539 research outputs found

    Comparison of Molecular Structures Determined by Electron Diffraction and Spectroscopy. Ethane and Diborane

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    Gas‐phase average structures for the ground‐vibrational state (rz)(rz) for ethane and diborane have been determined by a critical comparison of the experimental results obtained from electron diffraction (average internuclear distances rgrg) and those obtained from high‐resolution infrared and Raman spectroscopy (rotational constants Bz(α)Bz(α)). Experimental values have been taken from the recent literature and converted into the average structure (rzorrα0)(rzorrα0). The rgrg and rα0rα0 distances determined from electron diffraction carry uncertainties less than those in the rzrz distances determined from rotational constants, because the latter structures are very sensitive to assumptions about the unknown isotope differences in the structures. On the other hand, the average moments of inertia from spectroscopy are much more precise than those calculated from diffraction internuclear distances. Examinations of the data have led to the following rzrz structures with standard errors: For C2H6, rz(C�H)=1.0957±0.002Å,rz(C�C)=1.5319±0.002Å,and∠C�C�H=111.5°±0.3°; for C2D6, rz(C�D)=1.0941±0.002Å,rz(C�C)=1.5300±0.002Å,and∠C�C�D=111.4°±0.3°; and for B2H6, rz(B�Ht)=1.192±0.01Å,rz(B�Hb)=1.329±0.005Å,rz(B�B)=1.770±0.005Å, ∠Ht�B�Ht=121.8°±3°,and∠Hb�B�Hb=96.5°±0.5°. It was possible to increase the resolving power of the diffraction analysis of diborane by inclusion of calculated B☒H mean amplitudes.The effective complementary use of electron‐diffraction and spectroscopic data for determining reliable gas‐phase structures and the relative merits of the two alternative representations of the average structure (rgandrz)(rgandrz) have been discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70372/2/JCPSA6-49-10-4456-1.pd

    Representations of molecular force fields. V. On the equilibrium structure of methane

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    The correction from mean (rg) to equilibrium (re) bond lengths in methane is reinvestigated in response to suggestions that the original Kuchitsu–Bartell (KB) corrections were excessive by as much as 0.006 Å. It is found that the KB model anharmonic force field with its original parameterization does overcorrect, relative to the new Pulay ab initio force field, but only by 10% of the original (rg−re) corrections, or 0.0022 Å for CH4 and 0.0015 Å for CD4. The KB model with the more recent MUB‐2 nonbonded parameterization gives results in close accord with the ab initio results, supporting the utility of the augmented anharmonic Urey–Bradley representation of force fields. Reported discrepancies between experimental and theoretical methane bond lengths cannot be due solely to the form of or original parameterization of the KB model field.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69576/2/JCPSA6-68-3-1213-1.pd

    Frost damage of bricks composing a railway tunnel monument in Central Japan: field monitoring and laboratory simulation

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    International audienceBricks of tunnels and bridges of Usui Pass railway (Japan) exposed to north are subject to frost damage. Average depth of erosion due to detachment of angular blocks is around 1-1.5 cm. In order to assess this weathering and to understand its mechanism, an experimental study was carried out in the field and laboratory. Field monitoring showed the combination of seasonal and diurnal freezing with a maximum of heave when the freezing front reached 5 cm depth. Bricks taken from the site were submitted to unidirectional freezing at capillary and vacuum saturation in the laboratory. Results showed that frost damage of bricks was favoured by high saturation level and repetition of freeze-thaw cycles

    Electron‐Diffraction Study of Ammonia and Deuteroammonia

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    The gas‐phase structures of NH3 and ND3 molecules were determined by the sector‐microphotometer method of electron diffraction. The following internuclear distances rgrg and mean amplitudes lele with estimated standard errors were obtained: For NH3, rg(N − H)  =  1.0302 ± 0.002Å,rg(H − H)  =  1.662 ± 0.010Å,le(N − H)  =  0.0731 ± 0.002Å,le(H − H)  =  0.125 ± 0.006Årg(N−H)=1.0302±0.002Å,rg(H−H)=1.662±0.010Å,le(N−H)=0.0731±0.002Å,le(H−H)=0.125±0.006Å, and for ND3, rg(N − D)  =  1.0266 ± 0.003Å,rg(D − D)  =  1.654 ± 0.008Å,le(N − D)  =  0.0611 ± 0.002Å,le(D − D)  =  0.101 ± 0.006Årg(N−D)=1.0266±0.003Å,rg(D−D)=1.654±0.008Å,le(N−D)=0.0611±0.002Å,le(D−D)=0.101±0.006Å, with the parameter κκ representing bond‐stretching anharmonicity fixed at 1.0 × 10−5 and 0.5 × 10−5 Å3 for N☒H and N☒D, respectively. Effects of anharmonicity and isotope differences in the structural parameters analogous to those in CH4 and CD4 were observed. The rα0rα0 and rere bond distances calculated from the above rgrg distances are found to be consistent with the corresponding rzrz and rere distances derived from the spectroscopic rotational constants of Benedict and Plyler. The isotope effects reported by Bell and by Halevi for the dipole moment and polarizability of ammonia are discussed briefly in the light of the present results.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69943/2/JCPSA6-49-6-2488-1.pd

    Activation of ADF/cofilin by phosphorylation-regulated Slingshot phosphatase is required for the meiotic spindle assembly in Xenopus laevis oocytes

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    We identify Xenopus ADF/cofilin (XAC) and its activator, Slingshot phosphatase (XSSH), as key regulators of actin dynamics essential for spindle microtubule assembly during Xenopus oocyte maturation. Phosphorylation of XSSH at multiple sites within the tail domain occurs just after germinal vesicle breakdown (GVBD) and is accompanied by dephosphorylation of XAC, which was mostly phosphorylated in immature oocytes. This XAC dephosphorylation after GVBD is completely suppressed by latrunculin B, an actin monomer-sequestering drug. On the other hand, jasplakinolide, an F-actin-stabilizing drug, induces dephosphorylation of XAC. Effects of latrunculin B and jasplakinolide are reconstituted in cytostatic factor-arrested extracts (CSF extracts), and XAC dephosphorylation is abolished by depletion of XSSH from CSF extracts, suggesting that XSSH functions as an actin filament sensor to facilitate actin filament dynamics via XAC activation. Injection of anti-XSSH antibody, which blocks full phosphorylation of XSSH after GVBD, inhibits both meiotic spindle formation and XAC dephosphorylation. Coinjection of constitutively active XAC with the antibody suppresses this phenotype. Treatment of oocytes with jasplakinolide also impairs spindle formation. These results strongly suggest that elevation of actin dynamics by XAC activation through XSSH phosphorylation is required for meiotic spindle assembly in Xenopus laevis

    Genetic screen in Drosophila muscle identifies autophagy-mediated T-tubule remodeling and a Rab2 role in autophagy.

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    Transverse (T)-tubules make-up a specialized network of tubulated muscle cell membranes involved in excitation-contraction coupling for power of contraction. Little is known about how T-tubules maintain highly organized structures and contacts throughout the contractile system despite the ongoing muscle remodeling that occurs with muscle atrophy, damage and aging. We uncovered an essential role for autophagy in T-tubule remodeling with genetic screens of a developmentally regulated remodeling program in Drosophila abdominal muscles. Here, we show that autophagy is both upregulated with and required for progression through T-tubule disassembly stages. Along with known mediators of autophagosome-lysosome fusion, our screens uncovered an unexpected shared role for Rab2 with a broadly conserved function in autophagic clearance. Rab2 localizes to autophagosomes and binds to HOPS complex members, suggesting a direct role in autophagosome tethering/fusion. Together, the high membrane flux with muscle remodeling permits unprecedented analysis both of T-tubule dynamics and fundamental trafficking mechanisms

    Angle-resolved photoemission spectroscopy from first-principles quantum Monte Carlo

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    Angle-resolved photoemission spectroscopy allows one to visualize in momentum space the probability weight maps of electrons subtracted from molecules deposited on a substrate. The interpretation of these maps usually relies on the plane wave approximation through the Fourier transform of single particle orbitals obtained from density functional theory. Here we propose a first-principle many-body approach based on quantum Monte Carlo (QMC) to directly calculate the quasi-particle wave functions (also known as Dyson orbitals) of molecules in momentum space. The comparison between these correlated QMC images and their single particle counterpart highlights features that arise from many-body effects. We test the QMC approach on the linear C2H2, CO2, and N2 molecules, for which only small amplitude remodulations are visible. Then, we consider the case of the pentacene molecule, focusing on the relationship between the momentum space features and the real space quasi-particle orbital. Eventually, we verify the correlation effects present in the metal CuCl42- planar complex