[Show abstract][Hide abstract] ABSTRACT: Tyrosine D (TyrD) is one of two well-studied redox active tyrosines in Photosystem II. TyrD shows in contrast to its homologue, TyrZ, much slower redox kinetics and is normally present as a stable deprotonated radical (TyrD•). We have used time resolved CW-EPR and ESEEM spectroscopy to show that deuterium exchangeable protons can access TyrD on a time scale that is much faster (50-100 times) than previously observed. The time of H/D exchange is strongly dependent on the redox state of TyrD. This finding can be related to a change in position of a water molecule close to TyrD.
[Show abstract][Hide abstract] ABSTRACT: The coordination compound of Ru(II) with three 2,2’-bipyridine ligands possesses a potent photosensitization capacity for electron- and energy-transfer processes. In combination with salts of peroxydisulfate acid as sacrificial electron acceptors, Ru(bpy)32+ is widely used for photocatalytic oxidative transformations in organic synthesis and water splitting. The drawback of this system is that bipyridine degrades in the resulting strongly oxidative conditions, the concentration of Ru(bpy)32+ diminishes, and the photocatalytic reaction eventually stops. A commonly employed assay for the determination of the Ru(bpy)32+, UV-Vis spectroscopy, has low selectivity and does not distinguish between the intact complex and its decayed forms. Here, we report a matrix assisted laser desorption/ionisation mass spectrometric method for quantitative analysis of Ru(bpy)32+ in photochemical reaction mixtures. The developed method was successfully used for the determination of intact Ru(bpy)32+ during the course of the water photooxidation reaction. The significant difference between the results of MALDI MS and UV-Vis analyses was observed.
[Show abstract][Hide abstract] ABSTRACT: We have earlier shown that all electron transfer reactions in Photosystem II are operational up to 800 nm at room temperature [Thapper et al. (2009), Plant Cell 21, 2391-2401]. This led us to suggest an alternative charge separation pathway for far-red excitation. Here we extend these studies to very low temperature (5 K). Illumination of photosystem II (PS II) with visible light at 5 K is known to result in oxidation of almost similar amounts of YZ and the Cyt b559/ChlZ/CarD2 pathway. This is reproduced here using laser flashes at 532 nm and we find the partition ratio between the two pathways to be 1:0.8 at 5 K (the partition ratio is here defined as (yield of YZ/CaMn4 oxidation):(yield of Cyt b559/ChlZ/CarD2 oxidation)). The result using far red laser flashes is very different. We find partition ratios of 1.8 at 730 nm; 2.7 at 740 nm and >2.7 at 750 nm. No photochemistry involving these pathways is observed above 750 nm at this temperature. Thus, far-red illumination preferentially oxidizes YZ while the Cyt b559/ChlZ/CarD2 pathway is hardly touched. We propose that the difference in the partition ratio between visible and far-red light at 5 K reflects the formation of different first stable charge pair. In visible light, the first stable charge pair is considered to be PD1+Qa-. In contrast, we propose that the electron hole is residing on the ChlD1 molecule after illumination by far red at light 5 K resulting in the first stable charge pair being ChlD1+QA-. ChlD1 is much closer to YZ (11.3 Å) than to any component in Cyt b559/ChlZ/CarD2 pathway (closest distance is ChlD1 - CarD2 is 28.8 Å). This would then explain that far-red illumination preferentially drives efficient electron transfer from YZ. We also discuss mechanisms to account for the absorption of the far-red light and the existence of a hitherto unobserved charge transfer states. The involvement of two or more of the porphyrin molecules in the core of the Photosystem II reaction center is proposed.
[Show abstract][Hide abstract] ABSTRACT: Singlet oxygen, a harmful reactive oxygen species, can be quantified with the substance 2,2,6,6-tetramethylpiperidine (TEMP) that reacts with singlet oxygen, forming a stable nitroxyl radical (TEMPO). TEMPO has earlier been quantified with electron paramagnetic resonance (EPR) spectroscopy. In the present study, we designed an ultra-high-performance liquid chromatographic – tandem mass spectrometric (UHPLC-ESI-MS/MS) quantification method for TEMPO and showed that the method based on multiple reaction monitoring (MRM) can be used for the measurements of singlet oxygen from both non-biological and biological samples. Results obtained with both UHPLC-ESI-MS/MS and EPR methods suggest that plant thylakoid membranes produce 3.7 x 10-7 molecules of singlet oxygen per chlorophyll molecule in a second when illuminated with the photosynthetic photon flux density of 2000 μmol m-2s-1.This article is protected by copyright. All rights reserved.
Photochemistry and Photobiology 05/2014; · 2.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two types of manganese oxides have been prepared by hydrolysis of tetranuclear Mn(iii) complexes in the presence or absence of phosphate ions. The oxides have been characterized structurally using X-ray absorption spectroscopy and functionally by O2 evolution measurements. The structures of the oxides prepared in the absence of phosphate are dominated by di-μ-oxo bridged manganese ions that form layers with limited long-range order, consisting of edge-sharing MnO6 octahedra. The average manganese oxidation state is +3.5. The structure of these oxides is closely related to other manganese oxides reported as water oxidation catalysts. They show high oxygen evolution activity in a light-driven system containing [Ru(bpy)3](2+) and S2O8(2-) at pH 7. In contrast, the oxides formed by hydrolysis in the presence of phosphate ions contain almost no di-μ-oxo bridged manganese ions. Instead the phosphate groups are acting as bridges between the manganese ions. The average oxidation state of manganese ions is +3. This type of oxide has much lower water oxidation activity in the light-driven system. Correlations between different structural motifs and the function as a water oxidation catalyst are discussed and the lower activity in the phosphate containing oxide is linked to the absence of protonable di-μ-oxo bridges.
Physical Chemistry Chemical Physics 03/2014; · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A novel approach to anchor a molecular photosensitizer onto a heterogeneous water oxidation catalyst via coordination bonds is presented. A photosensitizer () based on [Ru(bpy)3](2+) and decorated with two methylenediphosphonate (M2P) groups has been designed and synthesized for this purpose. The M2P groups in complex allow for coordination of cobalt ions to afford a novel molecular-heterogeneous hybrid material . Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize as an amorphous, non-uniform material that contains Ru and Co in a ratio of 1 : 2. A suspension of in a buffered aqueous solution is active as a light-driven water oxidation catalyst in the presence of persulfate (S2O8(2-)) as electron acceptor. The yield of oxygen is higher when is prepared in situ by mixing and illuminating and Co(2+) in the presence of S2O8(2-). After oxygen evolution ceases, a second material can be isolated from the reaction mixture. is characterized by a lower Ru content than , and contains Co in a higher oxidation state. Interestingly, as a freshly prepared suspension is also active for light-driven water oxidation. It is shown that resides in the interior of and , and is thus in a location where undesirable quenching pathways of the photo-excited state of limit the oxygen production yields for both and .
Physical Chemistry Chemical Physics 01/2014; · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In higher plants, thylakoid membrane protein complexes show lateral heterogeneity in their distribution: photosystem (PS) II complexes are mostly located in grana stacks, whereas PSI and ATP synthase are mostly found in the stroma-exposed thylakoids. However, recent research has revealed strong dynamics in distribution of photosystems and their light harvesting antenna along the thylakoid membrane. Here, the dark-adapted spinach (Spinacia oleracea L.) thylakoid network was mechanically fragmented and the composition of distinct PSII-related proteins in various thylakoid subdomains was analyzed in order to get more insights into the composition and localization of various PSII subcomplexes and auxiliary proteins during the PSII repair cycle. Most of the PSII subunits followed rather equal distribution with roughly 70 % of the proteins located collectively in the grana thylakoids and grana margins, however, the low molecular mass subunits PsbW and PsbX as well as the PsbS proteins were found to be more exclusively located in grana thylakoids. The auxiliary proteins assisting in repair cycle of PSII were mostly located in stroma-exposed thylakoids, with the exception of TLP18.3, which was more evenly distributed between the grana and stroma thylakoids. The TL29 protein was present exclusively in grana thylakoids. Intriguingly, PROTON GRADIENT REGULATION5 (PGR5) was found to be distributed quite evenly between grana- and stroma thylakoids, whereas PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1) was highly enriched in the stroma thylakoids and practically missing from the grana cores. Photosynthesis Research for Sustainability: Keys to Produce Clean Energy. Guest Editors: Suleyman Allakhverdiev and Jian-Ren Shen.
Biochimica et Biophysica Acta 11/2013; · 4.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Illuminating a photosystem II sample at low temperatures (here 5-10 K) yields so called split signals detectable with CW-EPR. These signals reflect the oxidized, deprotonated radical of D1-Tyr161 (YZ(•)) in a magnetic interaction with the CaMn4 cluster in a particular S state. The intensity of the split EPR signals are affected by the addition of the water substrate analogue methanol. This was previously shown by the induction of split EPR signals from the S1, S3 and S0 states [Su, J-H. et al. (2006) Biochemistry 45, 7617-7627.]. Here, we use two split EPR signals induced from photosystem II trapped in the S2 state to further probe the binding of methanol in an S state dependent manner. The signals are induced with either visible or near-infrared light illumination provided at 5-10 K where methanol cannot bind or un-bind from its site. The results imply that the binding of methanol not only changes the magnetic properties of the CaMn4 cluster but also the hydrogen bond network in the OEC, thereby affecting the relative charge of the S2 state. The induction mechanisms for the two split signals are different resulting in two different redox states, S2YZ(•) and S1YZ(•) respectively. The two states show different methanol dependence for their induction. This indicates the existence of two binding sites for methanol in the CaMn4 cluster. It is proposed that methanol binds to MnA with high affinity and to MnD with lower affinity. The molecular nature and S-state dependence of the methanol binding to each respective site is discussed.
[Show abstract][Hide abstract] ABSTRACT: Photobiological H2 production is an attractive option for renewable solar fuels. Sulfur-deprived cells of Chlamydomonas reinhardtii have been shown to produce hydrogen with the highest efficiency among photobiological systems. We have investigated the photosynthetic reactions during sulfur deprivation and H2 production in the wild-type and state transition mutant 6 (Stm6) mutant of Chlamydomonas reinhardtii. The incubation period (130 h) was dissected into different phases, and changes in the amount and functional status of photosystem II (PSII) were investigated in vivo by electron paramagnetic resonance spectroscopy and variable fluorescence measurements. In the wild type it was found that the amount of PSII is decreased to 25% of the original level; the electron transport from PSII was completely blocked during the anaerobic phase preceding H2 formation. This block was released during the H2 production phase, indicating that the hydrogenase withdraws electrons from the plastoquinone pool. This partly removes the block in PSII electron transport, thereby permitting electron flow from water oxidation to hydrogenase. In the Stm6 mutant, which has higher respiration and H2 evolution than the wild type, PSII was analogously but much less affected. The addition of the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea revealed that ∼80% of the H2 production was inhibited in both strains. We conclude that (i) at least in the earlier stages, most of the electrons delivered to the hydrogenase originate from water oxidation by PSII, (ii) a faster onset of anaerobiosis preserves PSII from irreversible photoinhibition, and (iii) mutants with enhanced respiratory activity should be considered for better photobiological H2 production.
Proceedings of the National Academy of Sciences 04/2013; · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A rare example of a "monomeric" triple transition-metal substituted Keggin anion has been synthesized and characterized by various methods including X-ray crystallography, ESI and MALDI mass spectrometry, electrochemistry, EPR, and SQUID.
[Show abstract][Hide abstract] ABSTRACT: The Mn(4)CaO(5) cluster of photosystem II (PSII) catalyzes the oxidation of water to molecular oxygen through the light-driven redox S-cycle. The water oxidizing complex (WOC) forms a triad with Tyrosine(Z) and P(680), which mediates electrons from water towards the acceptor side of PSII. Under certain conditions two other redox-active components, Tyrosine(D) (Y(D)) and Cytochrome b (559) (Cyt b (559)) can also interact with the S-states. In the present work we investigate the electron transfer from Cyt b (559) and Y(D) to the S(2) and S(3) states at 195 K. First, Y(D) (•) and Cyt b (559) were chemically reduced. The S(2) and S(3) states were then achieved by application of one or two laser flashes, respectively, on samples stabilized in the S(1) state. EPR signals of the WOC (the S(2)-state multiline signal, ML-S(2)), Y(D) (•) and oxidized Cyt b (559) were simultaneously detected during a prolonged dark incubation at 195 K. During 163 days of incubation a large fraction of the S(2) population decayed to S(1) in the S(2) samples by following a single exponential decay. Differently, S(3) samples showed an initial increase in the ML-S(2) intensity (due to S(3) to S(2) conversion) and a subsequent slow decay due to S(2) to S(1) conversion. In both cases, only a minor oxidation of Y(D) was observed. In contrast, the signal intensity of the oxidized Cyt b (559) showed a two-fold increase in both the S(2) and S(3) samples. The electron donation from Cyt b (559) was much more efficient to the S(2) state than to the S(3) state.
Journal of Bioenergetics 01/2013; 45(1-2):111-120. · 2.71 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cryogenic illumination of Photosystem II (PSII) can lead to the trapping of the metastable radical Y(Z)(•), the radical form of the redox-active tyrosine residue D1-Tyr161 (known as Y(Z)). Magnetic interaction between this radical and the CaMn(4) cluster of PSII gives rise to so-called split electron paramagnetic resonance (EPR) signals with characteristics that are dependent on the S state. We report here the observation and characterization of a split EPR signal that can be directly induced from PSII centers in the S(2) state through visible light illumination at 10 K. We further show that the induction of this split signal takes place via a Mn-centered mechanism, in the same way as when using near-infrared light illumination [Koulougliotis, D., et al. (2003) Biochemistry 42, 3045-3053]. On the basis of interpretations of these results, and in combination with literature data for other split signals induced under a variety of conditions (temperature and light quality), we propose a unified model for the mechanisms of split signal induction across the four S states (S(0), S(1), S(2), and S(3)). At the heart of this model is the stability or instability of the Y(Z)(•)(D1-His190)(+) pair that would be formed during cryogenic oxidation of Y(Z). Furthermore, the model is closely related to the sequence of transfers of protons and electrons from the CaMn(4) cluster during the S cycle and further demonstrates the utility of the split signals in probing the immediate environment of the oxygen-evolving center in PSII.
[Show abstract][Hide abstract] ABSTRACT: The period of four oscillation of the S state intermediates of the water oxidizing complex in Photosystem II (PSII) is commonly analyzed by the Kok parameters. The important miss factor determines the efficiency for each S transition. Commonly, an equal miss factor has been used in the analysis. We have used EPR signals which probe all S states in the same sample during S cycle advancement. This allows, for the first time, to measure directly the miss parameter for each S state transition. Experiments were performed in PSII membrane preparations from spinach in the presence of electron acceptor at 1 °C and 20 °C. The data show that the miss parameter is different in different transitions and shows different temperature dependence. We found no misses at 1 °C and 10% misses at 20 °C during the S(1)→S(2) transition. The highest miss factor was found in the S(2)→S(3) transition which decreased from 23% to 16% with increasing temperature. For the S(3)→S(0) transition the miss parameter was found to be 7% at 1 °C and decreased to 3% at 20 °C. For the S(0)→S(1) transition the miss parameter was found to be approximately 10% at both temperatures. The contribution from the acceptor side in the form of recombination reactions as well as from the donor side of PSII to the uneven misses is discussed. It is suggested that the different transition efficiency in each S transition partly reflects the chemistry at the CaMn(4)O(5) cluster. That consequently contributes to the uneven misses during S cycle turnover in PSII.
Journal of Biological Chemistry 02/2012; 287(16):13422-9. · 4.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The carboxylate stretching frequencies of two high-valent, di-μ-oxido bridged, manganese dimers has been studied with IR spectroscopy in three different oxidation states. Both complexes contain one monodentate carboxylate donor to each Mn ion, in one complex, the carboxylate is coordinated perpendicular to the Mn-(μ-O)(2)-Mn plane, and in the other complex, the carboxylate is coordinated in the Mn-(μ-O)(2)-Mn plane. For both complexes, the difference between the asymmetric and the symmetric carboxylate stretching frequencies decrease for both the Mn(2)(IV,IV) to Mn(2)(III,IV) transition and the Mn(2)(III,IV) to Mn(2)(III,III) transition, with only minor differences observed between the two arrangements of the carboxylate ligand versus the Mn-(μ-O)(2)-Mn plane. The IR spectra also show that both carboxylate ligands are affected for each one electron reduction, i.e., the stretching frequency of the carboxylate coordinated to the Mn ion that is not reduced also shifts. These results are discussed in relation to FTIR studies of changes in carboxylate stretching frequencies in a one electron oxidation step of the water oxidation complex in Photosystem II.
[Show abstract][Hide abstract] ABSTRACT: This contribution was presented as the closing lecture at the Faraday Discussion 155 on artificial photosynthesis, held in Edinburgh Scotland, September 5-7 2011. The world needs new, environmentally friendly and renewable fuels to exchange for fossil fuels. The fuel must be made from cheap and "endless" resources that are available everywhere. The new research area of solar fuels aims to meet this demand. This paper discusses why we need a solar fuel and why electricity is not enough; it proposes solar energy as the major renewable energy source to feed from. The scientific field concerning artificial photosynthesis expands rapidly and most of the different scientific visions for solar fuels are briefly overviewed. Research strategies and the development of artificial photosynthesis research to produce solar fuels are overviewed. Some conceptual aspects of research for artificial photosynthesis are discussed in closer detail.
[Show abstract][Hide abstract] ABSTRACT: The world needs new, environmentally friendly and renewable fuels to allow an exchange from fossil fuels. The fuel must be made from cheap and 'endless' resources that are available everywhere. The new research area on solar fuels, which are made from solar energy and water, aims to meet this demand. The paper discusses why we need a solar fuel and why electricity is not enough; it proposes solar energy as the major renewable energy source to feed from. The present research strategies, involving direct, semi-direct and indirect approaches to produce solar fuels, are overviewed.
AMBIO A Journal of the Human Environment 01/2012; 41 Suppl 2:156-62. · 2.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The stability of the S(3) and S(2) states of the oxygen evolving complex in photosystem II (PSII) was directly probed by EPR spectroscopy in PSII membrane preparations from spinach in the presence of the exogenous electron acceptor PpBQ at 1, 10, and 20 °C. The decay of the S(3) state was followed in samples exposed to two flashes by measuring the split S(3) EPR signal induced by near-infrared illumination at 5 K. The decay of the S(2) state was followed in samples exposed to one flash by measuring the S(2) state multiline EPR signal. During the decay of the S(3) state, the S(2) state multiline EPR signal first increased and then decreased in amplitude. This shows that the decay of the S(3) state to the S(1) state occurs via the S(2) state. The decay of the S(3) state was biexponential with a fast kinetic phase with a few seconds decay half-time. This occurred in 10-20% of the PSII centers. The slow kinetic phase ranged from a decay half-time of 700 s (at 1 °C) to ~100 s (at 20 °C) in the remaining 80-90% of the centers. The decay of the S(2) state was also biphasic and showed quite similar kinetics to the decay of the S(3) state. Our experiments show that the auxiliary electron donor Y(D) was oxidized during the entire experiment. Thus, the reduced form of Y(D) does not participate to the fast decay of the S(2) and S(3) states we describe here. Instead, we suggest that the decay of the S(3) and S(2) states reflects electron transfer from the acceptor side of PSII to the donor side of PSII starting in the corresponding S state. It is proposed that this exists in equilibrium with Y(Z) according to S(3)Y(Z) ⇔ S(2)Y(Z)(•) in the case of the S(3) state decay and S(2)Y(Z) ⇔ S(1)Y(Z)(•) in the case of the S(2) state decay. Two kinetic models are discussed, both developed with the assumption that the slow decay of the S(3) and S(2) states occurs in PSII centers where Y(Z) is also a fast donor to P(680)(+) working in the nanosecond time regime and that the fast decay of the S(3) and S(2) states occurs in centers where Y(Z) reduces P(680)(+) with slower microsecond kinetics. Our measurements also demonstrate that the split S(3) EPR signal can be used as a direct probe to the S(3) state and that it can provide important information about the redox properties of the S(3) state.
[Show abstract][Hide abstract] ABSTRACT: The geometry and electronic structure of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) and its higher oxidation state species up formally to Ru(VI) have been studied by means of UV-vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that, as the oxidation state increases, the Ru-O bond distance decreases, indicating increased degrees of Ru-O multiple bonding. In addition, the O-Ru-O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low-spin configurations are favored for all oxidation states. Thus, cis-[Ru(IV)(bpy)(2)(OH)(2)](2+) (d(4)) has a singlet ground state and is EPR-silent at low temperatures, while cis-[Ru(V)(bpy)(2)(O)(OH)](2+) (d(3)) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O-Ru-O fragment. In addition, the photochemical isomerization of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) to its trans-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) isomer has been fully characterized through quantum chemical calculations. The excited-state process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the trans isomer.
[Show abstract][Hide abstract] ABSTRACT: The multisubunit membrane protein complex Photosystem II (PSII) catalyzes one of the key reactions in photosynthesis: the light-driven oxidation of water. Here, we focus on the role of the Psb27 assembly factor, which is involved in biogenesis and repair after light-induced damage of the complex. We show that Psb27 is essential for the survival of cyanobacterial cells grown under stress conditions. The combination of cold stress (30 °C) and high light stress (1000 μmol of photons × m(-2) × s(-1)) led to complete inhibition of growth in a Δpsb27 mutant strain of the thermophilic cyanobacterium Thermosynechococcus elongatus, whereas wild-type cells continued to grow. Moreover, Psb27-containing PSII complexes became the predominant PSII species in preparations from wild-type cells grown under cold stress. Two different PSII-Psb27 complexes were isolated and characterized in this study. The first complex represents the known monomeric PSII-Psb27 species, which is involved in the assembly of PSII. Additionally, a novel dimeric PSII-Psb27 complex could be allocated in the repair cycle, i.e. in processes after inactivation of PSII, by (15)N pulse-label experiments followed by mass spectrometry analysis. Comparison with the corresponding PSII species from Δpsb27 mutant cells showed that Psb27 prevented the release of manganese from the previously inactivated complex. These results indicate a more complex role of the Psb27 protein within the life cycle of PSII, especially under stress conditions.
Journal of Biological Chemistry 08/2011; 286(34):29548-55. · 4.60 Impact Factor