Fattal DR, Ben-Shaul A
. Molecular Theory of Acyl Chain Packing in Lipid and Lipid-Protein Membranes
. In: Barenholz Y, Lasic DP
Nonmedical Applications of Liposomes:Vol. 1 Liposomes: Theory and Basic Science. Vol. 1. New York: CRC Press ; 1995. pp. 129-151. 1995.drfabs.pdf
SHARLOW MF, SELINGER RLB, Benshaul A, Gelbart WM
. CELL MODEL AND COMPUTER-SIMULATION STUDIES OF LAYERED AND HEXAGONAL STATES OF ALIGNED, HARD DISKS VERSUS RODS
. JOURNAL OF PHYSICAL CHEMISTRY. 1995;99 :2907-2914.Abstract
We consider the possibility of smectic and columnar phases in fluids and colloidal suspensions of aligned rods and disks interacting through excluded-volume forces. After briefly reviewing previous work, and applying known cell model techniques to compare the phase behaviors of disks and rods, we present and discuss the results from new Monte Carlo simulations of perfectly oriented rods (spherocylinders) and disks (torocylinders). We conclude tentatively that columnar phases are stable only in the case of disks.
WEAKLIEM CL, FUJII G, CHANG JE, Benshaul A, Gelbart WM
. EFFECT OF TENSION ON PORE FORMATION IN DRUG-CONTAINING VESICLES
. JOURNAL OF PHYSICAL CHEMISTRY. 1995;99 :7694-7697.Abstract
Pore formation in unilamellar lipid vesicles is believed to occur when the concentration of membrane-bound drug molecules exceeds a certain value. We treat this phenomenon in analogy with that of the micellization of surfactant in bulk aqueous solutions, thereby relating the threshold concentration of drug molecules to the free energy associated with transferring a molecule to a pore from its uniformly-dispersed state in the membrane. Incorporating the effect of lateral tension induced by osmotic pressure, we calculate the lowering of the pore-formation threshold with increasing tension. These predictions are tested by direct measurements on liposomal dispersions involving the antifungal drug amphotericin B.
Hamburger DA, Yinnon AT, Farbman I, Benshaul A, Gerber RB
. HE SCATTERING FROM COMPACT CLUSTERS AND FROM DIFFUSION-LIMITED AGGREGATES ON SURFACES - OBSERVABLE SIGNATURES OF STRUCTURE
. SURFACE SCIENCE. 1995;327 :165-191.Abstract
The angular intensity distribution of He beams scattered from compact clusters and from diffusion limited aggregates, epitaxially grown on metal surfaces, is investigated theoretically. The purpose is two-fold: to distinguish compact cluster structures from diffusion limited aggregates, and to find observable signatures that can characterize the compact clusters at the atomic level of detail. To simplify the collision dynamics, the study is carried out in the framework of the sudden approximation, which assumes that momentum changes perpendicular to the surface are targe compared with momentum transfer due to surface corrugation. The diffusion limited aggregates on which the scattering calculations were done, were generated by kinetic Monte Carlo simulations, It is demonstrated, by focusing on the example of compact Pt heptamers, that signatures of structure of compact clusters may indeed be extracted from the scattering distribution. These signatures enable both an experimental distinction between diffusion limited aggregates and compact clusters, and a determination of the cluster structure, The characteristics comprising the signatures are, to varying degrees, the rainbow, Fraunhofer, specular and constructive interference peaks, all seen in the intensity distribution, It is also shown, how the distribution of adsorbate heights above the metal surface can be obtained by an analysis of the specuIar peak attenuation. The results contribute to establishing He scattering as a powerful tool in the investigation of surface disorder and epitaxial growth on surfaces, alongside with STM.
Fattal DR, Benshaul A
. LIPID CHAIN PACKING AND LIPID-PROTEIN INTERACTION IN MEMBRANES
. PHYSICA A. 1995;220 :192-216.Abstract
This article describes briefly several applications of a molecular theory of lipid organization in membranes to systems of biophysical interest. After introducing the basic concepts of this mean field theory we outline three of its recent applications. i) Calculations of lipid chain conformational statistics in membrane bilayers, and comparison of the results (e.g. bond orientational order parameters) to experiment and molecular dynamics simulations. Good agreement is found. ii) A molecular model for lipid-protein interactions, which explicitly considers the effects of a rigid hydrophobic protein on the elastic (conformational) properties of the lipid bilayer. We also analyze the role of the `hydrophobic mismatch' between the protein and lipid bilayer thickness. iii) A molecular level calculation of the vesicle to micelle transition, attendant upon the addition of ('curvature loving') surfactant to a lipid bilayer vesicle. Future applications, e.g. to the calculation of the free energy barriers involved in membrane fusion are briefly mentioned.
Bohbot Y, Benshaul A, GRANEK R, Gelbart WM
. MONTE-CARLO AND MEAN-FIELD STUDIES OF PHASE EVOLUTION IN CONCENTRATED SURFACTANT SOLUTIONS
. JOURNAL OF CHEMICAL PHYSICS. 1995;103 :8764-8782.Abstract
A two-dimensional lattice model, originally introduced by Granek et al. [J. Chem. Phys. 101, 4331 (l994)], is used to demonstrate the intricate coupling between the intramicellar interactions that determine the optimal aggregation geometry of surfactant molecules in dilute solution, and the intermicellar interactions that govern the phase behavior at higher concentrations. Three very different scenarios of self-assembly and phase evolution are analyzed in detail, based on Monte Carlo studies and theoretical interpretations involving mean-field, Landau-Ginzburg, Bethe-Peierls, and virial expansion schemes. The basic particles in the model are `'unit micelles'' which, due to spontaneous self-assembly or because of excluded area interactions, can fuse td form larger aggregates; These aggregates are envisaged as hat micelles composed of a bilayerlike body surrounded: by a curved semitoroidal rim. The system's Hamiltonian involves one- through four-body potentials between the unit micelles, which account for their tendency to form aggregates of different shapes, e.g., elongated vs disklike micelles. Equivalently the configurational energy of the system is a sum of micellar self-energies involving the packing free energies of the constituent molecules in the bilayer body and in rim segments of different local curvature. The rim energy is a sum of a line tension term and a 1D curvature energy which depends on the rim spontaneous curvature and bending rigidity. Different combinations of these molecular parameters imply different optimal packing geometries and hence different self-assembly and phase behaviors. The emphasis in this paper Is on systems of `'curvature loving'' amphiphiles which, in our model, are characterized by negative line tension. The three systems studied are: (i) A dilute solution of stable disklike micelles which, upon increasing the concentration, undergoes a first-order phase transition to a continuous bilayer with isolated hole defects. An intermediate modulated `'checkerboard'' phase appears under certain conditions at low temperatures. (ii) A system of unit micelles which in dilute solution tend to associate into Linear micelles. These micelles are rodlike gt low temperatures, becoming increasingly more flexible as the temperature increases.-Upon increasing the concentration the micelles grow and undergo (in 2D) a continuous transition into nematic and `'stripe'' phases of long rods. At still higher concentrations the micellar stripes fuse into continuous sheets with line defects. (iii) A system in which, already in dilute solution, the micelles favor the formation of branched aggregates, analogous to the branched cylindrical micelles recently observed in certain surfactant solutions, As the concentration increases the micelles associate into networks (''gels'') composed of a mesh of linear micelles linked by `'T-like'' intermicellar junctions. The network may span the entire system or phase separate and coexist with a dilute micellar phase, depending on the details of the molecular packing parameters. (C) 1995 American Institute of Physics.
May S, Benshaul A
. SPONTANEOUS CURVATURE AND THERMODYNAMIC STABILITY OF MIXED AMPHIPHILIC LAYERS
. JOURNAL OF CHEMICAL PHYSICS. 1995;103 :3839-3848.Abstract
The elastic behavior of mixed bilayers composed of two amphiphilic components with different chain length (and identical head groups) is studied using two molecular level models. In both, the bilayer free energy is expressed as a sum of chain, head group and interfacial contributions as well as a mixing entropy term. The head group and interfacial terms are modeled using simple phenomenological but general expressions. The models differ in their treatment of the chain conformational free energy. In one it is calculated using a detailed mean-field molecular theory. The other is based on a simple `'compression'' model. Both models lead to similar conclusions. Expressing the bilayer free energy as a sum of its two monolayer contributions, a thermodynamic stability analysis is performed to examine the possibility of spontaneous vesicle formation. To this end, we expand the bilayer free energy as a power series (up to second order) in terms of the monolayer curvatures, their amphiphilic compositions and the average cross sectional areas per molecule; all variables are coupled, with the molecular composition and areas treated as degrees of freedom which are allowed to relax during bending. Using reasonable molecular interaction parameters we find that a second order transition from a planar to a curved (vesicle) geometry in a randomly mixed bilayer is unlikely. Most of our analysis is devoted to calculating the spontaneous curvature and the bending rigidity of the bilayer as a function of its amphiphile chain composition. We find that adding short chain amphiphiles to a layer of long chain molecules reduces considerably its bending rigidity, as already known from calculations involving only the chain contributions. However, we find that inclusion of head group and interfacial interactions moderates the effect of the added short chains. We also find that the bending rigidity Of pure monolayers is approximately linear in chain length, as compared to the nearly cubic dependence implied by the chain free energy alone (at constant head group area). Our main result involves the calculation of the spontaneous curvature as a function of composition. We find, for different chain mixtures, that upon adding short chains to long chain monolayers, the spontaneous curvature first increases nearly Linearly with composition and then (beyond mole fraction of about 0.5) begins to saturate towards the spontaneous curvature of a pure short chain layer. Qualitative arguments are provided to explain this behavior. (C) 1995 American Institute of physics.
Fattal DR, ANDELMAN D, Benshaul A
. THE VESICLE MICELLE TRANSITION IN MIXED LIPID SURFACTANT SYSTEMS - A MOLECULAR-MODEL
. LANGMUIR. 1995;11 :1154-1161.Abstract
A molecular model is used to calculate the free energy of mixed vesicles and cylindrical micelles, composed of lipid molecules and short chain surfactants. The free energy of both aggregates (modeled as an infinite planar bilayer and an infinite cylindrical aggregate) is represented as a sum of internal free energy and mixing entropy contributions. The internal free energy is treated as a sum of chain (conformational), head group, and surface tension terms. Calculating the free energy of each aggregation geometry as a function of lipid/surfactant composition and using common tangent construction we obtain the compositions of the bilayer and the micelle at the phase transition. By varying certain molecular parameters (such as the `'hard core'' area of the surfactant head group or the length of the surfactant tail) we study the role of molecular packing characteristics in determining the compositions at phase coexistence. We find, as expected, that upon increasing the preference of the surfactant for the micellar geometry (larger spontaneous curvature) the bilayer is solubilized at lower surfactant/lipid concentration ratios. For some typical values of the parameters used, reasonable agreement with experimental results for mixtures of egg phosphatidylcholine and octylglucoside is obtained.