8,814 research outputs found
Revisiting the anomalous bending elasticity of sharply bent DNA
Several recent experiments suggest that sharply bent DNA has a surprisingly
high bending flexibility, but the cause of this flexibility is poorly
understood. Although excitation of flexible defects can explain these results,
whether such excitation can occur with the level of DNA bending in these
experiments remains unclear. Intriguingly, the DNA contained preexisting nicks
in most of these experiments but whether nicks might play a role in flexibility
has never been considered in the interpretation of experimental results. Here,
using full-atom molecular dynamics simulations, we show that nicks promote DNA
basepair disruption at the nicked sites, which drastically reduces DNA bending
energy. In addition, lower temperatures suppress the nick-dependent basepair
disruption. In the absence of nicks, basepair disruption can also occur but
requires a higher level of DNA bending. Therefore, basepair disruption inside
B-form DNA can be suppressed if the DNA contains preexisting nicks. Overall,
our results suggest that the reported mechanical anomaly of sharply bent DNA is
likely dependent on preexisting nicks, therefore the intrinsic mechanisms of
sharply bent nick-free DNA remain an open question.Comment: 39 pages, 11 figures, 1 supporting materia
An ASCA Study of the High Luminosity SNR G349.7+0.2
We present ASCA observations of supernova remnant (SNR) G349.7+0.2. The
remnant has an irregular shell morphology and is interacting with a molecular
cloud, evident from the presence of OH(1720 MHz) masers and shocked molecular
gas. The X-ray morphology is consistent with that at radio wavelengths, with a
distinct enhancement in the south. The X-ray emission from the SNR is well
described by a model of a thermal plasma which has yet to reach ionization
equilibrium. The hydrogen column of ~6.0 X 10^{22} cm^{-2} is consistent with
the large distance to the remnant of ~22 kpc estimated from the maser
velocities. We derive an X-ray luminosity of L_x(0.5-10.0 keV)= 1.8 X 10^{37}
d_{22}^2 erg/s, which makes G349.7+0.2 one of the most X-ray luminous
shell-type SNRs known in the Galaxy. The age of the remnant is estimated to be
about 2800 yrs. The ambient density and pressure conditions appear similar to
those inferred for luminous compact SNRs found in starburst regions of other
galaxies, and provides support for the notion that these may be the result of
SNR evolution in the vicinity of dense molecular clouds.Comment: 5 pages, 3 figures. Accepted for publication in Ap
Anomaly-safe discrete groups
We show that there is a class of finite groups, the so-called perfect groups,
which cannot exhibit anomalies. This implies that all non-Abelian finite simple
groups are anomaly-free. On the other hand, non-perfect groups generically
suffer from anomalies. We present two different ways that allow one to
understand these statements.Comment: 11 page
Synthesis of Nonspherical Microcapsules through Controlled Polyelectrolyte Coating of Hydrogel Templates
We report a simple approach to fabricate custom-shape microcapsules using hydrogel templates synthesized by stop flow lithography. Cargo-containing microcapsules were made by coating hydrogel particles with a single layer of poly-l-lysine followed by a one-step core degradation and capsule cross-linking procedure. We determined appropriate coating conditions by investigating the effect of pH, ionic strength, and prepolymer composition on the diffusion of polyelectrolytes into the oppositely charged hydrogel template. We also characterized the degradation of the templating core by tracking the diffusivity of nanoparticles embedded within the hydrogel. Unlike any other technique, this approach allows for easy fabrication of microcapsules with internal features (e.g., toroids) and selective surface modification of Janus particles using any polyelectrolyte. These soft, flexible capsules may be useful for therapeutic applications as well as fundamental studies of membrane mechanics.United States. Army Research Office (Institute for Collaborative Biotechnologies. Grant W911NF-09-0001)National Science Foundation (U.S.) (Grants CMMI-1120724 and DMR-1006147
Decentralized Data Fusion and Active Sensing with Mobile Sensors for Modeling and Predicting Spatiotemporal Traffic Phenomena
The problem of modeling and predicting spatiotemporal traffic phenomena over
an urban road network is important to many traffic applications such as
detecting and forecasting congestion hotspots. This paper presents a
decentralized data fusion and active sensing (D2FAS) algorithm for mobile
sensors to actively explore the road network to gather and assimilate the most
informative data for predicting the traffic phenomenon. We analyze the time and
communication complexity of D2FAS and demonstrate that it can scale well with a
large number of observations and sensors. We provide a theoretical guarantee on
its predictive performance to be equivalent to that of a sophisticated
centralized sparse approximation for the Gaussian process (GP) model: The
computation of such a sparse approximate GP model can thus be parallelized and
distributed among the mobile sensors (in a Google-like MapReduce paradigm),
thereby achieving efficient and scalable prediction. We also theoretically
guarantee its active sensing performance that improves under various practical
environmental conditions. Empirical evaluation on real-world urban road network
data shows that our D2FAS algorithm is significantly more time-efficient and
scalable than state-of-the-art centralized algorithms while achieving
comparable predictive performance.Comment: 28th Conference on Uncertainty in Artificial Intelligence (UAI 2012),
Extended version with proofs, 13 page
Synthesis of a 2,4,6,8,10-dodecapentanoic acid thioester as a substrate for biosynthesis of heat stable antifungal factor (HSAF)
The N-acetylcystamine (SNAC) thioester of dodecapentaenoic acid, an analog of a putative intermediate in the biosynthesis of Heat Stable Antifungal Factor (HSAF), is synthesized. Key steps include sequential Horner–Emmons homologations with the Weinreb amide of diethylphosponoacetic acid, and thioesterification of an aldol-derived 3-hydroxyalkanoate, which serves as a stable precursor of the sensitive polyenoate. The thioester was investigated as a biosynthetic substrate using a purified nonribosomal peptide synthetase and was not incorporated in the observed products
A Search for Nitrogen Enriched Quasars in the Sloan Digital Sky Survey Early Data Release
A search for nitrogen-rich quasars in the Sloan Digital Sky Survey Early Data
Release (SDSS EDR) catalog has yielded 16 candidates, including five with very
prominent emission, but no cases with nitrogen emission as strong as in
Q0353-383. The quasar Q0353-383 has long been known to have extremely strong
nitrogen intercombination lines at lambda 1486 and lambda 1750 Angstroms,
implying an anomalously high nitrogen abundance of about 15 times solar. It is
still the only one of its kind known. A preliminary search through the EDR
using the observed property of the weak C IV emission seen in Q0353-383
resulted in a sample of 23 objects with unusual emission or absorption-line
properties, including one very luminous redshift 2.5 star-forming galaxy. We
present descriptions, preliminary emission-line measurements, and spectra for
all the objects discussed here.Comment: 20 pages, 5 figures, submitted to AJ; final refereed versio
First Operation of a Resistive Shell Liquid Argon Time Projection Chamber -- A new Approach to Electric-Field Shaping
We present a new technology for the shaping of the electric field in Time
Projection Chambers (TPCs) using a carbon-loaded polyimide foil. This
technology allows for the minimisation of passive material near the active
volume of the TPC and thus is capable to reduce background events originating
from radioactive decays or scattering on the material itself. Furthermore, the
high and continuous electric resistivity of the foil limits the power
dissipation per unit area and minimizes the risks of damages in the case of an
electric field breakdown. Replacing the conventional field cage with a
resistive plastic film structure called 'shell' decreases the number of
components within the TPC and therefore reduces the potential points of failure
when operating the detector. A prototype liquid argon (LAr) TPC with such a
resistive shell and with a cathode made of the same material was successfully
tested for long term operation with electric field values up to about 1.5
kV/cm. The experiment shows that it is feasible to successfully produce and
shape the electric field in liquefied noble-gas detectors with this new
technology.Comment: 13 page
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