Q-band (35 Ghz), X-band (9 GHz) and S-band (4 GHz) frequencies. The model used for the simulation assumes that the signal arises from an essentially magnetically isolated MnIII-MnIV dimer, with a ground state electronic spin ST = 1/2. The spectra are generated from exact numerical solution of a general spin Hamiltonian containing anisotropic hyperfine and quadrupolar interactions at both Mn nuclei. The features that distinguish the multiline from the EPR spectra of model manganese dimer complexes (additional width of the spectrum (195 mT), additional peaks (22), internal "superhyperfine" structure) are plausibly explained assuming an unusual ligand geometry at both Mn nuclei, giving rise to normally forbidden transitions from quadrupole interactions as well as hyperfine anisotropy. The fitted parameters indicate that the hyperfine and quadrupole interactions arise from Mn ions in low symmetry environments, corresponding approximately to the removal of one ligand from an octahedral geometry in both cases. For a quadrupole interaction of the magnitude indicated here to be present, the MnIII ion must be 5-coordinate and the MnIV 5-coordinate or possibly have a sixth, weakly bound ligand. The hyperfine parameters indicate a quasi-axial anisotropy at MnIII, which while consistent with Jahn-Teller distortion as expected for a d4 ion, corresponds here to the unpaired spin being in the ligand deficient, z direction of the molecular reference axis. The fitted parameters for MnIV are very unusual, showing a high degree of anisotropy not expected in a d3 ion. This degree of anisotropy could be qualitatively accounted for by a histidine ligand providing pi backbonding into the metal dxy orbital, together with a weakly bound or absent ligand in the x direction.

Temperature-dependent pulsed electron paramagnetic resonance studies of the S2 state multiline signal of the photosynthetic oxygen-evolving complex

The electron spin-lattice relaxation rate (1/T1) of the g = 2 "multiline" manganese electron paramagnetic resonance (EPR) signal arising from the photosystem II oxygen-evolving complex poised in the S2 state has been directly measured over the temperature range of 4.2-11 K via the inversion-recovery pulsed EPR technique. The electron spin echo amplitude of the g = 2 "multiline" signal varies inversely with temperature over this range, indicating a ground spin state Curie law behavior in agreement with our previously reported work [Britt et al. (1992) Biochim. Biophys. Acta 1140, 95-101]. Results of a plot of the natural log of the electron spin-lattice relaxation rate versus reciprocal temperature are consistent with an Orbach mechanism serving as the dominant relaxation pathway for the "multiline" signal in this temperature range. The slope of the plot indicates that an excited spin state manifold exists 36.5 cm-1 above the ground-state manifold that gives rise to the "multiline" signal.

Angular orientation of the stable tyrosyl radical within photosystem II by high-field 245-GHz electron paramagnetic resonance

The 4 K 245-GHz/8.7-T electron paramagnetic resonance spectrum of the stable tyrosyl radical in photosystem II, known as TyrD., has been measured. Illumination at 200 K enhances the signal intensity of TyrD. by a factor of > 40 compared to the signal obtained from dark-adapted samples. This signal enhancement and the unusual line shape of the TyrD. resonance result from the magnetic dipolar coupling of the radical to the manganese cluster involved in oxygen evolution. The relative angular orientation of the manganese cluster with respect to TyrD. has been determined from line-shape analysis. The resonance arising from TyrD. in Tris-washed manganese-free photosystem II sample is also distorted. This effect probably originates from the influence of the nonheme iron on the spin relaxation of the tyrosyl radical. The relative angular orientation of the nonheme iron has also been determined. Oriented samples were used to determine the angular orientation of TyrD. with respect to the membrane plane. Combining angular data with published distances, we have constructed a three-dimensional picture of the relative positions of TyrD., the manganese cluster, and the nonheme iron. The data suggest a more symmetrical placement of the manganese relative to TyrD. and TyrZ, the tyrosine involved in electron transfer, than is usually assumed in current models of photosystem II.

Electron spin echo envelope modulation spectroscopy of the molybdenum center of xanthine oxidase

The pulsed EPR technique of electron spin echo envelope modulation (ESEEM) has been utilized to examined both the 'very rapid' and 'desulfo inhibited' Mo(V) signals of xanthine oxidase in order to probe for magnetic interactions with nitrogen, phosphorus and hydrogen nuclei. No 14N modulation is observed in the 'desulfo inhibited' EPR signal, indicating that histidine is unlikely to be a ligand to molybdenum. Strong 14N modulation is observed in the 'very rapid' EPR signal formed with 2-hydroxy-6-methylpurine substrate bound to molybdenum. We interpret this modulation as arising from nitrogens of the bound purine substrate. This interpretation is consistent with the present evidence indicating that the purine ring present in the species giving rise to the 'very rapid' EPR signal is coordinated to the molybdenum center through the catalytically introduced hydroxyl group. No modulation is observed from non-exchangeable deuterons in experiments performed with deuterated 2-hydroxy-6-methylpurine. Given the signal-to-noise level of the spectra, the lack of modulation indicates that each of the substrate methyl group deuterons is greater than 4.9 A from the Mo(V). The deuteron removed from the C8 position in the binding of the substrate is also exchanged to a site or sites greater than 4.9 A from the Mo(V) in the time-course of sample preparation. Moderately deep deuteron modulation arises from exchangeable sites. A large portion of this modulation can be accounted for by the exchangeable N7 deuteron of the 2-hydroxy-6-methylpurine substrate, which we estimate to be approximately 3.2 A from the molybdenum. Additional exchangeable deuterons on the protein or within the buffer must be present within 5 A of the molybdenum to account for the remaining modulation. No modulation from weakly-coupled 31P nuclei is observed in either the 'desulfo inhibited' or 'very rapid' EPR signal.

The manganese center of oxygen-evolving and Ca(2+)-depleted photosystem II: a pulsed EPR spectroscopy study

The environment of the multi-manganese center in the O2-evolving complex (OEC) of plant photosystem II (PS II) under conditions of Ca2+ depletion has been probed using pulsed electron paramagnetic resonance (EPR) spectroscopy, and the following results are reported: (1) In Ca(2+)-depleted PS II membranes treated with the chelator [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA), the modified Mn EPR signal arising from the OEC in the S2 state and the split EPR signal from the S3 state could be detected in the absorption mode by recording the amplitude of a two-pulse echo as a function of the external magnetic field. The formation of the S3 signal (g approximately 2.004; delta Hpp = 164 G) is not accompanied by the disappearance of the Mn EPR signal, although the signal becomes difficult to detect in CW EPR. This result supports the previous interpretation of the split S3 EPR signal as arising from the interaction of an organic radical with the Mn cluster [Boussac, A., Zimmermann, J. L., Rutherford, A. W., & Lavergne, J. (1990) Nature 347, 303-306]. (2) The two-pulse electron spin echo envelope modulation (ESEEM) spectra of the S2 state formed in Ca(2+)-depleted PS II membranes obtained from 14N- and 15N-labeled material are different. This indicates that nitrogen nuclei from nitrogen-containing protein residues are coupled to the Mn center in the S2 state of the inhibited enzyme. In addition, comparison with the two-pulse ESEEM data obtained for the S2 state in the untreated enzyme suggests that the coupling may be altered by the Ca2+ depletion and/or EGTA treatment. (3) The treatment of Ca(2+)-depleted PS II membranes with sodium pyrophosphate also induced a stable S2 state characterized by a modified multiline EPR signal that is similar to that obtained in EGTA-treated PS II membranes. Comparison of the ESEEM data obtained for the pyrophosphate and 14N and 15N samples treated with EGTA suggests that the modification induced by the EGTA treatment is accompanied by the binding of (an) EGTA molecule(s) to or near the Mn center. (4) ESEEM data obtained for the S3 state formed in the pyrophosphate or EGTA-treated enzyme are quite similar to those obtained for the corresponding S2 state. The data are also compared with ESEEM data obtained on oxidized 4(5)-methylimidazole obtained by UV irradiation. These results are discussed with respect to the current assignment of the S3 radical as arising from oxidation of a histidine residue.

Perspectives on the structure of the photosynthetic oxygen evolving manganese complex and its relation to the Kok cycle

This review describes the progress in our understanding of the structure of the Mn complex in Photosystem II over the last two decades. Emphasis is on the research from our laboratory, especially the results from X-ray absorption spectroscopy, low temperature electron paramagnetic resonance and electron spin echo envelope modulation studies. The importance of the interplay between electron paramagnetic resonance studies and X-ray absorption studies, which has led to a description of the oxidation states of manganese as the enzyme cycles through the Kok cycle, is described. Finally, the path, by which our group has utilized these two important methods to arrive at a working structural model for the manganese complex that catalyzes the oxidation of water to dioxygen in higher plants and cyanobacteria, is explained.