Etudes électrochimiques de chaînes de transferts d'électrons photosynthétiques (ou Vers une photoproduction biomimétique d'hydrogène)

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Conception et caractérisation de nouveaux catalyseurs pour la photolyse de l eau

La photosynthèse artificielle est considérée comme étant un atout capable de fournir des carburants alternatifs et renouvelables par conversion et stockage de l'énergie solaire. Une approche prometteuse consiste en un développement de photo-catalyseurs moléculaires inspirés par des enzymes photosynthétiques naturelles. La première partie de cette thèse concerne les modèles artificiels du photosystème II (qui catalyse l'oxydation d'eau), composé d'un chromophore et d un relais d électrons comme équivalent synthétique correspondant à l'ensemble P680-TyrZ/His190 du photosystème II. Trois complexes ruthénium polypyridyl - imidazole - phénol avec un groupe méthylique à différentes positions sur l'anneau phénolique (Ru-xMe) ont été synthétisés et caractérisés par des méthodes électrochimiques et photophysiques. L augmentation, comparée aux complexes précédents, du potentiel redox des groupes phénols (0.20 V->0.9 V par rapport à l électrode de ferrocène) rend leur fonction de relais d électron dans un système photocatalytique pour l'oxydation d'eau thermodynamiquement possible. Des études d absorption transitoire ont révélé que le transfert d électron intramoléculaire est rapide (5-10 s dans solvant aprotique et < 100 ns dans l'eau) malgré la faible force motrice, mettant en evidence l'importance de la liaison hydrogène entre le phénol et le groupe imidazole. Les légères différences entre les trois complexes Ru-xMe ainsi que l étude de l'effet de bases externes nous ont permis d établir un mécanisme dans laquelle l'imidazole est impliqué dans une réaction de transfert de proton en cascade. L'acceptation du proton phénolique durant l'oxydation du ligand rend son deuxième site azote plus acide et seulement la déprotonation de ce dernier bascule l équilibre réactionnel complétement vers l'oxydation du ligand. La deuxième partie de cette thèse consiste en la synthèse d un complexe chromophore-tryptophane en utilisant une approche de chimie dite click . On a montré que l'oxydation, induite par la lumière, du Trp au sein du complexe Ru-tryptophane suit un mécanisme ETPT. Selon le pH, les radicaux du tryptophane (Trp ou TrpH ) ont été détectés et les mesures spectrales à différents temps ont montrés la transition entre les deux formes radicalaires. La déprotonation du radical dépend de la concentration d'eau assurant la fonction d accepteur de proton. La dernière partie de la thèse concerne nos efforts à lier, par une liaison covalente, une unité catalytique au module de chromophore- relais électronique caractérisé précédemment. L'approche de chimie click n était pas efficace pour l obtention de l assemblage photocatalytique final. Donc, l'activation biomoléculaire d'un catalyseur Mn salen a été effectuée et la formation de l espèce Mn(IV) a été observée. Etant une étape vers l'utilisation de ces types de photocatalyseurs dans une cellule photoélectrochimique, un chromophore [Ru(bpy) ] avec des groupes d ancrage phosphonate a été synthétisé (Ru-phosphonate) et greffé sur la surface méso-poreuses d'un semi-conducteur de TiO pour effectuer des mesures du photocourant.Artificial photosynthesis is often considered to have great potential to provide alternative, renewable fuels by harvesting, conversion and storage of solar energy. One promising approach is the development of modular molecular photocatalysts inspired by natural photosynthetic enzymes. The first part of this thesis deals with artificial mimics of the water oxidizing photosystem II composed of a chromophore and an electron relay as synthetic counterpart of the P680-TyrZ/His190 ensemble of photosystem II. Three ruthenium polypyridyl imidazole - phenol complexes with varying position of a methyl group on the phenol ring (Ru-xMe) were synthesized and characterized by electrochemical and photophysical methods. As an improvement compared to earlier complexes the increased redox potential (~0.9 V vs. Ferrocene) of the phenol groups makes their function as an electron relay in a photocatalytic system for water oxidation thermodynamically possible. Time-resolved absorption studies revealed fast intramolecular electron transfer (<5-10 s in aprotic solvent and <100 ns in water) despite the low driving force and the importance of the hydrogen bond between the phenol and the imidazole group was put in evidence. Slight differences between the three Ru-xMe complexes and investigation of the effect of external bases allowed to derive a mechanistic picture in which the imidazole is involved in a proton domino reaction. Accepting the phenolic proton upon ligand oxidation (within the H-bond) renders its second nitrogen site more acidic and only deprotonation of this site pulls the overall equilibrium completely towards oxidation of the ligand. Another part of this thesis comprises a chromophore-tryptophan construct synthesized using a click chemistry approach. Light-induced oxidation of Trp in this Ru-tryptophan complex was shown to follow ETPT mechanism. Depending on the pH conditions tryptophan radicals, either Trp or TrpH were detected and spectral measurement at different time showed the transition between the two forms. Deprotonation of the radical was dependent on the concentration of water as proton acceptor. Later part of the thesis deals with efforts to covalently bind a catalytic unit to the previously characterized chromophore-electron relay module. The click chemistry approach was not successful to obtain the final photocatalytic assembly. Therefore bimolecular activation of a Mn salen catalyst was performed and formation of Mn(IV) species was observed. As a step towards utilization of these types of photocatalysts in a photoelectrochemical cell a [Ru(bpy) ] chromophore with phosphonate anchoring groups (Ru-Phosphonate) was synthesized and grafted on the surface of a TiO mesoporous semiconductor surface anode to perform photocurrent measurements.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Proton and electron transfer in wild-type and mutant reaction centers from <i>Rhodobacter sphaeroides</i> followed by rapid-scan FTIR spectroscopy

Rapid-scan FTIR difference spectroscopy was used to investigate proton and electron transfer reactions in photosynthetic reaction centers from Rhodobacter sphaeroides. Experiments at different temperatures and in the presence of D2O have provided strong indication that a transient band at 1707 cm−1 previously identified after both the 1st and the 2nd flashes [A. Mezzetti, W. Leibl, Eur. Biophys. J. 34 (2005) 921] is given by a transient protonation of the side chain of a Asp or Glu residue situated on the proton transfer pathway from the cytoplasm to the QB site. Experiments in D2O on a Asp-M17 → Asn mutant reaction center, where the proton and electron transfer reactions are slowed down compared to the wild-type, showed that the kinetic isotope effect induced by H/D exchange slows down the electron transfer reaction after the 1st flash, confirming the strong coupling between proton and electron transfer. Rapid-scan FTIR experiments on Cd2+-treated reaction centers, in agreement with previous UV–vis measurements [P. Adelroth, M.L. Paddock, L.B. Sagle, G. Feher, M.Y. Okamura, Proc. Natl. Acad. Sci. U.S.A. 97 (2000) 13086], showed that upon addition of Cd2+, which inhibits proton uptake, the QA−QB → QAQB− reaction is slowed. Interestingly, the transient 1707 cm−1 band is not visible in the first spectrum recorded early after the flash. This strongly suggests its identification with a residue situated on the proton transfer pathway, which is perturbed upon metal cation binding

Time-resolved infrared spectroscopy in the study of photosynthetic systems.

International audienceTime-resolved (TR) infrared (IR) spectroscopy in the nanosecond to second timescale has been extensively used, in the last 30 years, in the study of photosynthetic systems. Interesting results have also been obtained at lower time resolution (minutes or even hours). In this review, we first describe the used techniques-dispersive IR, laser diode IR, rapid-scan Fourier transform (FT)IR, step-scan FTIR-underlying the advantages and disadvantages of each of them. Then, the main TR-IR results obtained so far in the investigation of photosynthetic reactions (in reaction centers, in light-harvesting systems, but also in entire membranes or even in living organisms) are presented. Finally, after the general conclusions, the perspectives in the field of TR-IR applied to photosynthesis are described

Photoelectric Characterization of Forward Electron Transfer to Iron-Sulfur Centers in Photosystem I

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Identification of a Ubiquinone-Ubiquinol Quinhydrone Complex in Bacterial Photosynthetic Membranes and Isolated Reaction Centers by Time-Resolved Infrared Spectroscopy

International audienceUbiquinone redox chemistry is of fundamental importance in biochemistry, notably in bioenergetics. The bi-electronic reduction of ubiquinone to ubiquinol has been widely studied, including by Fourier transform infrared (FTIR) difference spectroscopy, in several systems. In this paper, we have recorded static and time-resolved FTIR difference spectra reflecting light-induced ubiquinone reduction to ubiquinol in bacterial photosynthetic membranes and in detergent-isolated photosynthetic bacterial reaction centers. We found compelling evidence that in both systems under strong light illumination—and also in detergent-isolated reaction centers after two saturating flashes—a ubiquinone–ubiquinol charge-transfer quinhydrone complex, characterized by a characteristic band at ~1565 cm−1, can be formed. Quantum chemistry calculations confirmed that such a band is due to formation of a quinhydrone complex. We propose that the formation of such a complex takes place when Q and QH2 are forced, by spatial constraints, to share a common limited space as, for instance, in detergent micelles, or when an incoming quinone from the pool meets, in the channel for quinone/quinol exchange at the QB site, a quinol coming out. This latter situation can take place both in isolated and membrane bound reaction centers Possible consequences of the formation of this charge-transfer complex under physiological conditions are discussed

Shaping the Electrocatalytic Performance of Metal Complexes for CO2 Reduction

International audienceThe mass scale catalytic transformation of carbon dioxide (CO2) into reduced forms of carbon is an imperative to address the ever-increasing anthropogenic emission. Understanding the mechanistic routes leading to the multi-electron-proton conversion of CO2 provides handles for chemists to overcome the kinetically and thermodynamically hard challenges and further optimize these processes. Through extensive electrochemical investigations, Prof. J-M. Savéant and coworkers have made accessible to chemists invaluable electro-analytical tools to address and position the electrocatalytic performance of molecular catalysts grounded on a theoretical basis. Furthermore, he has bequeathed lessons to future generations on ways to improve the catalytic activity and on the electrocatalytic zone we must target. As a tribute to his accomplishments, we recall here a few aspects on the tuning of iron porphyrin catalysts by playing on electronic effects, proton delivery, hydrogen bonding and electrostatic interactions and its implications to other catalytic systems