3,599 research outputs found
Sulfo-SMCC Prevents Annealing of Taxol-Stabilized Microtubules In Vitro
Microtubule structure and functions have been widely studied in vitro and in
cells. Research has shown that cysteines on tubulin play a crucial role in the
polymerization of microtubules. Here, we show that blocking sulfhydryl groups
of cysteines in taxol-stabilized polymerized microtubules with a commonly used
chemical crosslinker prevents temporal end-to-end annealing of microtubules in
vitro. This can dramatically affect the length distribution of the
microtubules. The crosslinker sulfosuccinimidyl
4-(N-maleimidomethyl)cyclohexane-1-carboxylate, sulfo-SMCC, consists of a
maleimide and an N-hydroxysuccinimide ester group to bind to sulfhydryl groups
and primary amines, respectively. Interestingly, addition of a maleimide dye
alone does not show the same interference with annealing in stabilized
microtubules. This study shows that the sulfhydryl groups of cysteines of
tubulin that are vital for the polymerization are also important for the
subsequent annealing of microtubules.Comment: 3 figure
A symmetrical method to obtain shear moduli from microrheology
Passive microrheology typically deduces shear elastic loss and storage moduli
from displacement time series or mean-squared displacement (MSD) of thermally
fluctuating probe particles in equilibrium materials. Common data analysis
methods use either Kramers-Kronig (KK) transformations or functional fitting to
calculate frequency-dependent loss and storage moduli. We propose a new
analysis method for passive microrheology that avoids the limitations of both
of these approaches. In this method, we determine both real and imaginary
components of the complex, frequency-dependent response function as direct integral
transforms of the MSD of thermal particle motion. This procedure significantly
improves the high-frequency fidelity of relative to the use of
KK transformation, which has been shown to lead to artifacts in
. We test our method on both model data and experimental
data. Experiments were performed on solutions of worm-like micelles and dilute
collagen solutions. While the present method agrees well with established
KK-based methods at low frequencies, we demonstrate significant improvement at
high frequencies using our symmetric analysis method, up to almost the
fundamental Nyquist limit.Comment: 8 pages, 4 figure
Topology counts: force distributions in circular spring networks
Filamentous polymer networks govern the mechanical properties of many
biological materials. Force distributions within these networks are typically
highly inhomogeneous and, although the importance of force distributions for
structural properties is well recognized, they are far from being understood
quantitatively. Using a combination of probabilistic and graph-theoretical
techniques we derive force distributions in a model system consisting of
ensembles of random linear spring networks on a circle. We show that
characteristic quantities, such as mean and variance of the force supported by
individual springs, can be derived explicitly in terms of only two parameters:
(i) average connectivity and (ii) number of nodes. Our analysis shows that a
classical mean-field approach fails to capture these characteristic quantities
correctly. In contrast, we demonstrate that network topology is a crucial
determinant of force distributions in an elastic spring network.Comment: 5 pages, 4 figures. Missing labels in Fig. 4 added. Reference fixe
Robust long-distance entanglement and a loophole-free Bell test with ions and photons
Two trapped ions that are kilometers apart can be entangled by the joint
detection of two photons, each coming from one of the ions, in a basis of
entangled states. Such a detection is possible with linear optical elements.
The use of two-photon interference allows entanglement distribution without
interferometric sensitivity to the path length of the photons. The present
method of creating entangled ions also opens up the possibility of a
loophole-free test of Bell's inequalities.Comment: published versio
Radiative Inflation and Dark Energy
We propose a model based on radiative symmetry breaking that combines
inflation with Dark Energy and is consistent with the WMAP 7-year regions. The
radiative inflationary potential leads to the prediction of a spectral index
0.955 \lesssim n_S \lesssim 0.967 and a tensor to scalar ratio 0.142 \lesssim r
\lesssim 0.186, both consistent with current data but testable by the Planck
experiment. The radiative symmetry breaking close to the Planck scale gives
rise to a pseudo Nambu-Goldstone boson with a gravitationally suppressed mass
which can naturally play the role of a quintessence field responsible for Dark
Energy. Finally, we present a possible extra dimensional scenario in which our
model could be realised.Comment: 15 pages, 4 figures; v2: references added, appendix added, Section 5
slightly modified; content matches published versio
Swelling and Softening of the Cowpea Chlorotic Mottle Virus in Response to pH Shifts
AbstractCowpea chlorotic mottle virus (CCMV) forms highly elastic icosahedral protein capsids that undergo a characteristic swelling transition when the pH is raised from 5 to 7. Here, we performed nano-indentation experiments using an atomic force microscope to track capsid swelling and measure the shells’ Young’s modulus at the same time. When we chelated Ca2+ ions and raised the pH, we observed a gradual swelling of the RNA-filled capsids accompanied by a softening of the shell. Control experiments with empty wild-type virus and a salt-stable mutant revealed that the softening was not strictly coupled to the swelling of the protein shells. Our data suggest that a pH increase and Ca2+ chelation lead primarily to a loosening of contacts within the protein shell, resulting in a softening of the capsid. This appears to render the shell metastable and make swelling possible when repulsive forces among the capsid proteins become large enough, which is known to be followed by capsid disassembly at even higher pH. Thus, softening and swelling are likely to play a role during inoculation
Combined macro- and micro-rheometer for use with Langmuir monolayers
A Langmuir monolayer trough that is equipped for simultaneous microrheology
and standard rheology measurements has been constructed. The central elements
are the trough itself with a full range of optical tools accessing the
air-water interface from below the trough and a portable knife-edge torsion
pendulum that can access the interface from above. The ability to
simultaneously measure the mechanical response of Langmuir monolayers on very
different lengths scales is an important step in for our understanding of the
mechanical response of such systems
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