37,085 research outputs found
Gap formation and soft phonon mode in the Holstein model
We investigate electron-phonon coupling in many-electron systems using
dynamical mean-field theory in combination with the numerical renormalization
group. This non-perturbative method reveals significant precursor effects to
the gap formation at intermediate coupling strengths. The emergence of a soft
phonon mode and very strong lattice fluctuations can be understood in terms of
Kondo-like physics due to the development of a double-well structure in the
effective potential for the ions
Isogrid design handbook
Handbook has been published which presents information needed for design of isogrid triangular integral-stiffened structures. It develops equations, methods, and graphs to handle wide variety of loadings, materials, and geometry. Handbook is divided into seven sections. Handbook may be used by marine and civil engineers and by students and designers without access to computers
Functional Sites Induce Long-Range Evolutionary Constraints in Enzymes
Functional residues in proteins tend to be highly conserved over evolutionary time. However, to what extent functional sites impose evolutionary constraints on nearby or even more distant residues is not known. Here, we report pervasive conservation gradients toward catalytic residues in a dataset of 524 distinct enzymes: evolutionary conservation decreases approximately linearly with increasing distance to the nearest catalytic residue in the protein structure. This trend encompasses, on average, 80% of the residues in any enzyme, and it is independent of known structural constraints on protein evolution such as residue packing or solvent accessibility. Further, the trend exists in both monomeric and multimeric enzymes and irrespective of enzyme size and/or location of the active site in the enzyme structure. By contrast, sites in protein–protein interfaces, unlike catalytic residues, are only weakly conserved and induce only minor rate gradients. In aggregate, these observations show that functional sites, and in particular catalytic residues, induce long-range evolutionary constraints in enzymes.Fil: Jack, Benjamin R.. University of Texas at Austin; Estados UnidosFil: Meyer, Austin G.. University of Texas at Austin; Estados UnidosFil: Echave, Julián. Universidad Nacional de San MartÃn. Escuela de Ciencia y TecnologÃa; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Wilke, Claus O.. University of Texas at Austin; Estados Unido
Lateral vibration effects in atomic-scale friction
The influence of lateral vibrations on the stick-slip motion of a nanotip
elastically pulled on a flat crystal surface is studied by atomic force
microscopy (AFM) measurements on a NaCl(001) surface in ultra-high vacuum. The
slippage of the nanotip across the crystal lattice is anticipated at increasing
driving amplitude, similarly to what is observed in presence of normal
vibrations. This lowers the average friction force, as explained by the
Prandtl-Tomlinson model with lateral vibrations superimposed at finite
temperature. Nevertheless, the peak values of the lateral force, and the total
energy losses, are expected to increase with the excitation amplitude, which
may limit the practical relevance of this effect.Comment: To appear in Applied Physics Letter
Facilitated diffusion of proteins on chromatin
We present a theoretical model of facilitated diffusion of proteins in the
cell nucleus. This model, which takes into account the successive
binding/unbinding events of proteins to DNA, relies on a fractal description of
the chromatin which has been recently evidenced experimentally. Facilitated
diffusion is shown quantitatively to be favorable for a fast localization of a
target locus by a transcription factor, and even to enable the minimization of
the search time by tuning the affinity of the transcription factor with DNA.
This study shows the robustness of the facilitated diffusion mechanism, invoked
so far only for linear conformations of DNA.Comment: 4 pages, 4 figures, accepted versio
Polarization phenomena in the reaction NN to NNpi near threshold
First calculations for spin-dependent observables of the reactions , and near threshold are presented,
employing the J\"ulich model for pion production. The influence of resonant
(via the excitation of the ) and non-resonant p-wave pion
production mechanisms on these observables is examined. For the reactions and nice agreement of our predictions with the
presently available data on spin correlation coefficents is observed whereas
for the description of the data is less satisfying.Comment: 10 pages, 4 figure
Exact calculations of first-passage quantities on recursive networks
We present general methods to exactly calculate mean-first passage quantities
on self-similar networks defined recursively. In particular, we calculate the
mean first-passage time and the splitting probabilities associated to a source
and one or several targets; averaged quantities over a given set of sources
(e.g., same-connectivity nodes) are also derived. The exact estimate of such
quantities highlights the dependency of first-passage processes with respect to
the source-target distance, which has recently revealed to be a key parameter
to characterize transport in complex media. We explicitly perform calculations
for different classes of recursive networks (finitely ramified fractals,
scale-free (trans)fractals, non-fractals, mixtures between fractals and
non-fractals, non-decimable hierarchical graphs) of arbitrary size. Our
approach unifies and significantly extends the available results in the field.Comment: 16 pages, 10 figure
Indomethacin decreases viscosity of gallbladder bile in patients with cholesterol gallstone disease
There is experimental evidence that inhibition of cyclooxygenase with nonsteroidal anti-inflammatory drugs may decrease cholesterol gall-stone formation and mitigate biliary pain in gall-stone patients. The mechanisms by which NSAIDs exert these effect are unclear. In a prospective, controlled clinical trial we examined the effects of oral indomethacin on the composition of human gall-bladder bile. The study included 28 patients with symptomatic cholesterol or mixed gallstones. Of these, 8 were treated with 3 × 25 mg indomethacin daily for 7 days prior to elective cholecystectomy while 20 received no treatment and served as controls. Bile and tissue samples from the gallbladder were obtained during cholecystectomy. Indomethacin tissue levels in the gallbladder mucosa, as assessed by HPLC, were 1.05±0.4 ng/mg wet weight, a concentration known to inhibit effectively cyclooxygenase activity. Nevertheless, no differences between the treated and untreated groups were found in the concentrations of biliary mucus glycoprotein (0.94±0.27 versus 0.93±0.32 mg/ml) or total protein (5.8±0.9 versus 6.4±1.3 mg/ml), cholesterol saturation (1.3±0.2 versus 1.5±0.2), or nucleation time (2.0±3.0 versus 1.5±2.0 days). However, biliary viscosity, measured using a low-shear rotation viscosimeter, was significantly lower in patients receiving indomethacin treatment (2.9±0.6 versus 5.6±1.2 mPa.s; P < 0.02). In conclusion, in man oral indomethacin decreases bile viscosity without alteration of bile lithogenicity or biliary mucus glycoprotein content. Since mucus glycoproteins are major determinants of bile viscosity, an alteration in mucin macromolecular composition may conceivably cause the indomethacin-induced decrease in biliary viscosity and explain the beneficial effects of nonsteroidal anti-inflammatory drugs in gallstone disease
Absolute sensitivity calibration of an extreme ultraviolet spectrometer for tokamak measurements
An extreme ultraviolet spectrometer installed on the Tore Supra tokamak has been calibrated in absolute units of brightness in the range 10-340 Ã…. This has been performed by means of a combination of techniques. The range 10-113 Ã… was absolutely calibrated by using an ultrasoft-X ray source emitting six spectral lines in this range. The calibration transfer to the range 113-182 Ã… was performed using the spectral line intensity branching ratio method. The range 182-340 Ã… was calibrated thanks to radiative-collisional modelling of spectral line intensity ratios. The maximum sensitivity of the spectrometer was found to lie around 100 Ã…. Around this wavelength, the sensitivity is fairly flat in a 80 Ã… wide interval. The spatial variations of sensitivity along the detector assembly were also measured. The observed trend is related to the quantum efficiency decrease as the angle of the incoming photon trajectories becomes more grazing
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