781 research outputs found
Dynamic Light Scattering from Semidilute Actin Solutions: A Study of Hydrodynamic Screening, Filament Bending Stiffness and the Effect of Tropomyosin/Troponin-Binding
Quasi-elastic light scattering (QELS) is applied to investigate the effect of
the tropomyosin/troponin complex (Tm/Tn) on the stiffness of actin filaments.
The importance of hydrodynamic screening in semidilute solutions is
demonstrated. A new concentration dependent expression for the dynamic
structure factor of semiflexible polymers in semidilute solutions
is used to analyze the experimental QELS data. A concentration independent
value for the bending modulus is thus obtained. It increases by 50\%
as a consequence of Tm/Tn binding in a 7:1:1 molar ratio of actin/Tm/Tn. In
addition a new expression for the initial slope of the dynamic structure factor
of a semiflexible polymer is used to determine the effective hydrodynamic
diameter of the actin filament. Our results confirm the general relevance of
the concept of (intrinsic) semiflexibility to polymer dynamics.Comment: 9 pages, RevTeX, 9 figures, all uuencoded gzipe
On Shape Transformations and Shape Fluctuations of Cellular Compartments and Vesicles
We discuss the shape formation and shape transitions of simple bilayer vesicles in context with their role in biology. In the first part several classes of shape changes of vesicles of one lipid component are described and it is shown that these can be explained in terms of the bending energy concept in particular augmented by the bilayer coupling hypothesis. In the second
part shape changes and vesicle fission of vesicles composed of membranes of lipid mixtures are reported. These are explained in terms of coupling between local curvature and phase separation
The Glassy Wormlike Chain
We introduce a new model for the dynamics of a wormlike chain in an
environment that gives rise to a rough free energy landscape, which we baptise
the glassy wormlike chain. It is obtained from the common wormlike chain by an
exponential stretching of the relaxation spectrum of its long-wavelength
eigenmodes, controlled by a single stretching parameter. Predictions for
pertinent observables such as the dynamic structure factor and the
microrheological susceptibility exhibit the characteristics of soft glassy
rheology and compare favourably with experimental data for reconstituted
cytoskeletal networks and live cells. We speculate about the possible
microscopic origin of the stretching, implications for the nonlinear rheology,
and the potential physiological significance of our results.Comment: 12 pages, 8 figures. Minor correction
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
Force-Extension Relation and Plateau Modulus for Wormlike Chains
We derive the linear force-extension relation for a wormlike chain of
arbitrary stiffness including entropy elasticity, bending and thermodynamic
buckling. From this we infer the plateau modulus of an isotropic
entangled solution of wormlike chains. The entanglement length is
expressed in terms of the characteristic network parameters for three different
scaling regimes in the entangled phase. The entanglement transition and the
concentration dependence of are analyzed. Finally we compare our findings
with experimental data.Comment: 5 pages, 1 eps-figure, to appear in PR
Polarisation selective magnetic vortex dynamics and core reversal in rotating magnetic fields
A magnetic vortex occurs as an equilibrium configuration in thin
ferromagnetic platelets of micron and sub-micron size and is characterised by
an in-plane curling magnetisation. At the centre, a magnetic singularity is
avoided by an out-of-plane magnetisation core. This core has a gyrotropic
excitation mode, which corresponds to a circular motion of the vortex around
its equilibrium position, where the rotation sense is determined by the
direction of the vortex core magnetisation, its polarisation. Unlike linear
fields or spin polarised currents, which excite both polarisation states, an
in-plane rotating field can selectively excite one of the polarisation states.
Here we report the observation of vortex dynamics in response to rotating
magnetic fields, imaged with time-resolved scanning X-ray microscopy. We
demonstrate that the rotating field only excites the gyrotropic mode if the
rotation sense of the field coincides with the vortex gyration sense and that
such a field can selectively reverse the vortex polarisation
Solar Model Uncertainties, MSW Analysis, and Future Solar Neutrino Experiments
Various theoretical uncertainties in the standard solar model and in the
Mikheyev-Smirnov-Wolfenstein (MSW) analysis are discussed. It is shown that two
methods of estimating the solar neutrino flux uncertainties are equivalent: (a)
a simple parametrization of the uncertainties using the core temperature and
the nuclear production cross sections; (b) the Monte Carlo method of Bahcall
and Ulrich. In the MSW analysis, we emphasize proper treatments of correlation
of theoretical uncertainties between flux components and between different
detectors, the Earth effect, and multiple solutions in a combined
procedure. The MSW solutions for various standard and nonstandard solar models
are also shown. The MSW predictions of the global solutions for the future
solar neutrino experiments are given, emphasizing the measurement of the energy
spectrum and the day-night effect in Sudbury Neutrino Observatory and
Super-Kamiokande to distinguish the two solutions.Comment: (Revtex 3.0, 43 pages + 26 figures (uuencoded ps files attached),
Easy way: ps files of entire text with embedded figures available by
anonymous ftp://upenn5.hep.upenn.edu/pub/hata/papers/msw_analysis.u
- …