| 1. |
- Ahmadova, Nigar, et al.
(författare)
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Formation of tyrosine radicals in photosystem II under far-red illumination
- 2018
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Ingår i: Photosynthesis Research. - : Springer Science and Business Media LLC. - 0166-8595 .- 1573-5079. ; 136:1, s. 93-106
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Tidskriftsartikel (refereegranskat)abstract
- Photosystem II (PS II) contains two redox-active tyrosine residues on the donor side at symmetrical positions to the primary donor, P680. TyrZ, part of the water-oxidizing complex, is a preferential fast electron donor while TyrD is a slow auxiliary donor to P680 +. We used PS II membranes from spinach which were depleted of the water oxidation complex (Mn-depleted PS II) to study electron donation from both tyrosines by time-resolved EPR spectroscopy under visible and far-red continuous light and laser flash illumination. Our results show that under both illumination regimes, oxidation of TyrD occurs via equilibrium with TyrZ • at pH 4.7 and 6.3. At pH 8.5 direct TyrD oxidation by P680 + occurs in the majority of the PS II centers. Under continuous far-red light illumination these reactions were less effective but still possible. Different photochemical steps were considered to explain the far-red light-induced electron donation from tyrosines and localization of the primary electron hole (P680 +) on the ChlD1 in Mn-depleted PS II after the far-red light-induced charge separation at room temperature is suggested.
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| 2. |
- Bellardita, Marianna, et al.
(författare)
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Highly stable defective TiO2-x with tuned exposed facets induced by fluorine : Impact of surface and bulk properties on selective UV/visible alcohol photo-oxidation
- 2020
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Ingår i: Applied Surface Science. - : ELSEVIER. - 0169-4332 .- 1873-5584. ; 510
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Tidskriftsartikel (refereegranskat)abstract
- Titanium dioxide samples were prepared in the presence of different amounts of fluorine via hydrothermal method. It has been found that the presence of fluoride influenced the physico-chemical properties of TiO2 in various ways as polymorphic form stability, surface hydroxylation, generation of hydroxyl radicals under irradiation and formation of Ti3+ centers and oxygen vacancies. The generation rate of (OH)-O-center dot radicals was investigated by the photoluminescence technique in the presence of terephthalic acid. X-ray diffractometry indicated that fluorine stabilized the anatase TiO2. X-Ray photoelectron spectroscopy (XPS) revealed the presence of fluorine on the surface and the shift of the valence band edge towards less negative potentials, electron paramagnetic resonance (EPR) confirmed the formation of Ti3+ in the bulk of the photocatalysts, UV-vis spectra showed the extension of the TiO2 photo-response in the visible light region. 2-Propanol degradation and 4-methoxybenzyl alcohol partial oxidation were studied as probe reactions by using the home prepared powders as photocatalysts. Surprisingly, the photocatalytic activity resulted to be mainly affected by (OH)-O-center dot radicals formation ability under irradiation, rather than by the presence of {0 0 1} facets, although it cannot be excluded that the latter could influence the ability to form radicals under irradiation.
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| 3. |
- Bhowmick, Asmit, et al.
(författare)
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Structural evidence for intermediates during O2 formation in photosystem II
- 2023
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Ingår i: Nature. - : Springer Nature. - 0028-0836 .- 1476-4687. ; 617:7961, s. 629-636
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Tidskriftsartikel (refereegranskat)abstract
- In natural photosynthesis, the light-driven splitting of water into electrons, protons and molecular oxygen forms the first step of the solar-to-chemical energy conversion process. The reaction takes place in photosystem II, where the Mn4CaO5 cluster first stores four oxidizing equivalents, the S0 to S4 intermediate states in the Kok cycle, sequentially generated by photochemical charge separations in the reaction center and then catalyzes the O–O bond formation chemistry. Here, we report room temperature snapshots by serial femtosecond X-ray crystallography to provide structural insights into the final reaction step of Kok’s photosynthetic water oxidation cycle, the S3→[S4]→S0 transition where O2 is formed and Kok’s water oxidation clock is reset. Our data reveal a complex sequence of events, which occur over micro- to milliseconds, comprising changes at the Mn4CaO5 cluster, its ligands and water pathways as well as controlled proton release through the hydrogen-bonding network of the Cl1 channel. Importantly, the extra O atom Ox, which was introduced as a bridging ligand between Ca and Mn1 during the S2→S3 transition, disappears or relocates in parallel with Yz reduction starting at approximately 700 μs after the third flash. The onset of O2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide.
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| 4. |
- Cheah, Mun Hon, et al.
(författare)
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Assessment of the manganese cluster’s oxidation state via photoactivation of photosystem II microcrystals
- 2020
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 117:1, s. 141-145
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Tidskriftsartikel (refereegranskat)abstract
- Knowledge of the manganese oxidation states of the oxygen-evolving Mn4CaO5 cluster in photosystem II (PSII) is crucial toward understanding the mechanism of biological water oxidation. There is a 4 decade long debate on this topic that historically originates from the observation of a multiline electron paramagnetic resonance (EPR) signal with effective total spin of S = 1/2 in the singly oxidized S2 state of this cluster. This signal implies an overall oxidation state of either Mn(III)3Mn(IV) or Mn(III)Mn(IV)3 for the S2 state. These 2 competing assignments are commonly known as “low oxidation (LO)” and “high oxidation (HO)” models of the Mn4CaO5 cluster. Recent advanced EPR and Mn K-edge X-ray spectroscopy studies converge upon the HO model. However, doubts about these assignments have been voiced, fueled especially by studies counting the number of flash-driven electron removals required for the assembly of an active Mn4CaO5 cluster starting from Mn(II) and Mn-free PSII. This process, known as photoactivation, appeared to support the LO model since the first oxygen is reported to evolve already after 7 flashes. In this study, we improved the quantum yield and sensitivity of the photoactivation experiment by employing PSII microcrystals that retained all protein subunits after complete manganese removal and by oxygen detection via a custom built thin-layer cell connected to a membrane inlet mass spectrometer. We demonstrate that 9 flashes by a nanosecond laser are required for the production of the first oxygen, which proves that the HO model provides the correct description of the Mn4CaO5 cluster’s oxidation states.
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| 5. |
- Chen, Yang-Er, et al.
(författare)
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The Low Molecular Mass Photosystem II Protein PsbTn is Important for Light Acclimation
- 2019
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Ingår i: Plant Physiology. - Rockville : American Society of Plant Biologists. - 0032-0889 .- 1532-2548. ; 179:4, s. 1739-1753
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Tidskriftsartikel (refereegranskat)abstract
- Photosystem II (PSII) is a supramolecular complex containing over 30 protein subunits and a large set of cofactors including various pigments and quinones as well as Mn, Ca, Cl, and Fe ions. Eukaryotic PSII complexes contain many subunits not found in their bacterial counterparts, including the proteins PsbP, PsbQ, PsbS, and PsbW, as well as the highly homologous, low molecular mass subunits PsbTn1 and PsbTn2 whose function is currently unknown. To determine the function of PsbTn1 and PsbTn2, we generated single and double psbTn1 and psbTn2 knock-out mutants in Arabidopsis thaliana. Cross-linking and reciprocal co-immunoprecipitation experiments revealed that PsbTn is a lumenal PSII protein situated next to the cytochrome b559 subunit PsbE. The removal of the PsbTn proteins decreased the oxygen evolution rate and PSII core phosphorylation level but increased the susceptibility of PSII to photoinhibition and the production of reactive oxygen species. The assembly and stability of PSII were unaffected, indicating that the deficiencies of the psbTn1 psbTn2 double mutants are due to structural changes. Double mutants exhibited a higher rate of non-photochemical quenching of excited states than the wild type and single mutants, as well as slower state transition kinetics and a lower quantum yield of PSII when grown in the field. Based on these results, we propose that the main function of the PsbTn proteins is to enable PSII to acclimate to light shifts or intense illumination.
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| 6. |
- de Lichtenberg, Casper, et al.
(författare)
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The D1-V185N mutation alters substrate water exchange by stabilizing alternative structures of the Mn4Ca-cluster in photosystem II
- 2021
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier. - 0005-2728 .- 1879-2650. ; 1862:1
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Tidskriftsartikel (refereegranskat)abstract
- In photosynthesis, the oxygen-evolving complex (OEC) of the pigment-protein complex photosystem II (PSII) orchestrates the oxidation of water. Introduction of the V185N mutation into the D1 protein was previously reported to drastically slow O2-release and strongly perturb the water network surrounding the Mn4Ca cluster. Employing time-resolved membrane inlet mass spectrometry, we measured here the H218O/H216O-exchange kinetics of the fast (Wf) and slow (Ws) exchanging substrate waters bound in the S1, S2 and S3 states to the Mn4Ca cluster of PSII core complexes isolated from wild type and D1-V185N strains of Synechocystis sp. PCC 6803. We found that the rate of exchange for Ws was increased in the S1 and S2 states, while both Wf and Ws exchange rates were decreased in the S3 state. Additionally, we used EPR spectroscopy to characterize the Mn4Ca cluster and its interaction with the redox active D1-Tyr161 (YZ). In the S2 state, we observed a greatly diminished multiline signal in the V185N-PSII that could be recovered by addition of ammonia. The split signal in the S1 state was not affected, while the split signal in the S3 state was absent in the D1-V185N mutant. These findings are rationalized by the proposal that the N185 residue stabilizes the binding of an additional water-derived ligand at the Mn1 site of the Mn4Ca cluster via hydrogen bonding. Implications for the sites of substrate water binding are discussed.
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| 7. |
- Ermakova, Maria, et al.
(författare)
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Upregulation of bundle sheath electron transport capacity under limiting light in C-4 Setaria viridis
- 2021
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Ingår i: The Plant Journal. - : John Wiley & Sons. - 0960-7412 .- 1365-313X. ; 106:5, s. 1443-1454
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Tidskriftsartikel (refereegranskat)abstract
- C-4 photosynthesis is a biochemical pathway that operates across mesophyll and bundle sheath (BS) cells to increase CO2 concentration at the site of CO2 fixation. C-4 plants benefit from high irradiance but their efficiency decreases under shade, causing a loss of productivity in crop canopies. We investigated shade acclimation responses of Setaria viridis, a model monocot of NADP-dependent malic enzyme subtype, focussing on cell-specific electron transport capacity. Plants grown under low light (LL) maintained CO2 assimilation rates similar to high light plants but had an increased chlorophyll and light-harvesting-protein content, predominantly in BS cells. Photosystem II (PSII) protein abundance, oxygen-evolving activity and the PSII/PSI ratio were enhanced in LL BS cells, indicating a higher capacity for linear electron flow. Abundances of PSI, ATP synthase, Cytochrome b(6)f and the chloroplast NAD(P)H dehydrogenase complex, which constitute the BS cyclic electron flow machinery, were also increased in LL plants. A decline in PEP carboxylase activity in mesophyll cells and a consequent shortage of reducing power in BS chloroplasts were associated with a more oxidised plastoquinone pool in LL plants and the formation of PSII - light-harvesting complex II supercomplexes with an increased oxygen evolution rate. Our results suggest that the supramolecular composition of PSII in BS cells is adjusted according to the redox state of the plastoquinone pool. This discovery contributes to the understanding of the acclimation of PSII activity in C-4 plants and will support the development of strategies for crop improvement, including the engineering of C-4 photosynthesis into C-3 plants.
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| 8. |
- Han, Guangye, et al.
(författare)
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Molecular basis for turnover inefficiencies (misses) during water oxidation in photosystem II
- 2022
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Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 13:29, s. 8667-8678
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Tidskriftsartikel (refereegranskat)abstract
- Photosynthesis stores solar light as chemical energy and efficiency of this process isv highly important. The electrons required for CO2 reduction are extracted from water in a reaction driven by light-induced charge separations in the Photosystem II reaction center and catalyzed by the CaMn4O5-cluster. This cyclic process involves five redox intermediates known as the S-0-S-4 states. In this study, we quantify the flash-induced turnover efficiency of each S state by electron paramagnetic resonance spectroscopy. Measurements were performed in photosystem II membrane preparations from spinach in the presence of an exogenous electron acceptor at selected temperatures between -10 degrees C and +20 degrees C and at flash frequencies of 1.25, 5 and 10 Hz. The results show that at optimal conditions the turnover efficiencies are limited by reactions occurring in the water oxidizing complex, allowing the extraction of their S state dependence and correlating low efficiencies to structural changes and chemical events during the reaction cycle. At temperatures 10 degrees C and below, the highest efficiency (i.e. lowest miss parameter) was found for the S-1 -> S-2 transition, while the S-2 -> S-3 transition was least efficient (highest miss parameter) over the whole temperature range. These electron paramagnetic resonance results were confirmed by measurements of flash-induced oxygen release patterns in thylakoid membranes and are explained on the basis of S state dependent structural changes at the CaMn4O5-cluster that were determined recently by femtosecond X-ray crystallography. Thereby, possible "molecular errors" connected to the e(-) transfer, H+ transfer, H2O binding and O-2 release are identified.
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| 9. |
- Khosravitabar, Fatemeh, et al.
(författare)
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Partial inhibition of the inter-photosystem electron transfer at cytochrome b6f complex promotes periodic surges of hydrogen evolution in Chlamydomonas reinhardtii
- 2023
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Ingår i: International journal of hydrogen energy. - : Elsevier. - 0360-3199 .- 1879-3487. ; 48:93, s. 36314-36326
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Tidskriftsartikel (refereegranskat)abstract
- Periodic surges of H2 evolution were observed in the wild-type strain CC-5325 of the green unicellular alga Chlamydomonas reinhardtii in the presence of the electron transport inhibitors dibromo-6-isopropyl-3-methyl-1,4-benzoquinone (DBMIB, 3.5 μM) and 2,4-dinitrophenylether and iodonitrothymol (DNP-INT, 0.6 μM). Addition of DBMIB partly inhibited the electron transfer from Cytochrome b6f complex to Photosystem I, over-reduced the plastoquinone pool, gradually inhibited photosystem II and created anoxic conditions in cells. During 30 days of anaerobic incubation, continues H2 photoproduction with a minimum rate of 1 ml/L of culture per day was accompanied with additional outbursts of H2 evolution. The first noticeable peak of H2 evolution was observed on day 6 of incubation, with maximum rate of 23 ml of H2 per L of culture per day. It was repeated on day 9 and day 22 with the 2 and 4 times lower rates respectively. Addition of DNP-NT showed similar effect by inducing the H2 photoproduction for 15 days, albeit at much lower rates. Contribution of the direct and indirect pathways to the H2 production is shown by fluorescence, thermoluminescence and electron paramagnetic resonance spectroscopy. It is proposed that photosynthetic electron transport in combination with photorespiration, chlororespiration and starch accumulation can switch on and off between photosynthetic, H2 producing and survival modes of cell metabolism. Controlled switching between these modes could potentially maintain the long lasting photosynthetic H2 production in the wild-type of Chlamydomonas.
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| 10. |
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| 11. |
- Krishna, Pilla Sankara, et al.
(författare)
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Characterization of the transient fluorescence wave phenomenon that occurs during H-2 production in Chlamydomonas reinhardtii
- 2019
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Ingår i: Journal of Experimental Botany. - : OXFORD UNIV PRESS. - 0022-0957 .- 1460-2431. ; 70:21, s. 6321-6336
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Tidskriftsartikel (refereegranskat)abstract
- The redox state of the plastoquinone (PQ) pool in sulfur-deprived, H-2-producing Chlamydomonas reinhardtii cells was studied using single flash-induced variable fluorescence decay kinetics. During H-2 production, the fluorescence decay kinetics exhibited an unusual post-illumination rise of variable fluorescence, giving a wave-like appearance. The wave showed the transient fluorescence minimum at 60 ms after the flash, followed by a rise, reaching the transient fluorescence maximum at 1 s after the flash, before decaying back to the initial fluorescence level. Similar wave-like fluorescence decay kinetics have been reported previously in anaerobically incubated cyanobacteria but not in green algae. From several different electron and proton transfer inhibitors used, polymyxin B, an inhibitor of type II NAD(P)H dehydrogenase (NDA2), had the effect of eliminating the fluorescence wave feature, indicating involvement of NDA2 in this phenomenon. This was further confirmed by the absence of the fluorescence wave in the Delta nda2 mutant lacking NDA2. Additionally, Delta nda2 mutants have also shown delayed and diminished H-2 production (only 23% if compared with the wild type). Our results show that the fluorescence wave phenomenon in C. reinhardtii is observed under highly reducing conditions and is induced by the NDA2-mediated electron flow from the reduced stromal components to the PQ pool. Therefore, the fluorescence wave phenomenon is a sensitive probe for the complex network of redox reactions at the PQ pool level in the thylakoid membrane. It could be used in further characterization and improvement of the electron transfer pathways leading to H-2 production in C. reinhardtii.
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| 12. |
- Krishna, Pilla Sankara, et al.
(författare)
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Photosystem ratio imbalance promotes direct sustainable H-2 production in Chlamydomonas reinhardtii
- 2019
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Ingår i: Green Chemistry. - : ROYAL SOC CHEMISTRY. - 1463-9262 .- 1463-9270. ; 21:17, s. 4683-4690
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Tidskriftsartikel (refereegranskat)abstract
- The green alga Chlamydomonas reinhardtii can photoproduce H-2 gas for only a few minutes under anaerobic conditions due to the inhibition of hydrogenase by O-2 produced by Photosystem II (PSII). A few days of sustained H-2 production can only be achieved when O-2 and H-2 production are temporally separated under two-stage processes such as sulfur deprivation. Under sulfur deprivation, H-2 production is initiated after the over-reduction of the plastoquinone pool and decreased PSII activity in the thylakoid membrane. As a result, activated hydrogenase consumes the excess of electrons produced by PSII [Volgusheva et al., Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 7223]. Here, we report that similar conditions can be achieved by simply altering the ratio between photosystem I (PSI) and PSII. In the C3 mutant of C. reinhardtii, we found a lower PSI/PSII ratio than in the wild type, 0.33 vs. 0.85, respectively. This imbalance of photosystems resulted in the over-reduced state of the plastoquinone pool and activation of hydrogenase in the C3 mutant that allowed the photoproduction of H-2 continuously for 42 days. This is an unprecedented duration of H-2 production in green algae under standard growth conditions without any nutrient limitation. Photosynthetic electron flow from PSII to hydrogenase was closely regulated during this long-term H-2 production. The amount of PSII was decreased and the amount of PSI was increased reaching a PSI/PSII ratio of more than 5 as shown by EPR and fluorescence spectroscopy. This fine-tuning of photosystems allows to sustain the long-term production of H-2 in C. reinhardtii by a direct photosynthetic pathway.
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| 13. |
- Magnuson, Ann, et al.
(författare)
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Toward Sustainable H-2 Production : Linking Hydrogenase with Photosynthesis
- 2020
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Ingår i: Joule. - : CellPress. - 2542-4351. ; 4:6, s. 1157-1159
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Molecular hydrogen, H-2, is an energy carrier that is increasing in popularity as an alternative fuel. Presently, the major part of commercially available H-2 is produced from fossil resources. To make H-2 a true contender to fossil fuels, a sustainable production via water splitting by renewable electricity in combination with earth-abundant catalysts or by photosynthetic microorganisms is required.
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| 14. |
- Miranda, Hélder, et al.
(författare)
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Sll1783, a monooxygenase associated with polysaccharide processing in the unicellular cyanobacterium Synechocystis PCC 6803
- 2017
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Ingår i: Physiologia Plantarum. - : Wiley. - 0031-9317 .- 1399-3054. ; 161:2, s. 182-195
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Tidskriftsartikel (refereegranskat)abstract
- Cyanobacteria play a pivotal role as the primary producer in many aquatic ecosystems. The knowledge on the interacting processes of cyanobacteria with its environment - abiotic and biotic factors - is still very limited. Many potential exocytoplasmic proteins in the model unicellular cyanobacterium Synechocystis PCC 6803 have unknown functions and their study is essential to improve our understanding of this photosynthetic organism and its potential for biotechnology use. Here we characterize a deletion mutant of Synechocystis PCC 6803, Δsll1783, a strain that showed a remarkably high light resistance which is related with its lower thylakoid membrane formation. Our results suggests Sll1783 to be involved in a mechanism of polysaccharide degradation and uptake and we hypothesize it might function as a sensor for cell density in cyanobacterial cultures.
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| 15. |
- Nilsson, Anders K., 1982, et al.
(författare)
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PSB33 protein sustains photosystem II in plant chloroplasts under UV-A light
- 2020
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Ingår i: Journal of Experimental Botany. - OXFORD ENGLAND : Oxford University Press (OUP). - 0022-0957 .- 1460-2431. ; 71:22, s. 7210-7223
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Tidskriftsartikel (refereegranskat)abstract
- Plants can quickly and dynamically respond to spectral and intensity variations of the incident light. These responses include activation of developmental processes, morphological changes, and photosynthetic acclimation that ensure optimal energy conversion and minimal photoinhibition. Plant adaptation and acclimation to environmental changes have been extensively studied, but many details surrounding these processes remain elusive. The photosystem II (PSII)-associated protein PSB33 plays a fundamental role in sustaining PSII as well as in the regulation of the light antenna in fluctuating light. We investigated how PSB33 knock-out Arabidopsis plants perform under different light qualities. psb33 plants displayed a reduction of 88% of total fresh weight compared to wild type plants when cultivated at the boundary of UV-A and blue light. The sensitivity towards UV-A light was associated with a lower abundance of PSII proteins, which reduces psb33 plants' capacity for photosynthesis. The UV-A phenotype was found to be linked to altered phytohormone status and changed thylakoid ultrastructure. Our results collectively show that PSB33 is involved in a UV-A light-mediated mechanism to maintain a functional PSII pool in the chloroplast.
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| 16. |
- Norrbo, Isabella, et al.
(författare)
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Solar UV index and UV dose determination with photochromic hackmanites : from the assessment of the fundamental properties to the device
- 2018
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Ingår i: Materials Horizons. - : Royal Society of Chemistry (RSC). - 2051-6347 .- 2051-6355. ; 5:3, s. 569-576
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Tidskriftsartikel (refereegranskat)abstract
- Extended exposure to sunlight or artificial UV sources is a major cause of serious skin and eye diseases such as cancer. There is thus a great need for convenient materials for the easy monitoring of UV doses. While organic photochromic molecules are tunable for responses under different wavelengths of UV radiation, they suffer from rather poor durability because the color changes involve drastic changes in molecular structure. Inorganic materials, on the other hand, are durable, but they have lacked tunability. Here, by combining computational and empirical data, we confirm the mechanism of coloration in the hackmanites, nature-based materials, and introduce a new technique called thermotenebrescence. With knowledge of the mechanism, we show that we can control and thus tune the energy of electronic states of synthetic hackmanites (Na,M)(8)Al6Si6O24(Cl,S)(2) so that their body color is sensitive to the solar UV index as well as UVA, UVB or UVC radiation levels. Finally, we demonstrate that it is possible to use images taken with an inexpensive cell phone to quantify the radiation dose or UV index. The hackmanite materials thus show great potential for use in portable healthcare both in everyday life and in laboratories.
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| 17. |
- Ozer, Lütfiye Y., et al.
(författare)
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Long-Lasting Non-hydrogenated Dark Titanium Dioxide : Medium Vacuum Anneal for Enhanced Visible Activity of Modified Multiphase Photocatalysts
- 2018
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Ingår i: ChemCatChem. - : Wiley. - 1867-3880 .- 1867-3899. ; 10:14, s. 2949-2954
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Tidskriftsartikel (refereegranskat)abstract
- Multiphase TiO2 was stably modified by vacuum treatment for a dramatic improvement in visible-light absorption and photocatalytic reactivity. The samples were made of rutile-brookite, bare or N doped, and were grown on reduced graphene oxide. The stable introduction of Ti3+ species and oxygen vacancies resulting in mitigated electron?hole recombination was identified as the main responsible factor, along with a change in surface charges.
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| 18. |
- Pavlou, Andrea, et al.
(författare)
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Far-red photosynthesis : Two charge separation pathways exist in plant Photosystem II reaction center
- 2023
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1864:4
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Tidskriftsartikel (refereegranskat)abstract
- An alternative charge separation pathway in Photosystem II under the far-red light was proposed by us on the basis of electron transfer properties at 295 K and 5 K. Here we extend these studies to the temperature range of 77-295 K with help of electron paramagnetic resonance spectroscopy. Induction of the S2 state multiline signal, oxidation of Cytochrome b559 and ChlorophyllZ was studied in Photosystem II membrane preparations from spinach after application of a laser flashes in visible (532 nm) or far-red (730-750 nm) spectral regions. Temperature dependence of the S2 state signal induction after single flash at 730-750 nm (Tinhibition - 240 K) was found to be different than that at 532 nm (Tinhibition - 157 K). No contaminant oxidation of the secondary electron donors cytochrome b559 or chlorophyllZ was observed. Photoaccumulation experiments with extensive flashing at 77 K showed similar results, with no or very little induction of the secondary electron donors. Thus, the partition ratio defined as (yield of YZ/CaMn4O5-cluster oxidation):(yield of Cytb559/ChlZ/CarD2 oxidation) was found to be 0.4 at under visible light and 1.7 at under far-red light at 77 K. Our data indicate that different products of charge separation after far-red light exists in the wide temperature range which further support the model of the different primary photochemistry in Photosystem II with localization of hole on the ChlD1 molecule.
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| 19. |
- Pavlou, Andrea, et al.
(författare)
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The wavelength of the incident light determines the primary charge separation pathway in Photosystem II
- 2018
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- Charge separation is a key component of the reactions cascade of photosynthesis, by which solar energy is converted to chemical energy. From this photochemical reaction, two radicals of opposite charge are formed, a highly reducing anion and a highly oxidising cation. We have previously proposed that the cation after far-red light excitation is located on a component different from P-D1, which is the location of the primary electron hole after visible light excitation. Here, we attempt to provide further insight into the location of the primary charge separation upon far-red light excitation of PS II, using the EPR signal of the spin polarized P-3(680) as a probe. We demonstrate that, under far-red light illumination, the spin polarized P-3(680) is not formed, despite the primary charge separation still occurring at these conditions. We propose that this is because under far-red light excitation, the primary electron hole is localized on Chl(D1), rather than on P-D1. The fact that identical samples have demonstrated charge separation upon both far-red and visible light excitation supports our hypothesis that two pathways for primary charge separation exist in parallel in PS II reaction centres. These pathways are excited and activated dependent of the wavelength applied.
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| 20. |
- Shevela, Dmitriy, 1979-, et al.
(författare)
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'Birth defects' of photosystem II make it highly susceptible to photodamage during chloroplast biogenesis
- 2019
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Ingår i: Physiologia Plantarum. - : Wiley-Blackwell. - 0031-9317 .- 1399-3054. ; 166:1, s. 165-180
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Tidskriftsartikel (refereegranskat)abstract
- High solar flux is known to diminish photosynthetic growth rates, reducing biomass productivity and lowering disease tolerance. Photosystem II (PSII) of plants is susceptible to photodamage (also known as photoinactivation) in strong light, resulting in severe loss of water oxidation capacity and destruction of the water‐oxidizing complex (WOC). The repair of damaged PSIIs comes at a high energy cost and requires de novo biosynthesis of damaged PSII subunits, reassembly of the WOC inorganic cofactors and membrane remodeling. Employing membrane‐inlet mass spectrometry and O2‐polarography under flashing light conditions, we demonstrate that newly synthesized PSII complexes are far more susceptible to photodamage than are mature PSII complexes. We examined these ‘PSII birth defects’ in barley seedlings and plastids (etiochloroplasts and chloroplasts) isolated at various times during de‐etiolation as chloroplast development begins and matures in synchronization with thylakoid membrane biogenesis and grana membrane formation. We show that the degree of PSII photodamage decreases simultaneously with biogenesis of the PSII turnover efficiency measured by O2‐polarography, and with grana membrane stacking, as determined by electron microscopy. Our data from fluorescence, QB‐inhibitor binding, and thermoluminescence studies indicate that the decline of the high‐light susceptibility of PSII to photodamage is coincident with appearance of electron transfer capability QA− → QB during de‐etiolation. This rate depends in turn on the downstream clearing of electrons upon buildup of the complete linear electron transfer chain and the formation of stacked grana membranes capable of longer‐range energy transfer.
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| 21. |
- Shylin, Sergii I., et al.
(författare)
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Efficient visible light-driven water oxidation catalysed by an iron(IV) clathrochelate complex
- 2019
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Ingår i: Chemical Communications. - 1359-7345 .- 1364-548X. ; 55:23, s. 3335-3338
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Tidskriftsartikel (refereegranskat)abstract
- A water-stable FeIV clathrochelate complex catalyzes fast and homogeneous photochemical oxidation of water to dioxygen with a turnover frequency of 2.27 s−1 and a maximum turnover number of 365. An FeV intermediate generated under catalytic conditions is trapped and characterised using EPR and Mössbauer spectroscopy.
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| 22. |
- Sterby, Mia, 1989-, et al.
(författare)
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Investigating electron transport in a PEDOT/Quinone conducting redox polymer with in situ methods
- 2019
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 308, s. 277-284
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Tidskriftsartikel (refereegranskat)abstract
- A conducting redox polymer is investigated in acidic electrolyte using various in situ methods, including electron paramagnetic resonance (EPR), UV–vis spectroscopy, and conductance measurements. The quinone redox active pendant group has a formal potential of 0.67 V (vs. standard hydrogen electrode) where a 2e2H process occurs. By analyzing the rate constant at different temperatures, the rate-limiting step in the redox reaction was found to be a thermally activated process with an activation energy of 0.3 eV. The electron transport through the conducting polymerwas found to be non-thermally activated and, hence, not redox rate-limiting. This is also the first time a negative temperature dependence has been reported for a conducting redox polymer in the same potential region where the redox active pendant group has its formal potential. EPR and conductance data indicated that the conductivity is governed by both polarons and bipolarons but their ratio is shifting during oxidation and reduction of the polymer.
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| 23. |
- Wang, Huan, et al.
(författare)
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A conducting additive-free high potential quinone-based conducting redox polymer as lithium ion battery cathode
- 2021
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Ingår i: Electrochimica Acta. - : Elsevier. - 0013-4686 .- 1873-3859. ; 391
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Tidskriftsartikel (refereegranskat)abstract
- Organic carbonyl compounds have been considered as promising alternatives to traditional inorganic battery materials due to their low-cost, sustainability and structural diversity. The development of carbonyl compounds as energy storage materials is, however, hampered by dissolution as well as by the low electronic conductivity of these materials. Herein a conducting redox polymer concept is employed where the carbonyl group is functionalized onto a conducting polymer. The utilization of a conducting polymer prevents the dissolution and provides electron transport pathways to support the carbonyl group redox reaction. A high potential quinizarin (Qz) is used as capacity-carrying group. It is functionalized onto a thiophene-based trimer unit which is polymerized through a post-deposition polymerization method. In the resulting material, Qz is redox-matched with the conducting polymer backbone and exhibits two reversible 1e/1Li + redox processes at 3.1 and 3.4 V vs. Li + /0 , respectively. Together with a lithium metal anode, a battery cell with an average discharge voltage of 3.3 V, a discharge capacity of 65 mAh/g at 1.5 C and a capacity retention of 74% after 500 cycles is assembled.
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| 24. |
- Wang, Huan, et al.
(författare)
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Redox-State-Dependent Interplay between Pendant Group and Conducting Polymer Backbone in Quinone-Based Conducting Redox Polymers for Lithium Ion Batteries
- 2019
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Ingår i: ACS Applied Energy Materials. - : American Chemical Society (ACS). - 2574-0962. ; 2:10, s. 7162-7170
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Tidskriftsartikel (refereegranskat)abstract
- Conducting redox polymers (CRPs) have attracted increased interest in recent years because of the possibility of combining the favorable electron-transport properties of conducting polymers with the additional functionality provided by the redox active pendant groups (PGs). Herein we present a series of quinone-substituted PEDOT-CRPs where the quinone PGs have been substituted by electron-withdrawing substituents. Introducing electron-withdrawing substituents leads to an increase of the quinone formal potential, making, for example, CF3-substituted CRPs, a promising high-voltage cathode material for lithium ion batteries with a well-defined charge/discharge plateau around 3 V vs Li+/Li0. Interestingly, we find a shift in conductance onset potential concomitant with the quinone formal potential shift, indicating that the polymer backbone conductance is intimately associated with the PG redox chemistry. Through in situ UV–vis, electron paramagnetic resonance (EPR), and electrochemical quartz crystal microbalance experiments as well as by experiments in lithium- and tert-butyl-ammonium-based electrolytes, we show that the conductance delay is caused by the reduced lithiated quinone state, most likely by localizing the polaron charge carrier as indicated by EPR and UV–vis experiments.
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