B. Zeks

Role of lamellar membrane structure in tether formation from bilayer vesicles

A theoretical analysis is presented of the formation of membrane tethers from micropipette-aspirated phospholipid vesicles. In particular, it is taken into account that the phospholipid membrane is composed of two layers which are in contact but unconnected. The elastic energy of the bilayer is taken to be the sum of contributions from area expansivity, relative expansivity of the two monolayers, and bending. The vesicle is aspirated into a pipette and a constant point force is applied at the opposite side in the direction away from the pipette. The shape of the vesicle in approximated as a cylindrical projection into the pipette with a hemispherical cap, a spherical section, and a cylindrical tether with a hemispherical cap. The dimensions of the different regions of the vesicle are obtained by minimizing its elastic energy subject to the condition that the volume of the vesicle is fixed. The range of values for the parameters of the system is determined at which the existence of a tether is possible. Stability analysis is performed showing which of these configurations are stable. The importance of the relative expansion and compression of the constituent monolayers is established by recognizing that local bending energy by itself does not stabilize the vesicle geometry, and that in the limit as the relative expansivity modulus becomes infinitely large, a tether cannot be formed. Predictions are made for the functional relationships among experimentally observable quantities. In a companion report, the results of this analysis are applied to experimental measurements of tether formation, and used to calculate values for the membrane material coefficients.

Flexoelectricity and piezoelectricity: the reason for the rich variety of phases in antiferroelectric smectic liquid crystals.

The free energy of antiferroelectric smectic liquid crystals which takes into account polar order explicitly is presented. Steric, van der Waals, piezoelectric, and flexoelectric interactions to the nearest layers, and dipolar electrostatic interactions to the nearest and to the next-nearest layers, induce indirect tilt interactions with chiral and achiral properties, which extend to the third- and to the fourth-nearest layers. Although the strength of microscopic interactions changes monotonically with decreasing temperature, the effective interlayer interactions change nonmonotonically and give rise to a nonmonotonic change of the modulation period through various phases. Increased chirality changes the phase sequence.

Shape behavior of lipid vesicles as the basis of some cellular processes

The basic principles that govern the shape behavior of phospholipid vesicle shapes are discussed. The important membrane parameters of the system are defined by presenting the expressions for the relevant contributions to the systems mechanical energy. In the description of the rather unique shape behavior of lipid vesicles, the emphasis is on providing a qualitative understanding of the dependence of vesicle shape on the parameters of the system. The vesicle shape behavior is then related to biologically important phenomena. Some examples are given of how the results of the shape behavior of lipid vesicles can be applied to the analysis of cellular systems. Red blood cell shape and shape transformations, vesicle fission and fusion processes, and the phenomenon of cellular polarity are considered. It is reasoned that the current biological processes that involve changes of membrane conformation may have their origin in the general shape behavior of closed lamellar membranes. Anat Rec 268:215–225, 2002.

The free energy,enthalpy and entropy of hydration of phospholipid bilayer membranes and their difference on the interfacial separation

Abstract The phenorncnological theory of water binding to non-charged phospholipid bilayer membranes is presented and the free energy, enthalpy, and entropy of hydration are calculated as a function ofinterfacial separation. Tlicsc estimated hydration quantities agree with our experimental data provided the interfacial effects arc described in terms of the sur- face water -orienting field of constant strength.

Positional, Reorientational, and Bond Orientational Order in DNA Mesophases

We investigate the orientational order of transverse polarization vectors of long, stiff polymer molecules and their coupling to bond orientational and positional order in high density mesophases. Homogeneous ordering of transverse polarization vector promotes distortions in the hexatic phase, whereas inhomogeneous ordering precipitates crystallization of the 2D sections with different orientations of the transverse polarization vector on each molecule in the unit cell. We propose possible scenarios for going from the hexatic phase, through the distorted hexatic phase, to the crystalline phase with an orthorhombic unit cell observed experimentally for the case of DNA.

Depletion of membrane skeleton in red blood cell vesicles

A possible physical interpretation of the partial detachment of the membrane skeleton in the budding region of the cell membrane and consequent depletion of the membrane skeleton in red blood cell vesicles is given. The red blood cell membrane is considered to consist of the bilayer part and the membrane skeleton. The skeleton is, under normal conditions, bound to the bilayer over its whole area. It is shown that, when in such conditions it is in the expanded state, some cell shape changes can induce its partial detachment. The partial detachment of the skeleton from the bilayer is energetically favorable if the consequent decrease of the skeleton expansion energy is larger than the corresponding increase of the bilayer-skeleton binding energy. The effect of shape on the skeleton detachment is analyzed theoretically for a series of the pear class shapes, having decreasing neck diameter and ending with a parent-daughter pair of spheres. The partial detachment of the skeleton is promoted by narrowing of the cell neck, by increasing the lateral tension in the skeleton and its area expansivity modulus, and by diminishing the attraction forces between the skeleton and the bilayer. If the radius of the daughter vesicle is sufficiently small relative to the radius of the parent cell, the daughter vesicle can exist either completely underlaid with the skeleton or completely depleted of the skeleton.

TfC23. Phase transitions in ferroelectric films

Abstract The system considered is a ferroelectric film between metal electrodes with polarization normal to the interfaces. The critical temperature Tc is depressed by the depolarization field, the film thickness lc for Tc to be reduced to zero being or order 103 times the Thomas-Fermi screening length of the electrodes. For a film with no electrodes the ferroelectric phase is completely suppressed. The variation of Tc with l > lc follows a simple hyperbolic law which should be open to experimental test.

Parametrization invariance and shape equations of elastic axisymmetric vesicles

The issue of different parameterizations of the axisymmetric vesicle shape addressed by Hu Jian-Guo and Ou-Yang Zhong-Can [ Phys.Rev. E 47 (1993) 461 ] is reassesed, especially as it transpires through the corresponding Euler - Lagrange equations of the associated elastic energy functional. It is argued that for regular, smooth contours of vesicles with spherical topology, different parameterizations of the surface are equivalent and that the corresponding Euler - Lagrange equations are in essence the same. If, however, one allows for discontinuous (higher) derivatives of the contour line at the pole, the differently parameterized Euler - Lagrange equations cease to be equivalent and describe different physical problems. It nevertheless appears to be true that the elastic energy corresponding to smooth contours remains a global minimum.

Theory of Ferroelectric Liquid Crystals

In the ferroelectric helicoidal smectic C* phase the chiral molecules are tilted with their long axes from the normal to smectic layers. The direction of the tilt precesses as one goes from one smectic layer to another resulting in a helicoidal structure. The order parameter of the phase transition from the smectic A to the smectic C* phase is the two component tilt vector, which breaks the axial symmetry around the long molecular axis and induces the ordering of transverse molecular dipoles. The extended thermodynamic model is reviewed, which includes the bilinear as well as the biquadratic coupling between the tilt and the transverse polarization and describes in a unified way the thermodynamic properties of these systems. A microscopic model is introduced, which is consistent with the extended thermodynamic model, and the effect of the shape of molecules on the macroscopic properties of ferroelectric liquid crystals is estimated.

Phenomenological model for antiferroelectric liquid crystals: phase sequences

The discrete phenomenological model of antiferroelectric liquid crystals is reviewed that takes into account interlayer interactions up to next nearest neighboring layers. Possible predicted tilted phases are presented and compared with experimentally observed phases: the ferroelectric SmC* phase, the antiferroelectric SmCA* phase, the ferrielectric SmC(gamma )* phase and the short pitch SmC(alpha )* phase. It is also shown that various observed sequences of phases in dependence of temperature can be obtained by properly choosing the model parameters.