789 research outputs found
Layer Analysis of the Structure of Water Confined in Vycor Glass
A Molecular Dynamics simulation of the microscopic structure of water
confined in a silica pore is presented. A single cavity in the silica glass has
been modeled as to reproduce the main features of the pores of real Vycor
glass. A layer analysis of the site-site radial distribution functions evidence
the presence in the pore of two subsets of water molecules with different
microscopic structure. Molecules which reside in the inner layer, close to the
center of the pore, have the same structure as bulk water but at a temperature
of 30 K higher. On the contrary the structure of the water molecules in the
outer layer, close to the substrate, is strongly influenced by the
water-substrate hydrophilic interaction and sensible distortions of the H-bond
network and of the orientational correlations between neighboring molecules
show up. Lowering the hydration has little effect on the structure of water in
the outer layer. The consequences on experimental determinations of the
structural properties of water in confinement are discussed.Comment: 6 pages, 8 figures included in the text, one figure added, changes in
the tex
Modifications of the hydrogen bond network of liquid water in a cylindrical SiO_2 pore
We present results of molecular dynamics simulations of water confined in a
silica pore. A cylindrical cavity is created inside a vitreous silica cell with
geometry and size similar to the pores of real Vycor glass. The simulations are
performed at different hydration levels. At all hydration levels water adsorbs
strongly on the Vycor surface; a double layer structure is evident at higher
hydrations. At almost full hydration the modifications of the
confinement-induced site-site pair distribution functions are in qualitative
agreement with neutron diffraction experiment. A decrease in the number of
hydrogen bonds between water molecules is observed along the pore radius, due
to the tendency of the molecules close to the substrate to form hydrogen-bonds
with the hydrophilic pore surface. As a consequence we observe a substrate
induced distortion of the H-bond tetrahedral network of water molecules in the
regions close to the surface.Comment: Talk presented at "Physics of Liquids: Foundations, Highlights,
Challenge", Murau Sept. 1998. To appear in J. Mol. Li
Dispersion Properties, Nonlinear Waves and Birefringence in Classical Nonlinear Electrodynamics
Using the very basic physics principles, we have studied the implications of
quantum corrections to classical electrodynamics and the propagation of
electromagnetic waves and pulses.
The initial nonlinear wave equation for the electromagnetic vector potential
is solved perturbatively about the known exact plane wave solution in both the
free vacuum case, as well as when a constant magnetic field is applied. A
nonlinear wave equation with nonzero convective part for the (relatively)
slowly varying amplitude of the first-order perturbation has been derived. This
equation governs the propagation of electromagnetic waves with a reduced speed
of light, where the reduction is roughly proportional to the intensity of the
initial pumping plane wave. A system of coupled nonlinear wave equations for
the two slowly varying amplitudes of the first-order perturbation, which
describe the two polarization states, has been obtained for the case of
constant magnetic field background.
Further, the slowly varying wave amplitude behavior is shown to be similar to
that of a cnoidal wave, known to describe surface gravity waves in shallow
water. It has been demonstrated that the two wave modes describing the two
polarization states are independent, and they propagate at different wave
frequencies. This effect is usually called nonlinear birefringence.Comment: 11 pages, 3 figure
A Unified Approach to Polycyclic Alkaloids of the Ingenamine Estate: Total Syntheses of Keramaphidin B, Ingenamine, and Nominal Njaoamine I
Many polycyclic marine alkaloids are thought to derive from partly reduced macrocyclic alkylpyridine derivatives via a transannular DielsâAlder reaction that forms their common etheno-bridged diaza-decaline core (âBaldwinâWhitehead hypothesisâ). Rather than trying to emulate this biosynthesis pathway, a route to these natural products following purely chemical logic was pursued. Specifically, a Michael/Michael addition cascade provided rapid access to this conspicuous tricyclic scaffold and allowed different handles to be introduced at the bridgehead quarternary center. This flexibility opened opportunities for the formation of the enveloping medium-sized and macrocyclic rings. Ring closing alkyne metathesis (RCAM) proved most reliable and became a recurrent theme en route to keramaphidin B, ingenamine, xestocyclamine A, and nominal njaoamine I (the structure of which had to be corrected in the aftermath of the synthesis). Best results were obtained with molybdenum alkylidyne catalysts endowed with (tripodal) silanolate ligands, which proved fully operative in the presence of tertiary amines, quinoline, and other Lewis basic sites. RCAM was successfully interlinked with macrolactamization, an intricate hydroboration/protonation/alkyl-Suzuki coupling sequence, or ring closing olefin metathesis (RCM) for the closure of the second lateral ring; the use of RCM for the formation of an 11-membered cycle is particularly noteworthy. Equally rare are RCM reactions that leave a pre-existing triple bond untouched, as the standard ruthenium catalysts are usually indiscriminative vis-Ă -vis the different Ï-bonds. Of arguably highest significance, however, is the use of two consecutive or even concurrent RCAM reactions en route to nominal njaoamine I as the arguably most complex of the chosen targets
Interchanging lexical resources on the Semantic Web
Lexica and terminology databases play a vital role in many NLP applications, but currently most such resources are published in application-specific formats, or with custom access interfaces, leading to the problem that much of this data is in ââdata silosââ and hence difficult to access. The Semantic Web and in particular the Linked Data initiative provide effective solutions to this problem, as well as possibilities for data reuse by inter-lexicon linking, and incorporation of data categories by dereferencable URIs. The Semantic Web focuses on the use of ontologies to describe semantics on the Web, but currently there is no standard for providing complex lexical information for such ontologies and for describing the relationship between the lexicon and the ontology. We present our model, lemon, which aims to address these gap
Supercooled confined water and the Mode Coupling crossover temperature
We present a Molecular Dynamics study of the single particle dynamics of
supercooled water confined in a silica pore. Two dynamical regimes are found:
close to the hydrophilic substrate molecules are below the Mode Coupling
crossover temperature, , already at ambient temperature. The water closer
to the center of the pore (free water) approaches upon supercooling as
predicted by Mode Coupling Theories. For free water the crossover temperature
and crossover exponent are extracted from power-law fits to both the
diffusion coefficient and the relaxation time of the late region.Comment: To be published, Phys. Rev. Lett., 4 pages, 3 figures, revTeX, minor
changes in the figures, references added, changes in the tex
First-Principles Evaluation of the Morphology of WS2 Nanotubes for Application as Visible-Light-Driven Water-Splitting Photocatalysts
This study was supported by the EC ERA.Net RUS Plus project No. 237 WATERSPLIT as well as Russian Basic Research Foundation No. 16-53-76019. S.K. and E.S. furthermore gratefully acknowledge computing time granted by the Center for Computational Sciences and Simulation (CCSS) of the Universitaáș Duisburg-Essen and the supercomputer magnitUDE (DFG grants INST 20876/209-1 FUGG, INST 20876/243-1 FUGG) provided by the Zentrum fĂŒr Informations-und Mediendienste (ZIM). E.S. is also grateful for support by the Cluster of Excellence RESOLV (EXC1069) funded by the Deutsche Forschungsgemeinschaft.One-dimensional tungsten disulfide (WS2) single-walled nanotubes (NTs) with either achiral, i.e., armchair (n, n) and zigzag-type (n, 0), or chiral (2n, n) configuration with diameters dNT > 1.9 nm have been found to be suitable for photocatalytic applications, since their band gaps correspond to the frequency range of visible light between red and violet (1.5 eV 1.9 nm, the condition Ï”VB < Ï”O2/H2O < Ï”H2/H2O < Ï”CB is fulfilled. The values of Ï”VB and Ï”CB have been found to depend only on the diameter and not on the chirality index of the nanotube. The reported structural and electronic properties have been obtained from either hybrid density functional theory and Hartree-Fock linear combination of atomic orbitals calculations (using the HSE06 functional) or the linear augmented cylindrical waves density functional theory method. In addition to single-walled NTs, we have investigated a number of achiral double-walled (m, m)at(n, n) and (m, 0)at(n, 0) as well as triple-walled (l, l)at(m, m)at(n, n) and (l, 0)at(m, 0)at(n, 0) nanotubes. All multiwalled nanotubes show a common dependence of their band gap on the diameter of the inner nanotube, independent of chirality index and number of walls. This behavior of WS2 NTs allows the exploitation of the entire range of the visible spectrum by suitably tuning the band gap.Deutsche Forschungsgemeinschaft INST 20876/243-1 FUGG,INST 20876/209-1 FUGG,EXC1069; Russian Foundation for Basic Research 16-53-76019; EC ERA.Net RUS Plus project No. 237 WATERSPLIT; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Unionâs Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART
X-ray Absorption Near-Edge Structure (XANES) at the O K-Edge of Bulk Co<sub>3</sub>O<sub>4</sub>: Experimental and Theoretical Studies
We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (Co3O4). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O 1s to O 2p states hybridized with the unoccu- pied 3d states of cobalt atoms. Also, since Co3O4 contains two types of Co atoms, i.e., Co3+ and Co2+, we find that contribution of Co2+ ions to the pre-edge peak is solely due to single spin-polarized t2g orbitals (dxz, dyz, and dxy) while that of Co3+ ions is due to spin-up and spin-down polarized eg orbitals (dx2ây2 and dz2 ). Furthermore, we deduce the magnetic moments on the Co3+ and Co2+ to be zero and 3.00 ÎŒB respectively. This is consistent with an earlier experimental study which found that the magnetic structure of Co3O4 consists of antiferromagnetically ordered Co2+ spins, each of which is surrounded by four nearest neighbours of oppositely directed spins
Gab2 deficiency prevents Flt3-ITD driven acute myeloid leukemia in vivo
Internal tandem duplications (ITD) of the FMS-like tyrosine kinase 3 (FLT3) predict poor prognosis in acute myeloid leukemia (AML) and often co-exist with inactivating DNMT3A mutations. In vitro studies implicated Grb2-associated binder 2 (GAB2) as FLT3-ITD effector. Utilizing a Flt3-ITD knock-in, Dnmt3a haploinsufficient mouse model, we demonstrate that Gab2 is essential for the development of Flt3-ITD driven AML in vivo, as Gab2 deficient mice displayed prolonged survival, presented with attenuated liver and spleen pathology and reduced blast counts. Furthermore, leukemic bone marrow from Gab2 deficient mice exhibited reduced colony-forming unit capacity and increased FLT3 inhibitor sensitivity. Using transcriptomics, we identify the genes encoding for Axl and the Ret co-receptor Gfra2 as targets of the Flt3-ITD/Gab2/Stat5 axis. We propose a pathomechanism in which Gab2 increases signaling of these receptors by inducing their expression and by serving as downstream effector. Thereby, Gab2 promotes AML aggressiveness and drug resistance as it incorporates these receptor tyrosine kinases into the Flt3-ITD signaling network. Consequently, our data identify GAB2 as a promising biomarker and therapeutic target in human AML
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