369 research outputs found
Electronic states in a disordered metal: Magnetotransport in doped germanium
We observe a sharp feature in the ultra-low-temperature magnetoconductivity of degenerately doped Ge:Sb at H∼25 kOe, which is robust up to at least three times the critical density for the insulator-metal transition. This field corresponds to a low-energy scale characteristic of the special nature of antimony donors in germanium. Its presence and sensitivity to uniaxial stress confirm the notion of metallic impurity bands in doped germanium
Shape-based peak identification for ChIP-Seq
We present a new algorithm for the identification of bound regions from
ChIP-seq experiments. Our method for identifying statistically significant
peaks from read coverage is inspired by the notion of persistence in
topological data analysis and provides a non-parametric approach that is robust
to noise in experiments. Specifically, our method reduces the peak calling
problem to the study of tree-based statistics derived from the data. We
demonstrate the accuracy of our method on existing datasets, and we show that
it can discover previously missed regions and can more clearly discriminate
between multiple binding events. The software T-PIC (Tree shape Peak
Identification for ChIP-Seq) is available at
http://math.berkeley.edu/~vhower/tpic.htmlComment: 12 pages, 6 figure
An isotopic analysis of ionising radiation as a source of sulphuric acid
Sulphuric acid is an important factor in aerosol nucleation and growth. It has been shown that ions enhance the formation of sulphuric acid aerosols, but the exact mechanism has remained undetermined. Furthermore some studies have found a deficiency in the sulphuric acid budget, suggesting a missing source. In this study the production of sulphuric acid from SO<sub>2</sub> through a number of different pathways is investigated. The production methods are standard gas phase oxidation by OH radicals produced by ozone photolysis with UV light, liquid phase oxidation by ozone, and gas phase oxidation initiated by gamma rays. The distributions of stable sulphur isotopes in the products and substrate were measured using isotope ratio mass spectrometry. All methods produced sulphate enriched in <sup>34</sup>S and we find an enrichment factor (&delta;<sup>34</sup>S) of 8.7 ± 0.4&permil; (1 standard deviation) for the UV-initiated OH reaction. Only UV light (Hg emission at 253.65 nm) produced a clear non-mass-dependent excess of <sup>33</sup>S. The pattern of isotopic enrichment produced by gamma rays is similar, but not equal, to that produced by aqueous oxidation of SO<sub>2</sub> by ozone. This, combined with the relative yields of the experiments, suggests a mechanism in which ionising radiation may lead to hydrated ion clusters that serve as nanoreactors for S(IV) to S(VI) conversion
Pulmonary embolism (PE) to chronic thromboembolic pulmonary disease (CTEPD): findings from a survey of UK physicians
Chronic thromboembolic pulmonary disease (CTEPD) is a complication of pulmonary embolism (PE). We conducted an online survey of UK PE-treating physicians to understand practices in the follow-up of PE and awareness of CTEPD. The physicians surveyed (N = 175) included 50 each from cardiology, respiratory and internal medicine, plus 25 haematologists. Most (89%) participants had local guidelines for PE management, and 65% reported a PE follow-up clinic, of which 69% were joint clinics. Almost half (47%) had a protocol for the investigation of CTEPD. According to participants, 129 (74%) routinely consider a diagnosis of CTEPD and 97 (55%) routinely investigate for CTEPD, with 76% of those 97 participants investigating in patients who are symptomatic at 3 months and 22% investigating in all patients. This survey demonstrated variability in the follow-up of PE and the awareness of CTEPD and its investigation. The findings support the conduct of a national audit to understand the barriers to the timely detection of CTEPD
Predictability of Self-Organizing Systems
We study the predictability of large events in self-organizing systems. We
focus on a set of models which have been studied as analogs of earthquake
faults and fault systems, and apply methods based on techniques which are of
current interest in seismology. In all cases we find detectable correlations
between precursory smaller events and the large events we aim to forecast. We
compare predictions based on different patterns of precursory events and find
that for all of the models a new precursor based on the spatial distribution of
activity outperforms more traditional measures based on temporal variations in
the local activity.Comment: 15 pages, plain.tex with special macros included, 4 figure
Transfer Functions for Protein Signal Transduction: Application to a Model of Striatal Neural Plasticity
We present a novel formulation for biochemical reaction networks in the
context of signal transduction. The model consists of input-output transfer
functions, which are derived from differential equations, using stable
equilibria. We select a set of 'source' species, which receive input signals.
Signals are transmitted to all other species in the system (the 'target'
species) with a specific delay and transmission strength. The delay is computed
as the maximal reaction time until a stable equilibrium for the target species
is reached, in the context of all other reactions in the system. The
transmission strength is the concentration change of the target species. The
computed input-output transfer functions can be stored in a matrix, fitted with
parameters, and recalled to build discrete dynamical models. By separating
reaction time and concentration we can greatly simplify the model,
circumventing typical problems of complex dynamical systems. The transfer
function transformation can be applied to mass-action kinetic models of signal
transduction. The paper shows that this approach yields significant insight,
while remaining an executable dynamical model for signal transduction. In
particular we can deconstruct the complex system into local transfer functions
between individual species. As an example, we examine modularity and signal
integration using a published model of striatal neural plasticity. The modules
that emerge correspond to a known biological distinction between
calcium-dependent and cAMP-dependent pathways. We also found that overall
interconnectedness depends on the magnitude of input, with high connectivity at
low input and less connectivity at moderate to high input. This general result,
which directly follows from the properties of individual transfer functions,
contradicts notions of ubiquitous complexity by showing input-dependent signal
transmission inactivation.Comment: 13 pages, 5 tables, 15 figure
Prediction of Large Events on a Dynamical Model of a Fault
We present results for long term and intermediate term prediction algorithms
applied to a simple mechanical model of a fault. We use long term prediction
methods based, for example, on the distribution of repeat times between large
events to establish a benchmark for predictability in the model. In comparison,
intermediate term prediction techniques, analogous to the pattern recognition
algorithms CN and M8 introduced and studied by Keilis-Borok et al., are more
effective at predicting coming large events. We consider the implications of
several different quality functions Q which can be used to optimize the
algorithms with respect to features such as space, time, and magnitude windows,
and find that our results are not overly sensitive to variations in these
algorithm parameters. We also study the intrinsic uncertainties associated with
seismicity catalogs of restricted lengths.Comment: 33 pages, plain.tex with special macros include
A Dynamic Model of Interactions of Ca^(2+), Calmodulin, and Catalytic Subunits of Ca^(2+)/Calmodulin-Dependent Protein Kinase II
During the acquisition of memories, influx of Ca^(2+) into the postsynaptic spine through the pores of activated N-methyl-D-aspartate-type glutamate receptors triggers processes that change the strength of excitatory synapses. The pattern of Ca^(2+) influx during the first few seconds of activity is interpreted within the Ca^(2+)-dependent signaling network such that synaptic strength is eventually either potentiated or depressed. Many of the critical signaling enzymes that control synaptic plasticity, including Ca^(2+)/calmodulin-dependent protein kinase II (CaMKII), are regulated by calmodulin, a small protein that can bind up to 4 Ca^(2+) ions. As a first step toward clarifying how the Ca^(2+)-signaling network decides between potentiation or depression, we have created a kinetic model of the interactions of Ca^(2+), calmodulin, and CaMKII that represents our best understanding of the dynamics of these interactions under conditions that resemble those in a postsynaptic spine. We constrained parameters of the model from data in the literature, or from our own measurements, and then predicted time courses of activation and autophosphorylation of CaMKII under a variety of conditions. Simulations showed that species of calmodulin with fewer than four bound Ca^(2+) play a significant role in activation of CaMKII in the physiological regime, supporting the notion that processing ofCa^(2+) signals in a spine involves competition among target enzymes for binding to unsaturated species of CaM in an environment in which the concentration of Ca^(2+) is fluctuating rapidly. Indeed, we showed that dependence of activation on the frequency of Ca^(2+) transients arises from the kinetics of interaction of fluctuating Ca^(2+) with calmodulin/CaMKII complexes. We used parameter sensitivity analysis to identify which parameters will be most beneficial to measure more carefully to improve the accuracy of predictions. This model provides a quantitative base from which to build more complex dynamic models of postsynaptic signal transduction during learning
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