Planar Bilayer Structures Research Articles

Dynamics insights into aggregation of phospholipid species with cholesterol and vitamin C

This paper provides dynamic insight into the aggregation profile of systems containing six different phospholipid species, cholesterol, and vitamin C thru Coarse-Grain Molecular Dynamics (CGMD) simulations. The simulation used 42 systems, and each system was composed of 220 molecules of each phospholipid species, a varied number of cholesterol molecules (0, 11, 22, 33, 66, 88), and 10 vitamin C molecules. The phospholipid species were DLPE, DOPE, DLiPE, DOPS, DLiPS, and DLiPC. We found curved bilayer, toroidal bilayer, concave micelle, disc-like bilayer, planar bilayer, and liposome structures in the systems during the 40 ns simulation. The systems with a ratio cholesterol:phospholipid between 15% and 40% formed liposomes regardless of the phospholipid species. Cholesterol is positioned in the liposome bilayer while vitamin C is encapsulated in the aqueous core of liposomes for all cholesterol compositions. The cholesterol influences the liposome formation of various phospholipid species and the encapsulation of vitamin C in the liposome structure.

Effects of ethanol and n-butanol on the fluidity of supported lipid bilayers

The interactions of molecules such as short-chain alcohols with the mammalian plasma membrane are thought to play a role in anesthetic effects. We have examined the concentration-dependent effects of ethanol and n-butanol on the fluidity of planar model lipid bilayer structures supported on mica. The supported model bilayer was composed of 1,2-dioleoyl-sn-phosphatidylcholine (DOPC), cholesterol, and sphingomyelin, and the bilayers were formed by vesicle fusion from extruded unilamellar vesicles (133 nm diameter, polydispersity index of 0.17). Controlled amounts of ethanol and n-butanol were added during vesicle deposition. Translational diffusion constants were obtained utilizing fluorescence recovery after photobleaching (FRAP) measurements on the micrometer scale with perylene as the fluorophore. The translational diffusion constants increased and then decreased with increasing ethanol concentration, with the bilayer structure degrading at ca. 0.8 M ethanol. A similar trend was observed for n-butanol at lower alcohol concentrations owing to greater interactions with phospholipid bilayer constituents. For n-butanol, the integrity of the planar bilayer structure deteriorated at ca. 0.4 M n-butanol. The results are consistent with bilayer interdigitation.

Soft, adhesive (+) alpha tocopherol phosphate planar bilayers that control oral biofilm growth through a substantive antimicrobial effect

‘Soft’ nanomaterials have the potential to produce substantive antibiofilm effects. The aim of this study was to understand the oral antimicrobial activity of soft nanomaterials generated from alpha-tocopherol (α-T) and alpha-tocopherol phosphate (α-TP). (+) α-TP formed planar bilayer islands (175 ± 21 nm, −14.9 ± 3.5 mV) in a Trizma® buffer, whereas (+) α-T formed spherical liposomes (563 ± 1 nm, −10.5 ± 0.2 mV). The (+) α-TP bilayers displayed superior Streptococcus oralis biofilm growth retardation, a more substantive action, generated a superior adsorption to hydroxyapatite and showed an enhanced inhibition of multi-species bacterial saliva biofilm growth (38 ± 7μm vs 58 ± 18 μm, P ˂ 0.05) compared to (+) α-T. Atomic force microscopy data indicated that the ability of the ‘soft’ α-TP nanomaterials to transition into planar bilayer structures upon contact with interfaces facilitated their adhesive properties and substantive antimicrobial effects.

Formation of supported lipid bilayers on porous polymeric substrates induced by hydrophobic interaction

This study presents a technique for forming a supported lipid bilayer (SLB) on flexible and porous polymeric substrates via a liposome rupture method induced by hydrophobic interaction. Porous polymeric substrates improve the mechanical strength and flexibility of SLBs and allow the molecular permeation through SLBs. Three types of commercial polymeric microfiltration membranes, regenerated cellulose (RC), nitrocellulose (NC) and polyvinylidene fluoride (PVDF), were used as porous substrates. First, the surfaces of the porous substrates were modified with alkyl chains using octadecyltrichlorosilane or palmitoyl chloride. The degree of the alkyl chains modified on the substrates was evaluated by x-ray photoelectron spectroscopy (XPS) measurement. The modified substrates were then immersed in a liposome suspension. The SLB formation was confirmed by evaluating the lateral diffusivity of lipid molecules on the substrate surfaces using a fluorescence recovery after photobleaching (FRAP) assay. Both the method used to modify the substrates with alkyl chains and the type of substrate material affected the SLB formation. When the degree of the alkyl chains modified on the substrate was high, the lateral diffusivity of lipids was also high and planar lipid bilayer structure was formed on the substrates. Furthermore, to show the applicability for platforms to immobilize various lipophilic biomolecules, streptolysin O (SLO) was incorporated into the formed SLB and was immunostained with fluorescently labeled antibodies.

Non-lamellar lipid assembly at interfaces: controlling layer structure by responsive nanogel particles.

Biological membranes do not only occur as planar bilayer structures, but depending on the lipid composition, can also curve into intriguing three-dimensional structures. In order to fully understand the biological implications as well as to reveal the full potential for applications, e.g. for drug delivery and other biomedical devices, of such structures, well-defined model systems are required. Here, we discuss the formation of lipid non-lamellar liquid crystalline (LC) surface layers spin-coated from the constituting lipids followed by hydration of the lipid layer. We demonstrate that hybrid lipid polymer films can be formed with different properties compared with the neat lipid LC layers. The nanostructure and morphologies of the lipid films formed reflect those in the bulk. Most notably, mixed lipid layers, which are composed of glycerol monooleate and diglycerol monooleate with poly(N-isopropylacrylamide) nanogels, can form films of reverse cubic phases that are capable of responding to temperature stimulus. Owing to the presence of the nanogel particles, changing the temperature not only regulates the hydration of the cubic phase lipid films, but also the lateral organization of the lipid domains within the lipid self-assembled film. This opens up the possibility for new nanostructured materials based on lipid-polymer responsive layers.

On the Formation of Lipid Nano-Scale Structures at Interfaces Beyond Planar Bilayers

Biological membranes do not only occur as planar bilayer structures, but bilayers have also been shown to, depending on the lipid composition, curve into intriguing 3D structures. Understanding the biological implication as well as the application of such interfaces, for e.g. drug delivery and other biomedical application, requires the development of well-defined model system.Here we demonstrate the formation of fluid supported bilayers on vertical gallium phosphide nanowire (NW) forests using self-assembly from lipid vesicular dispersions (1-2). The phospholipid mixture used had a composition that facilitates the formation of curved bilayers. By applying fluorescence recovery after photobleaching, we determined that the lipids are able to diffuse laterally within the NW-supported bilayer and that the bilayers were found to follow the contours of the nanowires to form continuous and locally highly curved model membranes. Membrane-anchoring proteins as well as tethered vesicles were found to be able to bind to these bilayers on the nanowire substrate.(1) Dabkowska, A.P., Niman, C.S., Piret, G., Persson, H., Wacklin, H.P., Linke, H., Prinz, C.N. & Nylander, T. Fluid and highly curved model membranes on vertical nanowire arrays. Nano Letters, 2014, 14, 4286-4292.(2) Dabkowska, A. P., Piret, G., Niman, C. S., Lard, M., Linke, H., Nylander, T., Prinz, C., N.: Surface nanostructures for fluorescence probing of supported lipid bilayers on reflective substrates. 2015, Accepted in Nanoscale

Immiscible solvents enabled nanostructure formation for efficient polymer photovoltaic cells

Organic photovoltaics (OPVs) fabricated via solution processing are an attractive way to realize low cost solar energy harvesting. Bulk heterojunction (BHJ) devices are the most successful design, but their morphology is less controllable. In this manuscript, we describe a simple approach to realize ‘ordered’ BHJ morphology using two immiscible solvents with different boiling point and a quasi-bilayer approach. Tunable fine structures were demonstrated in poly(3-hexylthiophene) (P3HT) and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) model systems, and the devices with optimized fine structure showed a 33% efficiency enhancement compared to those with a planar bilayer structure.

Dense network of O[sbnd]H⋯O and C[sbnd]H⋯O interactions in the solid state structure of n-pentyl-2-chloro-2-deoxy-α-d-manno-sept 3-uloside

Single crystal X-ray structural analysis of a septanoside, namely, n-pentyl-2-chloro-2-deoxy sept-3-uloside (1) provides many finer details of the molecular structure, in addition to its preferred twist-chair conformation, namely, 5,6TC3,4 conformation. Structural analysis reveals a dense network of OH⋯O, CH⋯O and van der Waals interactions that stabilize interdigitized, planar bi-layer structure of the crystal lattice.

Ethanol-Induced Perturbations to Planar Lipid Bilayer Structures

We report on the formation of planar lipid bilayer structures on mica where the bilayer contains the phosphocholine 1,2-dioleoyl-sn-phosphatidylcholine (DOPC), cholesterol, sphingomyelin and sulforhodamine-tagged-1,2-dioleoyl-sn-phosphatidylethanolamine (SR-DOPE). Phase separation is seen for the cholesterol domains within the bilayer structure, and exposure of this supported bilayer to controlled concentrations of ethanol reveals organizational changes on both the micrometer- and molecular-length scales. We report steady state fluorescence imaging, fluorescence lifetime imaging, and fluorescence anisotropy decay imaging for these bilayers. These data are complementary to existing information on the interactions of lipid bilayers with ethanol and point to subtle but important changes in the molecular-scale organization of these structures.

Label-Free Characterization of Peptide–Lipid Interactions Using Immobilized Lipodisks

Lipodisks, planar lipid bilayer structures stabilized by PEG-ylated lipids, were in the present study covalently bound and immobilized onto sensors for quartz crystal microbalance with dissipation monitoring (QCM-D) studies. It is shown that the modified sensors can be used to characterize the interaction of lipodisks with α-helical amphiphilic peptides with an accuracy similar to that obtained with well established fluorimetric approximations. The method presented has the great advantage that it can be used with peptides in their native form even if no fluorescent residues are present. The potential of the method is illustrated by determining the parameters describing the association of melittin, mastoparan X, and mastoparan with immobilized lipodisks. Both thermodynamic and kinetic analyses are possible. The presented method constitutes a useful tool for fundamental studies of peptide-membrane interactions and can also be applied to optimize the design of lipodisks, for example, for sustained release of antimicrobial peptides in therapeutic applications.

Effect of phospholipid composition on discoidal HDL formation

Discoidal high-density lipoprotein (HDL) particles are known to fractionalize into several discrete populations. Factors regulating their size are, however, less understood. To reveal the effect of lipid composition on their formation and characteristics, we prepared several reconstituted HDLs (rHDLs) with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), and sphingomyelin at phospholipid to apolipoprotein A-I ratios of 100 and 25. When reconstitution was conducted at 37°C, the efficiency of rHDL formation from POPC was decreased as compared with that conducted at 4°C. Moreover, large rHDLs with a Stokes diameter of 9.6nm became dominant over small rHDL with a diameter of 7.9nm, which was distinctly observed at 4°C. The aminophospholipids POPS and POPE promoted the formation of small rHDLs at 37°C, but fluorescence experiments revealed that they did so in a different fashion: Fluorescence lifetime data suggested that the head group of POPS reduces hydrophobic hydration, especially in small rHDLs, suggesting that this lipid stabilizes the saddle-shaped bilayer structure in small rHDLs. Fluorescence lifetime and anisotropy data showed that incorporation of POPE increases acyl chain order and water penetration into the head group region in large rHDLs, suggesting that POPE destabilizes the planar bilayer structure. These results imply that these aminophospholipids contribute to the formation of small rHDLs under biological conditions.

Mechanism of electric frequency tuning in composite resonators based on epitaxial ferrite films

A planar bilayer structure comprising an epitaxial film of yttrium iron garnet (YIG) ferrite film on a lead zirconate titanate (PZT) piezoelectric plate has been studied. It is shown both theoretically and experimentally that electric tuning of the resonant frequency is possible due to elastic-stress-induced variations of the field of uniaxial first-order anisotropy, while the field of uniaxial second-order anisotropy changes to a much lower degree. It is established that gluing of the ferrite and metal-coated piezoelectric layers rather insignificantly increases losses in the magnetostatic resonator.

Si/ZnO core–shell nanowire arrays for photoelectrochemical water splitting

n-Type Si nanowire (NW) arrays coated with a thin layer of ZnO were fabricated via a two-step route combined with metal-assisted chemical etching and sol-gel processes. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction were utilized to characterize the core/shell structure. The obtained Si/ZnO core/shell NW arrays exhibit about 8% optical reflectance in visible region, implying good optical absorption. Water splitting performance of Si/ZnO NW arrays was studied. The photoconversion efficiency for Si/ZnO NW arrays reaches 0.38% upon exposure to the illumination with a light intensity of 10 mW/cm 2, which is higher than those of the planar bilayer structure of Si/ZnO (0.19%) and the pure planar ZnO (0.09%). The relationship between the illumination intensity and the photoconversion efficiency of Si/ZnO NW arrays was also discussed.

Monodisperse Nonionic Isoprenoid-Type Hexahydrofarnesyl Ethylene Oxide Surfactants: High Throughput Lyotropic Liquid Crystalline Phase Determination

The neat and lyotropic phase behavior of eight new ethylene oxide amphiphiles (EO = 1-8) with a hexahydrofarnesyl chain (3,7,11-trimethyldodecyl) and narrow polydispersity (>98.5% purity) is reported. Below five EO units the behavior of the neat surfactants show only a glass transition, Tg ∼ -90 °C. Above four EO units, crystallization (Tcrys) and crystal-isotropic liquid (Tm) transitions are also observed that increase with degree of ethoxylation of the surfactant headgroup. The lyotropic liquid crystalline phase behavior spans a complex spectrum of surfactant-water interfacial curvatures. Specifically, inverse phases are present below ambient temperatures for EO < 4, with HFarn(EO)2 exhibiting an inverse hexagonal (H(II)) phase stable to dilution. The phase diagram of HFarn(EO)3 displays both the gyroid (Ia3d) and double diamond (Pn3m) inverse bicontinuous cubic phases, with the latter being thermodynamically stable in excess water within the physiological regime. There is a strong preference for planar bilayer structures at intermediate headgroup ethoxylation, with the crossover to normal phases occurring at HFarn(EO)(7-8) which exhibits normal hexagonal (H(I)) and cubic (Q(I)) phases at ambient temperatures. The toxicity of colloidal dispersions of these EO amphiphiles was assayed against normal breast epithelial (HMEpiC) and breast cancer (MCF7) cell lines. The IC50 of the EO amphiphiles was similar in both cell lines with moderate toxicity ranging from ca. <5 to 140 μM in an in vitro cell viability assay. Observations are qualitatively rationalized in terms of the molecular geometry of the surfactant. The physicochemical behavior of the HFarnesyl ethylene oxide amphiphiles is compared to other ethylene oxide surfactants.

Formation and Finite Element Analysis of Tethered Bilayer Lipid Structures

Rapid solvent exchange of an ethanolic solution of diphytanoyl phosphatidylcholine (DPhyPC) in the presence of a mixed self-assembled monolayer (SAM) [thiolipid/β-mercaptoethanol (βME) (3/7 mol/mol) on Au] shows a transition from densely packed tethered bilayer lipid membranes [(dp)tBLMs], to loosely packed tethered bilayer lipid membranes [(lp)tBLMs], and tethered bilayer liposome nanoparticles (tBLNs) with decreasing DPhyPC concentration. The tethered lipidic constructs in the aqueous medium were analyzed by atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). Finite element analysis (FEA) was applied to interpret spectral EIS features without referring to equivalent circuit modeling. Using structural data obtained earlier from neutron reflectometry and dielectric constants of lipid bilayers, we reproduced experimentally observed features of the electrochemical impedance (EI) spectra of complex surface constructs involving small pinhole defects, large membrane-free patches, and bound liposomes. We demonstrated by FEA that highly insulating (dp)tBLMs with low-defect density exhibit EI spectra in the shape of a perfect semicircle with or without low-frequency upward "tails" in the Cole-Cole representation. Such EI spectra were observed at DPhyPC concentrations of >5 × 10(-3) mol L(-1). While AFM was not able to visualize very small lateral defects in such films, EI spectra unambiguously signaled their presence by increased low frequency "tails". Using FEA we demonstrate that films with large diameter visible defects (>25 nm by AFM) produce EI spectral features consisting of two semicircles of comparable size. Such films were typically obtained at DPhyPC concentrations of <5 × 10(-3) mol L(-1). At DPhyPC concentrations of <1.0 × 10(-3) mol L(-1) the planar bilayer structures were replaced by ellipsoidal liposomes with diameters ranging from 50 to 500 nm as observed in AFM images. Despite the distinct surface morphology change, the EI curves exhibited two semicircle spectral features typical for the large size defects in planar tBLMs. FEA revealed that, to account for these EI features for bound liposome systems (50-500 nm diameter), one needs to assume much lower tBLM conductivities of the submembrane space, which separates the electrode surface and the phospholipid bilayer. Alternatively, FEA indicates that such features may also be observed on composite surfaces containing both bound liposomes and patches of planar bilayers. Triple semicircular features, observed in some of the experimental EI curves, were attributed to an increased complexity of the real tBLMs. The modeling demonstrated that such features are typical for heterogeneous tBLM surfaces containing large patches of different defectiveness levels. By integrating AFM, EIS, and FEA data, our work provides diagnostic criteria allowing the precise characterization of the properties and the morphology of surface supported bilayer systems.

Three-dimensionally interconnected organic nanocomposite thin films: implications for donor–acceptor photovoltaic applications

The formation of three-dimensionally interconnected donor–acceptor nanocomposites from copper(II) phthalocyanine tetrasulfonic tetrasodium salt (tsCuPc) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is reported. Thin films were formed using a multi-step template-directed growth method. The nanocomposites consist of crystalline, continuous and interpenetrating networks of tsCuPc and PCBM with an interfacial area to volume ratio 2–3 times greater than in a planar bilayer structure. This templating strategy allows the formation of engineered nanocomposites for optoelectronic applications whose structural properties can be easily tuned for increased efficiencies.

Fabrication and characterization of large-size electrolyte/anode bilayer structures for low-temperature solid oxide fuel cell stack based on gadolinia-doped ceria electrolyte

Preliminary progress is reported in this communication in building a planar anode-supported low-temperature solid oxide fuel cell (SOFC) stack based on gadolinia-doped ceria (GDC) electrolyte, i.e. fabrication and characterization of a Ø80 planar bilayer structure composed of GDC electrolyte film and Ni–GDC anode substrate. The anode substrates were prepared from mixtures of NiO, GDC, and carbon black by die-pressing. After pre-firing to remove the carbon black, the anode substrates were deposited with a GDC layer using a spray coating technique. The green bilayers of anode substrate and electrolyte film were then co-sintered at 1500 °C for 3 h. Through proper control of the sintering process, bilayer structures with excellent flatness were achieved after co-sintering. Scanning electron microscopy (SEM) observation indicated that the electrolyte film was about 22 μm in thickness, highly dense, crack-free, and well-bonded to the anode substrate. Small disks which were cut out from the Ø80 bilayer structure were electrochemically examined in a single button-cell mode incorporating a (LaSr)(CoFe)O 3–GDC composite cathode. With humidified hydrogen as the fuel and air as the oxidant, the cell demonstrated an open-circuit voltage of 0.884 V and a maximum power density of 562 mW/cm 2 at 600 °C. The results imply that high-quality anode-supported electrolyte/anode bilayer structures were successfully fabricated. Based on them, planar anode-supported SOFC stacks will be assembled in the future.

Planar complication schemes of binary ratios

The planar and linear complication schemes are constructed for the compositions corresponding to the phase diagram of BaO: BiO1.5, the compositions of borate radicals, the symbols of reflections with the intensity 100, the symbols of simple forms, the symbols of planes with a high density of atoms in planar bilayer structures, the theoretical and realized formulas of binary compounds, and the ratios between the chemical coefficients of two different cations in oxides. The schemes of silicate and borate radicals are compared. It is confirmed that the binary ratios obey the law of small numbers.

Lipid Phase Dependence of DNA−Cationic Phospholipid Bilayer Interactions Examined Using Atomic Force Microscopy

Magnetic AC mode atomic force microscopy was used to study DNA adsorption onto supported cationic phospholipid (CL) bilayers in the fluid (L α ) and gel (L β ) phases. A L α phase CL bilayer was used as a model system to study DNA-CL complex formation at the single molecule level. Pure and mixed bilayers of dioleoyltrimethylammonium propane (DOTAP), dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylcholine (DOPC), and dipalmitoyltrimethylammonium propane (DPTAP) were investigated. The adsorption of small oligodeoxynucleotide DNA (14 base pairs) and large (1000-3000 base pairs) was examined and revealed that the behavior of the DNA-CL complex depends on the length and density of DNA. Once adsorbed to the bilayer, large DNA induced changes in the bilayer coverage of the mica support, while short DNA did not destabilize the bilayer. There was a strong dependence of the bilayer destabilization on the type of neutral helper lipid mixed with the cationic lipid, with DOTAP/DOPE being much less stable to DNA adsorption than DOTAP/DOPC. The characteristics of pure and mixed DOTAP, DOPE, and DOPC supported phospholipid bilayers were investigated to understand the stability changes induced by DNA. Images of the DNA-free phospholipid bilayer revealed the dependence of supported planar bilayer structure and stability on the chemical nature of the headgroup and on the phase of the bilayer. Finally, it was illuminating to image DNA on DPTAP bilayers as a function of temperature. When the system was examined above the L α -L β phase transition, images similar to those recorded for DNA-DOTAP (L α ) and DNA-DOTAP/DOPC (L α ) were observed.

Photophysical behavior of a homologous series of amphiphilic hemicyanine dyes in thin AOT films

The photophysical behavior of a homologous series of amphiphilic hemicyanines, incorporated in thin films, made by dip-coating glass slides in bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT) reverse micellar solutions, has been studied by absorption spectrophotometry and computer simulation techniques. At relatively high dye concentration, a narrow band in the absorption spectrum was observed, corresponding to aggregated species. Aggregates are of the H-type, displaying a hypsochromic shift in their absorption spectra, except for the shortest chain homolog. The growth of aggregates in films has been studied in relation with the [water]/[dye] and the [surfactant]/[dye] ratios. Simulation of the aggregate formation by extended dipole interaction in spherical reverse micelles and planar bilayer structures in combination with experimental data, revealed that the films are structured in microheterogeneous assemblies analogous to the reverse micelles found in the original solutions.