14,773 research outputs found
The mean velocity of two-state models of molecular motor
The motion of molecular motor is essential to the biophysical functioning of
living cells. In principle, this motion can be regraded as a multiple chemical
states process. In which, the molecular motor can jump between different
chemical states, and in each chemical state, the motor moves forward or
backward in a corresponding potential. So, mathematically, the motion of
molecular motor can be described by several coupled one-dimensional hopping
models or by several coupled Fokker-Planck equations. To know the basic
properties of molecular motor, in this paper, we will give detailed analysis
about the simplest cases: in which there are only two chemical states.
Actually, many of the existing models, such as the flashing ratchet model, can
be regarded as a two-state model. From the explicit expression of the mean
velocity, we find that the mean velocity of molecular motor might be nonzero
even if the potential in each state is periodic, which means that there is no
energy input to the molecular motor in each of the two states. At the same
time, the mean velocity might be zero even if there is energy input to the
molecular motor. Generally, the velocity of molecular motor depends not only on
the potentials (or corresponding forward and backward transition rates) in the
two states, but also on the transition rates between the two chemical states
Persistence in the Voter model: continuum reaction-diffusion approach
We investigate the persistence probability in the Voter model for dimensions
d\geq 2. This is achieved by mapping the Voter model onto a continuum
reaction-diffusion system. Using path integral methods, we compute the
persistence probability r(q,t), where q is the number of ``opinions'' in the
original Voter model. We find r(q,t)\sim exp[-f_2(q)(ln t)^2] in d=2;
r(q,t)\sim exp[-f_d(q)t^{(d-2)/2}] for 2<d<4; r(q,t)\sim exp[-f_4(q)t/ln t] in
d=4; and r(q,t)\sim exp[-f_d(q)t] for d>4. The results of our analysis are
checked by Monte Carlo simulations.Comment: 10 pages, 3 figures, Latex, submitted to J. Phys. A (letters
Seasonal Movements, Migratory Behavior, and Site Fidelity of West Indian Manatees along the Atlantic Coast of the United States as Determined by Radio-telemetry
The study area encompassed the eastern coasts of Florida, Georgia, and South Carolina,
including inland waterways such as the St. Johns River (Fig. 1). Manatees inhabited the
relatively narrow band of water that lies between the barrier beaches and the mainland,
occasionally venturing into the ocean close to shore. Between Miami and Fernandina Beach,
Florida, 19 inlets provided manatees with corridors between the intracoastal waters and the
Atlantic Ocean; the distance between adjacent inlets averaged 32 km(SD = 24 km) and varied
from 3 to 88 km. Habitats used by manatees along this 900-km stretch ofcoastline varied
widely and included estuaries, lagoons, rivers and creeks, shallow bays and sounds, and ocean
inlets. Salinities in most areas were brackish, but ranged from completely fresh to completely
marine. The predominant communities of aquatic vegetation also varied geographically and
with salinity: seagrass meadows and mangrove swamps in brackish and marine waters along the
southern half of peninsular Florida; salt marshes in northeastern Florida and Georgia; benthic
macroalgae in estuarine and marine habitats; and a variety of submerged, floating, and emergent
vegetation in freshwater rivers, canals, and streams throughout the region.
Radio-telemetry has been used successfully to track manatees in other regions ofFlorida
(Bengtson 1981, Powell and Rathbun 1984, Lefebvre and Frohlich 1986, Rathbun et al. 1990)
and Georgia (Zoodsma 1991), but these early studies relied primarily on conventional VHF (very
high frequency) transmitters and were limited in their spatial and temporal scope (see O'Shea
and Kochman 1990 for overview). Typically, manatees were tagged at a thermal refuge in the
winter and then tracked until the tag detached, usually sometime between the spring and fall of
the same year. Our study differs from previous research on manatee movements in several
important respects. First, we relied heavily on data from satellite-monitored transmitters using
the Argos system, which yielded a substantially greater number of locations and more systematic
collection of data compared to previous VHF tracking studies (Deutsch et al. 1998). Second, our
tagging and tracking efforts encompassed the entire range of manatees along the Atlantic coast,
from the Florida Keys to South Carolina, so inferences were not limited to a small geographic
area. Third, we often used freshwater to lure manatees to capture sites, which allowed tagging
in all months of the year; this provided more information about summer movement patterns than
had previous studies which emphasized capture and tracking at winter aggregations. Finally, the
study spanned a decade, and success in retagging animals and in replacing transmitters allowed
long-term tracking ofmany individuals. This provided the opportunity to investigate variation in
seasonal movements, migratory behavior, and site fidelity across years for individual manatees.
(254 page document.
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Correlated modal mineralogy, aqueous alteration and oxygen isotope composition of CM Chondrites
In this study we move beyond defining alteration sequences in CM chondrites towards understanding the relationship between modal mineralogy, the extent of aqueous alteration and O-isotope compositions
Three Super-Earths Orbiting HD 7924
We report the discovery of two super-Earth mass planets orbiting the nearby
K0.5 dwarf HD 7924 which was previously known to host one small planet. The new
companions have masses of 7.9 and 6.4 M, and orbital periods of 15.3
and 24.5 days. We perform a joint analysis of high-precision radial velocity
data from Keck/HIRES and the new Automated Planet Finder Telescope (APF) to
robustly detect three total planets in the system. We refine the ephemeris of
the previously known planet using five years of new Keck data and high-cadence
observations over the last 1.3 years with the APF. With this new ephemeris, we
show that a previous transit search for the inner-most planet would have
covered 70% of the predicted ingress or egress times. Photometric data
collected over the last eight years using the Automated Photometric Telescope
shows no evidence for transits of any of the planets, which would be detectable
if the planets transit and their compositions are hydrogen-dominated. We detect
a long-period signal that we interpret as the stellar magnetic activity cycle
since it is strongly correlated with the Ca II H and K activity index. We also
detect two additional short-period signals that we attribute to
rotationally-modulated starspots and a one month alias. The high-cadence APF
data help to distinguish between the true orbital periods and aliases caused by
the window function of the Keck data. The planets orbiting HD 7924 are a local
example of the compact, multi-planet systems that the Kepler Mission found in
great abundance.Comment: Accepted to ApJ on 4/7/201
A Budding-Defective M2 Mutant Exhibits Reduced Membrane Interaction, Insensitivity To Cholesterol, And Perturbed Interdomain Coupling
Influenza A M2 is a membrane-associated protein with a C-terminal amphipathic helix that plays a cholesterol-dependent role in viral budding. An M2 mutant with alanine substitutions in the C-terminal amphipathic helix is deficient in viral scission. With the goal of providing atomic-level understanding of how the wild-type protein functions, we used a multipronged site-directed spin labeling electron paramagnetic resonance spectroscopy (SDSL-EPR) approach to characterize the conformational properties of the alanine mutant. We spin-labeled sites in the transmembrane (TM) domain and the C-terminal amphipathic helix (AH) of wild-type (WT) and mutant M2, and collected information on line shapes, relaxation rates, membrane topology, and distances within the homotetramer in membranes with and without cholesterol. Our results identify marked differences in the conformation and dynamics between the WT and the alanine mutant. Compared to WT, the dominant population of the mutant AH is more dynamic, shallower in the membrane, and has altered quaternary arrangement of the C-terminal domain. While the AH becomes more dynamic, the dominant population of the TM domain of the mutant is immobilized. The presence of cholesterol changes the conformation and dynamics of the WT protein, while the alanine mutant is insensitive to cholesterol. These findings provide new insight into how M2 may facilitate budding. We propose the AH–membrane interaction modulates the arrangement of the TM helices, effectively stabilizing a conformational state that enables M2 to facilitate viral budding. Antagonizing the properties of the AH that enable interdomain coupling within M2 may therefore present a novel strategy for anti-influenza drug design
Molecular Spiders with Memory
Synthetic bio-molecular spiders with "legs" made of single-stranded segments
of DNA can move on a surface which is also covered by single-stranded segments
of DNA complementary to the leg DNA. In experimental realizations, when a leg
detaches from a segment of the surface for the first time it alters that
segment, and legs subsequently bound to these altered segments more weakly.
Inspired by these experiments we investigate spiders moving along a
one-dimensional substrate, whose legs leave newly visited sites at a slower
rate than revisited sites. For a random walk (one-leg spider) the slowdown does
not effect the long time behavior. For a bipedal spider, however, the slowdown
generates an effective bias towards unvisited sites, and the spider behaves
similarly to the excited walk. Surprisingly, the slowing down of the spider at
new sites increases the diffusion coefficient and accelerates the growth of the
number of visited sites.Comment: 10 pages, 3 figure
Hydrodynamic induced deformation and orientation of a microscopic elastic filament
We describe simulations of a microscopic elastic filament immersed in a fluid
and subject to a uniform external force. Our method accounts for the
hydrodynamic coupling between the flow generated by the filament and the
friction force it experiences. While models that neglect this coupling predict
a drift in a straight configuration, our findings are very different. Notably,
a force with a component perpendicular to the filament axis induces bending and
perpendicular alignment. Moreover, with increasing force we observe four shape
regimes, ranging from slight distortion to a state of tumbling motion that
lacks a steady state. We also identify the appearance of marginally stable
structures. Both the instability of these shapes and the observed alignment can
be explained by the combined action of induced bending and non-local
hydrodynamic interactions. Most of these effects should be experimentally
relevant for stiff micro-filaments, such as microtubules.Comment: three figures. To appear in Phys Rev Let
Response to pulmonary arterial hypertension drug therapies in patients with pulmonary arterial hypertension and cardiovascular risk factors.
The age at diagnosis of pulmonary arterial hypertension (PAH) and the prevalence of cardiovascular (CV) risk factors are increasing. We sought to determine whether the response to drug therapy was influenced by CV risk factors in PAH patients. We studied consecutive incident PAH patients (n = 146) between January 1, 2008, and July 15, 2011. Patients were divided into two groups: the PAH-No CV group included patients with no CV risk factors (obesity, systemic hypertension, type 2 diabetes mellitus, permanent atrial fibrillation, mitral and/or aortic valve disease, and coronary artery disease), and the PAH-CV group included patients with at least one. The response to PAH treatment was analyzed in all the patients who received PAH drug therapy. The PAH-No CV group included 43 patients, and the PAH-CV group included 69 patients. Patients in the PAH-No CV group were younger than those in the PAH-CV group (P < 0.0001). In the PAH-No CV group, 16 patients (37%) improved on treatment and 27 (63%) did not improve, compared with 11 (16%) and 58 (84%) in the PAH-CV group, respectively (P = 0.027 after adjustment for age). There was no difference in survival at 30 months (P = 0.218). In conclusion, in addition to older age, CV risk factors may predict a reduced response to PAH drug therapy in patients with PAH
Effect of single versus antibiotic combinations on Staphylococcus epidermidis biofilm viability and on genetic expression of some virulence genes
In this study five clinical isolates strains were used, and nine antibiotics at breakpoint concentrations:
vancomycin, tetracycline, rifampicin, gentamicin, cefazolin, cephalothin, levofloxacine, daptomycin and
clindamycin were tested. 48 hours biofilms were grown on Calgary Biofilm Device (CBD) and challenged
overnight with antibiotics alone and in combination. Biofilm cells viability was determined by colony forming
units (cfu). Afterwards, the effect of the most active antibiotics combinations against S. epidermidis biofilm on
genetic expression of some genes of interest such as: icaA, icaR, sarA and rsbU was determined by real-time PCR.
Although biofilms were generally insensitive to individual antibiotics, they were more susceptible to
combinations. Levofloxacine was a constituent of almost all the combinations active against S. epidermidis
biofilm pointing to be part of any antibiotic therapy directed against biofilms of these organisms
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