Antenne collectrice

Résumé

A light-harvesting complex consists of a number of chromophores which are complex subunit proteins that may be part of a larger super complex of a photosystem, the functional unit in photosynthesis. It is used by plants and photosynthetic bacteria to collect more of the incoming light than would be captured by the photosynthetic reaction center alone. The light which is captured by the chromophores is capable of exciting molecules from their ground state to a higher energy state, known as the excited state. This excited state does not last very long and is known to be short-lived. Light-harvesting complexes are found in a wide variety among the different photosynthetic species, with no homology among the major groups. The complexes consist of proteins and photosynthetic pigments and surround a photosynthetic reaction center to focus energy, attained from photons absorbed by the pigment, toward the reaction center using Förster resonance energy transfer. Photosynthesis is a process where light is absorbed or harvested by pigment protein complexes which are able to turn sunlight into energy. Absorption of a photon by a molecule takes place when pigment protein complexes harvest sunlight leading to electronic excitation delivered to the reaction centre where the process of charge separation can take place. when the energy of the captured photon matches that of an electronic transition. The fate of such excitation can be a return to the ground state or another electronic state of the same molecule. When the excited molecule has a nearby neighbour molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another. This process is called resonance energy transfer, and the rate depends strongly on the distance between the energy donor and energy acceptor molecules. Before an excited photon can transition back to ground state, the energy needs to be harvested. This excitation is transferred among chromophores where it is delivered to the reaction centre.

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https://fr.wikipedia.org/wiki/Antenne_collectrice

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Two new dyes based on a dinuclear rhenium complex and (E)-3-(5-(4-(bis(2,4-dibutoxy-[1,1-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid (namely D35) have been investigated as sensitizers for dye sensitized solar cells (DSSCs). Two different pyridazine ligands have been used, namely 4-pyridazine-carboxylic acid for dye 2 ([Re-2(-H)(-D35)(CO)(6)(-pyridazine-4-COOH)]) and 4-pyridazinyl-butanoic acid for dye 3 ([Re-2(-H)(-D35)(CO)(6)(-pyridazine-4-C3H6-COOH)]). The performances of these new dyes have been compared with those of the dye containing the bare 4-diphenylaminobenzoic acid, namely TPA, as the ancillary ligand (dye 1). Compared to dye 1, dyes 2 and 3 show an impressive tenfold increase in the absorption intensity in the range of 487-493 nm on TiO2 films, with great improvement of the light harvesting. Cyclic voltammetry experiments, performed on derivatives containing the methyl ester of the pyridazine ligands, show narrow electrochemical band gaps in the range of 1.36-1.84 eV. Solar cells with each dye have been prepared, using both iodide/triiodide and cobalt redox couples as the electrolytes, platinum or carbon as the counter electrodes, and TiO2 or SnO2 as the metal oxide photoelectrodes, respectively. The best DSSC results have been obtained using dye 3, with an overall solar-to-electric conversion efficiency of 3.5%, which greatly overcomes the previous result of 1.0% obtained for dye 1 in a not-optimized setup of the device. The performances of dye 3 are due to the presence of D35 ligand, which further suppresses the recombination of the injected electron with the electrolyte and with the oxidized state of the dye.

ROYAL SOC CHEMISTRY2019

Structure of a PSI-LHCI-cyt b(6)f supercomplex in Chlamydomonas reinhardtii promoting cyclic electron flow under anaerobic conditions

Henning Paul-Julius Stahlberg

Photosynthetic linear electron flow (LEF) produces ATP and NADPH, while cyclic electron flow (CEF) exclusively drives photophosphorylation to supply extra ATP. The fine-tuning of linear and cyclic electron transport levels allows photosynthetic organisms to balance light energy absorption with cellular energy requirements under constantly changing light conditions. As LEF and CEF share many electron transfer components, a key question is how the same individual structural units contribute to these two different functional modes. Here, we report the structural identification of a photosystem I (PSI)-light harvesting complex I (LHCI)-cytochrome (cyt) b(6)f supercomplex isolated from the unicellular alga Chlamydomonas reinhardtii under anaerobic conditions, which induces CEF. This provides strong evidence for the model that enhanced CEF is induced by the formation of CEF supercomplexes, when stromal electron carriers are reduced, to generate additional ATP. The additional identification of PSI-LHCI-LHCII complexes is consistent with recent findings that both CEF enhancement and state transitions are triggered by similar conditions, but can occur independently from each other. Single molecule fluorescence correlation spectroscopy indicates a physical association between cyt b(6)f and fluorescent chlorophyll containing PSI-LHCI supercomplexes. Single particle analysis identified top-view projections of the corresponding PSI-LHCI-cyt b(6)f supercomplex. Based on molecular modeling and mass spectrometry analyses, we propose a model in which dissociation of LHCA2 and LHCA9 from PSI supports the formation of this CEF supercomplex. This is supported by the finding that a Delta lhca2 knockout mutant has constitutively enhanced CEF.

Proceedings of the National Academy of Sciences2018