Photosynthetic Oxygen-evolving Complex Research Articles

Influence of Thiolate versus Alkoxide Ligands on the Stability of Crystallographically Characterized Mn(III)-Alkylperoxo Complexes.

The work described herein demonstrates the exquisite control that the inner coordination sphere of metalloenzymes and transition-metal complexes can have on reactivity. We report one of few crystallographically characterized Mn-peroxo complexes and show that the tight correlations between metrical and spectroscopic parameters, established previously by our group for thiolate-ligated RS-Mn(III)-OOR complexes, can be extended to include an alkoxide-ligated RO-Mn(III)-OOR complex. We show that the alkoxide-ligated RO-Mn(III)-OOR complex is an order of magnitude more stable (t1/2298K = 6730 s, kobs298K = 1.03 × 10-4 s-1) than its thiolate-ligated RS-Mn(III)-OOR derivative (t1/2293K = 249 s, k1293K = 2.78 × 10-3 s-1). Electronic structure calculations provide insight regarding these differences in stability. The highest occupied orbital of the thiolate-ligated derivative possesses significant sulfur character and π-backdonation from the thiolate competes with π-backdonation from the peroxo π*(O-O). DFT-calculated Mulliken charges show that the Mn ion Lewis acidity of alkoxide-ligated RO-Mn(III)-OOR (+0.451) is greater than that of thiolate-ligated RS-Mn(III)-OOR (+0.306), thereby facilitating π-backdonation from the antibonding peroxo π*(O-O) orbital and increasing its stability. This helps to explain why the photosynthetic oxygen-evolving Mn complex, which catalyzes O-O bond formation as opposed to cleavage, incorporates O- and/or N-ligands as opposed to cysS-ligands.

Evaluation of new low-valent computational models for the oxygen-evolving complex of photosystem II

The oxidation states of the Mn4CaO5 cluster of the photosynthetic oxygen-evolving complex remain controversial. New quantum chemical models for the dark-stable S1 state suggest that a low-valent Mn(III-IV-III-II) form, opposed to the commonly accepted high-valent Mn(III-IV-IV-III) assignment, agrees with available structural data. We examine the magnetic properties of the models and the consequences for electronic structure and catalytic progression of assuming a neutral second-sphere residue D1-His337. The low-valent model reproduces the experimental diamagnetic ground spin-state. However, the protonation assignment introduces a critical flaw because neutral His337 becomes itself the locus of subsequent oxidation, in contradiction to physiological catalytic progression.

Orientation of Ligand Field for Dangling Manganese in Photosynthetic Oxygen-Evolving Complex of Photosystem II.

The spin structure in the S2 state and the crystal structure of the manganese cluster of the oxygen-evolving complex of plant photosystem II were combined by the quantitative evaluation of the magnetic anisotropy of the g = 4 signal. The g-values of 3.93 and 4.13 were obtained for the g = 4 signal in the directions parallel and perpendicular to the membrane normal, respectively. The peak-to-peak separations were 270 and 420 G for the parallel and perpendicular orientations to the membrane, respectively. By comparison with the crystal structure, the z-axis of the zero-field splitting was ascribed to the direction of the dangling Mn connecting water oxygen, Mn4-O(W1), in the manganese cluster. The results give the first experimental evidence that the valence of the dangling Mn is Mn(III) in the S2 high-spin state. We showed that the strong exchange coupling of Mn4 to Mn3 was required for g = 4.1 spin state in the four-spin couplings, estimated as > ∼|-30 cm-1|, indicating that the present closed cubane model in quantum mechanics/molecular mechanics (QM/MM) calculation cannot explain the g = 4.1 spin structure. The onsite zero-field splitting of the dangling Mn was evaluated as -2.3 cm-1 under the strong antiferromagnetic couplings (-50 cm-1) with the dangling Mn to the cubane frame in the four coupled spin state. From the viewpoint of the arrangement of the Mn valences in the cluster, a closed cubane model is effective, but without a large structural deviation from the S1 state crystal structure.

Kα X-ray Emission Spectroscopy on the Photosynthetic Oxygen-Evolving Complex Supports Manganese Oxidation and Water Binding in the S3 State.

The unique manganese-calcium catalyst in photosystem II (PSII) is the natural paragon for efficient light-driven water oxidation to yield O2. The oxygen-evolving complex (OEC) in the dark-stable state (S1) comprises a Mn4CaO4 core with five metal-bound water species. Binding and modification of the water molecules that are substrates of the water-oxidation reaction is mechanistically crucial but controversially debated. Two recent crystal structures of the OEC in its highest oxidation state (S3) show either a vacant Mn coordination site or a bound peroxide species. For purified PSII at room temperature, we collected Mn Kα X-ray emission spectra of the S0, S1, S2, and S3 intermediates in the OEC cycle, which were analyzed by comparison to synthetic Mn compounds, spectral simulations, and OEC models from density functional theory. Our results contrast both crystallographic structures. They indicate Mn oxidation in three S-transitions and suggest additional water binding at a previously open Mn coordination site. These findings exclude Mn reduction and render peroxide formation in S3 unlikely.

Carbonate Ions Induce Highly Efficient Electrocatalytic Water Oxidation by Cobalt Oxyhydroxide Nanoparticles.

Synthetic models of oxygen evolving complex (OEC) are used not only to gain better understanding of the mechanism and the roles of cofactors for water oxidation in photosynthesis, but also as water oxidation catalysts to realize artificial photosynthesis, which is anticipated as a promising solar fuel production system. However, although much attention has been paid to the composition and structure of active sites for development of heterogeneous OEC models, the cofactors, which are essential for water oxidation by the photosynthetic OEC, remain little studied. The high activity of CoO(OH) nanoparticles for electrocatalytic water oxidation is shown to be induced by a CO32- cofactor. The possibility of CO32- ions acting as proton acceptors for O-O bond formation based on the proton-concerted oxygen atom transfer mechanism is proposed. The O-O bond formation is supposed to be accelerated due to effective proton acceptance by adjacent CO32- ions coordinated on the CoIV center in the intermediate, which is consistent with Michaelis-Menten-type kinetics and the significant H/D isotope effect observed in electrocatalysis.

Geometric and electronic structures of the synthetic Mn₄CaO₄ model compound mimicking the photosynthetic oxygen-evolving complex.

Water oxidation by photosystem II (PSII) converts light energy into chemical energy with the concomitant production of molecular oxygen, both of which are indispensable for sustaining life on Earth. This reaction is catalyzed by an oxygen-evolving complex (OEC) embedded in the huge PSII complex, and its mechanism remains elusive in spite of the extensive studies of the geometric and electronic structures. In order to elucidate the water-splitting mechanism, synthetic approaches have been extensively employed to mimic the native OEC. Very recently, a synthetic complex [Mn4CaO4(Bu(t)COO)8(py)(Bu(t)COOH)2] (1) closely mimicking the structure of the native OEC was obtained. In this study, we extensively examined the geometric, electronic and spin structures of 1 using the density functional theory method. Our results showed that the geometric structure of 1 can be accurately reproduced by theoretical calculations, and revealed many similarities in the ground valence and spin states between 1 and the native OEC. We also revealed two different valence states in the one-electron oxidized state of 1 (corresponding to the S2 state), which lie in the lower and higher ground spin states (S = 1/2 and S = 5/2), respectively. One remarkable difference between 1 and the native OEC is the presence of a non-negligible antiferromagnetic interaction between the Mn1 and Mn4 sites, which slightly influenced their ground spin structures (spin alignments). The major reason causing the difference can be attributed to the short Mn1-O5 and Mn1-Mn4 distances in 1. The introduction of the missing O4 atom and the reorientation of the Ca coordinating ligands improved the Mn1-O5 and Mn1-Mn4 distances comparable to the native OEC. These modifications will therefore be important for the synthesis of further advanced model complexes more closely mimicking the native OEC beyond 1.

'Photosystem II: the water splitting enzyme of photosynthesis and the origin of oxygen in our atmosphere'.

About 3 billion years ago an enzyme emerged which would dramatically change the chemical composition of our planet and set in motion an unprecedented explosion in biological activity. This enzyme used solar energy to power the thermodynamically and chemically demanding reaction of water splitting. In so doing it provided biology with an unlimited supply of reducing equivalents needed to convert carbon dioxide into the organic molecules of life while at the same time produced oxygen to transform our planetary atmosphere from an anaerobic to an aerobic state. The enzyme which facilitates this reaction and therefore underpins virtually all life on our planet is known as Photosystem II (PSII). It is a pigment-binding, multisubunit protein complex embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Today we have detailed understanding of the structure and functioning of this key and unique enzyme. The journey to this level of knowledge can be traced back to the discovery of oxygen itself in the 18th-century. Since then there has been a sequence of mile stone discoveries which makes a fascinating story, stretching over 200 years. But it is the last few years that have provided the level of detail necessary to reveal the chemistry of water oxidation and O-O bond formation. In particular, the crystal structure of the isolated PSII enzyme has been reported with ever increasing improvement in resolution. Thus the organisational and structural details of its many subunits and cofactors are now well understood. The water splitting site was revealed as a cluster of four Mn ions and a Ca ion surrounded by amino-acid side chains, of which seven provide direct ligands to the metals. The metal cluster is organised as a cubane structure composed of three Mn ions and a Ca2+ linked by oxo-bonds with the fourth Mn ion attached to the cubane. This structure has now been synthesised in a non-protein environment suggesting that it is a totally inorganic precursor for the evolution of the photosynthetic oxygen-evolving complex. In summary, the overall structure of the catalytic site has given a framework on which to build a mechanistic scheme for photosynthetic dioxygen generation and at the same time provide a blue-print and incentive to develop catalysts for artificial photo-electrochemical systems to split water and generate renewable solar fuels.

First Site-Specific Incorporation of a Noncanonical Amino Acid into the Photosynthetic Oxygen-Evolving Complex

In photosystem II (PSII), water is oxidized at the oxygen-evolving complex. This process occurs through a light-induced cycle that produces oxygen and protons. While coupled proton and electron transfer reactions play an important role in PSII and other proteins, direct detection of internal proton transfer reactions is challenging. Here, we demonstrate that the unnatural amino acid, 7-azatryptophan (7AW), has unique, pH-sensitive vibrational frequencies, which are sensitive markers of proton transfer. The intrinsically disordered, PSII subunit, PsbO, which contains a single W residue (Trp241), was engineered to contain 7AW at position 241. Fluorescence shows that 7AW-241 is buried in a hydrophobic environment. Reconstitution of 7AW(241)PsbO to PSII had no significant impact on oxygen evolution activity or flash-dependent protein dynamics. We conclude that directed substitution of 7AW into other structural domains is likely to provide a nonperturbative spectroscopic probe, which can be used to define internal proton pathways in PsbO.

Calcium and the Hydrogen-Bonded Water Network in the Photosynthetic Oxygen-Evolving Complex.

In photosynthesis, photosystem II evolves oxygen from water at a Mn4CaO5 cluster (OEC). Calcium is required for biological oxygen evolution. In the OEC, a water network, extending from the calcium to four peptide carbonyl groups, has recently been predicted by a high-resolution crystal structure. Here, we use carbonyl vibrational frequencies as reporters of electrostatic changes to test the presence of this water network. A single flash, oxidizing Mn(III) to Mn(IV) (the S1 to S2 transition), upshifted the frequencies of peptide C═O bands. The spectral change was attributable to a decrease in C═O hydrogen bonding. Strontium, which supports a lower level of steady state activity, also led to an oxidation-induced shift in C═O frequencies, but treatment with barium and magnesium, which do not support activity, did not. This work provides evidence that calcium maintains an electrostatically responsive water network in the OEC and shows that OEC peptide carbonyl groups can be used as solvatochromic markers.

Redox Control and Hydrogen Bonding Networks: Proton-Coupled Electron Transfer Reactions and Tyrosine Z in the Photosynthetic Oxygen-Evolving Complex

In photosynthetic oxygen evolution, redox active tyrosine Z (YZ) plays an essential role in proton-coupled electron transfer (PCET) reactions. Four sequential photooxidation reactions are necessary to produce oxygen at a Mn(4)CaO(5) cluster. The sequentially oxidized states of this oxygen-evolving cluster (OEC) are called the S(n) states, where n refers to the number of oxidizing equivalents stored. The neutral radical, YZ•, is generated and then acts as an electron transfer intermediate during each S state transition. In the X-ray structure, YZ, Tyr161 of the D1 subunit, is involved in an extensive hydrogen bonding network, which includes calcium-bound water. In electron paramagnetic resonance experiments, we measured the YZ• recombination rate, in the presence of an intact Mn(4)CaO(5) cluster. We compared the S(0) and S(2) states, which differ in Mn oxidation state, and found a significant difference in the YZ• decay rate (t(1/2) = 3.3 ± 0.3 s in S(0); t(1/2) = 2.1 ± 0.3 s in S(2)) and in the solvent isotope effect (SIE) on the reaction (1.3 ± 0.3 in S(0); 2.1 ± 0.3 in S(2)). Although the YZ site is known to be solvent accessible, the recombination rate and SIE were pH independent in both S states. To define the origin of these effects, we measured the YZ• recombination rate in the presence of ammonia, which inhibits oxygen evolution and disrupts the hydrogen bond network. We report that ammonia dramatically slowed the YZ• recombination rate in the S(2) state but had a smaller effect in the S(0) state. In contrast, ammonia had no significant effect on YD•, the stable tyrosyl radical. Therefore, the alterations in YZ• decay, observed with S state advancement, are attributed to alterations in OEC hydrogen bonding and consequent differences in the YZ midpoint potential/pK(a). These changes may be caused by activation of metal-bound water molecules, which hydrogen bond to YZ. These observations document the importance of redox control in proton-coupled electron transfer reactions.

Proton coupled electron transfer and redox-active tyrosine Z in the photosynthetic oxygen-evolving complex.

Proton coupled electron transfer (PCET) reactions play an essential role in many enzymatic processes. In PCET, redox-active tyrosines may be involved as intermediates when the oxidized phenolic side chain deprotonates. Photosystem II (PSII) is an excellent framework for studying PCET reactions, because it contains two redox-active tyrosines, YD and YZ, with different roles in catalysis. One of the redox-active tyrosines, YZ, is essential for oxygen evolution and is rapidly reduced by the manganese-catalytic site. In this report, we investigate the mechanism of YZ PCET in oxygen-evolving PSII. To isolate YZ(•) reactions, but retain the manganese-calcium cluster, low temperatures were used to block the oxidation of the metal cluster, high microwave powers were used to saturate the YD(•) EPR signal, and YZ(•) decay kinetics were measured with EPR spectroscopy. Analysis of the pH and solvent isotope dependence was performed. The rate of YZ(•) decay exhibited a significant solvent isotope effect, and the rate of recombination and the solvent isotope effect were pH independent from pH 5.0 to 7.5. These results are consistent with a rate-limiting, coupled proton electron transfer (CPET) reaction and are contrasted to results obtained for YD(•) decay kinetics at low pH. This effect may be mediated by an extensive hydrogen-bond network around YZ. These experiments imply that PCET reactions distinguish the two PSII redox-active tyrosines.

Differential proteomic analysis of Arabidopsis thaliana genotypes exhibiting resistance or susceptibility to the insect herbivore, Plutella xylostella.

A proteomic study was conducted to investigate physiological factors affecting feeding behaviour by larvae of the insect, Plutella xylostella, on herbivore-susceptible and herbivore-resistant Arabidopsis thaliana. The leaves of 162 recombinant inbred lines (Rils) were screened to detect genotypes upon which Plutella larvae fed least (P. xylostella-resistant) or most (P. xylostella-susceptible). 2D-PAGE revealed significant differences in the proteomes between the identified resistant and susceptible Rils. The proteomic results, together with detection of increased production of hydrogen peroxide in resistant Rils, suggest a correlation between P. xylostella resistance and the production of increased levels of reactive oxygen species (ROS), in particular H2O2, and that this was expressed prior to herbivory. Many of the proteins that were more abundant in the Plutella-resistant Rils are known in other biological systems to be involved in limiting ROS damage. Such proteins included carbonic anhydrases, malate dehydrogenases, glutathione S-transferases, isocitrate dehydrogenase-like protein (R1), and lipoamide dehydrogenase. In addition, patterns of germin-like protein 3 isoforms could also be indicative of higher levels of reactive oxygen species in the resistant Rils. Consistent with the occurrence of greater oxidative stress in the resistant Rils is the observation of greater abundance in susceptible Rils of polypeptides of the photosynthetic oxygen-evolving complex, which are known to be damaged under oxidative stress. The combined results suggest that enhanced production of ROS may be a major pre-existing mechanism of Plutella resistance in Arabidopsis, but definitive corroboration of this requires much further work.

Structural and Physical Characterization of Tetranuclear [MnII3MnIV] and [MnII2MnIII2] Valence-Isomer Manganese Complexes

Two tetranuclear Mn complexes with an average Mn oxidation state of +2.5 have been prepared. These valence isomers have been characterized by a combination of X-ray crystallography, X-ray absorption spectroscopy, and magnetic susceptibility. The Mn(II)3Mn(IV) tetramer has the Mn ions arranged in a distorted tetrahedron, with an S = 6 ground spin state, dominated by ferromagnetic exchange among the manganese ions. The Mn(II)2Mn(III)2 tetramer also has a distorted tetrahedral arrangement of Mn ions but shows magnetic behavior, suggesting that it is a single-molecule magnet. The X-ray absorption near-edge structure (XANES) spectra for the two complexes are similar, suggesting that, while Mn XANES has sufficient sensitivity to distinguish between trinuclear valence isomers (Alexiou et al. Inorg. Chem. 2003, 42, 2185), similar distinctions are difficult for tetranuclear complexes such as that found in the photosynthetic oxygen-evolving complex.

FTIR Detection of Structural Changes in a Histidine Ligand during S-State Cycling of Photosynthetic Oxygen-Evolving Complex

Changes in structural coupling between the Mn cluster and a putative histidine ligand during the S-state cycling of the oxygen-evolving complex (OEC) have been detected directly by Fourier transform infrared (FTIR) spectroscopy in photosystem (PS) II core particles from the cyanobacterium Synechocystis sp. PCC6803, in which histidine residues were selectively labeled with l-[(15)N(3)]histidine. The bands sensitive to the histidine-specific isotope labeling appeared at 1120-1090 cm(-)(1) in the spectra induced upon the first-, second-, and fourth-flash illumination, for the S(2)/S(1), S(3)/S(2), and S(1)/S(0) differences, at similar frequencies with different sign and/or intensity depending on the respective S-state transitions. However, no distinctive band was observed in the third-flash induced spectrum for the S(0)/S(3) difference. The results indicate that a single histidine residue coupled with the structural changes of the OEC during the S-state cycling is responsible for the observed histidine bands, in which the histidine modes changed during the S(0)-to-S(1) transition are reversed upon the S(1)-to-S(2) and S(2)-to-S(3) transitions. The 1186(+)/1178(-) cm(-)(1) bands affected by l-[(15)N(3)]histidine labeling were observed only for the S(2)/S(1) difference, but those affected by universal (15)N labeling appeared prominently showing a clear S-state dependency. Possible origins of these bands and changes in the histidine modes during the S-state cycling are discussed.

Studies on photosynthetic oxygen-evolving complex by means of Fourier transform infrared spectroscopy: calcium and chloride cofactors

Structural roles of functional Ca2+ and Cl- ions in photosynthetic oxygen-evolving complexes (OEC) were studied using low- (640-350 cm-1) and mid- (1800-1200 cm-1) frequency S2/S1 Fourier transform infrared (FTIR) difference spectroscopy. Studies using highly active Photosystem (PS) II core particles from spinach enabled the detection of subtle spectral changes. Ca2+-depleted and Ca2+-reconstituted particles produced very similar mid- and low-frequency spectra. The mid-frequency spectrum was not affected by reconstitution with 44Ca isotope. In contrast, Sr2+-substituted particles showed unique spectral changes in the low-frequency Mn-O-Mn mode at 606 cm-1 as well as in the mid-frequency carboxylate stretching modes. The mid-frequency spectrum of Cl- -depleted OEC exhibited marked changes in the carboxylate stretching modes and the suppression of protein modes compared with that of Cl- -reconstituted OEC. However, Cl- -depletion did not exert significant effects on the low-frequency spectrum.

Structural Changes of D1 C-terminal α-Carboxylate during S-state Cycling in Photosynthetic Oxygen Evolution

Changes in the chemical structure of alpha-carboxylate of the D1 C-terminal Ala-344 during S-state cycling of photosynthetic oxygen-evolving complex were selectively measured using light-induced Fourier transform infrared (FTIR) difference spectroscopy in combination with specific [(13)C]alanine labeling and site-directed mutagenesis in photosystem II core particles from Synechocystis sp. PCC 6803. Several bands for carboxylate symmetric stretching modes in an S(2)/S(1) FTIR difference spectrum were affected by selective (13)C labeling of the alpha-carboxylate of Ala with l-[1-(13)C]alanine, whereas most of the isotopic effects failed to be induced in a site-directed mutant in which Ala-344 was replaced with Gly. Labeling of the alpha-methyl of Ala with l-[3-(13)C]alanine had much smaller effects on the spectrum to induce isotopic bands due to a symmetric CH(3) deformation coupled with the alpha-carboxylate. The isotopic bands for the alpha-carboxylate of Ala-344 showed characteristic changes during S-state cycling. The bands appeared prominently upon the S(1)-to-S(2) transition and to a lesser extent upon the S(2)-to-S(3) transition but reappeared at slightly upshifted frequencies with the opposite sign upon the S(3)-to-S(0) transition. No obvious isotopic band appeared upon the S(0)-to-S(1) transition. These results indicate that the alpha-carboxylate of C-terminal Ala-344 is structurally associated with a manganese ion that becomes oxidized upon the S(1)-to-S(2) transition and reduced reversely upon the S(3)-to-S(0) transition but is not associated with manganese ion(s) oxidized during the S(0)-to-S(1) (and S(2)-to-S(3)) transition(s). Consistently, l-[1-(13)C]alanine labeling also induced spectral changes in the low frequency (670-350 cm(-1)) S(2)/S(1) FTIR difference spectrum.

Impact of Replacement of D1 C-terminal Alanine with Glycine on Structure and Function of Photosynthetic Oxygen-evolving Complex

The C-terminal alanine 344 (Ala-344) in the D1 protein of photosystem II is conserved in all of the organisms performing oxygenic photosynthesis. A free alpha-COO(-) of Ala-344 has been proposed to be responsible for ligating the Mn cluster. Here, we constructed a mutant having D1 in which D1-Ala-344 was replaced with glycine (Gly) in cyanobacterium Synechocystis sp. PCC 6803. The effects of this minimal change in the side group from methyl to hydrogen on the properties of the oxygen-evolving complex were comprehensively investigated using purified core particles. The mutant grew photoautotrophically, and little change was observed in the protein composition of the oxygen-evolving core particles. The Gly-substituted oxygen-evolving complex showed small but normal S(2) multiline and enhanced g = 4.1 electron spin resonance signals and S(2)-state thermoluminescence bands with slightly elevated peak temperature. The Gly substitution resulted in distinct but relatively small changes in a few bands arising from the putative carboxylate ligand for the Mn cluster in the mid-frequency (1800-1000 cm(-1)) S(2)/S(1) Fourier transform infrared difference spectrum. In contrast, the low frequency (670-350 cm(-1)) S(2)/S(1) Fourier transform infrared difference spectrum was markedly changed by the substitution. The results indicate that the internal structure of the Mn cluster and/or the interaction between the Mn cluster and its ligand are considerably altered by a simple change in the side group, from methyl to hydrogen, at the C-terminal of the D1 protein.

XANES evidence against a manganyl species in the S3 state of the oxygen-evolving complex.

Manganyl Mn=O species have been suggested as possible intermediates in photosynthetic water oxidation and as the reactive species in asymmetric olefin epoxidation. The first X-ray absorption spectrum for a MnV=O complex is reported. Comparison of the EXAFS data for Na[MnV=O(HMPAB) with those for lower-valent Mn complexes suggests that EXAFS measurements and edge-energy measurements are unlikely to have sufficient sensitivity to reliably reflect the presence of Mn=O species. In contrast, the preedge transition for Na[MnV=O(HMPAB) is 4-fold more intense than the most intense preedge transition observed for nonmanganyl complexes. This increase in intensity is shown to be sufficiently sensitive to allow detection of a manganyl species if it is formed. These data together with published data for the photosynthetic oxygen-evolving complex provide strong evidence against the presence of manganyl Mn=O species in the oxygen-evolving complex through the S3 state.

Chloride ligation in inorganic manganese model compounds relevant to photosystem II studied using X-ray absorption spectroscopy.

Chloride ions are essential for proper function of the photosynthetic oxygen-evolving complex (OEC) of Photosystem II (PS II). Although proposed to be directly ligated to the Mn cluster of the OEC, the specific structural and mechanistic roles of chloride remain unresolved. This study utilizes X-ray absorption spectroscopy (XAS) to characterize the Mn-Cl interaction in inorganic compounds that contain structural motifs similar to those proposed for the OEC. Three sets of model compounds are examined; they possess core structures Mn(IV)(3)O(4)X (X=Cl, F, or OH) that contain a di-micro-oxo and two mono-micro-oxo bridges or Mn(IV)(2)O(2)X (X=Cl, F, OH, OAc) that contain a di-micro-oxo bridge. Each set of compounds is examined for changes in the XAS spectra that are attributable to the replacement of a terminal OH or F ligand, or bridging OAc ligand, by a terminal Cl ligand. The X-ray absorption near edge structure (XANES) shows changes in the spectra on replacement of OH, OAc, or F by Cl ligands that are indicative of the overall charge of the metal atom and are consistent with the electronegativity of the ligand atom. Fourier transforms (FTs) of the extended X-ray absorption fine structure (EXAFS) spectra reveal a feature that is present only in compounds where chloride is directly ligated to Mn. These FT features were simulated using various calculated Mn-X interactions (X=O, N, Cl, F), and the best fits were found when a Mn-Cl interaction at a 2.2-2.3 A bond distance was included. There are very few high-valent Mn halide complexes that have been synthesized, and it is important to make such a comparative study of the XANES and EXAFS spectra because they have the potential for providing information about the possible presence or absence of halide ligation to the Mn cluster in PS II.

Functional and structural study on chelator-induced suppression of S 2/S 1 FTIR spectrum in photosynthetic oxygen-evolving complex

Chelating agents have been shown to induce characteristic changes in the light-minus-dark Fourier transform infrared (FTIR) difference spectrum for the S 2/S 1 difference in the oxygen-evolving complex (OEC). Addition of various ethylenediamine- N, N, N′, N′-tetraacetic acid (EDTA)-type chelators, such as EDTA, O,O′-bis(2-aminoethyl)ethyleneglycol- N, N, N′, N′-tetraacetic acid (EGTA), trans-1,2-diaminocyclohexane- N, N, N′, N′-tetraacetic acid (CyDTA), or N-(2-hydroxyethyl)ethylenediamine -N, N′, N′-triacetic acid (HEDTA), to Ca 2+-depleted PS II membranes resulted in the suppression of typical S 2/S 1 vibrational features, including the symmetric (1365(+)/1404(−) cm −1) and the asymmetric (1587(+)/1566(−) cm −1) carboxylate stretching vibrations, as well as the amide I and II modes of the backbone polypeptides. In contrast, the addition of ethylenediamine- N, N′-diacetic acid (EDDA) showed less inhibitory effects. The effects of the chelators depended on the number of the carboxylate groups; chelators with more than two carboxymethyl groups were effective in altering the FTIR spectrum. The bridging structure that connects the two nitrogen atoms also influenced the inhibitory effects. However, the effects were not necessarily correlated with the stability constants of the chelators to Mn 2+. The vibrational modes that were suppressed by EDTA were almost completely restored by subsequent washing with Chelex-treated Ca 2+-free buffer medium, indicating that the spectral changes are attributable to the reversible association of chelators with the Ca 2+-depleted OEC. Nevertheless, prolonged incubation with chelators led to the impairment of the O 2-evolving capability, with differences in the effectiveness, in the order that is consistent with that for the suppression effects on FTIR spectra. Chelators with carboxylate and/or carboxymethyl groups bound to a single nitrogen [nitrilotriacetic acid (NTA) and iminodiacetic acid (IDA)] or carbon (citric acid) were relatively ineffective for the suppression. A chelator that includes four phosphate groups, ethylenediamine -N, N, N′, N′-tetrakis(methylenephosphonic) acid (EDTPO), also showed suppression effects on both the carboxylate and amide modes. Based on these findings, a possible mode of interaction between the chelators and the Mn cluster is discussed.