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1.
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
2.
R. Menes N. Grønbech-Jensen P.A. Pincus 《The European physical journal. E, Soft matter》2000,1(4):345-349
Using both theoretical modeling and computer simulations we study a model system for DNA interactions in the vicinity of charged
membranes. We focus on the polarization of the mobile charges in the membranes due to the nearby charged rods (DNA) and the
resulting screening of their fields and inter-rod interactions. We find, both within a Debye-Hückel model and in Brownian
dynamics simulations, that the confinement of the mobile charges to the surface leads to a qualitative reduction in their
ability to screen the charged rods to the degree that the fields and resulting interactions are not finite-ranged as in systems
including a bulk salt concentration, but rather decay algebraically and the screening effect is more like an effective increase
in the multipole moment of the charged rod.
Received 28 September 1999 相似文献
3.
The density of the elastic energy of a deformed membrane in a liquid state is calculated. The thermodynamic equilibrium of
its different parts is taken into account. The shape equation of a closed membrane is deduced. The quantity which keeps its
value, when the variations of the energy of the system are calculated, is not the area of the deformed membrane, but its area
in the flat tension free state. Because of this, additional terms appear in the second variation around the stable state.
The case of a lipid bilayer and its fluctuations is examined for both free and blocked exchange of molecules between the monolayers,
comprising the bilayer.
Received 4 February 2002 / Received in final form 15 April 2002 Published online 2 October 2002
RID="a"
ID="a"e-mail: bivas@issp.bas.bg 相似文献
4.
Perino-Gallice L Fragneto G Mennicke U Salditt T Rieutord F 《The European physical journal. E, Soft matter》2002,8(3):275-282
Thin multilamellar assemblies of neutral lipid bilayers deposited on silicon substrates are shown to be unstable upon hydration.
We analyze the stability of these systems taking into account a reduction of the fluctuation-related components of the bilayer
interaction potential. The sizes of the patterns observed are consistent with a spinodal dewetting process.
Received 27 November 2001 相似文献
5.
6.
We present a phase field model on buckling membranes to analyze phase separation and budding on soft membranes. By numerically
integrating dynamic equations, it turns out that the formation of caps is greatly influenced by the presence of a little excess
area due to the surface area constraint. When cap-shaped domains are created, domain coalescence is mainly observed not between
domains with same budding directions, but between domains with opposite budding directions, because the bending energy between
two domains is larger in the former case. Although we do not introduce spontaneous curvature like Helfrich model, we obtain
some suggestions related to the slow dynamics of the phase separation on vesicles. 相似文献
7.
R. Capovilla C. Chryssomalakos J. Guven 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(2):163-166
We examine the equilibria of a rigid loop in the plane, characterized by an energy functional quadratic in the curvature,
subject to the constraints of fixed length and fixed enclosed area. Whereas the only non self-intersecting equilibrium corresponding
to the fixed length constraint is the circle, the area constraint gives rise to distinct equilibria labeled by an integer.
These configurations exhibit self-intersections and bifurcations as the area is reduced. In addition, not only can the Euler-Lagrange
equation be integrated to provide a quadrature for the curvature but the embedding itself can be expressed as a local function
of the curvature. Perturbations connecting equilibria are shown to satisfy a first order ODE which is readily solved. Analytical
expressions for the energy as a function of the area are obtained in the limiting regimes.
Received 18 October 2001 / Received in final form 31 May 2002 Published online 2 October 2002
RID="a"
ID="a"e-mail: capo@fis.cinvestav.mx
RID="b"
ID="b"e-mail: chryss@nuclecu.unam.mx
RID="c"
ID="c"e-mail: jemal@nuclecu.unam.mx 相似文献
8.
Conditions for self-reproduction are sought for a growing vesicle with its growth defined by an exponential increase of vesicle
membrane area and by adequate flow of the solution across the membrane. In the first step of the presumed vesicle self-reproduction
process, the initially spherical vesicle must double its volume in the doubling time of the membrane area and, through the
appropriate shape transformations, attain the shape of two equal spheres connected by an infinitesimally thin neck. The second
step involves separation of the two spheres and relies on conditions that cause the neck to be broken. In this paper we consider
the first step of this self-reproduction process for a vesicle suspended in a solution whose solute can permeate the vesicle
membrane. It is shown that vesicle self-reproduction occurs only for certain combinations of the values of membrane hydraulic
and solute permeabilities and the external solute concentration, these quantities being related to the mechanical properties
of the membrane and the membrane area doubling time. The analysis includes also the relaxation of a perturbed system towards
stationary self-reproduction behavior and the case where the final shape consists of two connected spheres of different radii. 相似文献
9.
In this paper, we study the equilibrium properties of polymer chains end-tethered to a fluid membrane. The loss of conformational
entropy of the polymer results in an inhomogeneous pressure field that we calculate for Gaussian chains. We estimate the effects
of excluded volume through a relation between pressure and concentration. Under the polymer pressure, a soft surface will
deform. We calculate the deformation profile for a fluid membrane and show that close to the grafting point, this profile
assumes a cone-like shape, independently of the boundary conditions. Interactions between different polymers are also mediated
by the membrane deformation. This pair-additive potential is attractive for chains grafted on the same side of the membrane
and repulsive otherwise.
Received 20 April 2000 相似文献
10.
K. Kunz S.H. Anastasiadis M. Stamm T. Schurrat F. Rauch 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(3):411-419
The effect of the ratio of block lengths on the interfacial partitioning of poly(styrene-block-1,4 isoprene) diblock copolymers
from their mixtures with polystyrene homopolymer melt is investigated utilizing a series of copolymers with almost constant
molecular weight but different compositions. The concentration profile of the copolymer is measured directly using the nuclear reaction analysis technique; a segregation of the diblock is found at both the air/polymer surface, due to the lower
surface energy of polyisoprene, and at the substrate/polymer interface. No significant effect of the block length ratio on
the free-surface excess was observed. The block molecular weights have apparently led to dangling chain conformations in the
non-overlapping mushroom and in the overlapping mushroom regimes whereas the brush regime was not accessible; no indications
of a real border between the two former regimes was found.
Received: 20 July 1998 / Received in final form and Accepted: 11 September 1998 相似文献
11.
12.
T. Kerle J. Klein K. Binder 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(3):401-410
The capillary broadening of a 2-phase interface is investigated both experimentally and theoretically. When a binary mixture
in a thin film with thickness D segregates into two coexisting phases the interface between the two phases may form parallel to the substrate due to preferential
surface attraction of one of the components. We show that the interfacial profile (of intrinsic width w0) is broadened due to capillary waves, which lead to fluctuations, of correlation length of the local interface positions in the directions parallel to the confining walls. We postulate that acts as an upper cutoff for the spectrum of capillary waves on the interface, so that the effective mean square interfacial
width w varies as . In the limit of large D this yields or respectively for the case of short- or long-range forces between walls and the interface. We used the Nuclear Reaction Analysis
depth profiling technique, to investigate this broadening effect directly in two binary polymer mixtures. Our results reveal
that the interfacial width indeed increases with film thickness D, though the observed interfacial width is lower than the predicted w. This is probably due to surface tension effects imposed by the confining surfaces which are not taken into account in our
model.
Received: 19 February 1998 / Received in final form: 2 September 1998 / Accepted: 8 September 1998 相似文献
13.
B. Božič V. Heinrich S. Svetina B. Žekš 《The European physical journal. E, Soft matter》2001,6(1):91-98
Shapes of nearly cylindrical sections of axisymmetric phospholipid membranes are studied theoretically. Describing the shape
of such sections by their deviation from a reference cylinder, the well-established shape equation for axisymmetric bilayer
membranes is expanded in terms of this deviation, and it is then solved analytically. The phase diagram shows the resulting
stationary shapes as functions of system parameters and external conditions, i.e., the pressure difference across the membrane, the membrane tension, the difference between the tensions of the two monolayers,
and the axial force acting on the vesicle. The accuracy of the approximate analytical solution is demonstrated by comparison
with numerical results. The obtained analytical solution allows to extend the analysis to include shapes where numerical methods
have failed.
Received 27 September 2000 and Received in final form 26 March 2001 相似文献
14.
O. Dhez S. König D. Roux F. Nallet O. Diat 《The European physical journal. E, Soft matter》2000,3(4):377-388
A lyotropic system, consisting of a lecithin (DMPC) and a non-ionic surfactant (C12E5) in water was studied. The system exhibits a lamellar-to-nematic phase transition. The nematic phase appears as the temperature
is decreased and only exists in a very limited temperature and concentration range, for specific lipid-to-surfactant ratios.
While a lamellar phase is found at higher temperatures in both mixed and pure C12E5 systems, the transition to the nematic phase at lower temperatures coincides with a micellar phase in the pure C12E5 system. The transition appears to be driven by the strong temperature dependence of the surfactant film spontaneous curvature.
The structural properties of the lamellar phase close to the lamellar-to-nematic boundary have been studied by polarised light
microscopy and small-angle neutron and X-ray scattering experiments. The signature of a helical defect with Burgers vector
of magnitude 2 is apparent in our data, close to the lamellar-to-nematic phase transition. The proliferation of screw dislocations
in the lamellar phase might be a plausible mechanism for driving this transition.
Received 6 July 1999 and Received in final form 17 April 2000 相似文献
15.
16.
Wetting of structured or imprinted surfaces which leads to a variety of different morphologies such as droplets, channels
or thin films is studied theoretically using the general framework of surface or interface thermodynamics. The first variation
of the interfacial free energy leads to the well-known Laplace equation and a generalized Young equation which involves spatially
dependent interfacial tensions. Furthermore, we perform the second variation of the free energy for arbitrary surface patterns
and arbitrary shape of the wetting morphology in order to derive a new and general stability criterion. The latter criterion
is then applied to cylindrical segments or channels on homogeneous and structured surfaces.
Received 4 August 1999 相似文献
17.
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
18.
19.
Weikl TR Andelman D Komura S Lipowsky R 《The European physical journal. E, Soft matter》2002,8(1):59-66
Biomimetic membranes in contact with a planar substrate or a second membrane are studied theoretically. The membranes contain
specific adhesion molecules (stickers) which are attracted by the second surface. In the absence of stickers, the trans-interaction
between the membrane and the second surface is assumed to be repulsive at short separations. It is shown that the interplay
of specific attractive and generic repulsive interactions can lead to the formation of a potential barrier. This barrier induces
a line tension between bound and unbound membrane segments which results in lateral phase separation during adhesion. The
mechanism for adhesion-induced phase separation is rather general, as is demonstrated by considering two distinct cases involving:
i) stickers with a linear attractive potential, and ii) stickers with a short-ranged square-well potential. In both cases,
membrane fluctuations reduce the potential barrier and, therefore, decrease the tendency of phase separation.
Received 24 January 2002 and Received in final form 24 April 2002 相似文献
20.
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