2,781 research outputs found
On the efficient numerical solution of lattice systems with low-order couplings
We apply the Quasi Monte Carlo (QMC) and recursive numerical integration
methods to evaluate the Euclidean, discretized time path-integral for the
quantum mechanical anharmonic oscillator and a topological quantum mechanical
rotor model. For the anharmonic oscillator both methods outperform standard
Markov Chain Monte Carlo methods and show a significantly improved error
scaling. For the quantum mechanical rotor we could, however, not find a
successful way employing QMC. On the other hand, the recursive numerical
integration method works extremely well for this model and shows an at least
exponentially fast error scaling
Time-resolved charge translocation by the Ca-ATPase from sarcoplasmic reticulum after an ATP concentration jump
Time-resolved measurements of currents generated by Ca-ATPase from fragmented sarcoplasmic reticulum (SR) are described. SR vesicles spontaneously adsorb to a black lipid membrane acting as a capacitive electrode. Charge translocation by the enzyme is initiated by an ATP concentration jump performed by the light-induced conversion of an inactive precursor (caged ATP) to ATP with a time constant of 2.0 ms at pH 6.2 and 24 degrees C. The shape of the current signal is triphasic, an initial current flow into the vesicle lumen is followed by an outward current and a second slow inward current. The time course of the current signal can be described by five relaxation rate constants, lambda1 to lambda5 plus a fixed delay D approximately 1–3 ms. The electrical signal shows that 1) the reaction cycle of the Ca-ATPase contains two electrogenic steps; 2) positive charge is moved toward the luminal side in the first rapid step and toward the cytoplasmic side in the second slow step; 3) at least one electroneutral reaction precedes the electrogenic steps. Relaxation rate constant lambda3 reflects ATP binding, with lambda(3,max) approximately 100 s(-1). This step is electroneutral. Comparison with the kinetics of the reaction cycle shows that the first electrogenic step (inward current) occurs before the decay of E2P. Candidates are the formation of phosphoenzyme from E1ATP (lambda2 approximately 200 s[-1]) and the E1P --> E2P transition (D approximately 1 ms or lambda1 approximately 300 s[-1]). The second electrogenic transition (outward current) follows the formation of E2P (lambda4 approximately 3 s[-1]) and is tentatively assigned to H+ countertransport after the dissociation of Ca2+. Quenched flow experiments performed under the conditions of the electrical measurements 1) demonstrate competition by caged ATP for ATP-dependent phosphoenzyme formation and 2) yield a rate constant for phosphoenzyme formation of 200 s(-1). These results indicate that ATP and caged ATP compete for the substrate binding site, as suggested by the ATP dependence of lambda3 and favor correlation of lambda2 with phosphoenzyme formation
Simple t-distribution Based Tests for Meta-Analysis
The variance function of the optimal estimator of the overall mean in a heteroscedastic one-way ANOVA model is dominated by positive semi-definite quadratic functions. This makes it possible to develop closely related tests on the nullity of the overall mean parameter, in one-way fifixed and random effects ANOVA models, which make use of the quantiles of the t-distribution. These tests are founded on the convexity arguments similar to Hartung (1976). Simulation results indicate that the proposed tests attain type I error rates which are far more acceptable than those of the commonly used tests
Diastereofacial selectivity in reactions of substituted cyclohexyl radicals. An experimental and theoretical study
Polymorphism of the tumor necrosis factor beta gene in systemic lupus erythematosus
We investigated the Nco I restriction fragment
length polymorphism (RFLP) of the tumor necrosis
factor beta (TNFB) gene in 173 patients with systemic
lupus erythematosus (SLE), 192 unrelated
healthy controls, and eleven panel families, all of German
origin. The phenotype frequency of the TNFB*I
allele was significantly increased in patients compared
to controls (63.6% vs 47.1%, RR = 1.96, p <0.002).
The results of a two-point haplotype statistical analysis
between TNFB and HLA alleles show that there is linkage
disequilibrium between TNFB*I and HLA-A1,
Cw7, B8, DR3, DQ2, and C4A DE. The frequency of
TNFB*I was compared in SLE patients and controls in
the presence or absence of each of these alleles.
TNFB*I is increased in patients over controls only in
the presence of the mentioned alleles. Therefore, the
whole haplotypeA1, Cw7, B8, TNFB* I, C4A DE, DR3,
DQ2 is increased in patients and it cannot be determined
which of the genes carried by this haplotype is
responsible for the susceptibility to SLE. In addition,
two-locus associations were analyzed in 192 unrelated
healthy controls for TNFB and class I alleles typed by
serology, and for TNFB and class II alleles typed by
polymerase chain reaction/oligonucleotide probes. We
found positive linkage disequilibrium between
TNFB*I and the following alleles: HLA-A24, HLA-B8,
DRBI*0301, DRBI*ll04, DRBI*1302, DQAI*0501, DQBI*0201, DQBI*0604, and DPBI*OIO1. TNFB*2
is associated with HLA-B7, DRBI*1501, and
DQB I *0602
Enantioselective Olefin Metathesis with Cyclometalated Ruthenium Complexes
The success of enantioselective olefin metathesis relies on the design of enantioenriched alkylidene complexes capable of transferring stereochemical information from the catalyst structure to the reactants. Cyclometalation of the NHC ligand has proven to be a successful strategy to incorporate stereogenic atoms into the catalyst structure. Enantioenriched complexes incorporating this design element catalyze highly Z- and enantioselective asymmetric ring opening/cross metathesis (AROCM) of norbornenes and cyclobutenes, and the difference in ring strain between these two substrates leads to different propagating species in the catalytic cycle. Asymmetric ring closing metathesis (ARCM) of a challenging class of prochiral trienes has also been achieved. The extent of reversibility and effect of reaction setup was also explored. Finally, promising levels of enantioselectivity in an unprecedented Z-selective asymmetric cross metathesis (ACM) of a prochiral 1,4-diene was demonstrated
Hybridization of electron subbands in a double quantum well at quantizing magnetic field
We employ magnetocapacitance and far-infrared spectroscopy techniques to
study the spectrum of the double-layer electron system in a parabolic quantum
well with a narrow tunnel barrier in the centre. For gate-bias-controlled
asymmetric electron density distributions in this soft two-subband system we
observe both individual subband gaps and double layer gaps at integer filling
factor . The bilayer gaps are shown to be either trivial common for two
subbands or caused by hybridization of electron subbands in magnetic field. We
describe the observed hybrid gaps at and within a simple model
for the modified bilayer spectrum.Comment: REVTeX, 24 pages, 9 figures included. Submitted to Phys. Rev.
VLT Diffraction Limited Imaging and Spectroscopy in the NIR: Weighing the black hole in Centaurus A with NACO
We present high spatial resolution near-infrared spectra and images of the
nucleus of Centaurus A (NGC 5128) obtained with NAOS-CONICA at the VLT. The
adaptive optics corrected data have a spatial resolution of 0.06" (FWHM) in K-
and 0.11" in H-band, four times higher than previous studies. The observed gas
motions suggest a kinematically hot disk which is orbiting a central object and
is oriented nearly perpendicular to the nuclear jet. We model the central
rotation and velocity dispersion curves of the [FeII] gas orbiting in the
combined potential of the stellar mass and the (dominant) black hole. Our
physically most plausible model, a dynamically hot and geometrically thin gas
disk, yields a black hole mass of M_bh = (6.1 +0.6/-0.8) 10^7 M_sun. As the
physical state of the gas is not well understood, we also consider two limiting
cases: first a cold disk model, which completely neglects the velocity
dispersion; it yields an M_bh estimate that is almost two times lower. The
other extreme case is to model a spherical gas distribution in hydrostatic
equilibrium through Jeans equation. Compared to the hot disk model the best-fit
black hole mass increases by a factor of 1.5. This wide mass range spanned by
the limiting cases shows how important the gas physics is even for high
resolution data. Our overall best-fitting black hole mass is a factor of 2-4
lower than previous measurements. With our revised M_bh estimate, Cen A's
offset from the M_bh-sigma relation is significantly reduced; it falls above
this relation by a factor of ~2, which is close to the intrinsic scatter of
this relation. (Abridged)Comment: 12 pages, 14 figures, including minor changes following the referee
report; accepted for publication in The Astrophysical Journa
Detection of antibiotics and antibiotic resistance genes in manures from different pig producing systems and in biogas plants
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Diagnosing topographic forcing in an atmospheric dataset: the case of the North American Cordillera
It is well known from modelling studies that surface topography influences the large-scale atmospheric circulation and that several model biases are associated with incorrect representation of topography. The textbook explanation of topographic effects on large-scale circulation appeals to the theoretical relationship between surface forcing and vortex stretching along trajectories in single-layer models. The goal of this study is to design and use a simple diagnostic of the large-scale forcing on the atmosphere when air is passing over topography, directly from atmospheric fields, based on this theoretical relationship. The study examines the interaction of the atmosphere with the North American Cordillera and samples the flow by means of trajectories during Northern Hemisphere winter. We detect a signal of topographic forcing in the atmospheric dataset, which, although much less distinct than in the theoretical relationship, nevertheless exhibits a number of expected properties. Namely, the signal increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge, for example during periods of positive PNA. Furthermore, a connection is found between an enhanced signal of topographic forcing downslope of the North American Cordillera and periods of more frequent downstream European blocking
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