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Time-resolved infrared spectroscopic studies of ligand dynamics in the active site from cytochrome c oxidase
No full textInternational audienceThe catalytic site of heme–copper oxidases encompasses two close-lying ligand binding sites: the heme, where oxygen is bound and reduced and the CuB atom, which acts as ligand entry and release port. Diatomic gaseous ligands with a dipole moment, such as the signaling molecules carbon monoxide (CO) and nitric oxide (NO), carry clear infrared spectroscopic signatures in the different states that allow characterization of the dynamics of ligand transfer within, into and out of the active site using time-resolved infrared spectroscopy. We review the nature and diversity of these processes that have in particular been characterized with CO as ligand and which take place on time scales ranging from femtoseconds to milliseconds. These studies have advanced our understanding of the functional ligand pathways and reactivity in enzymes and more globally represent intriguing model systems for mechanisms of ligand motion in a confined protein environment. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems
Advancing Astrometry: Revisiting the VLBA Calibrator Surveys
No full textInternational audienceThe original VLBA Calibrator Surveys (VCS) were a series of 6 VLBA campaigns from 1994 to 2007 in which more than 2000 compact extragalactic radio sources were observed at X/S bands. The goals were to obtain precise positions (uncertainties ~1 mas or better) of many hundreds of new sources for use as VLBI phase referencing calibrators, and to make snapshot images of them for morphological studies. These VCS campaigns were highly successful, resulting in accurate positions and scientific-grade images maps for a majority of sources. These observations were later used to approximately triple the number of sources contained in the second realization of the International Celestial Reference Frame by VLBI (ICRF2). However, the VCS sources in ICRF2 represent a class of 'single epoch' sources with average position uncertainties ~5 times greater than the other ~1200 ICRF2 sources which are observed much more frequently in geodetic VLBI sessions. In an attempt to greatly reduce their position uncertainties, we are re-observing ~2400 VCS sources at X/S bands on the VLBA in 8 24-hr sessions. With the recent VLBA sensitivity upgrade, the sensitivity is now nearly 5 and 3 times greater at X and S bands than the original VCS sessions. Five of these sessions have been run so far, re-observing 1500 sources. Preliminary analysis shows an improvement in the average position uncertainties by a factor of ~3.2 times for these re-observed sources. We will present and discuss these improvements and their implication for ICRF3 development (planned for 2018). Images are also being produced and examples will be presented.Additional references:VLBA Calibrator Survey : Beasley et al. 2002 2002ApJS..141...13BICRF1 : 2009ITN....35....1MICRF2 : 1998AJ....116..516MICRF3 : 2014rfag.confE...1JGAIA : 2008IAUS..248..217
An assessment of the impact of local processes on dust lifting in martian climate models
No full textInternational audienceSimulation of the lifting of dust from the planetary surface is of substantially greater importance on Mars than on Earth, due to the fundamental role that atmospheric dust plays in the former’s climate, yet the dust emission parameterisations used to date in Martian global climate models (MGCMs) lag, understandably, behind their terrestrial counterparts in terms of sophistication. Recent developments in estimating surface roughness length over all Martian terrains and in modelling atmospheric circulations at regional to local scales (less than O(100 km)) presents an opportunity to formulate an improved wind stress lifting parameterisation. We have upgraded the conventional scheme by including the spatially varying roughness length in the lifting parameterisation in a fully consistent manner (thereby correcting a possible underestimation of the true threshold level for wind stress lifting), and used a modification to account for deviations from neutral stability in the surface layer. Following these improvements, it is found that wind speeds at typical MGCM resolution never reach the lifting threshold at most gridpoints — winds fall particularly short in the southern midlatitudes, where mean roughness is large. Sub-grid scale variability, manifested in both the near-surface wind field and the surface roughness, is then considered, and is found to be a crucial means of bridging the gap between model winds and thresholds. Both forms of small-scale variability contribute to the formation of dust emission ‘hotspots’: areas within the model gridbox with particularly favourable conditions for lifting, namely a smooth surface combined with strong near-surface gusts. Such small-scale emission could in fact be particularly influential on Mars, due both to the intense positive radiative feedbacks that can drive storm growth and a strong hysteresis effect on saltation. By modelling this variability, dust lifting is predicted at the locations at which dust storms are frequently observed, including the flushing storm sources of Chryse and Utopia, and southern midlatitude areas from which larger storms tend to initiate, such as Hellas and Solis Planum. The seasonal cycle of emission, which includes a double-peaked structure in northern autumn and winter, also appears realistic. Significant increases to lifting rates are produced for any sensible choices of parameters controlling the sub-grid distributions used, but results are sensitive to the smallest scale of variability considered, which high-resolution modelling suggests should be O(1 km) or less. Use of such models in future will permit the use of a diagnosed (rather than prescribed) variable gustiness intensity, which should further enhance dust lifting in the southern hemisphere in particular
Finding scattering data for a time-harmonic wave equation with first order perturbation from the Dirichlet-to-Neumann map
No full textInternational audienceWe present formulas and equations for finding scattering data from the Dirichlet-to-Neumann map for a time-harmonic wave equation with first order perturbation with compactly supported coefficients. We assume that the coefficients are matrix-valued in general. To our knowledge, these results are new even for the general scalar case
Construction of indistinguishable conductivity perturbations for the point electrode model in electrical impedance tomography
No full textInternational audienceWe explain how to build invisible isotropic conductivity perturbations of the unit conductivity in the framework of the point electrode model for two-dimensional electrical impedance tomography. The theoretical approach, based on solving a fixed point problem, is constructive and allows the implementation of an algorithm for approximating the invisible perturbations. The functionality of the method is demonstrated via numerical examples
Genomic sequencing reveals historical, demographic and selective factors associated with the diversification of the fire-associated fungus Neurospora discreta
No full textBGPI : équipe 5International audienceDelineating microbial populations, discovering ecologically relevant phenotypes and identifying migrants, hybrids or admixed individuals have long proved notoriously difficult , thereby limiting our understanding of the evolutionary forces at play during the diversification of microbial species. However, recent advances in sequencing and computational methods have enabled an unbiased approach whereby incipient species and the genetic correlates of speciation can be identified by examining patterns of genomic variation within and between lineages. We present here a population genomic study of a phy-logenetic species in the Neurospora discreta species complex, based on the resequencing of full genomes (~37 Mb) for 52 fungal isolates from nine sites in three continents. Population structure analyses revealed two distinct lineages in South–East Asia, and three lin-eages in North America/Europe with a broad longitudinal and latitudinal range and limited admixture between lineages. Genome scans for selective sweeps and comparisons of the genomic landscapes of diversity and recombination provided no support for a role of selection at linked sites on genomic heterogeneity in levels of divergence between lineages. However, demographic inference indicated that the observed genomic heterogeneity in divergence was generated by varying rates of gene flow between lin-eages following a period of isolation. Many putative cases of exchange of genetic material between phylogenetically divergent fungal lineages have been discovered, and our work highlights the quantitative importance of genetic exchanges between more closely related taxa to the evolution of fungal genomes. Our study also supports the role of allo-patric isolation as a driver of diversification in saprobic microbes
Mesure de la masse du boson de Higgs se désintégrant en quatre leptons dans l'expérience CMS auprès du LHC.
No full textLe sujet de ce mémoire est la mesure de la masse du boson de Higgs dans l'expérience CMS auprès du LHC, au travers du canal de désintégration en quatre leptons, avec un point de vue volontairement accentué vers les électrons. Ce document n’est ni une liste exhaustive de mes implications sur CMS, ni une revue générale de la reconstruction des électrons et de l’analyse du boson de Higgs en quatre leptons, mais une mixture des deux. La trame est structurée autour de la notion d’ingrédients, qui, pas à pas, sont nécessaires à la réalisation de la mesure finale
Micromécanismes actifs dans le sel gemme : quantifications expérimentales pour une modélisation pertinente
No full textInternational audienceLa formulation pertinente d'une loi de comportement représentative du comportement des matériaux resteun enjeu majeur pour garantir la capacité predictive des calculs numériques de structures, notamment lorsque l'onconsidère des réponses à long terme, ou sous des modes de sollicitation difficilement accessibles à l'expérimentationen laboratoire. Dans ce contexte, les approches multiéchelles reposant sur la connaissance des mécanismes physiquesélémentaires actifs à l'échelle d'un grain constitutif et l'utilisation de lois de changement d'échelle adéquates peuventêtre une réponse. Le comportement de la halite a fait et fait encore l'objet de nombreuses études dans ce sens,notamment en vue d'applications de stockage en cavités souterraines. Les modèles proposés reposent pour la plupartsur la connaissance des mécanismes de glissement plastique intracristallins, étudiés par exemple sur des monocristauxsous diverses conditions thermodynamiques. Il s'avère toutefois qu'une prévision raisonnable des réponsesmacroscopiques reposant sur ces seuls mécanismes de glissement cristallin conduit à des contradiction sur la naturedes systèmes activés et/ou les valeurs de leurs paramètres constitutifs. On montrera dans cet exposé comment uneanalyse expérimentale quantitative à l'échelle d'un ensemble de grains constitutifs, combinant expérimentation in situ sous MEB ou tomographe, à température ambiante et haute température, et traitement d'image par corrélation,permet de revisiter ces hypothèses de modélisation. On établit en particulier, d'une part, l'importance d'unmécanismes de déformation par glissement aux joints de grains, qui, certes secondaire en termes de contributionglobale à la déformation, s'avère indispensable à l'écoulement ductile macroscopique. D'autre part, l'analyse permetaussi une quantification des contributions relatives de mécanismes de glissement plastique et de leurs conditionsd'activation au sein du polycristal. Cette vision rénovée du comportement de cette roche est plus généralementsusceptible d'expliquer certaines observations paradoxales et illustre le danger potentiel d'une extrapolation hâtive
Brick polytopes of spherical subword complexes and generalized associahedra
No full textInternational audienceWe generalize the brick polytope of V. Pilaud and F. Santos to spherical subword complexes for finite Coxeter groups. This construction provides polytopal realizations for a certain class of subword complexes containing all cluster complexes of finite types. For the latter, the brick polytopes turn out to coincide with the known realizations of generalized associahedra, thus opening new perspectives on these constructions. This new approach yields in particular the vertex description of generalized associahedra, a Minkowski sum decomposition into Coxeter matroid polytopes, and a combinatorial description of the exchange matrix of any cluster in a finite type cluster algebra
O-Ethyl-S-(1,2,2,2-Tetrafluoroethyl)-Dithiocarbonate: A Convenient Reagent for the Generation and Capture of 1,2,2,2-Tetrafluoroethyl Radicals
No full textInternational audienceThe synthesis of a new fluorinated reagent O-ethyl S-(1,2,2,2-tetrafluoroethyl) carbonodithioate (xanthate) and its use for the generation and capture of the hitherto unexplored 1,2,2,2-tetrafluoroethyl radicals is described. This reagent allows the convenient introduction of the rare CF3CHF motif onto various olefinic substrates with a high functional-group tolerance