993 resultados para hyperpolarisiertes, xenon, Polarisator, GE180, T1 xenon


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The modes of binding of adenosine 2'-monophosphate (2'-AMP) to the enzyme ribonuclease (RNase) T1 were determined by computer modelling studies. The phosphate moiety of 2'-AMP binds at the primary phosphate binding site. However, adenine can occupy two distinct sites--(1) The primary base binding site where the guanine of 2'-GMP binds and (2) The subsite close to the N1 subsite for the base on the 3'-side of guanine in a guanyl dinucleotide. The minimum energy conformers corresponding to the two modes of binding of 2'-AMP to RNase T1 were found to be of nearly the same energy implying that in solution 2'-AMP binds to the enzyme in both modes. The conformation of the inhibitor and the predicted hydrogen bonding scheme for the RNase T1-2'-AMP complex in the second binding mode (S) agrees well with the reported x-ray crystallographic study. The existence of the first mode of binding explains the experimental observations that RNase T1 catalyses the hydrolysis of phosphodiester bonds adjacent to adenosine at high enzyme concentrations. A comparison of the interactions of 2'-AMP and 2'-GMP with RNase T1 reveals that Glu58 and Asn98 at the phosphate binding site and Glu46 at the base binding site preferentially stabilise the enzyme-2'-GMP complex.

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Using a lattice model for adsorption in microporous materials, pure component adsorption isotherms are obtained within a mean field approximation for methane at 300 K and xenon at 300 and 360 K in zeolite NaA. It is argued that the increased repulsive adsorbate-adsorbate interactions at high coverages must play an important role in determining the adsorption behavior. Therefore, this feature is incorporated through a "coverage-dependent interaction'' model, which introduces a free, adjustable parameter. Another important feature, the site volume reduction, has been treated in two ways: a van der Waal model and a 1D hard-rod theory [van Tassel et al., AIChE J. 40, 925 (1994)]; we have also generalized the latter to include all possible adsorbate overlap scenarios. In particular, the 1D hard-rod model, with our coverage-dependent interaction model, is shown to be in best quantitative agreement with the previous grand canonical Monte Carlo isotherms. The expressions for the isosteric heats of adsorption indicate that attractive and repulsive adsorbate-adsorbate interactions increase and decrease the heats of adsorption, respectively. It is concluded that within the mean field approximation, our simple model for repulsive interactions and the 1D hard-rod model for site volume reduction are able to capture most of the important features of adsorption in confined regions. (C) 1999 American Institute of Physics. [S0021-9606(99)70515-5].

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The statistical thermodynamics of adsorption in caged zeolites is developed by treating the zeolite as an ensemble of M identical cages or subsystems. Within each cage adsorption is assumed to occur onto a lattice of n identical sites. Expressions for the average occupancy per cage are obtained by minimizing the Helmholtz free energy in the canonical ensemble subject to the constraints of constant M and constant number of adsorbates N. Adsorbate-adsorbate interactions in the Brag-Williams or mean field approximation are treated in two ways. The local mean field approximation (LMFA) is based on the local cage occupancy and the global mean field approximation (GMFA) is based on the average coverage of the ensemble. The GMFA is shown to be equivalent in formulation to treating the zeolite as a collection of interacting single site subsystems. In contrast, the treatment in the LMFA retains the description of the zeolite as an ensemble of identical cages, whose thermodynamic properties are conveniently derived in the grand canonical ensemble. For a z coordinated lattice within the zeolite cage, with epsilon(aa) as the adsorbate-adsorbate interaction parameter, the comparisons for different values of epsilon(aa)(*)=epsilon(aa)z/2kT, and number of sites per cage, n, illustrate that for -1 <epsilon(aa)(*)< 0 and n greater than or equal to 10, the adsorption isotherms and heats of adsorption predicted with the two approaches are similar. In general, the deviation between the LMFA and GMFA is greater for smaller n and less sensitive to n for epsilon(aa)(*)> 0. We compare the isotherms predicted with the LMFA with previous GMFA predictions [K. G. Ayappa, C. R. Kamala, and T. A. Abinandanan, J. Chem. Phys. 110, 8714 (1999)] (which incorporates both the site volume reduction and a coverage-dependent epsilon(aa)) for xenon and methane in zeolite NaA. In all cases the predicted isotherms are very similar, with the exception of a small steplike feature present in the LMFA for xenon at higher coverages. (C) 1999 American Institute of Physics. [S0021-9606(99)70333-8].

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Thiolases catalyze the degradation and synthesis of 3-ketoacyl-CoA molecules. Here, the crystal structures of a T1-like thiolase (MSM-13 thiolase) from Mycobacterium smegmatis in apo and liganded forms are described. Systematic comparisons of six crystallographically independent unliganded MSM-13 thiolase tetramers (dimers of tight dimers) from three different crystal forms revealed that the two tight dimers are connected to a rigid tetramerization domain via flexible hinge regions, generating an asymmetric tetramer. In the liganded structure, CoA is bound to those subunits that are rotated towards the tip of the tetramerization loop of the opposing dimer, suggesting that this loop is important for substrate binding. The hinge regions responsible for this rotation occur near Val123 and Arg149. The L alpha 1-covering loop-L alpha 2 region, together with the N beta 2-N alpha 2 loop of the adjacent subunit, defines a specificity pocket that is larger and more polar than those of other tetrameric thiolases, suggesting that MSM-13 thiolase has a distinct substrate specificity. Consistent with this finding, only residual activity was detected with acetoacetyl-CoA as the substrate in the degradative direction. No activity was observed with acetyl-CoA in the synthetic direction. Structural comparisons with other well characterized thiolases suggest that MSM-13 thiolase is probably a degradative thiolase that is specific for 3-ketoacyl-CoA molecules with polar, bulky acyl chains.

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Photoelectron angular distributions produced in above-threshold ionization (ATI) are analysed using a nonperturbative scattering theory. The numerical results are in good qualitative agreement with recent measurements. Our study shows that the origin of the jet-like structure arises from the inherent properties of the ATI process and not from the angular momentum of either the initial or the excited states of the atom.

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The dynamic interaction processes between a nano-second laser pulse and a gas-puff target, such as those of plasma formation, laser heating, and x-ray emission, have been investigated quantitatively. Time and space-resolved x-ray and optical measurement techniques were used in order to investigate time-resolved laser absorption and subsequent x-ray generation. Efficient absorption of the incident laser energy into the gas-puff target of 17%, 12%, 38%, and 91% for neon, argon, krypton, and xenon, respectively, was shown experimentally. It was found that the laser absorption starts and, simultaneously, soft x-ray emission occurs. The soft x-ray lasts much longer than the laser pulse due to the recombination. Temporal evolution of the soft x-ray emission region was analyzed by comparing the experimental results to the results of the model calculation, in which the laser light propagation through a gas-puff plasma was taken into account. (C) 2003 American Institute of Physics.

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<p>Pulse-height and time-of-flight methods have been used to measure the electronic stopping cross sections for projectiles of <sup>12</sup>C, <sup>16</sup>O, <sup>19</sup>F, <sup>23</sup>Na, <sup>24</sup>Mg, and <sup>27</sup>Al, slowing in helium, neon, argon, krypton, and xenon. The ion energies were in the range 185 keV E 2560 keV.</p> <p>A semiempirical calculation of the electronic stopping cross section for projectiles with atomic numbers between 6 and 13 passing through the inert gases has been performed using a modification of the Firsov model. Using Hartree-Slater-Fock orbitals, and summing over the losses for the individual charge states of the projectiles, good agreement has been obtained with the experimental data. The main features of the stopping cross section seen in the data, such as the Z<sub>1</sub> oscillation and the variation of the velocity dependence on Z<sub>1</sub> and Z<sub>2</sub>, are present in the calculation. The inclusion of a modified form of the Bethe-Bloch formula as an additional term allows the increase of the velocity dependence for projectile velocities above v<sub>o</sub> to be reproduced in the calculation.</p>

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<p>The principle aims of this thesis include the development of models of sublimation and melting from first principles and the application of these models to the rare gases.</p> <p>A simple physical model is constructed to represent the sublimation of monatomic elements. According to this model, the solid and gas phases are two states of a single physical system. The nature of the phase transition is clearly revealed, and the relations between the vapor pressure, the latent heat, and the transition temperature are derived. The resulting theory is applied to argon, krypton, and xenon, and good agreement with experiment is found.</p> <p>For the melting transition, the solid is represented by an anharmonic model and the liquid is described by the Percus-Yevick approximation. The behavior of the liquid at high densities is studied on the isotherms kT/ = 1.3, 1.8, and 2.0, where k is Boltzmann's constant, T is the temperature, and e is the well depth of the Lennard-Jones 12-6 pair potential. No solutions of the PercusYevick equation were found for <sup>3</sup> above 1.3, where is the particle density and is the radial parameter of the Lennard-Jones potential. The liquid structure is found to be very different from the solid structure near the melting line. The liquid pressures are about 50 percent low for experimental melting densities of argon. This discrepancy gives rise to melting pressures up to twice the experimental values.</p>

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<p>The isotopic and elemental abundances of noble gases in the solar system are investigated, using simple mixing models and mass-spectrometric measurements of the noble gases in meteorites and terrestrial rocks and minerals.</p> <p>Primordial neon is modeled by two isotopically distinct components from the interstellar gas and dust. Neon from the gas dominates solar neon, which contains about ten times more <sup>20</sup>Ne than <sup>22</sup>Ne. Neon from the dust is represented in meteorites by neon-E, with <sup>20</sup>Ne/<sup>22</sup>Ne less than 0.6. Isotopic variations in meteorites require neon from both dust and gas to be present. Mixing dust and gas without neon loss generates linear correlation lines on three-isotope and composition-concentration diagrams. A model for solar wind implantation predicts small deviations from linear mixing, due to preferential sputtering of the lighter neon isotopes.</p> <p>Neon in meteorites consists of galactic cosmic ray spallation neon and at least two primordial components, neon-E and neon-S. Neon was measured in several meteorites to investigate these end- members. Cosmogenic neon produced from sodium is found to be strongly enriched in <sup>22</sup>Ne. Neon measurements on sodium-rich samples must be interpreted with care so not to confuse this source of <sup>22</sup>Ne with neon-E, which is also rich in <sup>22</sup>Ne.</p> <p>Neon data for the carbonaceous chondrite Mokoia show that the end member composition of neon-Si in meteorites is <sup>20</sup>Ne/<sup>22</sup>Ne = 13.7, the same as the present solar wind. The solar wind composition evidently has remained constant since before the compaction of Mokoia.</p> <p>Ca, Al-rich inclusions from the Allende meteorite were examined for correlation between neon-E and oxygen or magnesium isotopic anomalies. <sup>22</sup>Ne and <sup>36</sup>Ar enrichments found in some inclusions are attributed to cosmic- ray-induced reactions on Na and Cl, not to a primordial component. Neon-E is not detectably enriched in Allende.</p> <p> Measurements were made to determine the noble gas contents of various terrestrial rocks and minerals, and to investigate the cycling of noble gases between different terrestrial reservoirs. Beryl crystals contain a characteristic suite of magmatic gases including nucleogenic <sup>21</sup>Ne and <sup>22</sup>Ne from (,n) reactions, radiogenic <sup>40</sup>Ar, and fissiogenic <sup>131-136</sup>Xe from the decay of K and U in the continental crust. Significant concentrations of atmospheric noble gases are also present in beryl.</p> <p>Both juvenile and atmospheric noble gases are found in rocks from the Skaergaard intrusion. The ratio <sup>40</sup>Ar/<sup>36</sup>Ar (corrected for in situ decay of <sup>40</sup>K) correlates with <sup>18</sup>O in plagioclase. Atmospheric argon has been introduced into samples that have experienced oxygen-isotope exchange with circulating meteoric hydrothermal fluids. Unexchanged samples contain juvenile argon with <sup>40</sup>Ar/<sup>36</sup>Ar greater than 6000 that was trapped from the Skaergaard magma.</p> <p>Juvenile and atmospheric gases have been measured in the glassy rims of mid-ocean ridge (MOR) pillow basalts. Evidence is presented that three samples contain excess radiogenic <sup>129</sup>Xe and fission xenon, in addition to the excess radiogenic <sup>40</sup>Ar found in all samples. These juvenile gases are being outgassed from the upper-mantle source region of the MOR magma. No isotopic evidence has been found here for juvenile primordial noble gases accompanying the juvenile radiogenic gases in the MOR glasses. Large argon isotopic variations in a single specimen provide a clear indication of the late-stage addition of atmospheric argon, probably from seawater.</p> <p>The Skaergaard data demonstrate that atmospheric noble gases dissolved in ground water can be transferred into crustal rocks. Subduction of oceanic crust altered by seawater can transport atmospheric noble gases into the upper mantle. A substantial portion of the noble gases in mantle derived rocks may represent subducted gases, not a primordial component as is often assumed.</p>

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320nm

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ICFm16mm1140mmm17mm1270mm

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. Bi2O3,ZnO,Al2O3,MgO,CaCO3,SiO2,BaO,H3BO3,P2O5,Na2O . ,(Differential Thermal AnalysisDTA) X (X-ray diffraction XRD). . 1#2#4#10-6 Pa.

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The effect of weathering on the wear resistance of automotive clearcoats has been evaluated. Acrylic-based and urethane-based coatings were exposed in Florida, Belgium and Australia and also under accelerated conditions to SAE J1960 with 0.55 W m -2 borosilicate/borosilicate filtered xenon arc light. Weathering caused a significant reduction in the abrasion and erosion resistance of the clearcoats and large increases in their hardness. Accelerated weathering produced different effects from natural exposure.

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HIV-1, HIV-1, T1(tsCycT1)HIV-1 Tat . , RT-PCR , 2175bpts CycT1 cD NA. CycT1(hCycT1) cDNA 92.6%; 94.1%. , hCycT1 tsCycT1 127298.8%, 261. , tsCycT1 HIV-1 Tat , , HIV-1

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Hybrid bulk heterojunction solar cells based on blend of poly(3-hexylthiophene) (P3HT) and TiO2 nanotubes or dye(N719) modified TiO2 nanotubes were processed from solution and characterized to research the nature of organic/inorganic hybrid materials. Compared with the pristine polymer P3HT and TiO2 nanoparticles/P3HT solar cells, the TiO2 nanotubes/P3HT hybrid solar cells show obvious performance improvement, due to the formation of the bulk heterojunction and charge transport improvement. A further improvement in the device performance can be achieved by modifying TiO2 nanotube surface with a standard dye N719 which can play a role in the improvement of both the light absorption and charge dissociation. Compared with the non-modified TiO2 nanotubes solar cells, the modified ones have better power conversion efficiency under 100 mW/cm(2) illumination with 500W Xenon lamp. (C) 2008 Elsevier B. V. All rights reserved.