Droplet shape and reorientation fields in nematic droplet/polymer films

Polymer films containing droplets of nematic liquid crystal form an important class of new electro-optic light valves and displays. While previous work has shown that the nematic droplet size is an important factor in the electro-optic properties of these films, here we report that the droplet shape is equally important in determing the electro-optics of the film. Electron micrographs show that for films using polyvinyl alcohol as the polymeric binder the cavities formed by the polymer matrix are oblate in nature, and aligned with the minor axis perpendicular to the film plane. In oblate cavities the elastic-deformation free energy is minimized when the director field in the droplet is aligned along a major axis of the spheroid; the electric field performs work on the nematic in reorienting the nematic into a higher-energy state, equal to the elastic-free-energy difference between the two configurations. Calculations and experiment are used to estimate the elastic and electric field free-energy-density changes that occur upon reorientation of the nematic droplet. The general agreement between these two values is used to indicate that droplet shape anisotropy is a major factor in determining the electrooptic properties of these films.     

Droplet shape and reorientation fields in nematic droplet/polymer films

Polymer films containing droplets of nematic liquid crystal form an important class of new electro-optic light valves and displays. While previous work has shown that the nematic droplet size is an important factor in the electro-optic properties of these films, here we report that the droplet shape is equally important in determing the electro-optics of the film. Electron micrographs show that for films using polyvinyl alcohol as the polymeric binder the cavities formed by the polymer matrix are oblate in nature, and aligned with the minor axis perpendicular to the film plane. In oblate cavities the elastic-deformation free energy is minimized when the director field in the droplet is aligned along a major axis of the spheroid; the electric field performs work on the nematic in reorienting the nematic into a higher-energy state, equal to the elastic-free-energy difference between the two configurations. Calculations and experiment are used to estimate the elastic and electric field free-energy-density changes that occur upon reorientation of the nematic droplet. The general agreement between these two values is used to indicate that droplet shape anisotropy is a major factor in determining the electrooptic properties of these films.     

Formation of artificial lipid bilayers using droplet dielectrophoresis

We describe the formation of artificial bilayer lipid membranes (BLMs) by the controlled, electrical manipulation of aqueous droplets immersed in a lipid-alkane solution. Droplet movement was generated using dielectrophoresis on planar microelectrodes covered in a thin insulator. Droplets, surrounded by lipid monolayers, were brought into contact and spontaneously formed a BLM. The method produced BLMs suitable for single-channel recording of membrane protein activity and the technique can be extended to create programmable BLM arrays and networks.     

Thermal fluctuations in shape, thickness, and molecular orientation in lipid bilayers

We present a unified continuum-level model for bilayer energetics that includes the effects of bending, compression, lipid orientation (tilting relative to the monolayer surface normal), and microscopic noise (protrusions). Expressions for thermal fluctuation amplitudes of several physical quantities are derived. These predictions are shown to be in good agreement with molecular simulations.     

The effect of lipid oxidation on the water permeability of phospholipids bilayers

The effect of lipid oxidation on water permeability of phosphatidylcholine membranes was investigated by means of both scattering stopped flow experiments and atomistic molecular dynamics simulations. Formation of water pores followed by a significant enhancement of water permeability was observed. The molecules of oxidized phospholipids facilitate pore formation and subsequently stabilize water in the membrane interior. A wide range of oxidation ratios, from 15 to 100 mol%, was considered. The degree of oxidation was found to strongly influence the time needed for the opening of a pore. In simulations, the oxidation ratio of 75 mol% was found to be a threshold for spontaneous pore formation in the tens of nanosecond timescale, whereas 15 mol% of oxidation led to significant water permeation in the timescale of seconds. Once a pore was formed, the water permeability was found to be virtually independent of the oxidation ratio.     

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.     

Droplet size effects on film drainage between droplet and substrate

When a droplet approaches a solid surface, the thin liquid film between the droplet and the surface drains until an instability forms and then ruptures. In this study, we utilize microfluidics to investigate the effects of film thickness on the time to film rupture for water droplets in a flowing continuous phase of silicone oil deposited on solid poly(dimethylsiloxane) (PDMS) surfaces. The water droplets ranged in size from millimeters to micrometers, resulting in estimated values of the film thickness at rupture ranging from 600 nm down to 6 nm. The Stefan-Reynolds equation is used to model film drainage beneath both millimeter- and micrometer-scale droplets. For millimeter-scale droplets, the experimental and analytical film rupture times agree well, whereas large differences are observed for micrometer-scale droplets. We speculate that the differences in the micrometer-scale data result from the increases in the local thin film viscosity due to confinement-induced molecular structure changes in the silicone oil. A modified Stefan-Reynolds equation is used to account for the increased thin film viscosity of the micrometer-scale droplet drainage case.     

Multinuclear NMR studies of single lipid bilayers supported in cylindrical aluminum oxide nanopores

Lipid bilayers were deposited inside the 0.2 microm pores of anodic aluminum oxide (AAO) filters by extrusion of multilamellar liposomes and their properties studied by 2H, 31P, and 1H solid-state NMR. Only the first bilayer adhered strongly to the inner surface of the pores. Additional layers were washed out easily by a flow of water as demonstrated by 1H magic angle spinning NMR experiments with addition of Pr3+ ions to shift accessible lipid headgroup resonances. A 13 mm diameter Anopore filter of 60 microm thickness oriented approximately 2.5 x 10(-7) mol of lipid as a single bilayer, corresponding to a total membrane area of about 500 cm2. The 2H NMR spectra of chain deuterated POPC are consistent with adsorption of wavy, tubular bilayers to the inner pore surface. By NMR diffusion experiments, we determined the average length of those lipid tubules to be approximately 0.4 microm. There is evidence for a thick water layer between lipid tubules and the pore surface. The ends of tubules are well sealed against the pore such that Pr3+ ions cannot penetrate into the water underneath the bilayers. We successfully trapped poly(ethylene glycol) (PEG) with a molecular weight of 8000 in this water layer. From the quantity of trapped PEG, we calculated an average water layer thickness of 3 nm. Lipid order parameters and motional properties are unperturbed by the solid support, in agreement with existence of a water layer. Such unperturbed, solid supported membranes are ideal for incorporation of membrane-spanning proteins with large intra- and extracellular domains. The experiments suggest the promise of such porous filters as membrane support in biosensors.     

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.     

Microfluidic patterning of nanodisc lipid bilayers and multiplexed analysis of protein interaction

We report a microfluidic method for precisely patterning lipid bilayers and a multiplexed assay to examine the interaction between the lipids and protein analytes. The lipids were packaged into nanoscale lipid bilayer particles known as Nanodiscs and delivered to surfaces using microfluidic channels. Two types of lipids were used in this study: biontinylated lipids and phosphoserine lipids. The deposition of biotinylated lipids on a glass surface was confirmed by attaching streptavidin coated quantum dots to the lipids, followed by fluorescent imaging. Using this multiplexed grid assay, we examined binding of annexin to phosphoserine lipids, and compared these results to similar analysis performed by surface plasmon resonance.     

Impedance analysis of ion transport through gramicidin channels in supported lipid bilayers

Selectivity between monovalent cations and its sequence of conductivity in lipid bilayers doped with the antibiotic Gramicidin D (GD) were examined using EIS. Experiments were performed using lipid bilayers obtained from a lipid mixture of phosphatidylcholine and dimethyldioctadecylammonium chloride (DODAC). Lipid bilayers were supported on gold surfaces modified with a mercapto-carboxylic acid. The bilayers were formed by chemisorption of this last species to form the first monolayer on gold and subsequent fusion of unilamellar vesicles to form an external bilayer attached by electrostatic interactions. A mathematical expression for the impedance of the membrane processes was derived. Some predictions of the presented model were checked after fitting the experimental results in various electrolyte compositions.     

Redox reactions in lipid bilayers and membrane bioenergetics

This paper begins with a consideration of electronic processes in ultrathin artificial bilayer lipid membranes (BLM) and biomembranes of chloroplasts and mitochondria. Methods and materials used in the study of BLMs are described next. The results of recent experiments, as obtained by voltammetric techniques, are summarized. The remaining sections of the paper deal with the application of basic electrochemistry to membrane research, in particular, the Eyring equation, the Butler-Volmer equation and the Tafel equation, and the origin of electronic processes in membranes in terms of electrostenolysis. The paper concludes with a discussion on the determination of standard potentials (U_o′) of redox protein components in the absence of the lipid bilayer, and finally a suggestion of the future experiments in relation to electron-transfer and bioredox reactions in bilayer lipid membranes.     

376 - Relaxation studies on cell membranes and lipid bilayers in the high electric field range

Charge pulse experiments were performed on cells of the giant alga Valonia utricularis and on lipid bilayer membranes. For low electric fields (in the order of 100 mV) the resulting membrane voltage U_m is a linear function of the injected charge. For high voltages U_m (in the order of one volt) the relation between injected charge Q and resulting voltage U_m is no longer seen for both systems and a state of high conductance is observed without mechanical damage of the membranes. Because of the high conductance the membranes cannot be charged to voltages higher than a critical one, which is denoted as breakdown voltage U_t.The breakdown voltage U_c was found to be a function of the charging time needed to obtain breakdown. In the experiments with cells of Valonia utricularis, where presumably the breakdown occurs in both tonoplast and plasmalemma membrane, the breakdown voltage varies at 18 °C between 2.4 V (charging time 800 ns) and 750 mV (charging time 500 μs). A similar pulse length dependence was also found for lipid bilayer membranes. Between 300 ns and 5 μs at 25 °C and between 100 ns and 2 μs at 40 °C, U_c showed a strong dependence on charging time of the membrane and decreased from 1.2 V to 0.5 V (25 °C) and from 1 to 0.4 V (40 °C). For other charging times below and above these ranges the breakdown voltage seemed to be constant. The results indicate that the breakdown phenomenon occurs in less than 10 ns. The pulse length dependence of the breakdown voltage found for both cell membrane and lipid bilayer is consistent with the electro-mechanical model proposed earlier. However, it seems possible that at short charging times (where the breakdown voltage reaches a plateau) other processes (involving the Born energy) become possible.     

Tension-induced morphological transition in mixed lipid bilayers

Recently, Rozovsky et al. reported on the morphology and dynamics of superstructures in three-component lipid bilayers containing saturated lipid, unsaturated lipid, and cholesterol (Rozovsky, S.; Kaizuka, Y.; Groves, J. T. J. Am. Chem. Soc. 2005, 127, 36). We suggest that the observed sequence of the striped-to-hexagonal morphological transition in mixed bilayers can be attributed to an enhanced membrane surface tension that is induced by the vesicle adhesion onto the solid surface.     

PEG-covered lipid surfaces: bilayers and monolayers

In this review paper we survey the ways in which various micropipet techniques have been used to study the mechanochemical and interactive features of lipid bilayer vesicles and monolayer-coated gas bubbles. Special emphasis will be made on characterizing the barrier properties of grafted PEG layers and how a hierarchical approach that uses a short barrier and extended ligand allows us to start to mimic nature's own solution to the problem of ubiquitous repulsion and specific attraction. The information gained from such studies not only characterizes the membrane and other lipid surfaces and their intersurface interactions from a fundamental materials science perspective, but also provides essential materials property data that are required for the successful design and deployment of lipid-based carriers and other capsules in applications involving this so-called ‘stealthy’ surface.     

Synthetic functional pi-stack architecture in lipid bilayers

Neglected until recently, pi-stack architecture is rapidly emerging as a powerful strategy to create function in lipid bilayer membranes. Recent reports describe supramolecular rosettes acting as hosts of intercalating guests, to assemble in bilayer membranes and, in the case of stacked guanosine and folate quartets, to form ion channels. The introduction of rigid-rod pi-stack architecture allowed us to address one of the great challenges in the field, i.e. ligand gating. Inspiring pi-stack chemistry from related fields, covering rainbow coloration, conductivity, as well as the critical dependence of charge mobilities on the precision of supramolecular organization is summarized to zoom in on arguably the most promising application of functional pi-stack architecture in lipid bilayers, that is the creation of multifunctional photosystems.     

Activating mechanosensitive channels embedded in droplet interface bilayers using membrane asymmetry

Droplet microcompartments linked by lipid bilayers show great promise in the construction of synthetic minimal tissues. Central to controlling the flow of information in these systems are membrane proteins, which can gate in response to specific stimuli in order to control the molecular flux between membrane separated compartments. This has been demonstrated with droplet interface bilayers (DIBs) using several different membrane proteins combined with electrical, mechanical, and/or chemical activators. Here we report the activation of the bacterial mechanosensitive channel of large conductance (MscL) in a dioleoylphosphatidylcholine:dioleoylphosphatidylglycerol DIB by controlling membrane asymmetry. We show using electrical measurements that the incorporation of lysophosphatidylcholine (LPC) into one of the bilayer leaflets triggers MscL gating in a concentration-dependent manner, with partial and full activation observed at 10 and 15 mol% LPC respectively. Our findings could inspire the design of new minimal tissues where flux pathways are dynamically defined by lipid composition.

Electrophysiology shows asymmetric lysophosphatidylcholine-containing DIBs trigger mechanosensitive channel gating, enabling user-designed, autonomous flux pathways in droplet networks.     

Ultra-high vacuum surface analysis study of rhodopsin incorporation into supported lipid bilayers

Planar supported lipid bilayers that are stable under ambient atmospheric and ultra-high-vacuum conditions were prepared by cross-linking polymerization of bis-sorbylphosphatidylcholine (bis-SorbPC). X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were employed to investigate bilayers that were cross-linked using either redox-initiated radical polymerization or ultraviolet photopolymerization. The redox method yields a more structurally intact bilayer; however, the UV method is more compatible with incorporation of transmembrane proteins. UV polymerization was therefore used to prepare cross-linked bilayers with incorporated bovine rhodopsin, a light-activated, G-protein-coupled receptor (GPCR). A previous study (Subramaniam, V.; Alves, I. D.; Salgado, G. F. J.; Lau, P. W.; Wysocki, R. J.; Salamon, Z.; Tollin, G.; Hruby, V. J.; Brown, M. F.; Saavedra, S. S. J. Am. Chem. Soc. 2005, 127, 5320-5321) showed that rhodopsin retains photoactivity after incorporation into UV-polymerized bis-SorbPC, but did not address how the protein is associated with the bilayer. In this study, we show that rhodopsin is retained in supported bilayers of poly(bis-SorbPC) under ultra-high-vacuum conditions, on the basis of the increase in the XPS nitrogen concentration and the presence of characteristic amino acid peaks in the ToF-SIMS data. Angle-resolved XPS data show that the protein is inserted into the bilayer, rather than adsorbed on the bilayer surface. This is the first study to demonstrate the use of ultra-high-vacuum techniques for structural studies of supported proteolipid bilayers.     

Chain packing modes in crystalline surfactant and lipid bilayers

The main features of a subcell catalogue for the packing of molecules with long alkyl chains were developed in the period of 1950–1975 based on single crystal studies. In the last number of years many deviations from these typical packing modes in a subcell lattice were found and analyzed, particularly for surfactant molecules. From early on, energy approximations were already presented, but these became more dominant with the progress in hardware and software developments. Recently, new inputs are coming from the calculation of lattice energies for chain packing modes in connection with new experimental results. In this contribution, the most common chain packing modes already suggested in published papers are presented. The different packing modes are analyzed using lattice energy calculations. The results are discussed using a presentation method that allows us to find out interrelations between various packing modes.     

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.