465 research outputs found
Heavy Quarkonia in Quark-Gluon Plasma
Using the color-singlet free energy F_1 and total internal energy U_1
obtained by Kaczmarek et al. for a static quark Q and an antiquark Qbar in
quenched QCD, we study the binding energies and wave functions of heavy
quarkonia in a quark-gluon plasma. By minimizing the grand potential in a
simplified schematic model, we find that the proper color-singlet Q-Qbar
potential can be obtained from the total internal energy U_1 by subtracting the
gluon internal energy contributions. We carry out this subtraction in the local
energy-density approximation in which the gluon energy density can be related
to the local gluon pressure by the quark-gluon plasma equation of state. We
find in this approximation that the proper color-singlet Q-Qbar potential is
approximately F_1 for T ~ T_c and it changes to (3/4)F_1+(1/4)U_1 at high
temperatures. In this potential model, the J/psi is weakly bound above the
phase transition temperature T_c, and it dissociates spontaneously above 1.62
T_c, while chi_c and psi' are unbound in the quark-gluon plasma. The bottomium
states Upsilon, chi_b and Upsilon' are bound in the quark-gluon plasma and they
dissociate at 4.10 T_c, 1.18 T_c, and 1.38 T_c respectively. For comparison, we
evaluate the heavy quarkonium binding energies also in other models using the
free energy F_1 or the total internal energy U_1 as the Q-Qbar potential. The
comparison shows that the model with the new Q-Qbar potential proposed in this
manuscript gives dissociation temperatures that agree best with those from
spectral function analyses. We evaluate the cross section for
sigma(g+J/psi->c+cbar) and its inverse process, in order to determine the J/psi
dissociation width and the rate of J/psi production by recombining c and cbar
in the quark gluon plasma.Comment: 30 pages, in Late
Illusion and Reality in the Atmospheres of Exoplanets
The atmospheres of exoplanets reveal all their properties beyond mass,
radius, and orbit. Based on bulk densities, we know that exoplanets larger than
1.5 Earth radii must have gaseous envelopes, hence atmospheres. We discuss
contemporary techniques for characterization of exoplanetary atmospheres. The
measurements are difficult, because - even in current favorable cases - the
signals can be as small as 0.001-percent of the host star's flux. Consequently,
some early results have been illusory, and not confirmed by subsequent
investigations. Prominent illusions to date include polarized scattered light,
temperature inversions, and the existence of carbon planets. The field moves
from the first tentative and often incorrect conclusions, converging to the
reality of exoplanetary atmospheres. That reality is revealed using transits
for close-in exoplanets, and direct imaging for young or massive exoplanets in
distant orbits. Several atomic and molecular constituents have now been
robustly detected in exoplanets as small as Neptune. In our current
observations, the effects of clouds and haze appear ubiquitous. Topics at the
current frontier include the measurement of heavy element abundances in giant
planets, detection of carbon-based molecules, measurement of atmospheric
temperature profiles, definition of heat circulation efficiencies for tidally
locked planets, and the push to detect and characterize the atmospheres of
super-Earths. Future observatories for this quest include the James Webb Space
Telescope, and the new generation of Extremely Large Telescopes on the ground.
On a more distant horizon, NASA's concepts for the HabEx and LUVOIR missions
could extend the study of exoplanetary atmospheres to true twins of Earth.Comment: Invited Review for the 25th Anniversary issue of JGR Planets, in
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New insights into stop codon recognition by eRF1
In eukaryotes, translation termination is performed by eRF1, which recognizes stop codons via its N-terminal domain. Many previous studies based on point mutagenesis, cross-linking experiments or eRF1 chimeras have investigated the mechanism by which the stop signal is decoded by eRF1. Conserved motifs, such as GTS and YxCxxxF, were found to be important for termination efficiency, but the recognition mechanism remains unclear. We characterized a region of the eRF1 N-terminal domain, the P1 pocket, that we had previously shown to be involved in termination efficiency. We performed alanine scanning mutagenesis of this region, and we quantified in vivo readthrough efficiency for each alanine mutant. We identified two residues, arginine 65 and lysine 109, as critical for recognition of the three stop codons. We also demonstrated a role for the serine 33 and serine 70 residues in UGA decoding in vivo. NMR analysis of the alanine mutants revealed that the correct conformation of this region was controlled by the YxCxxxF motif. By combining our genetic data with a structural analysis of eRF1 mutants, we were able to formulate a new model in which the stop codon interacts with eRF1 through the P1 pocket
Brownian motion: a paradigm of soft matter and biological physics
This is a pedagogical introduction to Brownian motion on the occasion of the
100th anniversary of Einstein's 1905 paper on the subject. After briefly
reviewing Einstein's work in its contemporary context, we pursue some lines of
further developments and applications in soft condensed matter and biology.
Over the last century Brownian motion became promoted from an odd curiosity of
marginal scientific interest to a guiding theme pervading all of the modern
(live) sciences.Comment: 30 pages, revie
Scale-free static and dynamical correlations in melts of monodisperse and Flory-distributed homopolymers: A review of recent bond-fluctuation model studies
It has been assumed until very recently that all long-range correlations are
screened in three-dimensional melts of linear homopolymers on distances beyond
the correlation length characterizing the decay of the density
fluctuations. Summarizing simulation results obtained by means of a variant of
the bond-fluctuation model with finite monomer excluded volume interactions and
topology violating local and global Monte Carlo moves, we show that due to an
interplay of the chain connectivity and the incompressibility constraint, both
static and dynamical correlations arise on distances . These
correlations are scale-free and, surprisingly, do not depend explicitly on the
compressibility of the solution. Both monodisperse and (essentially)
Flory-distributed equilibrium polymers are considered.Comment: 60 pages, 49 figure
Phosphoregulation of the budding yeast EB1 homologue Bim1p by Aurora/Ipl1p
EB1 (end binding 1) proteins have emerged as central regulators of microtubule (MT) plus ends in all eukaryotes, but molecular mechanisms controlling the activity of these proteins are poorly understood. In this study, we show that the budding yeast EB1 protein Bim1p is regulated by Aurora B/Ipl1p-mediated multisite phosphorylation. Bim1p forms a stable complex with Ipl1p and is phosphorylated on a cluster of six Ser residues in the flexible linker connecting the calponin homology (CH) and EB1 domains. Using reconstitution of plus end tracking in vitro and total internal reflection fluorescence microscopy, we show that dimerization of Bim1p and the presence of the linker domain are both required for efficient tip tracking and that linker phosphorylation removes Bim1p from static and dynamic MTs. Bim1 phosphorylation occurs during anaphase in vivo, and it is required for normal spindle elongation kinetics and an efficient disassembly of the spindle midzone. Our results define a mechanism for the use and regulation of CH domains in an EB1 protein
Short-term geriatric assessment units: 30 years later
<p>Abstract</p> <p>Background</p> <p>The increasing number of hospitalized elderly persons has greatly challenged decision makers to reorganize services so as to meet the needs of this clientele. Established progressively over the last 30 years, the short-term Geriatric Assessment Unit (GAU) is a specialized care program, now implemented in all the general hospital centres in Quebec. Within the scope of a broader reflection upon the appropriate care delivery for elderly patients in our demographic context, there is a need to revisit the role of GAU within the hospital and the continuum of care. The objective of this project is to describe the range of activities offered by Quebec GAU and the resources available to them.</p> <p>Methods</p> <p>In 2004, 64 managers of 71 GAU answered a mail questionnaire which included 119 items covering their unit's operation and resources in 2002-2003. The clinical and administrative characteristics of the clientele admitted during this period were obtained from the provincial database Med-Echo. The results were presented according to the geographical location of GAU, their size, their university academic affiliation, the composition of their medical staff, and their clinical care profile.</p> <p>Results</p> <p>Overall, GAU programs admitted 9% of all patients aged 65 years and older in the surveyed year. GAU patients presented one or more geriatric syndromes, including dementia. Based on their clientele, three distinct clinical care profiles of GAU were identified. Only 19% of GAU were focused on geriatric assessment and acute care management; 23% mainly offered rehabilitation care, and the others offered a mix of both types. Thus, there was a significant heterogeneity in GAU's operation.</p> <p>Conclusions</p> <p>The GAU is at the cutting edge of geriatric services in hospital centres. Given the scarcity of these resources, it would be appropriate to better target the clientele that may benefit from them. Standardizing and promoting GAU's primary role in acute care must be reinforced. In order to meet the needs of the frail elderly not admitted in GAU, alternative care models centered on prevention of functional decline must be applied throughout all hospital wards.</p
Hydrazone chelators for the treatment of iron overload disorders: iron coordination chemistry and biological activity
The potentially tridentate ligand 2-pyridinecarbaldehyde isonicotinoyl hydrazone (HPCIH) and its analogues are an emerging class of orally effective Fe chelators that show great promise for the treatment of Fe overload diseases. Herein, we present an extensive study of the Fe coordination chemistry of the HPCIH analogues including the first crystallographically characterised Fe-II complex of these chelators. Unlike most other clinically effective Fe chelators, the HPCIH analogues bind Fe-II and not F-III. In fact, these chelators form low-spin bis-ligand F-II complexes, although NMR suggests that the complexes are close to the high-spin/low-spin crossover. All the Fe complexes show a high potential Fe-III/(II) redox couple (> 500 mV vs. NHE) and cyclic voltammetry in aqueous or mixed aqueous/organic solvents is irreversible as a consequence of a rapid hydration reaction that occurs upon oxidation. A number of the HPCIH analogues show high activity at inducing Fe efflux from cells and also at preventing Fe uptake by cells from the serum Fe transport protein transferrin. As a class of ligands, these chelators are more effective at reducing Fe uptake from transferrin than inducing Fe mobilisation from cells. This may be related to their ability to intercept Fe-II after its release from transferrin within the cell. Our studies indicate that their Fe chelation efficacy is due, at least in part, to the fact that these ligands and their Fe-II complexes are neutral at physiological pH (7.4) and sufficiently lipophilic to permeate cell membranes
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