22,819 research outputs found
Wide-Field Multi-Parameter FLIM: Long-Term Minimal Invasive Observation of Proteins in Living Cells.
Time-domain Fluorescence Lifetime Imaging Microscopy (FLIM) is a remarkable tool to monitor the dynamics of fluorophore-tagged protein domains inside living cells. We propose a Wide-Field Multi-Parameter FLIM method (WFMP-FLIM) aimed to monitor continuously living cells under minimum light intensity at a given illumination energy dose. A powerful data analysis technique applied to the WFMP-FLIM data sets allows to optimize the estimation accuracy of physical parameters at very low fluorescence signal levels approaching the lower bound theoretical limit. We demonstrate the efficiency of WFMP-FLIM by presenting two independent and relevant long-term experiments in cell biology: 1) FRET analysis of simultaneously recorded donor and acceptor fluorescence in living HeLa cells and 2) tracking of mitochondrial transport combined with fluorescence lifetime analysis in neuronal processes
A Physicsl Model of Electron According to the Basic Structures of Matter Hypothesis
A physical model of the electron is suggested according to the basic structures of
matter (BSM) hypothesis. BSM is based on an alternative concept about the
physical vacuum, assuming that space contains an underlying grid structure of
nodes formed of superdense subelementary particles, which are also involved in
the structure of the elementary particles. The proposed grid structure is formed of
vibrating nodes that possess quantum features and energy well. It is admitted that
this hypothetical structure could account for the missing âdark matterâ in the
universe. The signature of this dark matter is apparent in the galactic rotational
curves and in the relation between masses of the supermassive black hole in the
galactic center and the host galaxy. The suggested model of the electron possesses
oscillation features with anomalous magnetic moment and embedded signatures
of the Compton wavelength and the fine-structure constant. The analysis
of the interactions between the oscillating electron and the nodes of the vacuum
grid structure allows us to obtain physical meaning for some fundamental constants
Airborne chemical sensing with mobile robots
Airborne chemical sensing with mobile robots has been an active research areasince the beginning of the 1990s. This article presents a review of research work in this field,including gas distribution mapping, trail guidance, and the different subtasks of gas sourcelocalisation. Due to the difficulty of modelling gas distribution in a real world environmentwith currently available simulation techniques, we focus largely on experimental work and donot consider publications that are purely based on simulations
Matter-wave entanglement and teleportation by molecular dissociation and collisions
We propose dissociation of cold diatomic molecules as a source of atom pairs
with highly correlated (entangled) positions and momenta, approximating the
original quantum state introduced by Einstein, Podolsky and Rosen (EPR) [Phys.
Rev. 47, 777 (1935)]. Wavepacket teleportation is shown to be achievable by its
collision with one of the EPR correlated atoms and manipulation of the other
atom in the pair.Comment: REVTeX, 4 pages, 3 figures. Text reformulated, modified figs. 1 and
2. Accepted by Phys. Rev. Let
Inference in particle tracking experiments by passing messages between images
Methods to extract information from the tracking of mobile objects/particles
have broad interest in biological and physical sciences. Techniques based on
simple criteria of proximity in time-consecutive snapshots are useful to
identify the trajectories of the particles. However, they become problematic as
the motility and/or the density of the particles increases due to uncertainties
on the trajectories that particles followed during the images' acquisition
time. Here, we report an efficient method for learning parameters of the
dynamics of the particles from their positions in time-consecutive images. Our
algorithm belongs to the class of message-passing algorithms, known in computer
science, information theory and statistical physics as Belief Propagation (BP).
The algorithm is distributed, thus allowing parallel implementation suitable
for computations on multiple machines without significant inter-machine
overhead. We test our method on the model example of particle tracking in
turbulent flows, which is particularly challenging due to the strong transport
that those flows produce. Our numerical experiments show that the BP algorithm
compares in quality with exact Markov Chain Monte-Carlo algorithms, yet BP is
far superior in speed. We also suggest and analyze a random-distance model that
provides theoretical justification for BP accuracy. Methods developed here
systematically formulate the problem of particle tracking and provide fast and
reliable tools for its extensive range of applications.Comment: 18 pages, 9 figure
Secretory vesicles are preferentially targeted to areas of low molecular SNARE density
Intercellular communication is commonly mediated by the regulated fusion, or exocytosis, of vesicles with the cell surface. SNARE (soluble N-ethymaleimide sensitive factor attachment protein receptor) proteins are the catalytic core of the secretory machinery, driving vesicle and plasma membrane merger. Plasma membrane SNAREs (tSNAREs) are proposed to reside in dense clusters containing many molecules, thus providing a concentrated reservoir to promote membrane fusion. However, biophysical experiments suggest that a small number of SNAREs are sufficient to drive a single fusion event. Here we show, using molecular imaging, that the majority of tSNARE molecules are spatially separated from secretory vesicles. Furthermore, the motilities of the individual tSNAREs are constrained in membrane micro-domains, maintaining a non-random molecular distribution and limiting the maximum number of molecules encountered by secretory vesicles. Together our results provide a new model for the molecular mechanism of regulated exocytosis and demonstrate the exquisite organization of the plasma membrane at the level of individual molecular machines
Inferring Latent States and Refining Force Estimates via Hierarchical Dirichlet Process Modeling in Single Particle Tracking Experiments
Optical microscopy provides rich spatio-temporal information characterizing
in vivo molecular motion. However, effective forces and other parameters used
to summarize molecular motion change over time in live cells due to latent
state changes, e.g., changes induced by dynamic micro-environments,
photobleaching, and other heterogeneity inherent in biological processes. This
study focuses on techniques for analyzing Single Particle Tracking (SPT) data
experiencing abrupt state changes. We demonstrate the approach on GFP tagged
chromatids experiencing metaphase in yeast cells and probe the effective forces
resulting from dynamic interactions that reflect the sum of a number of
physical phenomena. State changes are induced by factors such as microtubule
dynamics exerting force through the centromere, thermal polymer fluctuations,
etc. Simulations are used to demonstrate the relevance of the approach in more
general SPT data analyses. Refined force estimates are obtained by adopting and
modifying a nonparametric Bayesian modeling technique, the Hierarchical
Dirichlet Process Switching Linear Dynamical System (HDP-SLDS), for SPT
applications. The HDP-SLDS method shows promise in systematically identifying
dynamical regime changes induced by unobserved state changes when the number of
underlying states is unknown in advance (a common problem in SPT applications).
We expand on the relevance of the HDP-SLDS approach, review the relevant
background of Hierarchical Dirichlet Processes, show how to map discrete time
HDP-SLDS models to classic SPT models, and discuss limitations of the approach.
In addition, we demonstrate new computational techniques for tuning
hyperparameters and for checking the statistical consistency of model
assumptions directly against individual experimental trajectories; the
techniques circumvent the need for "ground-truth" and subjective information.Comment: 25 pages, 6 figures. Differs only typographically from PLoS One
publication available freely as an open-access article at
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.013763
Formation of plasma around a small meteoroid: 2. Implications for radar head echo
This paper calculates the spatial distribution of the plasma responsible for radar head echoes by applying the kinetic theory developed in the companion paper. This results in a set of analytic expressions for the plasma density as a function of distance from the meteoroid. It shows that at distances less than a collisional mean free path from the meteoroid surface, the plasma density drops in proportion to 1/R where R is the distance from the meteoroid center; and, at distances much longer than the meanâfreeâpath behind the meteoroid, the density diminishes at a rate proportional to 1/R2. The results of this paper should be used for modeling and analysis of radar head echoes.This work was supported by NSF grant AGS-1244842. (AGS-1244842 - NSF
Electrostatic correlations in inhomogeneous charged fluids beyond loop expansion
Electrostatic correlation effects in inhomogeneous symmetric electrolytes are
investigated within a previously developed electrostatic self-consistent (SC)
theory (R.R. Netz and H. Orland, Eur. Phys.J. E 11, 301 (2003)). To this aim,
we introduce two computational approaches that allow to solve the SC equations
beyond the loop expansion. Both approaches can handle the case of
dielectrically discontinuous boundaries where the one-loop theory is known to
fail. By comparing the theoretical results obtained from these schemes with the
results of the MC simulations that we ran for ions at neutral single dielectric
interfaces as well as with previous MC data for charged interfaces, we first
show that the weak coupling (WC) Debye-Huckel (DH) theory remains
quantitatively accurate up to the bulk ion density rhob=0.01 M, whereas the SC
theory exhibits a good quantitative accuracy up to rhob=0.2 M. Then, we derive
from the perturbative SC scheme the one-loop theory of asymmetrically
partitioned salt systems around a dielectrically homogeneous charged surface.
It is shown that correlation effects originate in these systems from a
competition between the salt screening loss at the interface driving the ions
to the bulk region, and the interfacial counterion screening excess attracting
them towards the surface. In the case of weak surface charges, the interfacial
salt screening loss is the dominant effect. As a result, correlations decrease
the MF density of both coions and counterions. With increasing surface charge,
the surface-attractive counterion screening excess starts to dominate, and
correlation effects amplify in this regime the MF density of both type of ions.
We also show that at a characteristic value of the electrostatic coupling
parameter, electrostatic correlations result in a charge inversion effect
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