644 research outputs found
Entropic Tension in Crowded Membranes
Unlike their model membrane counterparts, biological membranes are richly
decorated with a heterogeneous assembly of membrane proteins. These proteins
are so tightly packed that their excluded area interactions can alter the free
energy landscape controlling the conformational transitions suffered by such
proteins. For membrane channels, this effect can alter the critical membrane
tension at which they undergo a transition from a closed to an open state, and
therefore influence protein function \emph{in vivo}. Despite their obvious
importance, crowding phenomena in membranes are much less well studied than in
the cytoplasm.
Using statistical mechanics results for hard disk liquids, we show that
crowding induces an entropic tension in the membrane, which influences
transitions that alter the projected area and circumference of a membrane
protein. As a specific case study in this effect, we consider the impact of
crowding on the gating properties of bacterial mechanosensitive membrane
channels, which are thought to confer osmoprotection when these cells are
subjected to osmotic shock. We find that crowding can alter the gating energies
by more than in physiological conditions, a substantial fraction of
the total gating energies in some cases.
Given the ubiquity of membrane crowding, the nonspecific nature of excluded
volume interactions, and the fact that the function of many membrane proteins
involve significant conformational changes, this specific case study highlights
a general aspect in the function of membrane proteins.Comment: 20 pages (inclduing supporting information), 4 figures, to appear in
PLoS Comp. Bio
Flicker Noise Induced by Dynamic Impurities in a Quantum Point Contact
We calculate low-frequency noise (LFN) in a quantum point contact (QPC) which
is electrostatically defined in a 2D electron gas of a GaAs-AlGaAs
heterostructure. The conventional source of LFN in such systems are scattering
potentials fluctuating in time acting upon injected electrons. One can
discriminate between potentials of different origin -- noise may be caused by
the externally applied gate- and source-drain voltages, the motion of defects
with internal degrees of freedom close to the channel, electrons hopping
between localized states in the doped region, etc. In the present study we
propose a model of LFN based upon the assumption that there are many dynamic
defects in the surrounding of a QPC. A general expression for the
time-dependent current-current correlation function is derived and applied to a
QPC with quantized conductance. It is shown that the level of LFN is
significantly different at and between the steps in a plot of the conductance
vs. gate voltage. On the plateaus, the level of noise is found to be low and
strongly model-dependent. At the steps, LFN is much larger and only weakly
model-dependent. As long as the system is biased to be at a fixed position
relative the conductance step,Comment: 26 revtex APR 94-4
Muscle dysmorphia: Could it be classified as an addiction to body image?
BACKGROUND: Muscle dysmorphia (MD) describes a condition characterised by a misconstrued body image in which individuals who interpret their body size as both small or weak even though they may look normal or highly muscular. MD has been conceptualized as a type of body dysmorphic disorder, an eating disorder, and obsessiveâcompulsive disorder symptomatology. METHOD AND AIM: Through a review of the most salient literature on MD, this paper proposes an alternative classification of MD â the âAddiction to Body Imageâ (ABI) model â using Griffiths (2005) addiction components model as the framework in which to define MD as an addiction. RESULTS: It is argued the addictive activity in MD is the maintaining of body image via a number of different activities such as bodybuilding, exercise, eating certain foods, taking specific drugs (e.g., anabolic steroids), shopping for certain foods, food supplements, and the use or purchase of physical exercise accessories). In the ABI model, the perception of the positive effects on the self-body image is accounted for as a critical aspect of the MD condition (rather than addiction to exercise or certain types of eating disorder). CONCLUSIONS: Based on empirical evidence to date, it is proposed that MD could be re-classified as an addiction due to the individual continuing to engage in maintenance behaviours that may cause long-term harm
Zero-bias anomalies of point contact resistance due to adiabatic electron renormalization of dynamical defects
We study effect of the adiabatic electron renormalization on the parameters
of the dynamical defects in the ballistic metallic point contact. The upper
energy states of the ``dressed'' defect are shown to give a smaller
contribution to a resistance of the contact than the lower energy ones. This
holds both for the "classical" renormalization related to defect coupling with
average local electron density and for the "mesoscopic" renormalization caused
by the mesoscopic fluctuations of electronic density the dynamical defects are
coupled with. In the case of mesoscopic renormalization one may treat the
dynamical defect as coupled with Friedel oscillations originated by the other
defects, both static and mobile. Such coupling lifts the energy degeneracy of
the states of the dynamical defects giving different mesoscopic contribution to
resistance, and provides a new model for the fluctuator as for the object
originated by the electronic mesoscopic disorder rather than by the structural
one. The correlation between the defect energy and the defect contribution to
the resistance leads to zero-temperature and zero-bias anomalies of the point
contact resistance.
A comparison of these anomalies with those predicted by the Two Channel Kondo
Model (TCKM) is made. It is shown, that although the proposed model is based on
a completely different from TCKM physical background, it leads to a zero-bias
anomalies of the point contact resistance, which are qualitatively similar to
TCKM predictions.Comment: 6 pages, to be published in Phys. Rev.
PINT: A Modern Software Package for Pulsar Timing
Over the past few decades, the measurement precision of some pulsar-timing
experiments has advanced from ~10 us to ~10 ns, revealing many subtle
phenomena. Such high precision demands both careful data handling and
sophisticated timing models to avoid systematic error. To achieve these goals,
we present PINT (PINT Is Not Tempo3), a high-precision Python pulsar timing
data analysis package, which is hosted on GitHub and available on Python
Package Index (PyPI) as pint-pulsar. PINT is well-tested, validated,
object-oriented, and modular, enabling interactive data analysis and providing
an extensible and flexible development platform for timing applications. It
utilizes well-debugged public Python packages (e.g., the NumPy and Astropy
libraries) and modern software development schemes (e.g., version control and
efficient development with git and GitHub) and a continually expanding test
suite for improved reliability, accuracy, and reproducibility. PINT is
developed and implemented without referring to, copying, or transcribing the
code from other traditional pulsar timing software packages (e.g., TEMPO and
TEMPO2) and therefore provides a robust tool for cross-checking timing analyses
and simulating pulse arrival times. In this paper, we describe the design,
usage, and validation of PINT, and we compare timing results between it and
TEMPO and TEMPO2.Comment: Re-submitted to the Astrophysical Journal at December 31st, 202
Measurement of the rare decay pi0 -> e+ e-
The branching ratio of the rare decay pi0 -> e+ e- has been measured
precisely, using the complete data set from the KTeV E799-II experiment at
Fermilab. We observe 794 candidate pi0 -> e+ e- events using K0_L -> 3pi0 as a
source of tagged pi0's. The expected background is 52.7 +- 11.2 events,
predominantly from high e+ e- mass pi0 -> e+ e- gamma decays. We have measured
B[(pi0 -> e+ e-), (m_e+e-/m_pi0)^2 > 0.95] = 6.44 +- 0.25(stat) +- 0.22(syst)
x10^(-8), which is above the unitary bound from pi0 -> gamma gamma and within
the range of theoretical expectations from the standard model.Comment: 6 pages, 4 figure
A conceptual classification of parentsâ attributions of the role of food advertising in childrenâs diets
Background: High levels of child obesity are triggering growing concerns about the prevalence and effects of food advertising targeted at children. Efforts to address this advertising are confounded by the expanding repertoire of media and promotional techniques used to reach and attract children. The present study explored parentsâ views on food marketing and the strategies parents employ when attempting to ameliorate its effects. As part of an online survey of Australian parentsâ attitudes towards a range of food advertisements, respondents were invited to provide additional comment in an open-ended question. The question was optional and asked âAre there any other comments you would like to make?â. One in five of the survey respondents (18%; n = 235) elected to answer this question by discussing their views on food advertising and childrenâs diets. The responses were imported into NVivo10 for coding and analysis. A grounded approach was used to draw meaning from the data and develop a proposed conceptual classification of parentsâ attributions relating to food advertising and its consequences.Results: The majority of responses related to the negative perceived effects of unhealthy food advertising on childrenâs diets, with few respondents considering such advertisements to be innocuous. The responses were classified into four conceptual categories reflecting differing attitudes to advertising (negative to neutral) and varying levels of locus of control (low to high). The typical characteristics of parents allocated to the four categories exhibited variation according to weight status, television viewing habits, education level, and family size. The largest number of responses was coded to the category characterized by a negative attitude toward food advertising and a low locus of control. Parents in this category were more likely than others to be overweight/obese and heavy television viewers. Parents in the negative attitude to advertising and high locus of control category nominated a variety of parenting practices that could form the basis of parent education interventions. Conclusions: The results suggest that many Australian parents may feel disempowered in the face of high levels of advertising for unhealthy foods. The current voluntary regulatory code appears to be inadequate in scope and coverage to address this situation
Precise Measurements of Direct CP Violation, CPT Symmetry, and Other Parameters in the Neutral Kaon System
We present precise tests of CP and CPT symmetry based on the full dataset of
K to pipi decays collected by the KTeV experiment at Fermi National Accelerator
Laboratory during 1996, 1997, and 1999. This dataset contains 16 million K to
2pi0 and 69 million K to pi+pi- decays. We measure the direct CP violation
parameter Re(epsilon'/epsilon) = (19.2 pm 2.1)x10-4. We find the KL-KS mass
difference Deltam = (5270 pm 12)x10^6 hbar/s and the KS lifetime tauS = (89.62
pm 0.05)x10-12 s. We also measure several parameters that test CPT invariance.
We find the difference between the phase of the indirect CP violation
parameter, epsilon, and the superweak phase, phi_epsilon - phi_SW = (0.40 pm
0.56) degrees. We measure the difference of the relative phases between the CP
violating and CP conserving decay amplitudes for K to pi+pi- (phi+-) and for K
to 2pi0 (phi00), Delta phi = (0.30 pm 0.35) degrees. From these phase
measurements, we place a limit on the mass difference between K0 and K0bar,
DeltaM < 4.8 x 10-19 GeV/c^2 at 95% C.L. These results are consistent with
those of other experiments, our own earlier measurements, and CPT symmetry.Comment: 28 pages, 30 figures; removed extra figur
Measurement of Direct Emission in the KL->pi+pi-gamma Decay Mode
In this paper the KTeV collaboration reports the analysis of 112.1*10^3
candidate KL->pi+pi-gamma decays including a background of 671+/-41 events with
the objective of determining the photon production mechanisms intrinsic to the
decay process. These decays have been analyzed to extract the relative
contributions of the CP violating bremsstrahlung process and the CP conserving
M1 and CP violating E1 direct photon emission processes. The M1 direct photon
emission amplitude and its associated vector form factor parameterized as
|g_M1|(1+ (a_1/a_2)/(M(rho)^2-M(K)^2+2M(K)*E(gamma)) have been measured to be
|g_M1|=1.198 +/- 0.035(stat) +/- 0.086(syst) and a_1/a_2 = -0.738 +/-
0.007(stat) +/- 0.018 (syst) GeV^2/c^2 respectively. An upper limit for the CP
violating E1 direct emission amplitude |g_E1| < 0.21 (90% CL) has been found.
The overall ratio of direct photon emission (DE) to total photon emission
including the bremsstrahlung process (IB) has been determined to be DE/(DE +IB)
= 0.689 +/- 0.021 for E(gamma) > 20 MeV.Comment: REVTeX 3.1, 4 pages, 4 figures submitted to Physical Review Letter
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