[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: 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.
Photochemistry and Photobiology 05/2014; 90(5). DOI:10.1111/php.12291 · 2.27 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.
[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 .
[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.
Proceedings of the National Academy of Sciences 04/2013; 110(18). DOI:10.1073/pnas.1220645110 · 9.67 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 Mn4CaO5 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 TyrosineZ and P680, which mediates electrons from water towards the acceptor side of PSII. Under certain conditions two other redox-active components, TyrosineD (YD) 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 YD to the S2 and S3 states at 195 K. First, YD• and Cyt b
559 were chemically reduced. The S2 and S3 states were then achieved by application of one or two laser flashes, respectively, on samples stabilized in the S1 state. EPR signals of the WOC (the S2-state multiline signal, ML-S2), YD• 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 S2 population decayed to S1 in the S2 samples by following a single exponential decay. Differently, S3 samples showed an initial increase in the ML-S2 intensity (due to S3 to S2 conversion) and a subsequent slow decay due to S2 to S1 conversion. In both cases, only a minor oxidation of YD was observed. In contrast, the signal intensity of the oxidized Cyt b
559 showed a two-fold increase in both the S2 and S3 samples. The electron donation from Cyt b
559 was much more efficient to the S2 state than to the S3 state.
Journal of Bioenergetics 02/2013; 45(1-2):111-120. DOI:10.1007/s10863-012-9482-8 · 3.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: On the path to an energy transition away from fossil fuels to sustainable sources, the European Union is for the moment keeping pace with the objectives of the Strategic Energy Technology-Plan. For this trend to continue after 2020, scientific breakthroughs must be achieved. One main objective is to produce solar fuels from solar energy and water in direct processes to accomplish the efficient storage of solar energy in a chemical form. This is a grand scientific challenge. One important approach to achieve this goal is Artificial Photosynthesis. The European Energy Research Alliance has launched the Joint Programme “Advanced Materials & Processes for Energy Applications” (AMPEA) to foster the role of basic science in Future Emerging Technologies. European researchers in artificial photosynthesis recently met at an AMPEA organized workshop to define common research strategies and milestones for the future. Through this work artificial photosynthesis became the first energy research sub-field to be organised into what is designated “an Application” within AMPEA. The ambition is to drive and accelerate solar fuels research into a powerful European field – in a shorter time and with a broader scope than possible for individual or national initiatives. Within AMPEA the Application Artificial Photosynthesis is inclusive and intended to bring together all European scientists in relevant fields. The goal is to set up a thorough and systematic programme of directed research, which by 2020 will have advanced to a point where commercially viable artificial photosynthetic devices will be under development in partnership with industry.
[Show abstract][Hide abstract] ABSTRACT: Electron transfer from the reduced tyrosine YD and cytochrome b559 (Cyt b559) to the S2 and S3 states of photosystem II was investigated at the temperature of 195 K. Electron transfer reactions were followed by measuring EPR signals of tyrosine YD·, oxidized Cyt b559 and the S2-state multiline signal. Long term incubation (∼90 days) at 195 K causes decay of the majority of S2 centers up to ∼40% of initial value, while in this time scale the intensity of YD· radical increases less than 10%. Samples advanced to S3 state demonstrates an increasing behavior of the S2-state multiline signal intensity in the beginning of incubation (∼20 days) and slow decay up to 40% of maximal amplitude during further incubation of the samples. Similarly to the S2 sample, small increase in YD· radical signal was observed during the S3 decay. However, in both types of samples prepared in S2 and S3 states after 90 days of incubation the signal of oxidized Cyt b559 is increased from 45%–50% up to 100% maximal intensity. The results obtained in this study support the conclusion of our early investigations which claimed the reduced Cyt b559 as electron source for the S2 and S3 states.
Photosynthesis Research for Food, Fuel and the Future, 01/2013: pages 218-221;
[Show abstract][Hide abstract] ABSTRACT: The world needs new, environmentally friendly, and renewable fuels to exchange for fossil fuels. The fuel must be made from cheap, abundant, and renewable resources. The research area of solar fuels aims to meet this demand. This paper discusses why we need a solar fuel, and proposes solar energy as the major renewable energy source to feed from. The scientific field concerning artificial photosynthesis is expanding rapidly and most of the different scientific visions for solar fuels are briefly reviewed. Research strategies for the development of artificial photosynthesis to produce solar fuels are overviewed, with some critical concepts 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 03/2012; 41 Suppl 2(S2):156-62. DOI:10.1007/s13280-012-0273-6 · 2.29 Impact Factor