Electrostatics of curved fluid membranes: The interplay of direct interactions and fluctuations in charged lamellar phases (original) (raw)
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Experimental Evidence of the Electrostatic Contribution to the Bending Rigidity of Charged Membranes
Journal of Physical Chemistry B, 2007
We have investigated the thermal fluctuations of giant unilamellar dimyristoylphosphatidlycholine vesicles in the presence of both non-ionic and ionic surfactants (peptides) with identical apolar chains. Using vesicle fluctuation analysis, the effects of ionic and non-ionic surfactants upon membrane bending rigidity in the case of no added salt have been determined and the electrostatic contribution thereby isolated. We interpret these experimental findings in terms of a mean-field free-energy model for the adsorption of charged surfactants to a lipid bilayer and couple these results to describe the electrostatic contribution to membrane bending rigidity. This experimental study demonstrates how electrostatics affect the elastic properties of unilamellar bilayers. H 3 C H 3 C H 3 C CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OCH 3 CH 2 OCH 3 CHC CHC CHC CHC CHC CHC CHC
Experimental evidence of the electrostatic contribution to membrane bending rigidity
Europhysics Letters (EPL), 2004
We have investigated the thermal fluctuations of giant unilamellar dimyristoylphosphatidlycholine vesicles in the presence of both non-ionic and ionic surfactants (peptides) with identical apolar chains. Using vesicle fluctuation analysis, the effects of ionic and non-ionic surfactants upon membrane bending rigidity in the case of no added salt have been determined and the electrostatic contribution thereby isolated. We interpret these experimental findings in terms of a mean-field free-energy model for the adsorption of charged surfactants to a lipid bilayer and couple these results to describe the electrostatic contribution to membrane bending rigidity. This experimental study demonstrates how electrostatics affect the elastic properties of unilamellar bilayers. H 3 C H 3 C H 3 C CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OCH 3 CH 2 OCH 3 CHC CHC CHC CHC CHC CHC CHC
Lamellar versus isotropic structures in dilute phases of fluid membranes
Physica A: Statistical Mechanics and its Applications, 1991
phase (L3) is isotropic and shows no long range positional order. Scattering patterns and transport properties strongly suggest that its structure consists of a randomly multiconnected bilayer separating two equivalent subvolumes of solvent. We here discuss the relative stability of these two structures in connection with the elastic properties of the amphiphilic membrane. A general scaling law for the free energy of the L 3 phase as function of the degree of dilution is proposed and is checked against experimental measurements of some of its static and dynamic physical properties.
Fluctuations and defects in lamellar stacks of amphiphilic bilayers
Computer Physics Communications, 2005
We review recent molecular dynamics simulations of thermally activated undulations and defects in the lamellar Lα phase of a binary amphiphile-solvent mixture, using an idealized molecular coarse-grained model: Solvent particles are represented by beads, and amphiphiles by bead-and-spring tetramers. We find that our results are in excellent agreement with the predictions of simple mesoscopic theories: An effective interface model for the undulations, and a line tension model for the (pore) defects. We calculate the binding rigidity and the compressibility modulus of the lamellar stack as well as the line tension of the pore rim. Finally, we discuss implications for polymer-membrane systems.
Bending of ionic surfactant monolayers
Physical Review E, 1996
The electrostatic contribution to the bending moduli and spontaneous curvature of monolayers formed by ionic surfactants in solution is calculated for all salt concentrations, ranging from no added salt ͑counterions only͒ to excess salt. This is accomplished using a perturbative expansion in curvature of the free energy of the Poisson-Boltzmann cell model, which is shown to give precisely the same results for the moduli as would an alternative calculation employing moments of the transverse pressure profile of the electric double layer. With this treatment, it is possible to quantify the dependence of the moduli and spontaneous curvature on surfactant concentration alone ͑i.e., with fixed average salt concentration͒, a point of central importance to the validity of the flexible surface model in the description of ionic surfactant systems. A manifestation of the counterion condensation phenomenon is also observed, as the monolayer rigidity saturates and becomes independent of the surface area per ionic headgroup. ͓S1063-651X͑96͒03810-X͔
Ion induced lamellar-lamellar phase transition in charged surfactant systems
Chemical Physics, 2006
We propose a model for the liquid-liquid ͑L ␣ → L ␣ Ј ͒ phase transition observed in osmotic pressure measurements of certain charged lamellae-forming amphiphiles. The model free energy combines mean-field electrostatic and phenomenological nonelectrostatic interactions, while the number of dissociated counterions is treated as a variable degree of freedom that is determined self-consistently. The model, therefore, joins two well-known theories: the Poisson-Boltzmann theory for ionic solutions between charged lamellae and the Langmuir-Frumkin-Davies adsorption isotherm modified to account for charged adsorbing species. Minimizing the appropriate free energy for each interlamellar spacing, we find the ionic density profiles and the resulting osmotic pressure. While in the simple Poisson-Boltzmann theory the osmotic pressure isotherms are always smooth, we observe a discontinuous liquid-liquid phase transition when the Poisson-Boltzmann theory is self-consistently augmented by the Langmuir-Frumkin-Davies adsorption. This phase transition depends on the area per amphiphilic head group, as well as on nonelectrostatic interactions of the counterions with the lamellae and interactions between counterion-bound and counterion-dissociated surfactants. Coupling the lateral phase transition in the bilayer plane with electrostatic interactions in the bulk, our results offer a qualitative explanation for the existence of the L ␣ → L ␣ Ј phase transition of didodecyldimethylammonium bromide ͑DDABr͒, but the transition's apparent absence for the chloride and the iodide homologs. More quantitative comparisons with experiment require better understanding of the microscopic basis of the phenomenological model parameters.
Bending Energy of Amphiphilic Films at the Nanometer Scale
Physical Review Letters, 1997
The diffuse scattering of x rays by the thermally excited out-of-plane fluctuations of different amphiphilic films was measured for in-plane wavelengths down to the nanometer range, giving access to nontrivial bending effects. The Helfrich Hamiltonian applies on pure water and in the solid phase of an arachidic acid monolayer a large bending rigidity constant was measured. When formed on a subphase containing divalent cadmium ions, the height-height fluctuation spectrum ͗z͑q͒z͑2q͒͘ is greatly modified: no longer consistent with a q 24 law at large wavelengths but rather with a q 23.360.2 law, revealing a very different physical mechanism whose origin is discussed. [S0031-9007 02940-2] PACS numbers: 68.10.Et, 61.10.-i, 68.60.Bs
Phase behavior of a charged, fluid lipid bilayer
2013
Lateral organization of cell membranes is an important problem in Biophysics. A large number of experimental studies points to the existence of lateral domains, sometimes called membrane rafts, and their relation with many biological processes, including signal transduction, membrane trafficking, cell adhesion, membrane fusion etc. Important aspects to consider are: electrostatic interaction, since charged lipids are ever present in biomembranes, the compositional lipid asymmetry and the coupling between the two faces of the membrane. In the present work, we study the lateral phase separation in an asymmetrically charged lipid bilayer consisting of neutral and negatively charged lipids that are in contact with an ionic solution. The two asymmetric monolayers are coupled only trough electrostatic interactions. We have describe the bilayer on the mean-field level by splitting the free energy into non-electrostatic and electrostatic contributions, the former using the Bragg-Williams approximation (random mixing approximation) of a binary lattice gas and the latter using the non-linear Poisson-Boltzmann theory. With the goal of modeling bilayers where only one monolayer tends to phase separate we have assigned different values of non-ideality parameter χ to each one. We calculated the entire phase diagram containing binodal and spinodal lines as a function of the of non-ideality parameter χ and monolayer-monolayer electrostatic coupling. It is shown that the domain formation may be induced or inhibited in the apposed monolayer depending on the location within the phase diagram.
The Journal of Chemical Physics, 2003
We investigate thermal fluctuations in a smectic A phase of an amphiphile-solvent mixture with molecular dynamics simulations. We use an idealized model system, where solvent particles are represented by simple beads, and amphiphiles by bead-and-spring tetramers. At a solvent bead fraction of 20 % and sufficiently low temperature, the amphiphiles self-assemble into a highly oriented lamellar phase. Our study aims at comparing the structure of this phase with the predictions of the elastic theory of thermally fluctuating fluid membrane stacks [Lei et al., J. Phys. II 5,1155]. We suggest a method which permits to calculate the bending rigidity and compressibility modulus of the lamellar stack from the simulation data. The simulation results are in reasonable agreement with the theory.
Statistical mechanics of bilayer membranes in troubled aqueous media
Physica A: Statistical Mechanics and its Applications, 2010
We consider a bilayer membrane surrounded by small impurities, assumed to be attractive or repulsive. The purpose is a quantitative study of the effects of these impurities on the statistical properties of the supported membrane. Using the replica trick combined with a variational method, we compute the membrane mean-roughness and the height correlation function for almost-flat membranes, as functions of the primitive elastic constants of the membrane and some parameter that is proportional to the volume fraction of impurities and their interaction strength. As results, the attractive impurities increase the shape fluctuations due to the membrane undulations, while repulsive ones suppress these fluctuations. Second, we compute the equilibrium diameter of (spherical) vesicles surrounded by small random particles starting from the curvature equation. Third, the study is extended to a lamellar phase composed of two parallel fluid membranes, which are separated by a finite distance. This lamellar phase undergoes an unbinding transition. We demonstrate that the attractive impurities increase the unbinding critical temperature, while repulsive ones decrease this temperature. Finally, we say that the presence of small impurities in an aqueous medium may be a mechanism to suppress or to produce an unbinding transition, even the temperature and polarizability of the aqueous medium are fixed, in lamellar phases formed by parallel lipid bilayers.