共查询到20条相似文献,搜索用时 500 毫秒
1.
Motivated by numerous X-ray scattering studies of lamellar phases with membrane proteins, amphiphilic peptides, polymers,
or other inclusions, we have determined the modifications of the classical Caillé law for a smectic phase as a function of
the nature and concentration of inclusions added to it. Besides a fundamental interest on the behavior of fluctuating systems
with inclusions, a precise characterization of the action of a given protein on a lipid membrane (anchoring, swelling, stiffening
...) is of direct biological interest and could be probed by way of X-ray measurements. As a first step we consider three
different couplings involving local pinching (or swelling), stiffening or curvature of the membrane. In the first two cases
we predict that independent inclusions induce a simple renormalization of the bending and compression moduli of the smectic
phase. The X-ray experiments may also be used to probe correlations between inclusions. Finally we show that asymmetric coupling
(such as a local curvature of the membrane) results in a modification of the usual Caillé law.
Received 10 March 2000 and Received in final form 30 August 2000 相似文献
2.
Tilt of hydrocarbon chains of lipid molecules with respect to membrane plane is commonly considered to characterize the internal
structure of a membrane in the crystalline state. However, membranes in the liquid state can also exhibit tilt resulting from
packing constraints imposed on the lipid molecules in diverse biologically relevant structures such as intermediates of membrane
fusion, pores in lipid bilayers and others. We analyze the energetics of tilt in liquid membranes and its coupling with membrane
bending. We consider three contributions to the elastic energy: constant tilt, variation of tilt along the membrane surface
and membrane bending. The major assumption of the model is that the core of a liquid membrane has the common properties of
an elastic continuum. We show that the variation of tilt and membrane bending are additive and that their energy contributions
are determined by the same elastic coefficient: the Helfrich bending modulus, the modulus of Gaussian curvature and the spontaneous
curvature known from previous studies of pure bending. The energy of a combined deformation of bending and varying tilt is
determined by an effective tensor accounting for the two factors. In contrast, the deformation of constant tilt does not couple
with bending and its contribution to the elastic energy is determined by an independent elastic constant. While accurate determination
of this constant requires additional measurements, we estimate its value using a simplified approach. We discuss the relationships
between the obtained elastic Hamiltonian of a membrane and the previous models of membrane elasticity.
Received 10 February 2000 and Received in final form 19 June 2000 相似文献
3.
The effect of rigid inclusions on the phase behavior of a film containing a mixture of lipid molecules is investigated. In
the proposed model, the inclusion-induced deformation of the film, and the resulting energy cost are strongly dependent upon
the spontaneous curvature of the mixed film. The spontaneous curvature is in turn strongly influenced by the composition of
film. This coupling between the film composition and the energy per inclusion leads to a lateral modulation of the composition,
which follows the local curvature of the membrane. In particular, it is shown that inclusions may induce a global phase separation
in a film which would otherwise be homogeneously mixed. The mixed film is then composed of patches of different average composition,
separated by the inclusions. This process may be of relevance to explain some aspects of lipid-protein association in biological
membranes.
Received 8 April 1999 and Received in final form 4 October 1999 相似文献
4.
W. Helfrich 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):481-489
The problem of membrane softening by thermal undulations is revisited. In contrast to general belief, fluid membranes are
predicted to be stiffened, not softened, by their undulations. Equal values of the effective bending rigidity are calculated
from the interplay of local mean curvature modes (hats) on the basically flat membrane and from the coupling of spherical
harmonic modes with spherical curvature. In addition, a conjecture is made on the entropy of membrane closure. It relies on
a similarity of membrane closure to periodic boundary conditions.
Received: 10 June 1997 / Revised: 7 October 1997 / Accepted: 2 December 1997 相似文献
5.
Inclusions embedded in lipid membranes undergo a mediated force, due to the tendency of the membrane to relax its excess of
elastic energy. In this paper we determine the exact shape of a two-dimensional vesicle hosting two different inclusions,
and we analyse how the inclusion conformation influences the mediated interaction. We find non-trivial equilibrium configurations
for the inclusions along the hosting membrane, and we derive the complete phase diagram of the mediated interaction. In particular,
we find a non-vanishing mediated force even when the distance between the inclusions is much greater than their size. Our
model can be applied to describe the mediated interactions of parallel, elongated inclusions embedded in three-dimensional
membranes.
Received 22 October 2001 and Received in final form 8 March 2002 相似文献
6.
P.G. Dommersnes J.-B. Fournier 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(1):9-12
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.
Received 30 July 1999 and Received in final form 8 September 1999 相似文献
7.
J. Prost J.-B. Manneville R. Bruinsma 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):465-480
Membranes in thermal equilibrium are well known to exhibit Brownian motion type shape fluctuations. Membranes containing active
force centers -- such as chemically active membrane proteins -- suffer additional non-equilibrium shape fluctuations due to
the activity of these force centers. We demonstrate, using scaling arguments, that non-equilibrium shape fluctuations are
in general greatly amplified by the presence of a nearby wall or membrane due to the absence of a fluctuation-dissipation
theorem. For adhesive membranes, this fluctuation magnification effect may facilitate the establishment of bonding. For non-adhesive
membranes, fluctuation magnification produces a long-range repulsive pressure which can exceed the well known Helfrich repulsion
due to purely thermal fluctuations.
Received: 1 September 1997 / Accepted: 3 December 1997 相似文献
8.
T. Ohta M. Nonomura 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(1):57-68
We study the domain morphology in a phase separated state of diblock copolymer-homopolymer mixtures. In the situation that
one of the blocks of copolymers is incompatible with homopolymers, formation of a bilayer membrane of block copolymers is
shown to be possible in the matrix of homopolymers. Starting with the free energy functional in terms of the local volume
fractions of each monomer, we derive the bending and curvature rigidities in the Helfrich free energy for a membrane. It is
found that the curvature modulus is negative only in a limited region of the parameters, where a closed shape of a membrane
like a vesicle is possibly formed. We establish a method to calculate the rigidities without a molecular picture in a consistent
way with the field theoretic model free energy. Stability of a bilayer membrane compared with micelles is also investigated.
Received: 22 July 1997 / Received in final form: 1 December 1997 / Accepted: 22 January 1998 相似文献
9.
S. May 《The European physical journal. E, Soft matter》2000,3(1):37-44
The line tension of a symmetric, lipid bilayer in its liquid-crystalline state is calculated on the basis of a molecular lipid
model. The lipid model extends the opposing forces model by an expression for the conformational free energy of the hydrocarbon
chains. We consider a membrane edge that consists of a perturbed bilayer covered by a section of a cylinder-like micelle.
The structural rearrangement of the lipids implies an excess free energy which we minimize with respect to the cross-sectional
shape of the membrane edge, including both the micellar and the bilayer region. The line tension is derived as a function
of molecular lipid properties, like the lipid chain length or the head group interaction strength. We also relate it to the
spontaneous curvature of the lipid layer. We find the line tension to become smaller for lipid layers that tend to curve more
towards the hydrophobic core. Our predictions for the line tension and their relation to experimentally derived values are
discussed.
Received 2 January 2000 相似文献
10.
Adhesion between membranes is studied using a phenomenological model, where the inter-membrane distance is coupled to the
concentration of sticker molecules on the membranes. The model applies to both adhesion of two flexible membranes and to adhesion
of one flexible membrane onto a second membrane supported on a solid substrate. We mainly consider the case where the sticker
molecules form bridges and adhere directly to both membranes. The calculated mean-field phase diagrams show an upward shift
of the transition temperature indicating that the lateral phase separation in the membrane is enhanced due to the coupling effect. Hence the possibility of adhesion-induced lateral phase separation is predicted. For a particular
choice of the parameters, the model exhibits a tricritical behavior. We also discuss the non-monotonous shape of the inter-membrane
distance occurring when the lateral phase separation takes place. The inter-membrane distance relaxes to the bulk values with
two symmetric overshoots. Adhesion mediated by other types of stickers is also considered.
Received 12 January 2000 and Received in final form 15 May 2000 相似文献
11.
Equilibrium phase coexistence between two chemical species implies the equality of the chemical potentials and of the osmotic
pressures. We study this problem on a deformable membrane when one type of the molecules serves as anchor for polymeric chains
immersed in the surrounding medium (considered as a good solvent). We derive the general conditions for phase coexistence
when both the curvature of the membrane and the density field of the anchor molecule are free to adjust themselves. We show
that curvature favors phase segregation. Our model predicts that membranes decorated with polymeric chains exhibit new shape
bifurcations without equivalent in fixed density systems.
Received: 26 November 2002 / Accepted: 2 April 2003 / Published online: 12 May 2003
RID="a"
ID="a"e-mail: nicolas@drfmc.ceng.cea.fr
RID="b"
ID="b"e-mail: bfourcade@cea.fr 相似文献
12.
R. Holzlöhner M. Schoen 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(3):413-419
The fluctuation-induced interaction between two rod-like, rigid inclusions in a fluid vesicle is studied by means of canonical
ensemble Monte-Carlo simulations. The vesicle membrane is represented by a triangulated network of hard spheres. Five rigidly
connected hard spheres form rod-like inclusions that can leap between sites of the triangular network. Their effective interaction
potential is computed as a function of mutual distance and angle of the inclusions. On account of the hard-core potential
among these, the nature of the potential is purely entropic. Special precaution is taken to reduce lattice artifacts and the
influence of finite-size effects due to the spherical geometry. Our results show that the effective potential is attractive
and short-range compared with the rod length L. Its well depth is of the order of , where is the bending modulus.
Received 5 February 1999 and Received in final form 14 May 1999 相似文献
13.
R.R. Netz H. Orland 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(1):81-98
Variational methods are applied to a single polyelectrolyte chain. The polymer is modeled as a Gaussian chain with screened
electrostatic repulsion between all monomers. As a variational Hamiltonian, the most general Gaussian kernel, including the
possibility of a classical or mean polymer path, is employed. The resulting self-consistent equations are systematically solved
both for large and small monomer-monomer separations along the chain. In the absence of screening, the polymer is stretched
on average. It is described by a straight classical path with Gaussian fluctuations around it. If the electrostatic repulsion
is screened, the polymer is isotropically swollen for large separations, and for small separations the polymer correlation
function is calculated as an analytic expansion in terms of the monomer-monomer separation along the chain. The electrostatic
persistence length and the electrostatic blobsize are inferred from the crossover between distinct scaling ranges. We perform
a global analysis of the scaling behavior as a function of the screening length and electrostatic interaction strength , where is the Bjerrum length and A is the distance of charges along the polymer chain. We find three different scaling regimes. i) A Gaussian-persistent regime with Gaussian behavior at small, persistent behavior at intermediate, and isotropically swollen behavior at large length
scales. This regime occurs for weakly charged polymers and only for intermediate values of the screening length. The electrostatic
persistence length is defined as the crossover length between the persistent and the asymptotically swollen behavior and is given by and thus disagrees with previous (restricted) variational treatments which predict a linear dependence on the screening length
.ii) A Gaussian regime with Gaussian behavior at small and isotropically swollen behavior at large length scales. This regime occurs for weakly
charged polymers and/or strong screening, and the electrostatic repulsion between monomers only leads to subfluent corrections
to Gaussian scaling at small separations. The concept of a persistence length is without meaning in this regime. iii) A persistent regime , where the chain resembles a stretched rod on intermediate and small scales. Here the persistence length is given by the
original Odijk prediction, , if the overstretching of the chain is avoided. We also investigate the effects of a finite polymer length and of an additional
excluded-volume interaction, which modify the resultant scaling behavior. Applications to experiments and computer simulations
are discussed.
Received 24 December 1997 相似文献
14.
Grain boundary lead inclusions formed by ion implantation of mazed bicrystal aluminum films have been investigated by transmission electron microscopy. The vapor-grown bicrystal films contained mainly 90°(110) tilt boundaries with fixed misorientation but variable inclination, as well as some growth twins with 70.5°(110) symmetrical tilt boundaries and a few small-angle boundaries. It was found that the shape, size and orientation of the inclusions in the grain boundaries depend on the orientation of the aluminum grain boundary plane. Inclusions at 90°(110) tilt boundaries were invariably sharply faceted toward one aluminum grain and more rounded toward the other grain. The faceted side was a section of the cuboctahedral equilibrium shape of bulk lead inclusions in parallel topotaxy with the aluminum matrix. The rounded side, where the aluminum grain was rotated by 90° with respect to the lead lattice, approximated a spherical cap. At specific low-energy segments of the grain boundary where a (100) plane in grain 1 meets an (011) plane in grain 2, only two of several possible shapes were observed. One of these was preferred in as-implanted samples while both types were found after melting and re-solidification of the lend inclusions. The observations are discussed in terms of a modified Wulff construction. 相似文献
15.
Effect of thermal undulations on the bending elasticity and spontaneous curvature of fluid membranes
We amplify previous arguments why mean curvature should be used as measure of integration in calculating the effective bending
rigidity of fluid membranes subjected to a weak background curvature. The stiffening of the membrane by its fluctuations,
recently derived for spherical shapes, is recovered for cylindrical curvature. Employing curvilinear coordinates, we then
discuss stiffening for arbitrary shapes, confirm that the elastic modulus of Gaussian curvature is not renormalized in the
presence of fluctuations, and show for the first time that any spontaneous curvature also remains unchanged.
Received 19 April 1999 and in Received in final form 7 January 2000 相似文献
16.
M.E.S. Borelli H. Kleinert A.M.J. Schakel 《The European physical journal. E, Soft matter》2001,4(2):217-221
A stack of tensionless membranes with nonlinear curvature energy and vertical harmonic interaction is studied. At low temperatures,
the system forms a lamellar phase. At a critical temperature, the stack disorders vertically in a melting-like transition.
Received 26 April 2000 and Received in final form 16 August 2000 相似文献
17.
G. Barbero L.R. Evangelista N.V Madhusudana 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):327-331
We analyse the influence of adsorbed ions and the resulting surface electric field and its gradient on the anchoring properties
of nematics with ionic conductivity. We take into account two physical mechanisms for the coupling of the nematic director
with the surface electric field: (i) the dielectric anisotropy and (ii) the coupling of the quadrupolar component of the flexoelectric
coefficient with the field gradient. It is shown that for sufficiently large fields near saturated coverage of the adsorbed
ions, there can be a spontaneous curvature distortion in the cell even when the anchoring energy is infinitely strong. We
also discuss the director distortion when the anchoring energy of the surface is finite.
Received: 29 September 1997 / Received in final from: 10 November 1997 / Accepted: 18 November 1997 相似文献
18.
K. Dworecki T. Kosztolowicz St. Mrówczynski S. Wasik 《The European physical journal. E, Soft matter》2000,3(4):389-394
The near membrane layer is a region where the concentration of the substance transported across the membrane is significantly
decreased. Its thickness is defined as a length over which the concentration drops k times with k being an arbitrary large number. The time evolution of such a layer is studied experimentally by means of the laser interferometric
method. It is shown that within the experimental errors the thickness of the near membrane layer grows in time for any k as with the coefficient a being independent of the initial concentration and the membrane permeability. Time evolution of the near membrane layers
is also analyzed theoretically. The regularities found experimentally are naturally described within the model which has been
earlier developed by one of us. In particular, a scales as .
Received 12 November 1999 and Received in final form 3 July 2000 相似文献
19.
D. Schlauf C. Bahr V.K. Dolganov J.W. Goodby 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(3):461-469
We report a detailed ellipsometric study of freely suspended films of chiral liquid-crystal compounds possessing smectic-A
and smectic-C phases. In the temperature region between the smectic-A - smectic-C bulk and surface transitions, a discontinuous
reconstruction of the tilt profile across the film is observed in the presence of a constant d.c. electric field. Comparison
of the measured ellipsometric quantities with values calculated from model tilt profiles reveals a competition between a structure
possessing a homogeneous tilt direction and large ferroelectric polarization and a structure with opposite tilt direction
in the two film halfs and large flexoelectric polarization.
Received 21 October 1998 相似文献
20.
M. Capone M. Grilli W. Stephan 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(4):551-557
We investigate polaron formation in a many-electron system in the presence of a local repulsion sufficiently strong to prevent
local-bipolaron formation. Specifically, we consider a Hubbard-Holstein model of interacting electrons coupled to dispersionless
phonons of frequency . Numerically solving the model in a small one-dimensional cluster, we find that in the nearly adiabatic case , the necessary and sufficient condition for the polaronic regime to occur is that the energy gain in the atomic (i.e., extremely localized) regime overcomes the energy of the purely electronic system . In the antiadiabatic case, , polaron formation is instead driven by the condition of a large ionic displacement (g being the electron-phonon coupling). Dynamical properties of the model in the weak and moderately strong coupling regimes
are also analyzed.
Received 15 February 1999 相似文献