Constituent Monolayers Research Articles

Flexibility and roughness of mixed and partially polymerized bilayers in terms of the hat model and local bending frustration

The flexibility of mixed fluid membranes is examined in terms of the so-called hat model that is based on local curvature fluctuations. For monolayers, the hat model reproduces the increase of the flexibility obtained previously with a continuum model. In bilayers, the bending frustration of the constituent monolayers causes equal molecules to avoid and unequal molecules to seek each other when they are in opposite monolayers. This local interaction across the middle surface of the bilayer leads to an additional increase of bilayer flexibility. The cooperative effect of local bending frustration in partially polymerized bilayers may result in the segregation ofpolymer chains belonging to opposite monolayers. Possible consequences of the segregation are a nearly divergent flexibility, a pronounced roughness of the bilayer, and budding of bilayer vesicles

Characteristics of Monolayers of Ferrocene Derivatives and Estimation of Electrical Double Layers Formed in Langmuir Films

Abstract In order to estimate the potential difference of the electrical double layer formed in heterogeneous Sensitizer (S)/Electron Donor (D) Langmuir–Blodgett (LB) films, the surface potentials of the constituent monolayers were measured simultaneously with the π–A isotherms. The S monolayer was a mixed [Ru(bpy)3]2+ complex monolayer with a long alkyl fatty acid, while the D monolayer was one of three kinds of ferrocene monolayers with different signs of the hydrophilic head groups and with the same redox potential. The formation of the potential difference between the charged head group of the amphiphile and its counterion at the water-air interface was confirmed; those values were consistent with the electrical double-layer effect on the luminescent lifetime of [Ru(bpy)3]2+ in the S monolayer in contact with the D monolayer.

Local and nonlocal curvature elasticity in bilayer membranes by tether formation from lecithin vesicles

Bilayer membranes exhibit an elastic resistance to changes in curvature. This resistance depends both on the intrinsic stiffness of the constituent monolayers and on the curvature-induced expansion or compression of the monolayers relative to each other. The monolayers are constrained by hydrophobic forces to remain in contact, but they are capable of independent lateral redistribution to minimize the relative expansion or compression of each leaflet. Therefore, the magnitude of the expansion and compression of the monolayers relative to each other depends on the integral of the curvature over the entire membrane capsule. The coefficient characterizing the membrane stiffness resulting from relative expansion is the nonlocal bending modulus kr. Both the intrinsic (local) bending modulus (kc) and the nonlocal bending modulus (kr) can be measured by the formation of thin cylindrical membrane strands (tethers) from giant phospholipid vesicles. Previously, we reported measurements of kc based on measurements of tether radius as a function of force (Song and Waugh, 1991, J. Biomech. Engr. 112:233). Further analysis has revealed that the contribution from the nonlocal bending stiffness can be detected by measuring the change in the aspiration pressure required to establish equilibrium with increasing tether length. Using this approach, we obtain a mean value for the nonlocal bending modulus kr of approximately 4.1 x 10(-19)J. The range of values is broad (1.1-10.1 x 10(-19)J) and could reflect contributions other than simple mechanical equilibrium. Inclusion of the nonlocal bending stiffness in the calculation of kc results in a value for that modulus of approximately 1.20 +/- 0.17 x 10(-19)J, in close agreement with values obtained by other methods.

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.

Amphiphilic membranes in thin films of a complex fluid: Statics and dynamics in lattice models

We investigate the equilibrium structure as well as a specific dynamical aspect of thin films of complex fluids containing amphiphilic membranes. We explore the structures of ternary microemulsions, confined in a pore, resulting from various different types of interactions of the confining wall with the molecules. We compute the concentration profiles of the three components, namely, oil, water and amphiphiles, by carrying out extensive Monte Carlo simulations of the Widom model and its recently proposed generalizations on a simple-cubic lattice of size Lx×Ly×Lz under appropriate boundary conditions. We investigate both the wide-gap geometry (Lx=Ly≂Lz≫1) and narrow-gap geometry (Lx=Ly≫Lz). We observe novel concentration profiles in the narrow-gap geometry when Lz is of the order of a few molecular layers (i.e., in micropores). We also compute the lifetime of thermodynamically unstable bilayers of a specific initial conformation in the Widom model of microemulsions. For identical initial conformations of these model bilayers, we study the dependence of the lifetime on (a) the initial concentration of the amphiphilic molecules, (b) the temperature, (c) the curvature elasticity of the amphiphilic membrane, and (d) the thickness of water layers inside the two constituent monolayers. We also compare the models and results in our computer experiment with those in the laboratory experiment on the rupture of the Newton black films and common black films.

Transport through liquid membranes generated by lecithin, cholesterol and lecithin-cholesterol mixtures in the presence of prostaglandins

Transport through liquid membranes generated by lecithin, cholesterol and lecithin-cholesterol mixtures has been studied in the presence of prostaglandins. The data indicate that prostaglandins in association with cholesterol may be responsible for the aqueous pores present in the lipid bilayers controlling passive transport through biomembranes. The data further indicate that the presence of cholesterol in each of the two constituent monolayers of the lipid bilayer is essential for pore formation by prostaglandins.

Determination of reflection coefficients of liposomes for some non-electrolytes by osmotic pressure measurement

The reflection coefficients of bilayer lipid vesicles (liposomes) of various compositions have been determined for a number of non-electrolytes. The solutes were the same and the method of measurement was essentially the same as those which have been used to estimate an equivalent pore radius for erythrocytes. The method involves matching the osmotic pressure of solutions of a permeant test solute with that of a known inpermeant solute. Reflection coefficients for cholesterol-containing liposomes and those of erythrocytes are, when account is taken of those solutes known to permeate the erythrocyte by specialized pathways, not greatly different. Lipid bilayers can thus account for most of the permeability characteristics of the cell originally interpreted as due to aqueous pores. Reflection coefficients are significantly higher for egg phosphatidylcholine membranes that contain cholesterol than those which do not. There is a strong correlation between relative permeabilities derived from reflection coefficients and oil-water partition coefficients. There is also good agreement betwen these permeabilities and permeabilities measured by others, which exhibit an inverse dependence on molecular size. It is suggested that this tendency of membranes to pass small molecules more readily than large molecules, other properties being equal, is a consequence of the surface pressure of the constituent monolayers of the membrane.

Curvature Elasticity Moduli of Bilayer Lipid Membranes

Abstract It has been shown how the sum of curvature elasticity moduli 2kc+[kbar]c of a bilayer lipid membrane (the symbols are the same as these used by Helfrich1) can be obtained, if the distribution of the lateral pressure along the thickness of the membrane is known, and the volume of the hydrophobic part of the membrane is kept constant. Cases of free and blocked flip-flop have been considered. A relation between the elasticity moduli of a bilayer and its constituent monolayers has been drawn for an arbitrary bilayer in which the chains of both monolayers do not interpenetrate each other. On the basis of a model previously worked out by us2, numerical results of (2kc+[kbar]c) have been obtained. A comparison has been carried out with existing experimental results.

Membrane bending energy in relation to bilayer couples concept of red blood cell shape transformations

A simple four parameter geometrical model is introduced to approximately simulate the axisymmetrically shaped red blood cells. Possible shapes are calculated according to the requirement that the cell volume and areas of the two constituent monolayers of the membrane are constant during cell shape transformations. The intervals for the geometrical parameters are determined within which the cell can have a series of rotationally symmetrical shapes. The cell shape at given values of the above three cell properties is obtained by finding the minimum value of the membrane bending energy. The presented picture of red blood cell shape transformations is shown to be in agreement with the concept of bending energy as well as with the bilayer couples concept.

Structure-Dependent Feature of Electron Transport in Langmuir Multilayer Assemblies

Abstract The behavior of dc and ac conductivities is studied in the Langmuir multilayer films sandwiched between aluminium electrodes. The structure-dependence of conductivities has been systematically examined on different film structures constructed by the Molecular Assembly Technique. The constituent monolayers used were Cd salts of fatty acids CH3-(CH2)n-2-COOH with n = 16, 18, 20 and a mixture (C16)x-(C20)1–x. The structure dependent aspects are summarized in the following items, which are reasonably interpreted by the model based on the hopping or thermally assisted tunneling of electrons via localized states assumed at each monolayer interface: 1) exponential decrease of dc conductivity with increasing chain length of the fatty acid radical: 2) the same exponential variation of dc conductivity for mixed monolayers; 3) reduction of dc conductivity caused by one single C20 monolayer introduced into C16 multilayer system; 4) frequency-dispersion of conductivity in heterogeneous multilayer assemblies i...

Quantum mechanical hopping in one-dimensional superstructure

The photo-induced increment of a.c. conductance, Δ G, has been studied in the dye-sensitized Langmuir-films with different structures and compositions. Each Δ G-ω characteristic exhibits humps at frequencies which decrease exponentially with increasing thicknesses of constituent monolayers. The humps are assumed to correspond to the characteristic relaxation times of quantum mechanical hopping governed by the one-dimensional superstructure associated with the film.