ETUDE DE MUTANTS DE L'ATPASE-CA 2 + DU RETICULUM SARCOPLASMIQUE DU MUSCLE SQUELETTIQUE DE LAPIN (CONSEQUENCES STRUCTURALES ET FONCTIONNELLES)

ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Effect of radiation-induced amorphization on smectite dissolution kinetics

International audienceIn the high level nuclear radioactive waste repositories (HLNWR) management, smectite (a major constituent of bentonite) is considered to limit the dissemination of radionuclide in the environment 1. Indeed, smectite exhibits interesting properties such as a high cation exchange capacity and swelling capacity that are expected to enhance the retention of radionuclides in a case of a leakage of the waste. For this reason, in France, smectite will be found in the near and far field of the waste. In the near field of the waste, bentonite will be a major constituent of the engineering barriers used mainly for backfilling and in the far field, deep argillaceous formations are the expected host rock. However, it was recently reported that the conditions for induced amorphization of smectite by heavy ions were consistent with a leakage scenario in a HLNWR 2. Such amorphization is expected to induce strong modifications of the mineral properties such as sorption capacity, swelling and dissolution kinetics. In this study, we used a purified smectite issued from the MX80 bentonite in order to evaluate the influence of amorphization on the dissolution kinetics. The sample was irradiated with 925 MeV xenon ions at the GANIL facility (Caen, France). Amorphization was controlled by X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectroscopy. Besides, an important coalescence of the smectite sheets was observed by scanning electron microscopy, which lead to a loss of interparticle porosity. Furthermore, it was observed that the dissolution rate far-from-equilibrium is two times larger in the amorphous sample than in the reference clay. We will discuss the result with previous observations on other minerals and the potential implications for the dissemination of radionuclides in the environment

Effect of Radiation-Induced Amorphization on Smectite Dissolution

International audienceEffects of radiation-induced amorphization of smectite were investigated using artificial irradiation. Beams of 925 MeV Xenon ions with radiation dose reaching 73 MGy were used to simulate the effects generated by alpha recoil nuclei or fission products in the context of high level nuclear waste repository. Amorphization was controlled by X-ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy. An important coalescence of the smectite sheets was observed which lead to a loss of interparticle porosity. The amorphization is revealed by a loss of long-range structure and accompanied by dehydroxylation. The dissolution rate far-from- equilibrium shows that the amount of silica in solution is two times larger in the amorphous sample than in the reference clay, a value which may be enhanced by orders of magnitude when considering the relative surface area of the samples. Irradiation-induced amorphization thus facilitates dissolution of the clay-derived material. This has to be taken into account for the safety assessment of high level nuclear waste repository, particularly in a scenario of leakage of the waste package which would deliver alpha emitters able to amorphize smectite after a limited period of time

Revealing forms of iron in river-borne material from major tropical rivers of the Amazon Basin (Brazil).

The present study deals with the direct determination of colloidal forms of iron in river-borne solids from main rivers of the Amazon Basin. The contribution of different forms of colloidal iron have been assessed using ultrafiltration associated with various techniques including electron paramagnetic resonance spectroscopy (EPR), high resolution transmission electron microscopy (HRTEM), and micro proton-induced X ray emission analysis (μPIXE). EPR shows the presence of Fe3+ bound to organic matter (Fe3+-OM) and colloidal iron oxides. Quantitative estimate of Fe3+-OM content in colloidal matter ranges from 0.1 to 1.6 weight % of dried solids and decreases as the pH of the river increases in the range 4 to 6.8. The modeling of the field data with the Equilibrium Calculation of Speciation and Transport (ECOSAT) code demonstrates that this trend is indicative of a geochemical control resulting from the solubility equilibrium of Fe oxyhydroxide phase and Fe binding to organic matter. Combining EPR and μPIXE data quantitatively confirms the presence of colloidal iron phase (min. 35 to 65% of iron content), assuming no divalent Fe is present. In the Rio Negro, HRTEM specifies the nature of colloidal iron phase mainly as ferrihydrite particles of circa 20 to 50 Å associated with organic matter. The geochemical forms of colloidal iron differentiate the pedoclimatic regions drained by the different rivers, corresponding to different major weathering/erosion processes. Modeling allows the calculation of the speciation of iron as mineral, organic and dissolved phases in the studied rivers

Experimental Colonization and Alteration of Orthopyroxene by the Pleomorphic Bacteria Ramlibacter tataouinensis.

The colonization of orthopyroxene crystals by a pleomorphic bacterium and the mineralogical products resulting from a prolonged interaction have been studied. We usedRamlibacter tataouinensis(strain TTB310), which is an aerobicβ-Proteobacterium moving over surfaces by gliding motility and whose life cycle includes rods and spherical cysts. Analysis of cultures grown on solid media with micrometer-sized pyroxene and quartz crystals scattered over the surface revealed a taxis of the bacteria toward the crystals. Given the mineralogical non-specificity of the interaction, a mechanism of elasticotaxis is inferred. After the rods had adhered to the pyroxene surface, they differentiated into cysts leading to the formation of microcolonies that were centered on a crystal grain. This suggests an original coupling between the life cycle ofR. tataouinensisand the interaction with the crystals. The alteration of orthopyroxene was studied by high-resolution transmission electron microscopy at the interface between cysts and pyroxene crystals. The pyroxene surface showed an amorphous layer that was more developed than that of abiotic control samples processed under the same conditions. Moreover, chemical analyses showed that the dissolution of pyroxene was reduced in the presence ofR. tataouinensis. The origin and the significance of the amorphous layers is discussed

A method to assess algicidal activity of microalgal extracts coupling microalgae produced in stirred closed photobioreactor operating in continuous with pulse amplitude modulated (PAM) fluorometry

We describe in the present study a quick and reliable method based on chlorophyll a fluorescence to assess putative algicidal effect of different microalgal extracts. We couple microalgal production under chemostat cultivation mode to continuously produce a given microalgae species (e.g. Dunaliella salina in this study) at a stable physiological state to ease comparison between extracts tested; with a non-destructive method based on chlorophyll a fluorescence. Pulse amplitude modulated (PAM) fluorometry was used to assess over time the effect of different microalgal crude extracts on the efficiency of the photosystem II (PSII) of a tested microalgae (Dunaliella salina). • Microalgal production at stationary phase in stirred closed photobioreactor (PBR) operating in continuous have stable photophysiological parameters, which is a prerequisite to compare the impact of different algicidal compounds. • The combination of both methods, allows to quickly assess the algicidal effect of diverse microalgal (crude) extracts on the PSII efficiency of a tested microalgae. • The method may be used to identify and isolate algicidal molecules affecting algal PSII using a bio-guided isolation protocol

Amorphous to Crystal Conversion as a Mechanism Governing the Structure of Luminescent YVO4:Eu Nanoparticles

International audienceThe development of functional materials by taking advantage of the physical properties of nanoparticles needs an optimal control over their size and crystal quality. In this context, the synthesis of crystalline oxide nanoparticles in water at room temperature is a versatile and industrially appealing process but lacks control especially for "large" nanoparticles (>30 nm), which commonly consist of agglomerates of smaller crystalline primary grains. Improvement of these syntheses is hampered by the lack of knowledge on possible intermediate, noncrystalline stages, although their critical importance has already been outlined in crystallization processes. Here, we show that during the synthesis of luminescent Eu-doped YVO4 nanoparticles a transient amorphous network forms with a two-level structuration. These two prestructuration scales constrain topologically the nucleation of the nanometer-sized crystalline primary grains and their aggregation in nanoparticles, respectively. This template effect not only clarifies why the crystal size is found independent of the nucleation rate, in contradiction with the classical nucleation models, but also supports the possibility to control the final nanostructure with the amorphous phase

Chemical Heterogeneities within the Disordered Mineral domains of Aragonite Platelets in nacre from the European Abalone Haliotis tuberculata

International audienceSince the observation in 2005 of a disordered mineral layer at the surface of aragonite platelets in abalone Haliotis laevigata nacre, the model of the organo–mineral interface in such biomineralized tissue has been challenged. As a direct interaction between the aragonite crystalline core and the organic matrix is no longer appropriate to describe such interface, a structural description of the disordered mineral domains at the atomic level is a key for a comprehensive view of nacre ultrastructure. Here, we use European abalone Haliotis tuberculata as a model to investigate aragonite nacre through high-resolution transmission electron microscopy (HR-TEM) and multinuclear solid state nuclear magnetic resonance (ssNMR). The presence of a disordered domain around aragonite crystals is shown through HR-TEM observations similarly to H. laevigata. The structure and the ionic composition of the disordered mineral environments of H. tuberculata nacre are investigated through 1H, 13C, 43Ca, and 23Na ssNMR. Interestingly, we demonstrate that the disordered mineral domains in nacre seem to be heterogeneous in terms of structure and chemical composition and do not match with amorphous calcium carbonate stricto sensu. At least three different carbonates species are evidenced, including CO32– and HCO3– present in the same mineral domain and closely associated with rigid H2O molecules. The local disorder around these ions is found to be inhomogeneous as some CO32– possess an aragonitic environment and are rather ordered (according to the position and line width of their 13C resonance) whereas, in opposition, the chemical environment around HCO3– is highly distributed. The analysis of potential cations as counterions revealed the presence of disordered Ca2+ and the presence of Na+ closely associated with HCO3–. On the basis of these structural data, we propose an atomic-level model for the disordered domains in abalone (H.tuberculata) nacre where the protonation level of carbonate ions, the proportion of sodium ions and the local disorder are increasing from the inner to the outer part of disordered domains. These results give an unprecedented structural view at the atomic scale of such disordered mineral domains in nacre aragonite tablets

Overproduction in yeast and rapid and efficient purification of the rabbit SERCA1a Ca2+-ATPase

Large amounts of heterologous C-terminally his-tagged SERCAla Ca2+-ATPase were expressed in yeast using a galactose-regulated promoter and purified by Ni2+ affinity chromatography followed by Reactive red chromatography. Optimizing the number of galactose inductions and increasing the amount of Ga14p transcription factor improved expression. Lowering the temperature from 28degreesC to 18degreesC during expression enhanced the recovery of solubilized and active Ca2+-ATPase. In these conditions, a 41 yeast culture produced 100 mg of Ca2+-ATPase, 60 and 22 mg being pelleted with the heavy and light membrane fractions respectively, representing 7 and 1.7% of total proteins. The Ca2+-ATPase expressed in light membranes was 100% solubilized with L-alpha-lysophosphatidylcholine (LPC), 50% with n-dodecyl beta-D-maltoside (DM) and 25% with octaethylene glycol mono-n-dodecyl ether (C12E8). Compared to LPC, DM preserved specific activity of the solubilized Ca2+-ATPase during the chromatographic steps. Starting from 1/6 (3.8 mg) of the total amount of Ca2+-ATPase expressed in light membranes, 800 mug could be routinely purified to 50% purity by metal affinity chromatography and then 200 mug to 70% with Reactive red chromatography. The purified Ca2+-ATPase displayed the same K-m for calcium and ATP as the native enzyme but a reduced specific activity ranging from 4.5 to 7.3 mumol ATP hydrolyzed/min/mg Ca2+-ATPase. It was stable and active for several days at 4degreesC or after removal of DM with Bio-beads and storage at -80degreesC. (C) 2002 Elsevier Science B.V. All rights reserved