- [Show abstract] [Hide abstract] ABSTRACT: Redox properties of the acceptor side of Photosystem II were studied during H2 gas production in cells of Chlamydomonas reinhardtii. Flash-induced variable fluorescence changes and thermoluminescence measurements were performed in wild type and Stm6 mutant cells during different stages of sulfur (S)-deprivation. Analysis of the fluorescence decay kinetics indicated that the forward electron transfer on the acceptor side of Photosystem II was dramatically slowed down during the O2 evolution and O2 consumption stages and was completely blocked in the anaerobic stage of S-deprivation, thus, indicating a complete reduction of the PQ-pool. During the H2 formation stage, the forward electron transfer kinetics in the μsec and msec time scale re-appeared indicating partially restored electron flow from QA⁻ to QB and the PQ-pool. Thermoluminescese measurements fully confirmed the fluorescence kinetic analysis. Activation of hydrogenase in the H2 formation stage is responsible for re-oxidation of the PQ pool and reactivation of the electron flow which was found to be faster and more efficient on the Stm6 mutant due to the higher amount of functionally preserved Photosystem II.
- [Show abstract] [Hide abstract] ABSTRACT: In oxygenic photosynthesis, water is oxidized and dioxygen is produced at a Mn4Ca complex bound to the proteins of photosystem II (PSII). Valence and coordination changes in its catalytic S-state cycle are of high interest. In room-temperature (in-situ) experiments, time-resolved energy-sampling X-ray emission spectroscopy of the Mn Kß1,3 line after laser-flash excitation of PSII membrane particles was applied to characterize the redox transitions in the S-state cycle. The Kß1,3 line energies suggest a high-valence configuration of the Mn4Ca complex with Mn(III)3Mn(IV) in S0, Mn(III)2Mn(IV)2 in S1, Mn(III)Mn(IV)3 in S2, and Mn(IV)4 in S3, and thus manganese oxidation in each of the three accessible oxidizing transitions of the water oxidizing complex. There are no indications for formation of a ligand radical, thus rendering partial water oxidation before reaching the S4 state unlikely. The difference spectra of both manganese Kß1,3 emission and K-edge X-ray absorption display different shapes for Mn(III) oxidation in the S2->S3 transition when compared to Mn(III) oxidation in S1->S2. Comparison to spectra of manganese compounds with known structures and oxidation states and varying metal coordination environment suggests a change in the manganese ligand environment in the S2->S3 transition, which could be oxidation of five-coordinated Mn(III) to six-coordinated Mn(IV). Conceivable options for the rearrangement of (substrate) water species and metal-ligand bonding patterns at the Mn4Ca complex in the S2->S3 transition are discussed.
- [Show abstract] [Hide abstract] ABSTRACT: While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.
- [Show abstract] [Hide abstract] ABSTRACT: Photosystem I (PSI) uses light energy and electrons supplied by photosystem II (PSII) to reduce NADP+ to NADPH. PSI is very tolerant of excess light but extremely sensitive to excess electrons from PSII. It has been assumed that PSI is protected from photoinhibition by strict control of the intersystem electron transfer chain (ETC). Here we demonstrate that the iron–sulphur (FeS) clusters of PSI are more sensitive to high light stress than previously anticipated, but PSI with damaged FeS clusters still functions as a non-photochemical photoprotective energy quencher (PSI-NPQ). Upon photoinhibition of PSI, the highly reduced ETC further triggers thylakoid phosphorylation-based mechanisms that increase energy flow towards PSI. It is concluded that the sensitivity of FeS clusters provides an additional photoprotective mechanism that is able to downregulate PSII, based on PSI quenching and protein phosphorylation.
- [Show abstract] [Hide abstract] ABSTRACT: The filamentous cyanobacterium Nostoc punctiforme ATCC 29133 produces hydrogen via nitrogenase in heterocysts upon onset of nitrogen-fixing conditions. N. punctiforme expresses concomitantly the uptake hydrogenase HupSL, which oxidizes hydrogen in an effort to recover some of the reducing power used up by nitrogenase. Eliminating uptake activity has been employed as a strategy for net hydrogen production in N. punctiforme (Lindberg et al., Int. J. Hydrogen Energy, 2002, 27, 1291–1296). However, nitrogenase activity wanes within a few days. In the present work, we modify the proximal iron-sulfur cluster in the hydrogenase small subunit HupS by introducing the designed mutation C12P in the fusion protein f-HupS for expression in E. coli (Raleiras et al., J. Biol. Chem., 2013, 288, 18345–18352), and in the full HupSL enzyme for expression in N. punctiforme. C12P f-HupS was investigated by EPR spectroscopy and found to form a new paramagnetic species at the proximal cluster site consistent with a [4Fe–4S] to [3Fe–4S] cluster conversion. The new cluster has the features of an unprecedented mixed-coordination [3Fe–4S] metal center. The mutation was found to produce stable protein in vitro, in silico and in vivo. When C12P HupSL was expressed in N. punctiforme, the strain had a consistently higher hydrogen production than the background ΔhupSL mutant. We conclude that the increase in hydrogen production is due to the modification of the proximal iron-sulfur cluster in HupS, leading to a turn of the electron flow in the enzyme.
- [Show abstract] [Hide abstract] ABSTRACT: Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogues like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron–nitrogen–heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of electron paramagnetic resonance, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chemical calculations, we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the 3MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements.
- [Show abstract] [Hide abstract] ABSTRACT: Co-oxides are promising water oxidation catalysts for artificial photosynthesis devices. Presently, several different proposals exist for how they catalyze O2 formation from water. Knowledge about this process at molecular detail will be required for their further improvement. Here we present time-resolved 18O-labelling isotope-ratio membrane-inlet mass spectrometry (MIMS) experiments to study the mechanism of water oxidation in Co/methylenediphosphonate (Co/M2P) oxide nanoparticles using [Ru(bpy)3]3+ (bpy = 2,2’-bipyridine) as chemical oxidant. We show that 16O-Co/M2P-oxide nanoparticles produce 16O2 during their first turnover after simultaneous addition of H218O and [Ru(bpy)3]3+, while sequential addition with a delay of 3 s yields oxygen reflecting bulk water 18O-enrichment. This result is interpreted to show that the O–O bond formation in Co/M2P-oxide nanoparticles occurs via intramolecular oxygen coupling between two terminal Co-OHn ligands that are readily exchangeable with bulk water in the resting state of the catalyst. Importantly, our data allow the determination of the number of catalytic sites within this amorphous nanoparticular material, to calculate the TOF per catalytic site and to derive the number of holes needed for the production of the first O2 molecule per catalytic site. We propose that the mechanism of O–O bond formation during bulk catalysis in amorphous Co-oxides may differ from that taking place at the surface of crystalline materials.
- [Show abstract] [Hide abstract] ABSTRACT: The small subunit from the NiFe uptake hydrogenase, HupSL, in the cyanobacterium Nostoc punctiforme ATCC 29133, has been isolated in the absence of the large subunit (P. Raleiras, P. Kellers, P. Lindblad, S. Styring, A. Magnuson, J. Biol. Chem. 288 (2013) 18,345-18,352). Here, we have used flash photolysis to reduce the iron-sulfur clusters in the isolated small subunit, HupS. We used ascorbate as electron donor to the photogenerated excited state of Ru(II)-trisbipyridine (Ru(bpy)3), to generate Ru(I)(bpy)3 as reducing agent. Our results show that the isolated small subunit can be reduced by the Ru(I)(bpy)3 generated through flash photolysis. Copyright © 2015 Elsevier Inc. All rights reserved.
- [Show abstract] [Hide abstract] ABSTRACT: In this paper a novel synthetic route, being a paradigm of the “direct synthesis” approach, is proposed for the preparation of heterometallic Mn/V compounds by a one-pot reaction. Two synthesized complexes, (NH4)2[Mn2(HGly)(H2O)10][V10O28]·(HGly)·2H2O (1) and (NH4)2[Mn(β-HAla)(H2O)5]2[V10O28]·2H2O (2) (HGly = glycine, β-HAla = β-alanine) have been fully characterized by elemental analysis, single-crystal X-ray diffraction, cyclic voltammetry, magnetic susceptibility, FTIR and EPR spectroscopy. Thermal degradation of these compounds lead to the formation of porous, solid mixed oxides V2O5/MnV2O6 in a ratio of 3:2, which were analyzed by X-ray phase analysis and scanning electron microscopy with energy dispersive X-ray microanalysis (SEM/EDX). Additionally the ability of 1 and 2 to act as oxygen evolving water oxidation catalysts under visible light-driven conditions have been studied in a Clark type cell and by ex situ EPR spectroscopy.
- [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.
- [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.
- [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 .
Chapter: Artificial Photosynthesis
- [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.
- [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.
- [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.
Uppsala UniversityUppsala, Uppsala, Sweden
Adam Mickiewicz UniversityPosen, Greater Poland Voivodeship, Poland
IT University of CopenhagenKøbenhavn, Capital Region, Denmark
Roskilde UniversityRoskilde, Zealand, Denmark
University of HelsinkiHelsinki, Uusimaa, Finland
Université Paris 13 NordÎle-de-France, France
Lund, Skåne, Sweden
- Center for Chemistry and Chemical Engineering
Tukholma, Stockholm, Sweden
- • Department of Organic Chemistry
- • Department of Biochemistry and Biophysics