Artículos de revistas: "Antenne monomeriche"

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Crepin, Aurélie, Erica Belgio, Barbora Šedivá, Eliška Kuthanová Trsková, Edel Cunill-Semanat y Radek Kaňa. "Size and Fluorescence Properties of Algal Photosynthetic Antenna Proteins Estimated by Microscopy". International Journal of Molecular Sciences 23, n.º 2 (11 de enero de 2022): 778. http://dx.doi.org/10.3390/ijms23020778.

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Antenna proteins play a major role in the regulation of light-harvesting in photosynthesis. However, less is known about a possible link between their sizes (oligomerization state) and fluorescence intensity (number of photons emitted). Here, we used a microscopy-based method, Fluorescence Correlation Spectroscopy (FCS), to analyze different antenna proteins at the particle level. The direct comparison indicated that Chromera Light Harvesting (CLH) antenna particles (isolated from Chromera velia) behaved as the monomeric Light Harvesting Complex II (LHCII) (from higher plants), in terms of their radius (based on the diffusion time) and fluorescence yields. FCS data thus indicated a monomeric oligomerization state of algal CLH antenna (at our experimental conditions) that was later confirmed also by biochemical experiments. Additionally, our data provide a proof of concept that the FCS method is well suited to measure proteins sizes (oligomerization state) and fluorescence intensities (photon counts) of antenna proteins per single particle (monomers and oligomers). We proved that antenna monomers (CLH and LHCIIm) are more “quenched” than the corresponding trimers. The FCS measurement thus represents a useful experimental approach that allows studying the role of antenna oligomerization in the mechanism of photoprotection.

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Miloslavina, Yuliya, Silvia de Bianchi, Luca Dall'Osto, Roberto Bassi y Alfred R. Holzwarth. "Quenching in Arabidopsis thaliana Mutants Lacking Monomeric Antenna Proteins of Photosystem II". Journal of Biological Chemistry 286, n.º 42 (15 de agosto de 2011): 36830–40. http://dx.doi.org/10.1074/jbc.m111.273227.

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Ballottari, Matteo, Milena Mozzo, Julien Girardon, Rainer Hienerwadel y Roberto Bassi. "Chlorophyll Triplet Quenching and Photoprotection in the Higher Plant Monomeric Antenna Protein Lhcb5". Journal of Physical Chemistry B 117, n.º 38 (8 de julio de 2013): 11337–48. http://dx.doi.org/10.1021/jp402977y.

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KUREISHI, YASUHIKO y HITOSHI TAMIAKI. "Synthesis and Self-aggregation of Zinc 20-Halogenochlorins as a Model for Bacteriochlorophylls c/d". Journal of Porphyrins and Phthalocyanines 02, n.º 02 (marzo de 1998): 159–69. http://dx.doi.org/10.1002/(sici)1099-1409(199803/04)2:2<159::aid-jpp62>3.0.co;2-q.

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Zinc 20-halogenochlorins 2(20- F ), 3(20- Cl ) and 4(20- Br ) were synthesized by halogenation of a chlorophyll a derivative at the 20-position as a model for bacteriochlorophyll ( BChl )c, which possesses a methyl group at the 20-position and 20-unsubstituted BChl d. Visible spectra in a polar tetrahydrofuran ( THF ) solution showed that 2-4 were monomeric and the planarity of the chlorin ring, was distorted with increasing bulkiness of the 20-substituent. Visible, circular dichroism and IR spectra revealed that 2-4 self-aggregated to form oligomers similarly with 20-unsubstituted 1 and BChls c/d in the heterogeneous thin film as well as in homogeneous non-polar solvents (1% (v/v) THF-hexane). Therefore, the in vitro self-aggregates of 2-4 are good structural models for in vivo BChls c/d self-aggregates, the main antenna components of photosynthetic green bacteria. Fluorescence spectra showed that monomeric 3 and 4 were less emissive than 1 and 2 due to the heavy atom effect which could not be observed in the oligomeric species, indicating that the in vitro aggregates should be promising as functional (light-harvesting) models.

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Pi, Xiong, Songhao Zhao, Wenda Wang, Desheng Liu, Caizhe Xu, Guangye Han, Tingyun Kuang, Sen-Fang Sui y Jian-Ren Shen. "The pigment-protein network of a diatom photosystem II–light-harvesting antenna supercomplex". Science 365, n.º 6452 (1 de agosto de 2019): eaax4406. http://dx.doi.org/10.1126/science.aax4406.

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Diatoms play important roles in global primary productivity and biogeochemical cycling of carbon, in part owing to the ability of their photosynthetic apparatus to adapt to rapidly changing light intensity. We report a cryo–electron microscopy structure of the photosystem II (PSII)–fucoxanthin (Fx) chlorophyll (Chl) a/c binding protein (FCPII) supercomplex from the centric diatom Chaetoceros gracilis. The supercomplex comprises two protomers, each with two tetrameric and three monomeric FCPIIs around a PSII core that contains five extrinsic oxygen-evolving proteins at the lumenal surface. The structure reveals the arrangement of a huge pigment network that contributes to efficient light energy harvesting, transfer, and dissipation processes in the diatoms.

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Hofmann, E., T. Schulte, S. P. Sharples y R. G. Hiller. "High-salt peridinin-chlorophyll-protein fromA. carterae: the structure of the monomeric antenna protein complex". Acta Crystallographica Section A Foundations of Crystallography 62, a1 (6 de agosto de 2006): s137. http://dx.doi.org/10.1107/s0108767306097261.

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Mascoli, Vincenzo, Vladimir Novoderezhkin, Nicoletta Liguori, Pengqi Xu y Roberta Croce. "Design principles of solar light harvesting in plants: Functional architecture of the monomeric antenna CP29". Biochimica et Biophysica Acta (BBA) - Bioenergetics 1861, n.º 3 (marzo de 2020): 148156. http://dx.doi.org/10.1016/j.bbabio.2020.148156.

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Squires, Allison H. y W. E. Moerner. "Direct single-molecule measurements of phycocyanobilin photophysics in monomeric C-phycocyanin". Proceedings of the National Academy of Sciences 114, n.º 37 (28 de agosto de 2017): 9779–84. http://dx.doi.org/10.1073/pnas.1705435114.

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Phycobilisomes are highly organized pigment–protein antenna complexes found in the photosynthetic apparatus of cyanobacteria and rhodophyta that harvest solar energy and transport it to the reaction center. A detailed bottom-up model of pigment organization and energy transfer in phycobilisomes is essential to understanding photosynthesis in these organisms and informing rational design of artificial light-harvesting systems. In particular, heterogeneous photophysical behaviors of these proteins, which cannot be predicted de novo, may play an essential role in rapid light adaptation and photoprotection. Furthermore, the delicate architecture of these pigment–protein scaffolds sensitizes them to external perturbations, for example, surface attachment, which can be avoided by study in free solution or in vivo. Here, we present single-molecule characterization of C-phycocyanin (C-PC), a three-pigment biliprotein that self-assembles to form the midantenna rods of cyanobacterial phycobilisomes. Using the Anti-Brownian Electrokinetic (ABEL) trap to counteract Brownian motion of single particles in real time, we directly monitor the changing photophysical states of individual C-PC monomers from Spirulina platensis in free solution by simultaneous readout of their brightness, fluorescence anisotropy, fluorescence lifetime, and emission spectra. These include single-chromophore emission states for each of the three covalently bound phycocyanobilins, providing direct measurements of the spectra and photophysics of these chemically identical molecules in their native protein environment. We further show that a simple Förster resonant energy transfer (FRET) network model accurately predicts the observed photophysical states of C-PC and suggests highly variable quenching behavior of one of the chromophores, which should inform future studies of higher-order complexes.

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Caffarri, Stefano, Francesca Passarini, Roberto Bassi y Roberta Croce. "A specific binding site for neoxanthin in the monomeric antenna proteins CP26 and CP29 of Photosystem II". FEBS Letters 581, n.º 24 (4 de septiembre de 2007): 4704–10. http://dx.doi.org/10.1016/j.febslet.2007.08.066.

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10

Ballottari, Matteo, Julien Girardon, Nico Betterle, Tomas Morosinotto y Roberto Bassi. "Identification of the Chromophores Involved in Aggregation-dependent Energy Quenching of the Monomeric Photosystem II Antenna Protein Lhcb5". Journal of Biological Chemistry 285, n.º 36 (28 de junio de 2010): 28309–21. http://dx.doi.org/10.1074/jbc.m110.124115.

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11

Sumaoka, Jun, Hiroki Akiba y Makoto Komiyama. "Selective Sensing of Tyrosine Phosphorylation in Peptides Using Terbium(III) Complexes". International Journal of Analytical Chemistry 2016 (2016): 1–14. http://dx.doi.org/10.1155/2016/3216523.

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Phosphorylation of tyrosine residues in proteins, as well as their dephosphorylation, is closely related to various diseases. However, this phosphorylation is usually accompanied by more abundant phosphorylation of serine and threonine residues in the proteins and covers only 0.05% of the total phosphorylation. Accordingly, highly selective detection of phosphorylated tyrosine in proteins is an urgent subject. In this review, recent developments in this field are described. Monomeric and binuclearTbIIIcomplexes, which emit notable luminescence only in the presence of phosphotyrosine (pTyr), have been developed. There, the benzene ring of pTyr functions as an antenna and transfers its photoexcitation energy to theTbIIIion as the emission center. Even in the coexistence of phosphoserine (pSer) and phosphothreonine (pThr), pTyr can be efficintly detected with high selectivity. Simply by adding theseTbIIIcomplexes to the solutions, phosphorylation of tyrosine in peptides by protein tyrosine kinases and dephosphorylation by protein tyrosine phosphatases can be successfully visualized in a real-time fashion. Furthermore, the activities of various inhibitors on these enzymes are quantitatively evaluated, indicating a strong potential of the method for efficient screening of eminent inhibitors from a number of candidates.

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12

Kobuke, Yoshiaki. "Porphyrin supramolecules by complementary coordination for units constructing photosynthetic systems". Journal of Porphyrins and Phthalocyanines 08, n.º 02 (febrero de 2004): 156–74. http://dx.doi.org/10.1142/s1088424604000155.

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Among unique structural arrangements provided by nature, special pair and macroring antenna complexes in bacterial photosynthetic systems have been mimicked by simple organization of choromophores. The special pair was mimicked by imidazolyl-substituted porphyrinatozinc which gave a complementary coordination dimer of slipped cofacial orientation with an extremely large stability constant of 1010 M-1 in CHCl 3. When two imidazolylporphyrinatozinc units were linked directly at the meso positions, linear and continuous growth of the complementary coordination lead to porphyrin arrays of a few hundreds nanometer scale, corresponding to molecular weights of a few 105. At the same time, the porphyrin array could be dissociated into the monomeric unit by competitive coordination of solvents. This provided a way of terminating or initiating the oligomeric porphyrin array with appropriate chain terminals/initiators. Two imidazolylporphyrinatozinc complexes were then linked by a m-phenylene unit with an angle of 120°. The linear and macrocyclic oligomer mixtures initially obtained were converged smoothly by reorganization equilibrium into a mixture of hexameric and pentameric macrocycles under high-dilution conditions. The ring mimicked the structure and function of the light harvesting complexes of bacterial photosynthetic systems. The covalent linking of coordination organized porphyrins was also developed to maintain the structure even in highly coordinating solvents such as pyridine. The linear array formation and the facile introduction of specific terminal/initiator groups by complementary coordination were then applied to introduce antenna function onto solar cell. Through thiolate attachment on a gold surface, imidazolylporphyrinatozinc initiated the growth of meso-meso linked porphyrin arrays by successive complementary coordination. This methodology improved the efficiency of absorption of incident photon and increased significantly the total photocurrent generation efficiency.

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13

Betterle, Nico, Matteo Ballottari, Rainer Hienerwadel, Luca Dall’Osto y Roberto Bassi. "Dynamics of zeaxanthin binding to the photosystem II monomeric antenna protein Lhcb6 (CP24) and modulation of its photoprotection properties". Archives of Biochemistry and Biophysics 504, n.º 1 (diciembre de 2010): 67–77. http://dx.doi.org/10.1016/j.abb.2010.05.016.

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14

Dannenbauer, Nicole, Ana Kuzmanoski, Claus Feldmann y Klaus Müller-Buschbaum. "1,3-Thiazole as Suitable Antenna Ligand for Lanthanide Photoluminescence in [LnCl3(thz)4]·0.5thz, Ln = Sm, Eu, Gd, Tb, Dy". Zeitschrift für Naturforschung B 69, n.º 2 (1 de febrero de 2014): 255–62. http://dx.doi.org/10.5560/znb.2014-3292.

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The series of luminescent monomeric lanthanide thiazole complexes [LnCl3(thz)4]⋅0.5thz (Ln = Sm, Eu, Gd, Tb, Dy; thz=1,3-thiazole) has been synthesised and characterised by powder and singlecrystal X-ray diffraction, IR and photoluminescence spectroscopy, DTA/TG as well as elemental analysis. The colourless compounds exhibit photoluminescence in the visible region with varying quantum efficiencies up to QY = 48% for [TbCl3(thz)4]⋅0.5thz. Both, the lanthanide ions as well as the thiazole ligand contribute to the luminescence. Excitation can be achieved via intra-4 f transitions and by exciting the ligand, emission is observed mainly from the lanthanide ions again by 4 f transitions. Thiazole can transfer energy to the lanthanide ions, which further feeds the lanthanide emission by an efficient antenna effect even at room temperature. The lanthanide ions show pentagonalbipyramidal coordination by three chloride anions and four N atoms of 1,3-thiazole, which leads to a strong 5D0 →7F4 transition for europium. Significant differences arise as compared to thiophene complexes because no sulphur atom is involved in the metal coordination, as the thiazole ligand is solely coordinated via its nitrogen function.

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15

Gerotto, Caterina, Andrea Trotta, Azfar Ali Bajwa, Tomas Morosinotto y Eva-Mari Aro. "Role of Serine/Threonine Protein Kinase Stn7 in the Formation of Two Distinct Photosystem I Supercomplexes in Physcomitrium patens". Plant Physiology, 23 de junio de 2022. http://dx.doi.org/10.1093/plphys/kiac294.

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Abstract Reversible thylakoid protein phosphorylation provides most flowering plants with dynamic acclimation to short-term changes in environmental light conditions. Here, through generating Serine/Threonine protein kinase 7 (STN7)-depleted mutants in the moss Physcomitrella (Physcomitrium patens), we identified phosphorylation targets of STN7 kinase and their roles in short- and long-term acclimation of the moss to changing light conditions. Biochemical and mass spectrometry analyses revealed STN7-dependent phosphorylation of N-terminal Thr in specific Light Harvesting Complex II (LHCII) trimer subunits (LHCBM2 and LHCBM4/8) and provided evidence that phospho-LHCBM accumulation is responsible for the assembly of two distinct Photosystem I (PSI) supercomplexes, both of which are largely absent in STN7-depleted mutants. Besides the canonical state transition complex (PSI-LHCI-LHCII), we isolated the larger moss-specific PSI-Large (PSI-LHCI-LHCB9-LHCII) from stroma-exposed thylakoids. Unlike PSI-LHCI-LHCII, PSI-Large did not demonstrate short-term dynamics for balancing the distribution of excitation energy between PSII and PSI. Instead, PSI-Large contributed to a more stable increase in PSI antenna size in Physcomitrella, except under prolonged high irradiance. Additionally, the STN7-depleted mutants revealed altered light-dependent phosphorylation of a monomeric antenna protein, LHCB6, whose phosphorylation displayed a complex regulation by multiple kinases. Collectively, the unique phosphorylation plasticity and dynamics of Physcomitrella monomeric LHCB6 and trimeric LHCBM isoforms, together with the presence of PSI supercomplexes with different antenna size and responsiveness to light changes, reflect the evolutionary position of mosses between green algae and vascular plants, yet with clear moss-specific features emphasizing their adaptation to terrestrial low-light environments.

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Akhtar, Parveen, Avratanu Biswas, Fanny Balog-Vig, Ildikó Domonkos, László Kovács y Petar H. Lambrev. "Trimeric photosystem I facilitates energy transfer from phycobilisomes in Synechocystis sp. PCC 6803". Plant Physiology, 18 de marzo de 2022. http://dx.doi.org/10.1093/plphys/kiac130.

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Abstract In cyanobacteria, phycobilisomes (PBS) serve as peripheral light-harvesting complexes of the two photosystems, extending their antenna size and the wavelength range of photons available for photosynthesis. The abundance of PBS, the number of phycobiliproteins they contain, and their light-harvesting function are dynamically adjusted in response to the physiological conditions. PBS are also thought to be involved in state transitions that maintain the excitation balance between the two photosystems. Unlike its eukaryotic counterpart, PSI is trimeric in many cyanobacterial species and the physiological significance of this is not well understood. Here, we compared the composition and light-harvesting function of PBS in cells of Synechocystis sp. PCC 6803, which has primarily trimeric PSI, and the ΔpsaL mutant, which lacks the PsaL subunit of PSI and is unable to form trimers. We also investigated a mutant additionally lacking the PsaJ and PsaF subunits of PSI. Both strains with monomeric PSI accumulated significantly more allophycocyanin per chlorophyll, indicating higher abundance of PBS. On the other hand, a higher phycocyanin:allophycocyanin ratio in the wild type suggests larger PBS or the presence of APC-less PBS (CpcL-type) that are not assembled in cells with monomeric PSI. Steady-state and time-resolved fluorescence spectroscopy at room temperature and 77 K revealed that PSII receives more energy from the PBS at the expense of PSI in cells with monomeric PSI, regardless of the presence of PsaF. Taken together, these results show that the oligomeric state of PSI impacts the excitation energy flow in Synechocystis.

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Mazor, Yuval, Daniel Nataf, Hila Toporik y Nathan Nelson. "Crystal structures of virus-like photosystem I complexes from the mesophilic cyanobacterium Synechocystis PCC 6803". eLife 3 (28 de enero de 2014). http://dx.doi.org/10.7554/elife.01496.

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Oxygenic photosynthesis supports virtually all life forms on earth. Light energy is converted by two photosystems—photosystem I (PSI) and photosystem II (PSII). Globally, nearly 50% of photosynthesis takes place in the Ocean, where single cell cyanobacteria and algae reside together with their viruses. An operon encoding PSI was identified in cyanobacterial marine viruses. We generated a PSI that mimics the salient features of the viral complex, named PSIPsaJF. PSIPsaJF is promiscuous for its electron donors and can accept electrons from respiratory cytochromes. We solved the structure of PSIPsaJF and a monomeric PSI, with subunit composition similar to the viral PSI, providing for the first time a detailed description of the reaction center and antenna system from mesophilic cyanobacteria, including red chlorophylls and cofactors of the electron transport chain. Our finding extends the understanding of PSI structure, function and evolution and suggests a unique function for the viral PSI.

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Xu, Caizhe, Xiong Pi, Yawen Huang, Guangye Han, Xiaobo Chen, Xiaochun Qin, Guoqiang Huang et al. "Structural basis for energy transfer in a huge diatom PSI-FCPI supercomplex". Nature Communications 11, n.º 1 (8 de octubre de 2020). http://dx.doi.org/10.1038/s41467-020-18867-x.

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Abstract Diatom is an important group of marine algae and contributes to around 20% of the global photosynthetic carbon fixation. Photosystem I (PSI) of diatoms is associated with a large number of fucoxanthin-chlorophyll a/c proteins (FCPIs). We report the structure of PSI-FCPI from a diatom Chaetoceros gracilis at 2.38 Å resolution by single-particle cryo-electron microscopy. PSI-FCPI is a monomeric supercomplex consisting of 12 core and 24 antenna subunits (FCPIs), and 326 chlorophylls a, 34 chlorophylls c, 102 fucoxanthins, 35 diadinoxanthins, 18 β-carotenes and some electron transfer cofactors. Two subunits designated PsaR and PsaS were found in the core, whereas several subunits were lost. The large number of pigments constitute a unique and huge network ensuring efficient energy harvesting, transfer and dissipation. These results provide a firm structural basis for unraveling the mechanisms of light-energy harvesting, transfer and quenching in the diatom PSI-FCPI, and also important clues to evolutionary changes of PSI-LHCI.

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Tang-Siegel, Gaoyan G., David R. Danforth, Jake Tristano, Teresa Ruiz y Keith P. Mintz. "The serotype a-EmaA adhesin of Aggregatibacter actinomycetemcomitans does not require O-PS synthesis for collagen binding activity". Microbiology 168, n.º 5 (12 de mayo de 2022). http://dx.doi.org/10.1099/mic.0.001191.

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Aggregatibacter actinomycetemcomitans , a causative agent of periodontitis and non-oral diseases, synthesizes a trimeric extracellular matrix protein adhesin A (EmaA) that mediates collagen binding and biofilm formation. EmaA is found as two molecular forms, which correlate with the serotype of the bacterium. The canonical protein (b-EmaA), associated with serotypes b and c, has a monomeric molecular mass of 202 kDa. The collagen binding activity of b-EmaA is dependent on the presence of O-polysaccharide (O-PS), whereas biofilm activity is independent of O-PS synthesis. The EmaA associated with serotype a strains (a-EmaA) has a monomeric molecular mass of 173 kDa and differs in the amino acid sequence of the functional domain of the protein. In this study, a-emaA was confirmed to encode a protein that forms antenna-like appendages on the surface of the bacterium, which were found to be important for both collagen binding and biofilm formation. In an O-PS-deficient talose biosynthetic (tld) mutant strain, the electrophoretic mobility of the a-EmaA monomers was altered and the amount of membrane-associated EmaA was decreased when compared to the parent strain. The mass of biofilm formed remained unchanged. Interestingly, the collagen binding activity of the mutant strain was similar to the activity associated with the parent strain, which differs from that observed with the canonical b-EmaA isoform. These data suggest that the properties of the a-EmaA isoform are like those of b-EmaA, with the exception that collagen binding activity is independent of the presence or absence of the O-PS.

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20

Ilíková, Iva, Petr Ilík, Monika Opatíková, Rameez Arshad, Lukáš Nosek, Václav Karlický, Zuzana Kučerová et al. "Towards spruce-type photosystem II: consequences of the loss of light-harvesting proteins LHCB3 and LHCB6 in Arabidopsis". Plant Physiology, 1 de septiembre de 2021. http://dx.doi.org/10.1093/plphys/kiab396.

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Abstract The largest stable photosystem II (PSII) supercomplex in land plants (C2S2M2) consists of a core complex dimer (C2), two strongly (S2) and two moderately (M2) bound light-harvesting protein (LHCB) trimers attached to C2 via monomeric antenna proteins LHCB4–6. Recently, we have shown that LHCB3 and LHCB6, presumably essential for land plants, are missing in Norway spruce (Picea abies), which results in a unique structure of its C2S2M2 supercomplex. Here, we performed structure–function characterization of PSII supercomplexes in Arabidopsis (Arabidopsis thaliana) mutants lhcb3, lhcb6, and lhcb3 lhcb6 to examine the possibility of the formation of the “spruce-type” PSII supercomplex in angiosperms. Unlike in spruce, in Arabidopsis both LHCB3 and LHCB6 are necessary for stable binding of the M trimer to PSII core. The “spruce-type” PSII supercomplex was observed with low abundance only in the lhcb3 plants and its formation did not require the presence of LHCB4.3, the only LHCB4-type protein in spruce. Electron microscopy analysis of grana membranes revealed that the majority of PSII in lhcb6 and namely in lhcb3 lhcb6 mutants were arranged into C2S2 semi-crystalline arrays, some of which appeared to structurally restrict plastoquinone diffusion. Mutants without LHCB6 were characterized by fast induction of non-photochemical quenching and, on the contrary to the previous lhcb6 study, by only transient slowdown of electron transport between PSII and PSI. We hypothesize that these functional changes, associated with the arrangement of PSII into C2S2 arrays in thylakoids, may be important for the photoprotection of both PSI and PSII upon abrupt high-light exposure.

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"Zeolite Microcrystals as Hosts for Supramolecular Organization of Dye Molecules". CHIMIA 52, n.º 10 (28 de octubre de 1998): 525. http://dx.doi.org/10.2533/chimia.1998.525.

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Zeolite microcrystals can act as host for supramolecular organization of molecules, complexes, clusters, and quantum-size particles. They allow the design of precise and reversible functionalities. Techniques for arranging zeolite microcrystals of good quality and narrow size distribution as dense monograin layers on different substrates can be used to realize specific properties. The chemical reactivity between the intercalated molecules offers possibilities for in situ synthesis of molecular chains, clusters, and quantum-size particles, which might not be accessible otherwise. In some cases, guest-host reactivity must be considered. The reactivity of intercalated compounds with (small) molecules penetrating from the outside is an option for changing the composition of a material, i.e., molecules intercalated as monomers in a first step can be linked to form chains. New electronic structures are accessible either by specific geometrical arrangements made possible by the structure of the host and/or by explicitly involving its electronic properties. Some systems meet the conditions necessary for the occurrence of intrazeolite charge transport (ionic and electronic), realized by the guests in their ground state and in electronically excited states under high-vacuum conditions or in the presence of a solvent, depending on the composition and the structure of the material. In this article, we focus on organic dye molecules in the one-dimensional channels of zeolites with a hexagonal framework. This system consists of supramolecularly organized dye molecules. It is shown to provide fascinating possibilities for building an artificial antenna device which consists of highly concentrated monomeric dye molecules of up to 0.4M with a large Förster energy-transfer radius and a high luminescence quantum yield in an ideal geometrical arrangement of optimal size. Extremely fast electronic excitation-energy transport has been demonstrated by us in oxonine- and pyronine-dye-loaded zeolite L microcrystals. Many other highly organized dye-zeolite materials can be prepared, and they are expected to show a wide variety of challenging properties. We report on methods to distinguish between dye molecules which are inside of a microcrystal and those adsorbed on its outer surface, and we explain a demonstration experiment illustrating the intercalation of thionine into zeolite L and the thus resulting improved chemical stability of this dye.

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22

"The origin of chlorophyll fluorescence In vivo and its quenching by the photosystem II reaction centre". Philosophical Transactions of the Royal Society of London. B, Biological Sciences 323, n.º 1216 (12 de abril de 1989): 227–39. http://dx.doi.org/10.1098/rstb.1989.0006.

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Isolated chlorophyll a , in contrast to when it is dissolved in organic solvents, shows a lower and variable yield of fluorescence when bound to protein and embedded in the thylakoid membrane of photosynthetic organisms. There are two current theories that attempt to explain the origin of this variable yield of fluorescence, (i) It may be emitted directly from the photosystem II (PSII) antenna system and therefore in competition with photochemical trapping (prompt fluorescence), (ii) It may be derived from a recombination reaction between oxidized P 680 and reduced pheophytin within the PS II reaction centre (delayed fluorescence). We have isolated a PS II reaction centre complex that binds only four chlorophyll a molecules and can carry out primary charge separation. The complex contains no plastoquinone and therefore is devoid of the secondary electron acceptor Q A . It does, however, contain two pheophytin a molecules, and one of these acts as a primary electron acceptor. The electron donor is P 680 , which is either a monomeric or dimeric form of chlorophyll a . The isolated PS II reaction centre fluoresces at room temperature with a maximum at 683 nm, and the intensity of this emission is almost totally quenched when reduced pheophytin (bright light plus sodium dithionite) or oxidized P 680 (bright light plus silicomolybdate) is photoaccumulated. The photo-induced quenching of chlorophyll fluorescence when sodium dithionite is present is also observed in intact PS II preparations containing plastoquinone Q A . In the latter case Q A is chemically reduced in the dark by dithionite. Bearing in mind the above two postulates for the origin of variable chlorophyll fluorescence it has been possible to investigate the relative quantum yields for the photoproduction of the P 680 Pheo - state either in the absence (with isolated PS II reaction centres) or presence (with PSII-enriched membranes) of reduced Q A . It has been shown that in the absence of Q - A the quantum efficiency for production of the P 680 Pheo - is several orders of magnitude greater than when Q - A is present. This difference probably partly reflects the coulombic restraints on primary charge separation when Q A is reduced and would suggest that under these conditions the PS II reaction centre is a less efficient trap. Such a conclusion is therefore consistent with postulate (i) that the increase inyield of chlorophyll fluorescence as Q A becomes reduced is not due to a back reaction between P + 680 and Pheo - but rather to a decrease in competition between emission and trapping. The results do emphasize however, that the P 680 Pheo - and P + 680 Pheo states are quenchers of chlorophyll fluorescence. In addition to the above, it has been noted that at 77 K fluorescence from the isolated PS II reaction centre reaches a maximum at 685 nm and does not have a peak at 695 nm. This observation appears to invalidate the postulate that the 695 nm emission is from the pheophytin of the PS II reaction centre.

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Artículos de revistas sobre el tema "Antenne monomeriche"

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