15,734 research outputs found
Coupling between membrane tilt-difference and dilation: a new ``ripple'' instability and multiple crystalline inclusions phases
A continuum Landau theory for the micro-elasticity of membranes is discussed,
which incorporates a coupling between the bilayer thickness variation and the
difference in the two monolayers' tilts. This coupling stabilizes a new phase
with a rippled micro-structure. Interactions among membrane inclusions combine
a dilation-induced attraction and a tilt-difference-induced repulsion that
yield 2D crystal phases, with possible coexistence of different lattice
spacings for large couplings. Inclusions favoring crystals are those with
either a long-convex or a short-concave hydrophobic core.Comment: EURO LaTeX, 6 pages, 4 figures, to be published in Europhys. Let
Wormlike chain or tense string? A question of resolution
It is shown that a wormlike chain, i.e., a filament with a fixed
contour-length S and a bending elasticity kappa, attached to a frame of length
L, can be described--at low resolutions--by the same type of elastic
free-energy as a tense string. The corresponding tension is calculated as a
function of temperature, L, kappa and S.Comment: 13 pages, 3 figures. To appear in Continuum Mechanics and
Thermodynamic
Microscopic membrane elasticity and interactions among membrane inclusions: Interplay between the shape, dilation, tilt and tilt-difference modes
A phenomenological Landau elasticity for the shape, dilation, and lipid-tilt
of bilayer membranes is developed. The shape mode couples with the sum of the
monolayers' tilt, while the dilation mode couples with the difference of the
monolayers' tilts. Interactions among membrane inclusions within regular arrays
are discussed. Inclusions modifying the membrane thickness and/or inducing a
tilt-difference due to their convex or concave shape yield a dilation-induced
attraction and a tilt-difference-induced repulsion. The resulting interaction
can stabilize 2D crystal phases, with the possible coexistence of different
lattice spacings when the dilation-tilt-difference coupling is large.
Inclusions favoring crystals are those with either a long-convex or a
short-concave hydrophobic core. Inclusions inducing a local membrane curvature
due to their conical shape repel one another. At short inclusions separations,
a tilt comparable with the inclusion's cone angle develops: it relaxes the
membrane curvature and reduces the repulsion. At large separations the tilt
vanishes, whatever the value of the shape-tilt coupling.Comment: 13 pages, 19 figure
N-body Study of Anisotropic Membrane Inclusions: Membrane Mediated Interactions and Ordered Aggregation
We study the collective behavior of inclusions inducing local anisotropic
curvatures in a flexible fluid membrane. The N-body interaction energy for
general anisotropic inclusions is calculated explicitly, including multi-body
interactions. Long-range attractive interactions between inclusions are found
to be sufficiently strong to induce aggregation. Monte Carlo simulations show a
transition from compact clusters to aggregation on lines or circles. These
results might be relevant to proteins in biological membranes or colloidal
particles bound to surfactant membranes.Comment: 4 pages, 3 figs, LaTe
On the surface tension of fluctuating quasi-spherical vesicles
We calculate the stress tensor for a quasi-spherical vesicle and we thermally
average it in order to obtain the actual, mechanical, surface tension of
the vesicle. Both closed and poked vesicles are considered. We recover our
results for by differentiating the free-energy with respect to the
proper projected area. We show that may become negative well before the
transition to oblate shapes and that it may reach quite large negative values
in the case of small vesicles. This implies that spherical vesicles may have an
inner pressure lower than the outer one.Comment: To appear in Eur. Phys. J. E, revised versio
Dynamin recruitment by clathrin coats: a physical step?
Recent structural findings have shown that dynamin, a cytosol protein playing
a key-role in clathrin-mediated endocytosis, inserts partly within the lipid
bilayer and tends to self-assemble around lipid tubules. Taking into account
these observations, we make the hypothesis that individual membrane inserted
dynamins imprint a local cylindrical curvature to the membrane. This imprint
may give rise to long-range mechanical forces mediated by the elasticity of the
membrane. Calculating the resulting many-body interaction between a collection
of inserted dynamins and a membrane bud, we find a regime in which the dynamins
are elastically recruited by the bud to form a collar around its neck, which is
reminiscent of the actual process preempting vesicle scission. This physical
mechanism might therefore be implied in the recruitment of dynamins by clathrin
coats.Comment: 11 pages, 6 figures, to appear in C.R.A.S. ser II
Design aspects of a solar array drive for spot, with a high platform stability objective
A solar array drive mechanism (MEGS) for the SPOT platform, which is a prototype of a multimission platform, is described. High-resolution cameras and other optical instruments are carried by the platform, requiring excellent platform stability in order to obtain high-quality pictures. Therefore, a severe requirement for the MEGS is the low level of disturbing torques it may generate considering the 0.6 times 10 to the minus 3 power deg/sec stability required. The mechanical design aspects aiming at reducing the mean friction torque, and therefore its fluctuations, are described as well as the method of compensation of the motor imperfections. It was concluded, however, that this is not sufficient to reach the stability requirement
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