3,718 research outputs found
Path integral Monte Carlo simulations of silicates
We investigate the thermal expansion of crystalline SiO in the --
cristobalite and the -quartz structure with path integral Monte Carlo
(PIMC) techniques. This simulation method allows to treat low-temperature
quantum effects properly. At temperatures below the Debye temperature, thermal
properties obtained with PIMC agree better with experimental results than those
obtained with classical Monte Carlo methods.Comment: 27 pages, 10 figures, Phys. Rev. B (in press
Glass-Like Heat Conduction in High-Mobility Crystalline Semiconductors
The thermal conductivity of polycrystalline semiconductors with type-I
clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr
and/or Eu) exhibit lattice thermal conductivities typical of amorphous
materials. Remarkably, this behavior occurs in spite of the well-defined
crystalline structure and relatively high electron mobility (). The dynamics of dopant ions and their interaction with the
polyhedral cages of the structure are a likely source of the strong phonon
scattering.Comment: 4 pages, 3 postscript figures, to be published, Phys. Rev. Let
Mitochondrial peroxiredoxin functions as crucial chaperone reservoir in Leishmania infantum
Cytosolic eukaryotic 2-Cys-peroxiredoxins have been widely reported to act as dual-function proteins, either detoxifying reactive oxygen species or acting as chaperones to prevent protein aggregation. Several stimuli, including peroxide-mediated sulfinic acid formation at the active site cysteine, have been proposed to trigger the chaperone activity. However, the mechanism underlying this activation and the extent to which the chaperone function is crucial under physiological conditions in vivo remained unknown. Here we demonstrate that in the vector-borne protozoan parasite Leishmania infantum, mitochondrial peroxiredoxin (Prx) exerts intrinsic ATP-independent chaperone activity, protecting a wide variety of different proteins against heat stress-mediated unfolding in vitro and in vivo. Activation of the chaperone function appears to be induced by temperature-mediated restructuring of the reduced decamers, promoting binding of unfolding client proteins in the center of Prx's ringlike structure. Client proteins are maintained in a folding-competent conformation until restoration of nonstress conditions, upon which they are released and transferred to ATP-dependent chaperones for refolding. Interference with client binding impairs parasite infectivity, providing compelling evidence for the in vivo importance of Prx's chaperone function. Our results suggest that reduced Prx provides a mitochondrial chaperone reservoir, which allows L. infantum to deal successfully with protein unfolding conditions during the transition from insect to the mammalian hosts and to generate viable parasites capable of perpetuating infection.We thank Frederico Silva for help with size-exclusion chromatography experiments, and Ana G. Gomes-Alves and Ricardo Silva for constructing the pSSU-PHLEO-infantum-MTS.His.THR-mTXNPx plasmid. This work was supported by National Institutes of Health Grant GM065318 (to U.J.) and Project "NORTE-07-0124-FEDER-000002-Host-Pathogen Interactions" cofunded by Programa Operacional Regional do Norte under the Quadro de Referencia Estrategico Nacional, through Fundo Europeu de Desenvolvimento Regional, and by the Portuguese Foundation for Science and Technology (FCT) (A.M.T.). F.T. and H.C. were supported by Portuguese FCT Fellowships SFRH/BD/70438/2010 and SFRH/BPD/80836/2011, respectively
Longer-Term Postcure Measurement of Cuspal Deformation Induced by Dimensional Changes in Dental Materials
Aim. This paper presents a simple, versatile in vitro methodology that enables indirect quantification of shrinkage and expansion stresses under clinically relevant conditions without the need for a dedicated instrument. Methods. For shrinkage effects, resulting cusp deformation of aluminum blocks with MOD type cavity, filled with novel filling compositions and commercial cements, has been measured using a bench-top micrometer and a Linear Variable Differential Transformer (LVDT, a displacement transducer) based instrument. Results. The results demonstrated the validity of the proposed simple methodology. The technique was successfully used in longer-term measurements of shrinkage and expansion stress for several dental compositions. Conclusions. In contrast to in situ techniques where a measuring instrument is dedicated to the sample and its data collection, the proposed simple methodology allows for transfer of the samples to the environment of choice for storage and conditioning. The presented technique can be reliably used to quantify stress development of curing materials under clinically relevant (oral) conditions. This enables direct examination and comparison of structural properties corresponding to the final stage of formed networks. The proposed methodology is directly applicable to the study of self-curing systems as they require mouth-type conditions (temperature and humidity) to achieve their designed kinetics and reactions
The electronic structure of amorphous silica: A numerical study
We present a computational study of the electronic properties of amorphous
SiO2. The ionic configurations used are the ones generated by an earlier
molecular dynamics simulations in which the system was cooled with different
cooling rates from the liquid state to a glass, thus giving access to
glass-like configurations with different degrees of disorder [Phys. Rev. B 54,
15808 (1996)]. The electronic structure is described by a tight-binding
Hamiltonian. We study the influence of the degree of disorder on the density of
states, the localization properties, the optical absorption, the nature of
defects within the mobility gap, and on the fluctuations of the Madelung
potential, where the disorder manifests itself most prominently. The
experimentally observed mismatch between a photoconductivity threshold of 9 eV
and the onset of the optical absorption around 7 eV is interpreted by the
picture of eigenstates localized by potential energy fluctuations in a mobility
gap of approximately 9 eV and a density of states that exhibits valence and
conduction band tails which are, even in the absence of defects, deeply located
within the former band gap.Comment: 21 pages of Latex, 5 eps figure
Solid molecular hydrogen: The Broken Symmetry Phase
By performing constant-pressure variable-cell ab initio molecular dynamics
simulations we find a quadrupolar orthorhombic structure, of symmetry,
for the broken symmetry phase (phase II) of solid H2 at T=0 and P =110 - 150
GPa. We present results for the equation of state, lattice parameters and
vibronic frequencies, in very good agreement with experimental observations.
Anharmonic quantum corrections to the vibrational frequencies are estimated
using available data on H2 and D2. We assign the observed modes to specific
symmetry representations.Comment: 5 pages (twocolumn), 4 Postscript figures. To appear in Phys. Rev.
Let
Ice XII in its second regime of metastability
We present neutron powder diffraction results which give unambiguous evidence
for the formation of the recently identified new crystalline ice phase[Lobban
et al.,Nature, 391, 268, (1998)], labeled ice XII, at completely different
conditions. Ice XII is produced here by compressing hexagonal ice I_h at T =
77, 100, 140 and 160 K up to 1.8 GPa. It can be maintained at ambient pressure
in the temperature range 1.5 < T < 135 K. High resolution diffraction is
carried out at T = 1.5 K and ambient pressure on ice XII and accurate
structural properties are obtained from Rietveld refinement. At T = 140 and 160
K additionally ice III/IX is formed. The increasing amount of ice III/IX with
increasing temperature gives an upper limit of T ~ 150 K for the successful
formation of ice XII with the presented procedure.Comment: 3 Pages of RevTeX, 3 tables, 3 figures (submitted to Physical Review
Letters
Is mindfulness Buddhist? (and why it matters).
Modern exponents of mindfulness meditation promote the therapeutic effects of "bare attention"--a sort of non-judgmental, non-discursive attending to the moment-to-moment flow of consciousness. This approach to Buddhist meditation can be traced to Burmese Buddhist reform movements of the first half of the 20th century, and is arguably at odds with more traditional Theravāda Buddhist doctrine and meditative practices. But the cultivation of present-centered awareness is not without precedent in Buddhist history; similar innovations arose in medieval Chinese Zen (Chan) and Tibetan Dzogchen. These movements have several things in common. In each case the reforms were, in part, attempts to render Buddhist practice and insight accessible to laypersons unfamiliar with Buddhist philosophy and/or unwilling to adopt a renunciatory lifestyle. In addition, these movements all promised astonishingly quick results. And finally, the innovations in practice were met with suspicion and criticism from traditional Buddhist quarters. Those interested in the therapeutic effects of mindfulness and bare attention are often not aware of the existence, much less the content, of the controversies surrounding these practices in Asian Buddhist history
Tpeak-Tend, Tpeak-Tend/QT ratio and Tpeak-Tend dispersion for risk stratification in Brugada Syndrome:A systematic review and meta-analysis
Background: Brugada syndrome is an ion channelopathy that predisposes affected subjects to ventricular tachycardia/fibrillation (VT/VF), potentially leading to sudden cardiac death (SCD). Tpeak-Tend intervals, (Tpeak-Tend)/QT ratio and Tpeak-Tend dispersion have been proposed for risk stratification, but their predictive values in Brugada syndrome have been challenged recently. Methods: A systematic review and meta-analysis was conducted to examine their values in predicting arrhythmic and mortality outcomes in Brugada Syndrome. PubMed and Embase databases were searched until 1 May 2018, identifying 29 and 57 studies. Results: Nine studies involving 1740 subjects (mean age 45 years old, 80% male, mean follow-up duration was 68 ± 27 months) were included. The mean Tpeak-Tend interval was 98.9 ms (95% CI: 90.5-107.2 ms) for patients with adverse events (ventricular arrhythmias or SCD) compared to 87.7 ms (95% CI: 80.5-94.9 ms) for those without such events, with a mean difference of 11.9 ms (95% CI: 3.6-20.2 ms, P = 0.005; I2 = 86%). Higher (Tpeak-Tend)/QT ratios (mean difference = 0.019, 95% CI: 0.003-0.036, P = 0.024; I2 = 74%) and Tpeak-Tend dispersion (mean difference = 7.8 ms, 95% CI: 2.1-13.4 ms, P = 0.007; I2 = 80%) were observed for the event-positive group. Conclusion: Tpeak-Tend interval, (Tpeak-Tend)/QT ratio and Tpeak-Tend dispersion were higher in high-risk than low-risk Brugada subjects, and thus offer incremental value for risk stratification
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