347 research outputs found

    Produit de solubilité de la calcite et constantes de dissociation de CaHCO3+ et CaCO30 entre 5 et 75 °C

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    Les valeurs du produit de solubilitĂ© de la calcite et des constantes de dissociation de CaHCO3+ et CaCO30, notĂ©es K3 et K4, ont Ă©tĂ© dĂ©terminĂ©es Ă  diffĂ©rentes tempĂ©ratures comprises entre 5 et 75 °C (la calcite est instable aux tempĂ©ratures plus Ă©levĂ©es) Ă  partir des mesures [(Ca2+)T, pH] de solubilitĂ© de ce sel dans l'eau carboniquement pure. Les rĂ©sultats obtenus ont permis d'Ă©tablir les relations empiriques suivantes :pKs= 7,8156 + 0,03111 T + (1 502/T) - 5,518 log TpK3= 6,2447 + 0,00437 T + (864,479/T) - 0,363 log TpK4= 2,89636 + 0,00707 T + (102,87/T) - 0,44176 log Texpressions dans lesquelles T dĂ©signe la tempĂ©rature absolue (K) et log le logarithme dĂ©cimal.Des variations de pKS avec la tempĂ©rature nous avons dĂ©duit, Ă  25 °C, les grandeurs thermodynamiques relatives Ă  la dissolution de la calcite :∆H0 = -2510 cal. mol-1, ∆S0 = -47,2 cal. mol-1. K-1et ∆C∘p = -73,9 cal. mol-1. K-1The values of the solubility product of calcite and dissociation constants of CaHCO3+ and CaCO30, K3 and K4 respectively, were determined at several temperatures between 5 and 75 °C (calcite becomes unstable at higher temperatures) from measurements [(Ca2+)T, pH] of calcite solubility using carbonically pure water. The results obtained lead to the following empirical expressions for the dependence of equilibrium constants on the temperature :pKs= 7,8156 + 0,03111 T + (1 502/T) - 5,518 log TpK3= 6,2447 + 0,00437 T + (864,479/T) - 0,363 log TpK4= 2,89636 + 0,00707 T + (102,87/T) - 0,44176 log Twhere log T is the common logarithm of the absolute temperature T(K).Using this expression of pKS, the calculated thermodynamic properties of the calcite dissolution reaction at 25 °C are :∆H0 = -2510 cal. mol-1, ∆S0 = -47,2 cal. mol-1. K-1et ∆C∘p = -73,9 cal. mol-1. K-

    Stacking order dynamic in the quasi-two-dimensional dichalcogenide 1T-TaS2_2 probed with MeV ultrafast electron diffraction

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    Transitions between different charge density wave (CDW) states in quasi-two-dimensional materials may be accompanied also by changes in the inter-layer stacking of the CDW. Using MeV ultrafast electron diffraction, the out-of-plane stacking order dynamics in the quasi-two-dimensional dichalcogenide 1T-TaS2_2 is investigated for the first time. From the intensity of the CDW satellites aligned around the commensurate ll = 1/6 characteristic stacking order, it is found out that this phase disappears with a 0.5 ps time constant. Simultaneously, in the same experiment, the emergence of the incommensurate phase, with a slightly slower 2.0 ps time constant, is determined from the intensity of the CDW satellites aligned around the incommensurate ll = 1/3 characteristic stacking order. These results might be of relevance in understanding the metallic character of the laser-induced metastable "hidden" state recently discovered in this compound

    Production of ηâ€Č\eta\prime Mesons in the pp→ppηâ€Čpp \to pp\eta\prime Reaction at 3.67 GeV/c

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    The ratio of the total exclusive production cross sections for ηâ€Č\eta\prime and η\eta mesons has been measured in the pppp reaction at pbeam=3.67p_{beam}=3.67 GeV/c. The observed ηâ€Č/η\eta\prime/\eta ratio is (0.83±0.11−0.18+0.23)×10−2(0.83\pm{0.11}^{+0.23}_{-0.18})\times 10^{-2} from which the exclusive ηâ€Č\eta\prime meson production cross section is determined to be (1.12±0.15−0.31+0.42)ÎŒb(1.12\pm{0.15}^{+0.42}_{-0.31})\mu b. Differential cross section distributions have been measured. Their shape is consistent with isotropic ηâ€Č\eta\prime meson production.Comment: 14 pages, 5 figures, accepted by Phys.Lett.

    Phthiocerol Dimycocerosates of M. tuberculosis Participate in Macrophage Invasion by Inducing Changes in the Organization of Plasma Membrane Lipids

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    Phthiocerol dimycocerosates (DIM) are major virulence factors of Mycobacterium tuberculosis (Mtb), in particular during the early step of infection when bacilli encounter their host macrophages. However, their cellular and molecular mechanisms of action remain unknown. Using Mtb mutants deleted for genes involved in DIM biosynthesis, we demonstrated that DIM participate both in the receptor-dependent phagocytosis of Mtb and the prevention of phagosomal acidification. The effects of DIM required a state of the membrane fluidity as demonstrated by experiments conducted with cholesterol-depleting drugs that abolished the differences in phagocytosis efficiency and phagosome acidification observed between wild-type and mutant strains. The insertion of a new cholesterol-pyrene probe in living cells demonstrated that the polarity of the membrane hydrophobic core changed upon contact with Mtb whereas the lateral diffusion of cholesterol was unaffected. This effect was dependent on DIM and was consistent with the effect observed following DIM insertion in model membrane. Therefore, we propose that DIM control the invasion of macrophages by Mtb by targeting lipid organisation in the host membrane, thereby modifying its biophysical properties. The DIM-induced changes in lipid ordering favour the efficiency of receptor-mediated phagocytosis of Mtb and contribute to the control of phagosomal pH driving bacilli in a protective niche
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