2,245 research outputs found
Momentum Transfer to an Atom in a Molecule: Internal Excitation and Bond Dissociation
An atom will dissociate from a compound if the atom receives a recoil momentum greater than some average value Q0. Considering a polyatomic molecule as composed of point‐mass atoms, there is derived an equation which relates Q0 to the bond energy, bond angles and distances, and masses of the atoms in the molecule. The minimum net recoil energy required for bond rupture, the kinetic energy of the recoiling radicals, and the internal energy of the radical originally bonded to the activated atom are calculated for a series of simple alkyl halides.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70523/2/JCPSA6-36-4-947-1.pd
Characterization and subcellular targeting of GCaMP-type genetically-encoded calcium indicators
Genetically-encoded calcium indicators (GECIs) hold the promise of monitoring [Ca(2+)] in selected populations of neurons and in specific cellular compartments. Relating GECI fluorescence to neuronal activity requires quantitative characterization. We have characterized a promising new genetically-encoded calcium indicator-GCaMP2-in mammalian pyramidal neurons. Fluorescence changes in response to single action potentials (17+/-10% DeltaF/F [mean+/-SD]) could be detected in some, but not all, neurons. Trains of high-frequency action potentials yielded robust responses (302+/-50% for trains of 40 action potentials at 83 Hz). Responses were similar in acute brain slices from in utero electroporated mice, indicating that long-term expression did not interfere with GCaMP2 function. Membrane-targeted versions of GCaMP2 did not yield larger signals than their non-targeted counterparts. We further targeted GCaMP2 to dendritic spines to monitor Ca(2+) accumulations evoked by activation of synaptic NMDA receptors. We observed robust DeltaF/F responses (range: 37%-264%) to single spine uncaging stimuli that were correlated with NMDA receptor currents measured through a somatic patch pipette. One major drawback of GCaMP2 was its low baseline fluorescence. Our results show that GCaMP2 is improved from the previous versions of GCaMP and may be suited to detect bursts of high-frequency action potentials and synaptic currents in vivo
Muon-Induced Background Study for an Argon-Based Long Baseline Neutrino Experiment
We evaluated rates of transversing muons, muon-induced fast neutrons, and
production of Cl and other cosmogenically produced nuclei that pose as
potential sources of background to the physics program proposed for an
argon-based long baseline neutrino experiment at the Sanford Underground
Research Facility (SURF). The Geant4 simulations were carried out with muons
and muon-induced neutrons for both 800 ft (0.712 km.w.e.) and 4850 ft levels
(4.3 km.w.e.). We developed analytic models to independently calculate the
Cl production using the measured muon fluxes at different levels of the
Homestake mine. The muon induced Cl production rates through stopped
muon capture and the muon-induced neutrons and protons via (n,p) and (p,n)
reactions were evaluated. We find that the Monte Carlo simulated production
rates of Cl agree well with the predictions from analytic models. A
depth-dependent parametrization was developed and benchmarked to the direct
analytic models. We conclude that the muon-induced processes will result in
large backgrounds to the physics proposed for an argon-based long baseline
neutrino experiment at a depth of less than 4.0 km.w.e.Comment: 12 pages, 15 figure
Radial distribution function of semiflexible polymers
We calculate the distribution function of the end--to--end distance of a
semiflexible polymer with large bending rigidity. This quantity is directly
observable in experiments on single semiflexible polymers (e.g., DNA, actin)
and relevant to their interpretation. It is also an important starting point
for analyzing the behavior of more complex systems such as networks and
solutions of semiflexible polymers. To estimate the validity of the obtained
analytical expressions, we also determine the distribution function numerically
using Monte Carlo simulation and find good quantitative agreement.Comment: RevTeX, 4 pages, 1 figure. Also available at
http://www.cip.physik.tu-muenchen.de/tumphy/d/T34/Mitarbeiter/frey.htm
The spread of ras activity triggered by activation of a single dendritic spine
In neurons, individual dendritic spines isolate N-methyl-d-aspartate (NMDA) receptor-mediated calcium ion (Ca2+) accumulations from the dendrite and other spines. However, the extent to which spines compartmentalize signaling events downstream of Ca2+ influx is not known. We combined two-photon fluorescence lifetime imaging with two-photon glutamate uncaging to image the activity of the small guanosine triphosphatase Ras after NMDA receptor activation at individual spines. Induction of long-term potentiation (LTP) triggered robust Ca2+-dependent Ras activation in single spines that decayed in approximately 5 minutes. Ras activity spread over approximately 10 micrometers of dendrite and invaded neighboring spines by diffusion. The spread of Ras-dependent signaling was necessary for the local regulation of the threshold for LTP induction. Thus, Ca2+-dependent synaptic signals can spread to couple multiple synapses on short stretches of dendrite
Fluctuating-friction molecular motors
We show that the correlated stochastic fluctuation of the friction
coefficient can give rise to long-range directional motion of a particle
undergoing Brownian random walk in a constant periodic energy potential
landscape. The occurrence of this motion requires the presence of two
additional independent bodies interacting with the particle via friction and
via the energy potential, respectively, which can move relative to each other.
Such three-body system generalizes the classical Brownian ratchet mechanism,
which requires only two interacting bodies. In particular, we describe a simple
two-level model of fluctuating-friction molecular motor that can be solved
analytically. In our previous work [M.K., L.M and D.P. 2000 J. Nonlinear Opt.
Phys. Mater. vol. 9, 157] this model has been first applied to understanding
the fundamental mechanism of the photoinduced reorientation of dye-doped liquid
crystals. Applications of the same idea to other fields such as molecular
biology and nanotechnology can however be envisioned. As an example, in this
paper we work out a model of the actomyosin system based on the
fluctuating-friction mechanism.Comment: to be published in J. Physics Condensed Matter
(http://www.iop.org/Journals/JPhysCM
Stereotyped odor-evoked activity in the mushroom body of Drosophila revealed by green fluorescent protein-based Ca2+ imaging
To study the representation of olfactory information in higher brain centers, we expressed a green fluorescent protein-based Ca2+ sensor, G-CaMP, in the Drosophila mushroom body (MB). Using two-photon microscopy, we imaged odor-evoked G-CaMP fluorescence transients in MB neurons [Kenyon cells (KCs)] with single-cell resolution. Odors produced large fluorescence transients in a subset of KC somata and in restricted regions of the calyx, the neuropil of the MB. In different KCs, odor-evoked fluorescence transients showed diverse changes with odor concentration: in some KCs, fluorescence transients were evoked by an odor at concentrations spanning several orders of magnitude, whereas in others only at a narrow concentration range. Different odors produced fluorescence transients in different subsets of KCs. The spatial distributions of KCs showing fluorescence transients evoked by a given odor were similar across individuals. For some odors, individual KCs with fluorescence transients evoked by a particular odor could be found in similar locations in different flies with spatial precisions on the order of the size of KC somata. These results indicate that odor-evoked activity can have remarkable spatial specificity in the MB
Perturbation Theory for Path Integrals of Stiff Polymers
The wormlike chain model of stiff polymers is a nonlinear -model in
one spacetime dimension in which the ends are fluctuating freely. This causes
important differences with respect to the presently available theory which
exists only for periodic and Dirichlet boundary conditions. We modify this
theory appropriately and show how to perform a systematic large-stiffness
expansions for all physically interesting quantities in powers of ,
where is the length and the persistence length of the polymer. This
requires special procedures for regularizing highly divergent Feynman integrals
which we have developed in previous work. We show that by adding to the
unperturbed action a correction term , we can calculate
all Feynman diagrams with Green functions satisfying Neumann boundary
conditions. Our expansions yield, order by order, properly normalized
end-to-end distribution function in arbitrary dimensions , its even and odd
moments, and the two-point correlation function
Understanding Mechanochemical Coupling in Kinesins Using First-Passage Time Processes
Kinesins are processive motor proteins that move along microtubules in a
stepwise manner, and their motion is powered by the hydrolysis of ATP. Recent
experiments have investigated the coupling between the individual steps of
single kinesin molecules and ATP hydrolysis, taking explicitly into account
forward steps, backward steps and detachments. A theoretical study of
mechanochemical coupling in kinesins, which extends the approach used
successfully to describe the dynamics of conventional motor proteins, is
presented. The possibility of irreversible detachments of kinesins from the
microtubules is also explicitly taken into account. Using the method of first-
passage times, experimental data on the mechanochemical coupling in kinesins
are fully described using the simplest two-state model. It is shown that the
dwell times for the kinesin to move one step forward or backward, or to
dissociate irreversibly are the same, although the probabilities of these
events are different. It is concluded that the current theoretical view, that
only the forward motion of the motor protein molecule is coupled to ATP
hydrolysis, is consistent with all available experimental observations for
kinesins.Comment: Submitted to Biophysical Journa
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