Asymmetric distribution of anionic phospholipids in supported lipid bilayers

Lipid bilayers with a controlled content of anionic lipids are a prerequisite for the quantitative study of hydrophobic-electrostatic interactions of proteins with lipid bilayers. Here, the asymmetric distribution of zwitterionic and anionic lipids in supported lipid bilayers is studied by neutron reflectometry. We prepare POPC/POPS (3:1) unilamellar vesicles in a high-salt-concentration buffer. Initially, no fusion of the vesicles to a SiO(2) surface is observed over hours and days. Once the isotonic buffer is exchanged with hypotonic buffer, vesicle fusion and bilayer formation occur by osmotic shock. Neutron reflectivity on the bilayers formed this way reveals the presence of anionic lipids (d(31)-POPS) in the outer bilayer leaflet only, and no POPS is observed in the leaflet facing the SiO(2) substrate. We argue that this asymmetric distribution of POPS is induced by the electrostatic repulsion of the phosphatidylserines from the negatively charged hydroxy surface groups of the silicon block. Such bilayers with controlled and high contents of anionic lipids in the outer leaflet are versatile platforms for studying anionic lipid protein interactions that are key elements in signal transduction pathways in the cytoplasmic leaflet of eukaryotic cells.     

Droplet shape analysis and permeability studies in droplet lipid bilayers

We apply optical manipulation to prepare lipid bilayers between pairs of water droplets immersed in an oil matrix. These droplet pairs have a well-defined geometry allowing the use of droplet shape analysis to perform quantitative studies of the dynamics during bilayer formation and to determine time-dependent values for the droplet volumes, bilayer radius, bilayer contact angle, and droplet center-line approach velocity. During bilayer formation, the contact angle rises steadily to an equilibrium value determined by the bilayer adhesion energy. When there is a salt concentration imbalance between droplets, there is a measurable change in the droplet volume. We present an analytical expression for this volume change and use this expression to calculate the bilayer permeability to water.     

A molecular-dynamics study of lipid bilayers: effects of the hydrocarbon chain length on permeability

In this paper, we investigate the effects of the hydrocarbon chain length of lipid molecules on the permeation process of small molecules through lipid bilayers. We perform molecular-dynamics simulations using three kinds of lipid molecules with different chain length: dilauroylphosphatidylcholine, dimyristoylphosphatidylcholine, and dipalmiltoylphosphatidylcholine. Free-energy profiles of O2, CO, NO, and water molecules are calculated by means of the cavity insertion Widom method and the probability ratio method. We show that the lipid membrane with longer chains has a larger and wider energy barrier. The local diffusion coefficients of water across the bilayers are also calculated by the force autocorrelation function method and the velocity autocorrelation function method. The local diffusion coefficients in the bilayers are not altered significantly by the chain length. We estimate the permeability coefficients of water across the three membranes according to the solubility-diffusion model; we find that the water permeability decreases modestly with increasing chain length of the lipid molecules.     

High pressure effect on phase transition behavior of lipid bilayers

Phase behavior of lipid bilayers at high pressure is critical to biological processes. Using coarse grained molecular dynamic simulations, we report critical characteristics of dipalmitoylphosphatidylcholine bilayers with applied high pressure, and also show their phase transition by cooling bilayer patches. Our results indicate that the phase transition temperature of dipalmitoylphosphatidylcholine bilayers obviously shifts with pressure increasing in the rate of 37 °C kbar(-1), which are in agreement with experimental data. Moreover, the main phase transition is revealed to be strongly dependent on lipid area. A critical lipid area of ~0.57 nm(2) is found on the main phase transition boundary. Similar structures of acyl chains lead to the same sensitivity of phase transition temperature of different lipids to the pressure. Based on the lateral density and pressure profiles, we also discuss the different effects on bilayer structure induced by high temperature and high pressure, e.g., increasing temperature induces higher degree of interdigitation of lipid tails and thinner bilayers, and increasing pressure maintains the degree of interdigitation and bilayer thickness.     

The hydrophobic acyl-chain effect in the lipid domains appearance through phospholipid bilayers

An intermolecular interaction model for selective association processes of double-chain phospholipids in bilayer lipid membranes has been proposed, analysed and solved numerically. A large variety of binary mixtures of asymmetrical double-chain phospholipids with the cross-sectional areas of the polar headgroups a₁ = 40 Ų (the first component) and a₂ = 60 Ų (the second component) have been investigated. Changing the hydrophobic acyl-chain lengths of both mixture components, we found in all cases that the self-association probability (the association of like-pairs of phospholipids) of the first component in parallel alignment of the electric dipole moments of the polar headgroups is higher than the cross-association probability (the association of cross-pairs of phospholipids) and the self-association probability of the second component. This result is in good agreement with the experimental evidence that where the cross-sectional area of the polar headgroups matches the hydrocarbon chain-packing cross-sectional area (a 2Ξ 40 Ų), lipids possess a high tendency to aggregate into well packed bilayer structures with the acyl-chains oriented perpendicularly to the bilayer plane. Our theoretical data confirm that the double-chain phospholipids may associate themselves into anti-parallel alignment of the polar headgroups (P′₂₂) as well. The hydrophobic acyl-chain effect of phospholipids may modulate the distribution of lipid domains within bilayers that have a large variety of functional roles in cellular metabolism.     

Permeability of water and polar solutes in lipid bilayers

The three commonly used formalisms to describe water and solute permeation in lipid bilayers (namely, solubility-solute properties, activated rate processes and the thermodynamics of the irreversible process theory) are analyzed in the light of experimental results. These approaches are based on the consideration of the lipid bilayer as a composite membrane containing a hydrocarbon core, an H-bonded interfacial network and a fluctuating structure in which pores can appear. The particular structure of the lipid bilayer (i.e., a hydrophobic-hydrophilic leaflet) makes the permeation process of polar solutions more complicated than that occurring in inert polymeric membranes. Thus, the permeation theories of Fick, Henry and Kedem and Katchalsky should be adapted to introduce interfacial and elastic phenomena. A critical analysis of the experimental results available in the current literature opens the possibility to formulate a broader formalism for permeation in lipid membranes.     

The effect of interfacial microstructure on the lipid oxidation stability of oil-in-water emulsions

Novel rattle-type magnetic mesoporous carbon spheres are successfully prepared using composite spheres with Fe(3)O(4) as core and mesoporous SiO(2) as shell plus solid SiO(2) as a middle layer as templates. These rattle-type spheres possess the magnetization strength of as high as 37.5 emu/g, high and tunable specific surface areas (382-512.6 m(2)/g) due to mesoporous carbon shells. This magnetic rattle-type structure and the readily accessible mesoporous shell are very favoring for the fast adsorption and release of guest objects triggered by external stimulus, for example, the spheres showed very good adsorptive property to dye.     

Stretching effects on the permeability of water molecules across a lipid bilayer

Using a coarse grained molecular dynamics model of a solvent-surfactant system, we study the effects of stretching on the permeability of water across a lipid bilayer. The density profile, free energy profile, diffusion profile, and tail ordering parameter were computed for a set of stretched membranes maintained at constant area. We computed the water permeability across each membrane using the inhomogeneous solubility-diffusion model first proposed by Marrink and Berendsen [J. Phys. Chem. 98, 4155 (1994)]. We find that even though the resistance to permeation profile shows a great deal of qualitative change as the membranes are stretched, the overall permeability remains nearly constant within the relevant range of stretching. This is explained by the fact that the main barrier to permeation, located in the densest section of the tails, is insensitive to increased area per lipid, as a result of competing effects. Expansion leads to thinning and a higher density in the tail region, the latter leading to an increase in the free energy barrier. However, this is compensated by the reduction in the transverse distance to cross and a larger diffusion coefficient due to increased disordering in the chains.     

Flexoelectricity of lipid bilayers

Abstract

The direct flexoeffect in single lipid bilayers in the form of black lipid membranes has been investigated experimentally by the oscillating pressure technique in the regime of voltage measurement. Black lipid membranes of various composition have been studied in order to check the effect of lipid surface charge on the curvature-electric response and its frequency dependence; these include egg yolk lecithin (low negative charge); egg yolk lecithin plus phosphatidyl serine (high negative charge); egg yolk lecithin with surface adsorbed ions of uranyl acetate (high positive charge). An increase of the response has been found by increasing the surface charge and a reversal of the sign of the flexoelectric coefficient from positive to negative has been obtained by changing the sign of the surface charge from negative to positive. These results underline the leading role of the contribution of the surface charge to the flexoelectricity of lyotropics. Their theoretical interpretation provides further insight into the molecular mechanism of this phenomenon.     

Probing the hydration of lipid bilayers using a solvent isotope effect on phospholipid mixing

Nearest-neighbor recognition experiments, which have been carried out using exchangeable dimers derived from 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine,and 1-palmitoyl-2-oleoyl-sn-glycerophosphoethanolamine, indicate that replacement of H2O by D2O can significantly influence phospholipid mixing, but only in bilayers that are saturated and devoid of cholesterol. These findings, together with those of previous electron spin resonance spin-labeling studies,indicate that mammalian membranes, which are rich in cholesterol and unsaturated phospholipids, are ideal hydrophobic barriers.     

Solvent-free bilayers from squalene solutions of phospholipids

The approach proposed by White in 1978 for obtaining solvent-free bilayers from glycerylmonooleate is used for the formation of phospholipid membranes. High capacitance values of the bilayers from squalene solutions of azolectin and phosphatidylethanolamine (0.78 and 0.69 μF/cm², respectively) indicate that in this case, too, solvent-free bilayers are formed. A comparative investigation of the phospholipid membranes formed from squalene and decane solutions in terms of the capacitance, charge, tension, resistance and electromechanical stability is carried out. Possible fields of application of solvent-free squalene bilayers as an experimental model are discussed.     

Selective association of cholesterol with long-chain phospholipids in liquid-ordered bilayers: support for the existence of lipid rafts

Nearest-neighbor recognition experiments, which have been carried out under fluidizing and condensing conditions, using exchangeable dimers derived from 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine, and cholesterol, have provided strong evidence that sterol-phospholipid recognition is limited to the liquid-ordered phase.     

Stability and permeability of amphiphile bilayers

In this review the rupture and permeability of bilayers are considered on the basis of a mechanism of the formation of microscopic holes as fluctuations in the bilayers. The hole formation is treated as a nucleation process of a new phase in a two-dimensional system with short-range intermolecular forces. Free rupture and deliberate rupture (by α-particles) of foam bilayers (Newtonian black films) are discussed. A comparison is made between the rupture of foam and emulsion bilayers. Experimental methods for obtaining foam and emulsion bilayers from thin liquid films are considered. Methods for investigating the stability and permeability of foam bilayers, which are based on a microscopic model allowing the use of amphiphile solutions with very low concentrations, are described. Experimental dependences of the lifetime of bilayers, the probability of observing the foam bilayer in a foam film, the gas permeability of bilayers, etc. on the concentration of amphiphile molecules in the solution are reported. The influence of temperature and external impact (e.g. α-particle irradiation) have also been experimentally studied. A good agreement between theory and experiment is established, allowing determination of several characteristics of foam and emulsion bilayers obtained from ionics or non-ionics: the specific edge energy of bilayer holes, equilibrium surfactant concentration below which the bilayer is thermodynamically metastable, work for the formation of a nucleus hole, number of vacancies in the nucleus hole, coefficient of gas diffusion through the bilayer, etc. On the basis of the effect of temperature on the rupture of foam bilayers the binding energy of a surfactant molecule in the bilayer is determined. The adsorption isotherm of surfactant vacancies in the foam bilayer is obtained which shows a first-order phase transition. Some applications to scientific, technological and medical problems are considered. The foam bilayer is used as a model for investigating short-range forces in biological structures, the interaction between membranes and cell fusion. It is also shown that the foam bilayer is a suitable model for studying the alveolar surface and stability. On that basis a clinical diagnostic method is developed for assessment of the human foetal lung maturity.     

The interaction of double-charged metal ions with monolayers and bilayers of phospholipids

Electrophoretic mobilities of hexadecane/water emulsions containing dimyristoyl-phosphatidylcholine (DMPC) or egg yolk lecithin (EYL) monolayers at the interface and those of liposomes prepared from the same lipids were measured as functions of the concentrations of Ca²⁺, Mn²⁺, Cu²⁺, and Ni²⁺ cations in the aqueous phase. The surface potentials, surface charge densities (σ), and the Langmuir adsorption isotherms for various distances from the charged surface to the slip plane (d) were calculated on the basis of the Gouy-Chapman theory for 1∶2 electrolytes and the values of ζ-potentials. The binding constants (K) and parametersd were determined under the assumption that the maximum σ values correspond to one ion per phospholipid molecule at the interface. In the case of DMPC, the ion binding constants (L mol⁻¹) at 25°C are 230 and 87 for Ca²⁺, 31.5 and 21 for Mn²⁺, 11 and 6 for Cu²⁺, and 7.5 and 5.3 for Ni²⁺ in liposomes and emulsions, respectively. The affinities of Cu²⁺ and Ni²⁺ ions for EYL monolayers and bilayers are lower than those for DMPC mono- and bilayers. Thed parameters for all ions are smaller than the radii of the hydrated ions. In the case of Ca²⁺, Cu²⁺, and Ni²⁺, thed values for mono- and bilayers are different. The differences in K values between monolayers and bilayers as well as those between DMPC and EYL mono- and bilayers can be attributed to the differences in the local environment and orientation of the interfacial phosphate groups in these systems. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2490–2495, December, 1998.     

On the effect of the solid support on the interleaflet distribution of lipids in supported lipid bilayers

The adsorption and spreading of lipid vesicles on solid supports has become a popular way to create supported lipid bilayers (SLBs), but little attention has been paid to the possible redistribution of lipid material between the two leaflets of an SLB. We use the technique of quartz crystal microbalance with dissipation monitoring (QCM-D) to follow the adsorption of prothrombin on SLBs formed from sonicated unilamellar vesicles containing mixtures of dioleoylphosphatidylcholine (DOPC) and dioleoylphospatidylserine (DOPS). The specific interaction of prothrombin with negatively charged lipids is quantified and serves as a reporter of the content of accessible DOPS in SLBs. We compare results obtained on silica and mica and find that the underlying support can induce substantial redistribution of lipid material between the two leaflets. In particular, SLBs formed on mica showed a substantially depleted amount of accessible DOPS in the presence of calcium. The mechanisms that lead to the lipid redistribution process are discussed.     

Effect of lipid headgroup composition on the interaction between melittin and lipid bilayers

The effect of the lipid polar headgroup on melittin-phospholipid interaction was investigated by cryo-TEM, fluorescence spectroscopy, ellipsometry, circular dichroism, electrophoresis and photon correlation spectroscopy. In particular, focus was placed on the effect of the lipid polar headgroup on peptide adsorption to, and penetration into, the lipid bilayer, as well as on resulting colloidal stability effects for large unilamellar liposomes. The effect of phospholipid headgroup properties on melittin-bilayer interaction was addressed by comparing liposomes containing phosphatidylcholine, -acid, and -inositol at varying ionic strength. Increasing the bilayer negative charge leads to an increased liposome tolerance toward melittin which is due to an electrostatic arrest of melittin at the membrane interface. Balancing the electrostatic attraction between the melittin positive charges and the phospholipid negative charges through a hydration repulsion, caused by inositol, reduced this surface arrest and increased liposome susceptibility to the disruptive actions of melittin. Furthermore, melittin was demonstrated to induce liposome structural destabilization on a colloidal scale which coincided with leakage induction for both anionic and zwitterionic systems. The latter findings thus clearly show that coalescence, aggregation, and fragmentation contribute to melittin-induced liposome leakage, and that detailed molecular analyses of melittin pore formation are incomplete without considering also these colloidal aspects.