1,160 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
Shock-wave therapy of gastric outlet syndrome caused by a gallstone
A patient with gastric outlet syndrome (Bouveret's syndrome) caused by a large gallstone impacted in the duodenal bulb was successfully treated by extracorporeal shock-wave lithotripsy. Thus, open abdominal surgery could be avoided. For disintegration of the stone, three consecutive lithotripsy procedures were necessary. Thereafter, stone fragments could be extracted endoscopically. Extracorporeal shock-wave lithotripsy could become a non-surgical alternative in patients with obstruction of the duodenum caused by a gallstone
Attractive instability of oppositely charged membranes induced by charge density fluctuations
We predict the conditions under which two oppositely charged membranes show a
dynamic, attractive instability. Two layers with unequal charges of opposite
sign can repel or be stable when in close proximity. However, dynamic charge
density fluctuations can induce an attractive instability and thus facilitate
fusion. We predict the dominant instability modes and timescales and show how
these are controlled by the relative charge and membrane viscosities. These
dynamic instabilities may be the precursors of membrane fusion in systems where
artificial vesicles are engulfed by biological cells of opposite charge
Two-dimensional streptavidin crystals on giant lipid bilayer vesicles
Streptavidin was crystallized on giant bilayer vesicles (20-60 mum) in sucrose solution at various pH values. The streptavidin-coated vesicles exhibited unique roughened spherical and prolate ellipsoidal shapes, illustrating resistance to curvature of the two-dimensional crystals. Studies indicated that the spheroids and prolate ellipsoids correspond to different crystal morphologies. Through confocal microscopy, the various crystal morphologies on vesicle surfaces were observed under different solution conditions. Unlike two-dimensional (2D) streptavidin crystals grown in ionic buffer that assume the P1, P2, and C222 lattices at pH 4, 5.5, and 7, respectively (Wang et al. Langmuir 1999, 15, 154 1), crystals grown in sucrose with no added salt show only the lowest density C222 lattice due to strong electrostatic interactions
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
Phase ordering and shape deformation of two-phase membranes
Within a coupled-field Ginzburg-Landau model we study analytically phase
separation and accompanying shape deformation on a two-phase elastic membrane
in simple geometries such as cylinders, spheres and tori. Using an exact
periodic domain wall solution we solve for the shape and phase ordering field,
and estimate the degree of deformation of the membrane. The results are
pertinent to a preferential phase separation in regions of differing curvature
on a variety of vesicles.Comment: 4 pages, submitted to PR
Lateral phase separation in mixtures of lipids and cholesterol
In an effort to understand "rafts" in biological membranes, we propose phenomenological models for saturated and unsaturated lipid mixtures, and lipid-cholesterol mixtures. We consider simple couplings between the local composition and internal membrane structure, and their influence on transitions between liquid and gel membrane phases. Assuming that the gel transition temperature of the saturated lipid is shifted by the presence of the unsaturated lipid, and that cholesterol acts as an external field on the chain melting transition, a variety of phase diagrams are obtained. The phase diagrams for binary mixtures of saturated/unsaturated lipids and lipid/cholesterol are in semi-quantitative agreement with the experiments. Our results also apply to regions in the ternary phase diagram of lipid/lipid/cholesterol systems
Luminosities of AGB Variables
The prevailing evidence suggests that most large-amplitude AGB variables
follow the period luminosity (PL) relation that has been established for Miras
in the LMC and galactic globular clusters. Hipparcos observations indicate that
most Miras in the solar neighbourhood are consistent with such a relation.
There are two groups of stars with luminosities that are apparently greater
than the PL relation would predict: (1) in the LMC and SMC there are large
amplitude variables, with long periods, P> 420 days, which are probably
undergoing hot bottom burning, but which are very clearly more luminous than
the PL relation (these are visually bright and are likely to be among the first
stars discovered in more distant intermediate age populations); (2) in the
solar neighbourhood there are short period, P<235 days, red stars which are
probably more luminous than the PL relation. Similar short-period red stars,
with high luminosities, have not been identified in the Magellanic Clouds.Comment: 8 pages, 2 figure, to be published in Mass-Losing Pulsating Stars and
their Circumstellar Matter, Y. Nakada & M. Honma (eds) Kluwer ASSL serie
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