464 research outputs found
Theory of mechanical unfolding of homopolymer globule: all-or-none transition in force-clamp mode vs phase coexistence in position-clamp mode
Equilibrium mechanical unfolding of a globule formed by long flexible
homopolymer chain collapsed in a poor solvent and subjected to an extensional
force f (force-clamp mode) or extensional deformation D (position-clamp mode)
is studied theoretically. Our analysis, like all previous analysis of this
problem, shows that the globule behaves essentially differently in two modes of
extension. In the force-clamp mode, mechanical unfolding of the globule with
increasing applied force occurs without intramolecular microphase segregation,
and at certain threshold value of the pulling force the globule unfolds as a
whole ("all-or-none" transition). The value of the threshold force and the
corresponding jump in the distance between the chain ends increase with a
deterioration of the solvent quality and/or with an increase in the degree of
polymerization. In the position-clamp mode, the globule unfolding occurs via
intramolecular microphase coexistence of globular and extended microphases
followed by an abrupt unraveling transition. Reaction force in the microphase
segregation regime demonstrates an "anomalous" decrease with increasing
extension. Comparison of deformation curves in force and position-clamp modes
demonstrates that at weak and strong extensions the curves for two modes
coincide, differences are observed in the intermediate extension range. Another
unfolding scenario is typical for short globules: in both modes of extension
they unfold continuously, without jumps or intramolecular microphase
coexistence, by passing a sequence of uniformly elongated configurations.Comment: 19 pages, 13 figures, 1 tabl
Reversible stretching of homopolymers and random heteropolymers
We have analyzed the equilibrium response of chain molecules to stretching.
For a homogeneous sequence of monomers, the induced transition from compact
globule to extended coil below the -temperature is predicted to be
sharp. For random sequences, however, the transition may be smoothed by a
prevalence of necklace-like structures, in which globular regions and coil
regions coexist in a single chain. As we show in the context of a random
copolymer, preferential solvation of one monomer type lends stability to such
structures. The range of stretching forces over which necklaces are stable is
sensitive to chain length as well as sequence statistics.Comment: 14 pages, 4 figure
Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus)
The Eastern, migratory population of monarch butterflies (Danaus plexippus), an iconic North American insect, has declined by ~80% over the last decade. The monarch’s multi-generational migration between overwintering grounds in central Mexico and the summer breeding grounds in the northern U.S. and southern Canada is celebrated in all three countries and creates shared management responsibilities across North America. Here we present a novel Bayesian multivariate auto-regressive state-space model to assess quasi-extinction risk and aid in the establishment of a target population size for monarch conservation planning. We find that, given a range of plausible quasi-extinction thresholds, the population has a substantial probability of quasi-extinction, from 11–57% over 20 years, although uncertainty in these estimates is large. Exceptionally high population stochasticity, declining numbers, and a small current population size act in concert to drive this risk. An approximately 5-fold increase of the monarch population size (relative to the winter of 2014–15) is necessary to halve the current risk of quasi-extinction across all thresholds considered. Conserving the monarch migration thus requires active management to reverse population declines, and the establishment of an ambitious target population size goal to buffer against future environmentally driven variability
Mechanical response of random heteropolymers
We present an analytical theory for heteropolymer deformation, as exemplified
experimentally by stretching of single protein molecules. Using a mean-field
replica theory, we determine phase diagrams for stress-induced unfolding of
typical random sequences. This transition is sharp in the limit of infinitely
long chain molecules. But for chain lengths relevant to biological
macromolecules, partially unfolded conformations prevail over an intermediate
range of stress. These necklace-like structures, comprised of alternating
compact and extended subunits, are stabilized by quenched variations in the
composition of finite chain segments. The most stable arrangements of these
subunits are largely determined by preferential extension of segments rich in
solvophilic monomers. This predicted significance of necklace structures
explains recent observations in protein stretching experiments. We examine the
statistical features of select sequences that give rise to mechanical strength
and may thus have guided the evolution of proteins that carry out mechanical
functions in living cells.Comment: 10 pages, 6 figure
Impact of propofol on mid-latency auditory-evoked potentials in children†
Background Propofol is increasingly used in paediatric anaesthesia, but can be challenging to titrate accurately in this group. Mid-latency auditory-evoked potentials (MLAEPs) can be used to help titrate propofol. However, the effects of propofol on MLAEP in children are unclear. Therefore, we investigated the relationship between propofol and MLAEP in children undergoing anaesthesia. Methods Fourteen healthy children aged 4-16 yr received anaesthesia for elective surgery. Before surgery, propofol was administered in three concentrations (3, 6, 9 µg ml−1) through a target-controlled infusion pump using Kataria and colleagues' model. MLAEPs were recorded 5 min after having reached each target propofol concentration at each respective concentration. Additionally, venous propofol blood concentrations were assayed at each measuring time point. Results Propofol increased all four MLAEP peak latencies (peaks Na, Pa, Nb, P1) in a dose-dependent manner. In addition, the differences in amplitudes were significantly smaller with increasing propofol target concentrations. The measured propofol plasma concentrations correlated positively with the latencies of the peaks Na, Pa, and Nb. Conclusions Propofol affects MLAEP latencies and amplitudes in children in a dose-dependent manner. MLAEP measurement might therefore be a useful tool for monitoring depth of propofol anaesthesia in childre
Projet « cabinets de groupe » : collaboration entre médecine de premier recours et psychiatrie [« Group medical practices » project : collaboration between primary care medicine and institutional public psychiatry]
Collaboration between primary care medicine and psychiatry is a well-known challenge. In order to improve access to psychological care for patients undergoing primary care, the « group medical practices » project proposes a collaborative care model in which a psychiatrist employed by a public psychiatric institution integrates group medical practices in order to provide assistance to primary care physicians. It is thus able to evaluate patients directly in the practices and to offer supervision and consilium spaces to primary care physicians
Development of the fully Geant4 compatible package for the simulation of Dark Matter in fixed target experiments
The search for new comparably light (well below the electroweak scale) feebly
interacting particles is an exciting possibility to explain some mysterious
phenomena in physics, among them the origin of Dark Matter. The sensitivity
study through detailed simulation of projected experiments is a key point in
estimating their potential for discovery.
Several years ago we created the DMG4 package for the simulation of DM (Dark
Matter) particles in fixed target experiments. The natural approach is to
integrate this simulation into the same program that performs the full
simulation of particles in the experiment setup. The Geant4 toolkit framework
was chosen as the most popular and versatile solution nowadays.
The simulation of DM particles production by this package accommodates
several possible scenarios, employing electron, muon or photon beams and
involving various mediators, such as vector, axial vector, scalar,
pseudoscalar, or spin 2 particles. The bremsstrahlung, annihilation or
Primakoff processes can be simulated.
The package DMG4 contains a subpackage DarkMatter with cross section methods
weakly connected to Geant4. It can be used in different frameworks.
In this paper, we present the latest developments of the package, such as
extending the list of possible mediator particle types, refining formulas for
the simulation and extending the mediator mass range. The user interface is
also made more flexible and convenient.
In this work, we also demonstrate the usage of the package, the improvements
in the simulation accuracy and some cross check validations.Comment: 17 pages, 11 figures, 1 tabl
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