Tethered or adsorbed supported lipid bilayers in nanotubes characterized by deuterium magic angle spinning NMR spectroscopy

2H solid-state NMR experiments were performed under magic angle spinning on lipid bilayers oriented into nanotubes arrays, as a new method to assess the geometrical arrangement of the lipids. Orientational information is obtained from the intensities of the spinning sidebands. The lipid bilayers are formed by fusion of small unilamellar vesicles of DMPC-d54 inside a nanoporous anodic aluminum oxide, either by direct adsorption on the support or by tethering through a streptavidin/biotin linker. The results support that the quality of the lipid bilayers alignment is clearly in favor of the tethering rather than an adsorbed strategy.     

EPR and fluorescence study of molecular dynamics of lipids in bilayers adsorbed on porous silica gel

Lipid bilayers immobilized on highly porous silica gel monoliths (SLBs) have been proposed as templates for chemosensors. Procedures for preparation and characterization of SLBs have been developed and applied. Rotational dynamics and orientational ordering at different depths in a SLB membrane and, for comparison, in bilayer liposomes have been studied for the first time using n-PC phospholipid spin labels.     

DC photoelectric signals from bacteriorhodopsin adsorbed on lipid monolayers and thiol/lipid bilayers supported by mercury

Purple membrane (PM) fragments were adsorbed on a dioleoylphosphatidylcholine (DOPC) monolayer and on a mixed alkanethiol/DOPC bilayer supported by mercury to investigate the kinetics of light-driven proton transport by bacteriorhodopsin (bR). The light-on and light-off capacitive currents on an alkanethiol/DOPC bilayer at pH 6.4 were interpreted on the basis of a simple equivalent circuit. The pH dependence of the biphasic decay kinetics of the light-on currents was analyzed to estimate the pK(a) values for the transitions releasing protons to, and taking up protons from, the solution. The linear dependence of the stationary light-on current of bR on a DOPC monolayer self-assembled on mercury upon the applied potential was interpreted on the basis of an equivalent circuit.     

Flip-flop of glycosylphosphatidylinositols (GPI's) across the ER

The transbilayer flip-flop of early intermediates in the glycosylphosphatidylinositol (GPI) biosynthetic pathway has been demonstrated using novel fluorescent GPI probes and a biochemical reconstitution approach.     

Polymerisation in supported bilayers

The intercalation of the twin-tailed amphiphilic dioctadecyldimethylammonium (DODA) ions in smectite clay minerals provides a well-defined supported bilayer system. Polymerisation of styrene in these robust model bilayers allows one to focus on the effect of the constrained medium on the polymerisation process and the polymer. Small-angle X-ray scattering analysis and differential scanning calorimetry data indicated that the polymerisation of styrene in DODA–montmorillonite leads to phase separation between polystyrene and the surfactant matrix. Received: 14 April 2000/Accepted: 19 June 2000     

Flip-flop sensors: a new class of silicon sensors

Integrated sensors based on flip-flops have been succesfully fabricated for the measurement of several physical parameters and subsequently tested. The sensors consist of flip-flops in which one circuit element is sensitive to the desired physical parameter. The sensing action consists of alternately bringing the flip-flop into an unstable and a stable state and counting the resulting ones or zeros. This number is a measure for the asymmetry of the flip-flop and thus for the physical parameter. To illustrate the principle, a force sensor based on a flip-flop containing piezoresistors is presented.     

Correlations in simulated model bilayers

Molecular dynamics simulations of a model bilayer in a solvent are used to determine the structure factors S(q) and other correlations at vanishing lateral tension of the system and at positive and negative tensions as well. The role played by curvature in the absence of surface tension, metastability, and phase transitions, and the dependence on the specific area, are discussed. Possible height-height correlation functions S(q) are examined and tested.     

Dimetallic functionalities in liposome bilayers

A dicopper(II) complex can be covalently linked to palmitate/palmitoyl-oleoyl-phosphatidylcholine (PA/POPC) liposomes using the following one-pot strategy: preformed [Cu₂(bpbp)(PA)](ClO₄)₂ (bpbp^?  = 2,6-bis((N,N′-bis(2-picolyl)amino)methyl)-4-tertbutylphenolato) was incorporated into POPC liposomes with a loading of up to 10 mol%. Despite its shape and charge, the decoration of PA/POPC liposomes with {Cu₂(bpbp)}³⁺ did not disrupt the liposome structure; however, the mean liposome diameter increased from about 130 nm (0 mol% dicopper complex) to about 150 nm (10 mol% dicopper complex). Single-crystal X-ray structures furnish ‘snapshots’ of the pH-dependent solution state derivatives of {Cu₂(bpbp)}³⁺, and model the structure of the [Cu₂(bpbp)(PA)]²⁺ head group at the surface of the liposomes. An impressive plasticity in the intramolecular non-bonded Cu….Cu distance for these ions, ranging from 3 to 4 Å, in [Cu₂(bpbp)OH]²⁺, [Cu₂(bpbp)(OAc)(H₂O)]²⁺ and [Cu₂(bpbp)(H₂O)₂]³⁺ allows for their utility as labile reagents in water. Remarkably, the flexible dicopper site is selective for a single carboxylate ligand, so that [Cu₂(bpbp)(PA)]²⁺ is favoured even in the presence of other chemically similar oxoanions, such as CO₃^2 ? , HCO₃^? , NO₃^? , ClO₄^? , ReO₄^?  and CF₃SO₃^? .     

Lateral diffusion in sphingomyelin bilayers

Sphingomyelin (SM) is an important lipid of eukaryotic cellular membranes and neuronal tissues. We studied lateral diffusion in macroscopically oriented bilayers of synthetic palmitoylsphingomyelin (PSM) and natural sphingomyelins of egg yolk (eSM), bovine brain (bSM) and bovine milk (mSM) by pulsed field gradient NMR (PFG NMR) in the temperature range 45-60 °C. We found that the mean values of lateral diffusion coefficients (LDCs) of SMs are 1.9-fold lower compared with those of dipalmitoylphosphatidylcholine (DPPC), which is similar in molecular structure. This discrepancy could be explained by the characteristics of intermolecular SM interactions. The LDCs of different SMs differ: egg SM is most similar to PSM; both of them have a 10% higher LDC value compared with the other two natural SMs. Besides, all natural SMs show a complicated form of the spin-echo diffusion decay (DD), which is an indicator of a distribution of LDC values in bilayers. This peculiarity is explained by the broad distributions of hydrocarbon chain lengths of the natural SMs studied here, especially mSM and bSM. We confirmed the relationship between chain length and LDC in the bilayers by computer analysis of a set of (1)H NMR spectra obtained by scanning the value of the pulsed field gradient. There is a correlation between lower LDC values and SM molecules with longer acyl chains. The most probable mechanisms by which long-chain SM molecules decrease their lateral diffusion relative to the average value are protrusion into the other side of the bilayer or lateral separation into areas that diverge with their LDCs.     

Supported lipopolysaccharide bilayers

In this report, the formation of supported lipopolysaccharide bilayers (LPS-SLBs) is studied with extracted native and glycoengineered LPS from Escherichia coli ( E. coli ) and Salmonella enterica sv typhimurium ( S. typhimurium ) to assemble a platform that allows measurement of LPS membrane structure and the detection of membrane tethered saccharide-protein interactions. We present quartz crystal microbalance with dissipation monitoring (QCM-D) and fluorescence recovery after photobleaching (FRAP) characterization of LPS-SLBs with different LPS species, having, for example, different molecular weights, that show successful formation of SLBs through vesicle fusion on SiO(2) surfaces with LPS fractions up to 50 wt %. The thickness of the LPS bilayers were investigated with AFM force-distance measurements which showed only a slight thickness increase compared to pure POPC SLBs. The E. coli LPS were chosen to study the saccharide-protein interaction between the Htype II glycan epitope and the Ralstonia solanacearum lectin (RSL). RSL specifically recognizes fucose sugars, which are present in the used Htype II glycan epitope and absent in the epitopes LPS1 and EY2. We show via fluorescence microscopy that the specific, but weak and multivalent interaction can be detected and discriminated on the LPS-SLB platform.     

Dynamic adsorption and structure of interfacial bilayers adsorbed from lipopeptide surfactants at the hydrophilic silicon/water interface: effect of the headgroup length

Lipopeptides are an important group of biosurfactants expressed by microorganisms. Because they are well-known for being biocompatible, biodegradable, and highly surface active, they are attractive for a wide range of applications. Natural lipopeptide surfactants are however impure; it is hence difficult to use them for exploring the structure-function relation. In this work, a series of cationic lipopeptide surfactants, C(14)K(n) (n = 1-4), where C denotes the myristic acyl chain and K denotes lysine (Lys), have been synthesized, and their interfacial behavior has been characterized by studying their adsorption at the silicon/water interface (bearing a thin native oxide layer) using spectroscopic ellipsometry and neutron reflection (NR). The dynamic adsorption was marked by an initial fast step within the first 2-3 min followed by a slow molecular relaxation process over the subsequent 20-30 min. The initial rate of time-dependent adsorption and the equilibrated adsorbed amount showed a steady decrease with increasing n, indicating the impact of the molecular size, structure, and charge. NR revealed the formation of sandwiched bilayers from C(14)K(n), similar to conventional surfactants such as nonionic C(12)E(6) and cationic C(16)TAB. However, the electrostatic attraction between K and the silica surface caused confinement of the K groups, forcing the head segments into a predominantly flat-on conformation. This characteristic structural feature was confirmed by the almost constant thickness of the headgroup regions ranging from 8 to 11 ? as determined from NR combined with partial deuterium labeling to the acyl tail. An increase in area per molecular pair with n resulted directly from increasing the footprint. As a result, the hydrophobic back-to-back tail mixing and acyl chain tilting rose with n. The extent of chain-head intermixing became so intensified that the C(14)K(4) bilayer could be approximated to a uniform layer distribution.     

Selective sterol-phospholipid associations in fluid bilayers

The nearest-neighbor preferences of three exchangeable lipid monomers (two phospholipids that differ in length, A and B, and a derivative of cholesterol, C) have been quantified in the fluid bilayer state by use of the nearest-neighbor recognition method (Davidson, S. K. M.; Regen, S. L. Chem. Rev. 1997, 97, 1269). Thus, an analysis of the equilibrium dimer distributions has shown that (i) the sterol favors both phospholipids as nearest neighbors relative to other sterol molecules, (ii) that this recognition is selective (i.e., the sterol favors the longer phospholipid as a nearest neighbor over the shorter one, especially when the sterol concentrations in the bilayer is high (e.g., 40 mol %), and (iii) the phospholipids, themselves, are unable to recognize each other. Taken together, these findings indicate that the probable mechanism by which cholesterol induces homoassociation of A and B in analogous bilayers is one in which the sterol "pulls" two or more of the longer phospholipid monomers (B) out of a "sea" of randomly mixed A and B. These findings also lend support for the notion of cholesterol-phospholipid complexation in fluid bilayers. The biological implications of these findings are briefly discussed.     

Balance of forces in simulated bilayers

Two kinds of simulated bilayers are described, and the results are reported for lateral tension and for partial contributions of intermolecular forces to it. Data for the widest possible range of areas per surfactant head, from tunnel formation through tensionless state, transition to floppy bilayer, and its disintegration, are reported and discussed. The significance of the tensionless state is discussed.     

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.     

Asymmetric droplet interface bilayers

In cell membranes, the lipid compositions of the inner and outer leaflets differ. Therefore, a robust model system that enables single-channel electrical recording with asymmetric bilayers would be very useful. We and others recently developed the droplet interface bilayer (DIB), which is formed by connecting lipid monolayer-encased aqueous droplets submerged in an oil-lipid mixture. Here, we incorporate lipid vesicles of different compositions into aqueous droplets and immerse them in an oil bath to form asymmetric DIBs (a-DIBs). Both alpha-helical and beta-barrel membrane proteins insert readily into a-DIBs, and their activity can be measured by single-channel electrical recording. We show that the gating behavior of outer membrane protein G (OmpG) from Escherichia coli differs depending on the side of insertion in an asymmetric DIB with a positively charged leaflet opposing a negatively charged leaflet. The a-DIB system provides a general platform for studying the effects of bilayer leaflet composition on the behavior of ion channels and pores.     

Shape instabilities in adsorbed polymer condensates

A polymer chain in a poor solvent collapses to a spherical globule. If this globule is subsequently deformed, either by stretching it or if it has some non-zero net electric charge, it will become unstable to sinusoidal perturbations leading to novel final states. When such globules are imaged by direct methods such as atomic force microscopy the substrate on which they adsorb can affect the final morphological state. We therefore investigate strongly adsorbed polymers in poor solvents. We demonstrate that the real-space, Self-Consistent Field method is an ideal numerical tool in predicting equilibrium morphologies. New structures are predicted, which support previous explicit free energy calculations. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3327–3337, 2007     

Biomecular quenching in the adsorbed layer

The influence of heat treatment and of added coadsorbates on the surface photochemistry of pyrene and 1,2-benzanthracene on aerosil is reported. It is pointed out that the surface plays an active role in determining the primary quenching processes.