Phase transition of a single lipid bilayer measured by sum-frequency vibrational spectroscopy

In this communication, we demonstrate the first use of sum-frequency generation (SFG) vibrational spectroscopy to measure directly the phase transition temperature (Tm) of a single planar supported lipid bilayer (PSLB). Three saturated phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-diheptadecanoyl-sn-glycero-3-phosphocholine (DHPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), were studied. Lipid bilayer films were prepared by the the Langmuir-Blodgett method at a surface pressure of 30 nN/m. The symmetric nature of the bilayer was used to determine the Tm of bilayers by measuring the intensity of the symmetric methyl stretch at 2875 cm-1 from the lipid fatty acid chains as a function of temperature. A maximum in the CH3 symmetric stretch transition was observed at the Tm of the lipid film due to the reduction of symmetry in the bilayer. The SFG measured Tm for DPPC, DHPC, and DSPC were 41.0 +/- 0.4, 52.4 +/- 0.7, and 57.9 +/- 0.5 degrees C, respectively. These values correlate well with the literature values of 41.3 +/- 1.8, 49 +/- 3, and 54.5 +/- 1.5 degrees C for DPPC, DHPC, and DSPC, respectively obtained by differential scanning calorimetry (DSC) of lipid vesicles in solution. The high degree of correlation between the SFG spectroscopic measurements and the DSC results suggests the Tm of these lipids is not significantly altered upon immobilization on a surface.     

Direct measurement of the transbilayer movement of phospholipids by sum-frequency vibrational spectroscopy

The direct measurement of the transbilayer movement of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in a planar supported lipid bilayer (PSLB) at the fused silica/D2O interface was obtained with sum-frequency generation (SFG) vibrational spectroscopy. The intrinsic sensitivity of SFG to the symmetry of an interface was used to measure the asymmetric distribution of DSPC and perdeuterated DSPC (DSPC-d83) lipids in asymmetrically prepared DSPC/DSPC-d83 PSLBs. Changes in the membrane lipid composition due to exchange between leaflets was monitored by measuring the decay in the CH3 symmetric stretch intensity at 2875 cm-1 with time. The activation energy for transverse motion was determined directly from spectral relaxation measurements at several temperatures and was determined to be 206 +/- 18 kJ/mol. At room temperature (25 degrees C) the half-time of lipid flip-flop was calculated to be approximately 25 days. At 51 degrees C, only 7 degrees C below the main phase-transition temperature of DSPC, the half-time decreases to 25 min. These results have important implications for understanding the transbilayer movement of lipids in biological membranes.     

Electrochemical and PM-IRRAS a glycolipid-containing biomimetic membrane prepared using Langmuir-Blodgett/Langmuir-Schaefer deposition

Differential capacitance, chronocoulometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize the structure and orientation of a DMPC + cholesterol + GM 1 (60:30:10 mol %) bilayer supported at a Au(111) electrode surface prepared using combined Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) deposition. The electrochemical measurements indicate that the incorporation of ganglioside GM 1 into the membrane significantly improves the quality of the bilayer, reflected in the very low capacitance value of approximately 0.8 microF cm (-2). An analysis of the infrared data suggests that the incorporation of the glycolipid into the membrane changes both the orientation of the lipid acyl chains in the membrane and the hydration of the membrane, particularly with respect to the interfacial region of the lipids.     

Measuring lipid asymmetry in planar supported bilayers by fluorescence interference contrast microscopy

There is substantial scientific and practical interest in engineering supported lipid bilayers with asymmetric lipid distributions as models for biological cell membranes. In principle, it should be possible to make asymmetric supported lipid bilayers by either the Langmuir-Blodgett/Schafer (LB/LS) or Langmuir-Blodgett/vesicle fusion (LB/VF) techniques (Kalb et al. Biochim. Biophys. Acta 1992, 1103, 307-316). However, the retention of asymmetry in biologically relevant lipid bilayers has never been experimentally examined in any of these systems. In the present work, we developed a technique that is based on fluorescence interference contrast (FLIC) microscopy to measure lipid asymmetry in supported bilayers. We compared the final degree of lipid asymmetry in LB/LS and LB/VF bilayers with and without cholesterol in liquid-ordered (l(o)) and liquid-disordered (l(d)) phases. Of five different fluorescent lipid probes that were examined, 1,2-dipalmitoyl-phosphatidylethanolamine-N-[lissamine rhodamine B] was the best for studying supported bilayers of complex composition and phase by FLIC microscopy. An asymmetrically labeled bilayer made by the LB/LS method was found to be at best 70-80% asymmetric once completed. In LB/LS bilayers of either l(o) or l(d) phase, cholesterol increased the degree of lipid mixing between the opposing monolayers. The use of a tethered polymer support for the initial monolayer did not improve lipid asymmetry in the resulting bilayer. However, asymmetric LB/VF bilayers retained nearly 100% asymmetric label, with or without the use of a tethered polymer support. Finally, lipid mixing across the center of LB/LS bilayers was found to have drastic effects on the appearance of l(d)-l(o) phase coexistence as shown by epifluorescence microscopy.     

Asymmetry of lipid bilayers induced by monovalent salt: atomistic molecular-dynamics study

Interactions between salt ions and lipid components of biological membranes are essential for the structure, stability, and functions of the membranes. The specific ionic composition of aqueous buffers inside and outside of the cell is known to differ considerably. To model such a situation we perform atomistic molecular-dynamics (MD) simulations of a single-component phosphatidylcholine lipid bilayer which separates two aqueous reservoirs with and without NaCl salt. To implement the difference in electrolyte composition near two membrane sides, a double bilayer setup (i.e., two bilayers in a simulation box) is employed. It turns out that monovalent salt, being in contact with one leaflet only, induces a pronounced asymmetry in the structural, electrostatic, and dynamical properties of bilayer leaflets after 50 ns of MD simulations. Binding of sodium ions to the carbonyl region of the leaflet which is in contact with salt results in the formation of "Na-lipids" complexes and, correspondingly, reduces mobility of lipids of this leaflet. In turn, attractive interactions of chloride ions (mainly located in the aqueous phase close to the water-lipid interface) with choline lipid groups lead to a substantial (more vertical) reorientation of postphatidylcholine headgroups of the leaflet adjoined to salt. The difference in headgroup orientation on two sides of a bilayer, being coupled with salt-induced reorientation of water dipoles, leads to a notable asymmetry in the charge-density profiles and electrostatic potentials of bilayer constitutes of the two leaflets. Although the overall charge density of the bilayer is found to be almost insensitive to the presence of salt, a slight asymmetry in the charge distribution between the two bilayer leaflets results in a nonzero potential difference of about 85 mV between the two water phases. Thus, a transmembrane potential of the order of the membrane potential in a cell can arise without ionic charge imbalance between two aqueous compartments.     

Layer-by-layer PMIRRAS characterization of DMPC bilayers deposited on a Au111 electrode surface

A combination of Langmuir-Blodgett and Langmuir-Schaefer techniques was employed to deposit 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers at a gold electrode surface. One leaflet consisted of hydrogen-substituted acyl chains, and the second leaflet was composed of molecules with deuterium-substituted acyl chains. This architecture allowed for layer-by-layer analysis of the structure of the bilayer. Photon polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to determine the conformation and orientation of the acyl chains of DMPC molecules in the individual leaflets as a function of the potential applied to the gold electrode. The bilayer is adsorbed onto the metal surface when the field applied to the membrane does not exceed approximately 108 V/m. When adsorbed, the bottom leaflet is in contact with a hydrophobic metal surface, and the top leaflet is interacting with the aqueous solution. The asymmetry of the environment has an effect on the orientation of the DMPC molecules in each leaflet. The tilt angle of the acyl chains of the DMPC molecules in the bottom leaflet that is in contact with the gold is approximately 10 degrees smaller than that observed for the top leaflet that is exposed to the solution. These studies provide direct evidence that the structure of a phospholipid bilayer deposited at an electrode surface is affected by interaction with the metal.     

Homogeneity, electrical resistivity and lateral diffusion of lipid bilayers coupled to polyelectrolyte multilayers

The electrostatic coupling of charged phospholipid bilayers with polyelectrolyte multilayers is studied varying the lipid charge density, multilayer composition and preparation conditions. It is shown that in all cases the bilayer is insufficiently insulating for meaningful electrochemical studies. Homogeneity on a light microscopical length scale was obtained by two methods: vesicle fusion into bilayers and deposition from monolayers by the Langmuir–Schäfer (LB/LS) technique. Largest progress was achieved aiming for lateral diffusion comparable to an uncoupled bilayer. For this mixtures with 10% charged (DOPA) and 90% uncharged (DMPC) lipid were prepared that exhibited sufficient anchoring density and at the same time a fluid DMPC phase on going above the main phase transition at 24°C. This yielded diffusion coefficients in aqueous environment above 1 μm² s⁻¹ with almost no immobile fractions.     

Composition and asymmetry in supported membranes formed by vesicle fusion

The structure and formation of supported membranes at silica surfaces by vesicle fusion was investigated by neutron reflectivity and quartz crystal microbalance (QCM-D) measurements. The structure of equimolar phospholipid mixtures of DLPC-DPPC, DMPC-DPPC, and DOPC-DPPC depends intricately on the vesicle deposition conditions. The supported bilayer membranes exhibit varying degrees of compositional asymmetry between the monolayer leaflets, which can be modified by the deposition temperature as well as the salt concentration of the vesicle solution. The total lipid composition of the supported bilayers differs from the composition of the vesicles in solution, and the monolayer proximal to the silica surface is always enriched in DPPC compared to the distal monolayer. The results, which show unambiguougsly that some exchange and rearrangement of lipids occur during vesicle deposition, can be rationalized by considering the effects of salt screening and temperature on the rates of lipid exchange, rearrangement, and vesicle adsorption, but there is also an intricate dependence on the lipid-lipid interactions. Thus, although both symmetric and asymmetric supported bilayers can be prepared from vesicles, the optimal conditions are sensitive to the lipid composition of the system.     

Engineered lipids that cross-link the inner and outer leaflets of lipid bilayers

The application of supported lipid bilayer systems as molecular sensors, diagnostic devices, and medical implants is limited by their lack of stability. In an effort to enhance the stability of supported lipid bilayers, three pairs of phosphatidylcholine lipids were designed to cross-link at the termini of their 2-position acyl chain upon the formation of lipid bilayers. The cross-linked lipids span the lipid bilayer, resembling naturally occurring bolaamphiphiles that stabilize archaebacterial membranes against high temperatures. The three reactions investigated here include the acyl chain cross-linking between thiol and bromine groups, thiol and acryloyl groups, and cyclopentadiene and acryloyl groups. All three reactive lipid pairs were found to cross-link in liposomal membranes, as determined by thin-layer chromatography, ion-spray mass spectrometry, and 1H NMR. The monolayer film properties of the reactive amphiphiles were characterized by surface pressure-area isotherms and showed that stable monolayers formed at the air-water interface with limiting molecular areas comparable to that of pure saturated phosphatidylcholine lipids. Langmuir-Blodgett bilayers of dimyristoylphosphatidylcholine incorporating 15 mol % of the reactive thiol and acryloyl lipids had diffusion coefficients comparable with pure dimyristoylphosphatidylcholine, while bilayers with more than 25 mol % of the reactive lipids were immobile, suggesting that interleaflet cross-linking of the lipids inhibited membrane diffusion. Our results show that the reactive lipids can cross-link within a lipid bilayer and are suitable for assembling supported lipid bilayers using Langmuir-Blodgett deposition. By using terminally reactive amphiphiles to build up supported lipid bilayers with cross-linked leaflets, bolaamphiphiles can be incorporated into asymmetric solid supported membranes to increase their stability in biosensor and medical implant applications.     

Effects of lipid chain unsaturation and headgroup type on molecular interactions between paclitaxel and phospholipid within model biomembrane

Molecular interactions between paclitaxel, an anticancer drug, and phospholipids of various chain unsaturations and headgroup types were investigated in the present study by Langmuir film balance and differential scanning calorimetry. Both the lipid monolayer at the air-water interface and the lipid bilayer vesicles (liposomes) were employed as model cell membranes. It was found that, regardless of the difference in molecular structure of the lipid chains and headgroup, the drug can form nonideal, miscible systems with the lipids at the air-water interface over a wide range of paclitaxel mole fractions. The interaction between paclitaxel and phospholipid within the monolayer was dependent on the molecular area of the lipids at the interface and can be explained by intermolecular forces or geometric accommodation. Paclitaxel is more likely to form thermodynamically stable systems with 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) and 1,2-dielaidoyl-sn-glycero-3-phosphocholine (DEPC) than with 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC). Investigation of the drug penetration into the lipid monolayer showed that DPPC and DEPC have higher incorporation abilities for the drug than DPPE and DSPC. A similar trend was also evidenced by DSC investigation with liposomes. While little change of DSC profiles was observed for the DPPE/paclitaxel and DSPC/paclitaxel liposomes, paclitaxel caused noticeable changes in the thermographs of DPPC and DEPC liposomes. Paclitaxel was found to cause broadening of the main phase transition without significant change in the peak melting temperature of the DPPC bilayers, which demonstrates that paclitaxel was localized in the outer hydrophobic cooperative zone of the bilayer, i.e., in the region of the C1-C8 carbon atoms of the acyl chain or binding at the polar headgroup site of the lipids. However, it may penetrate into the deeper hydrophobic zone of the DEPC bilayers. These findings provide useful information for liposomal formulation of anticancer drugs as well as for understanding drug-cell membrane interactions.     

Directed assembly of surface-supported bilayers with transmembrane helices

The lateral assembly of transmembrane (TM) helices gives rise to membrane proteins with complex folds, which play important roles in biochemical processes. Therefore, the assembly of surface-supported bilayers containing TM helices is the first step toward the development of functional biomembrane mimetics. Here we report novel directed assembly of surface-supported lipid bilayers with laterally mobile TM helices. The TM helices were incorporated into lipid monolayers at the air/water interface, and the monolayers were then transferred onto glass substrates using Langmuir-Blodgett (LB) deposition. Finally, bilayers were assembled using lipid vesicle fusion on top of the LB monolayers. The novelty is the incorporation of the peptides into the monolayer at the first step of bilayer assembly, which allows control over the peptide concentration and orientation. The transmembrane orientation of the peptides was confirmed using oriented circular dichroism (OCD), lateral mobility was assessed using fluorescence recovery after photobleaching (FRAP), and diffusion coefficients were determined using a novel boundary profile evolution (BPE) method. The described directed-assembly approach can be used to develop versatile bilayer platforms for studying membrane proteins interactions in native bilayer environments.     

Interleaflet interaction and asymmetry in phase separated lipid bilayers: molecular dynamics simulations

In order to investigate experimentally inaccessible, molecular-level detail regarding interleaflet interaction in membranes, we have run an extensive series of coarse-grained molecular dynamics simulations of phase separated lipid bilayers. The simulations are motivated by differences in lipid and cholesterol composition in the inner and outer leaflets of biological membranes. Over the past several years, this phenomenon has inspired a series of experiments in model membrane systems which have explored the effects of lipid compositional asymmetry in the two leaflets. The simulations are directed at understanding one potential consequence of compositional asymmetry, that being regions of bilayers where liquid-ordered (L(o)) domains in one leaflet are opposite liquid-disordered (L(d)) domains in the other leaflet (phase asymmetry). The simulated bilayers are of two sorts: 1) Compositionally symmetric leaflets where each of the two leaflets contains an identical, phase separated (L(o)/L(d)) mixture of cholesterol, saturated and unsaturated phospholipid; and 2) Compositionally asymmetric leaflets, where one leaflet contains a phase separated (L(o)/L(d)) mixture while the other contains only unsaturated lipid, which on its own would be in the L(d) phase. In addition, we have run simulations where the lengths of the saturated lipid chains as well as the mole ratios of the three lipid components are varied. Collectively, we report on three types of interleaflet coupling within a bilayer. First, we show the effects of compositional asymmetry on acyl chain tilt and order, lipid rotational dynamics, and lateral diffusion in regions of leaflets that are opposite L(o) domains. Second, we show substantial effects of compositional asymmetry on local bilayer curvature, with the conclusion that phase separated leaflets resist curvature, while inducing large degrees of curvature in an opposing L(d) leaflet. Finally, in compositionally symmetric, phase separated bilayers, we find phase asymmetry (domain antiregistration) between the two leaflets occurs as a consequence of mismatched acyl chain-lengths in the saturated and unsaturated lipids.     

Electrical measurements of bilayer membranes formed by Langmuir-Blodgett deposition on single-crystal silicon

Bilayer membranes on solid supports are used for fundamental studies of biophysical properties and for the development of biosensors and other devices. Here we report on electrically addressable bilayer membranes formed by Langmuir-Blodgett (LB)-based deposition on single-crystal silicon. The incorporation of a polymer cushion ensures high lipid mobility in both the lower and upper leaflet, allowing the potential for combined investigations of electrical, structural, and dynamic characteristics of membrane-associated proteins. Impedance spectroscopy is used to demonstrate that the lipid bilayers are robust and reproducible with an impedance of about 10(4) Omega cm2 and a capacitance of about 0.8 microF cm(-2). The ability to characterize ion channels is demonstrated using the model system gramicidin. These results demonstrate that artificial bilayers formed by LB deposition have many unique advantages for electrical measurements of membranes and their components.     

Structure and thermotropic phase behavior of fluorinated phospholipid bilayers: a combined attenuated total reflection FTIR spectroscopy and imaging ellipsometry study

Lipid bilayers consisting of lipids with terminally perfluoroalkylated chains have remarkable properties. They exhibit increased stability and phase-separated nanoscale patterns in mixtures with nonfluorinated lipids. In order to understand the bilayer properties that are responsible for this behavior, we have analyzed the structure of solid-supported bilayers composed of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and of a DPPC analogue with 6 terminal perfluorinated methylene units (F6-DPPC). Polarized attenuated total reflection Fourier-transform infrared spectroscopy indicates that for F6-DPPC, the tilt of the lipid acyl chains to the bilayer normal is increased to 39 degrees as compared to 21 degrees for native DPPC, for both lipids in the gel phase. This substantial increase of the tilt angle is responsible for a decrease of the bilayer thickness from 5.4 nm for DPPC to 4.5 nm for F6-DPPC, as revealed by temperature-controlled imaging ellipsometry on microstructured lipid bilayers and solution atomic force microscopy. During the main phase transition from the gel to the fluid phase, both the relative bilayer thickness change and the relative area change are substantially smaller for F6-DPPC than for DPPC. In light of these structural and thermotropic data, we propose a model in which the higher acyl-chain tilt angle in F6-DPPC is the result of a conformational rearrangement to minimize unfavorable fluorocarbon-hydrocarbon interactions in the center of the bilayer due to chain staggering.     

The effect of electrolyte on the encapsulation efficiency of vesicles formed by the nonionic surfactant, 2C18E12

Encapsulation efficiencies of vesicles formed by the nonionic surfactant 1,2-dioctadecyl-rac-glycerol-3-omega-methoxydodecylethylene glycol (abbreviated as 2C18E12) and its phospholipid counterpart, distearoylphosphatidylcholine (DSPC) at 298 K, were determined by the entrapment of the water-soluble dye, carboxyfluorescein (CF) to be 0.045+/-0.001 and 0.03+/-0.04 L mol(-1) for 2C18E12 vesicles prepared using low osmolarity (270 m Osm) Krebs-Henseleit (K-H) buffer and a modified 'high salt' (1600 m Osm) variant of K-H buffer, respectively, and 0.64+/-0.01 and 0.31+/-0.04 Lmol(-1) for DSPC vesicles prepared under the same conditions and in the same buffers. Freeze fracture electron microscopy studies confirmed the presence of vesicles when 2C18E12 and DSPC were dispersed in water and both buffer solutions. Small angle neutron scattering (SANS) studies, using D2O in place of H2O, showed that when 2C18E12 vesicles were prepared in the 'high salt' variant of K-H buffer as opposed to K-H buffer or water, a higher proportion of multilamellar vesicles (MLV) were formed. Furthermore when prepared in the 'high salt' variant of K-H buffer, the 2C18E12 bilayers were thinner, and when present in the form of MLV exhibited a smaller layer of water separating the bilayers. However, even in the absence of electrolyte, 2C18E12 formed surprisingly thin bilayers due to the penetration of the polyoxyethylene chains into the hydrophobic chain region of the bilayer. Due to the dehydrating effect of the high concentration of electrolyte present in the 'high salt' variant of K-H, the polyoxyethylene head groups penetrated further into the hydrophobic region of the bilayer making the bilayer even thinner. In the case of the DSPC vesicles, although the SANS study showed an increase in the relative proportion of multilamellar to unilamellar vesicles when samples were prepared in the 'high salt' variant of K-H buffer, no differences were observed in the thickness and the d-spacing of the vesicle bilayers. Variable temperature turbidity measurements of 2C18E12, and DSPC vesicles prepared in water indicated phase changes at 320+/-0.5 and 327+/-0.5 K, respectively, and were unchanged when the 'high salt' variant of K-H buffer was used as hydrating medium. Taken together, these results suggest that a low phase transition temperature was not the reason for the poor entrapment efficiency of 2C18E12 vesicles but rather the very 'thin' hydrophobic barrier formed by the penetration of the polyoxyethylene chains into the hydrophobic region of the bilayer.     

Elastic deformation of membrane bilayers probed by deuterium NMR relaxation

In deuterium ((2)H) NMR spectroscopy of fluid lipid bilayers, the average structure is manifested in the segmental order parameters (S(CD)) of the flexible molecules. The corresponding spin-lattice relaxation rates (R(1Z) depend on both the amplitudes and the rates of the segmental fluctuations, and indicate the types of lipid motions. By combining (2)H NMR order parameter measurements with relaxation studies, we have obtained a more comprehensive picture of lipids in the liquid-crystalline (L(alpha)) state than formerly possible. Our data suggest that a lipid bilayer constitutes an ordered fluid, in which the phospholipids are grafted to the aqueous interface via their polar headgroups, whereas the fatty acyl chains are in effect liquid hydrocarbon. Studies of (2)H-labeled saturated lipids indicate their R(1Z) rates and S(CD) order parameters are correlated by a model-free, square-law functional dependence, signifying the presence of relatively slow bilayer fluctuations. A new composite membrane deformation model explains simultaneously the frequency (magnetic field) dependence and the angular anisotropy of the relaxation. The results imply the R(1Z) rates are due to a broad spectrum of 3-D collective bilayer excitations, together with effective axial rotations of the lipids. For the first time, NMR relaxation studies show that the viscoelastic properties of membrane lipids at megahertz frequencies are modulated by the lipid acyl length (bilayer thickness), polar headgroups (bilayer interfacial area), inclusion of a nonionic detergent (C(12)E(8)), and the presence of cholesterol, leading to a range of bilayer softness. Our findings imply the concept of elastic deformation is relevant on lengths approaching the bilayer thickness and less (the mesoscopic scale), and suggest that application of combined R(1Z) and S(CD) studies of phospholipids can be used as a simple membrane elastometer. Heuristic estimates of the bilayer bending rigidity kappa and the area elastic modulus K(a) enable comparison to other biophysical studies, involving macroscopic deformation of thin membrane lipid films. Finally, the bilayer softness may be correlated with the lipid diversity of biomembranes, for example, with regard to membrane curvature, repulsive interactions between bilayers, and lipid-protein interactions.     

Polarization modulation infrared reflection absorption spectroscopy investigations of thin silica films deposited on gold. 2. Structural analysis of a 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer

In this paper we report on the structural analysis of bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) using polarization modulation infrared reflection absorption spectroscopy (PM IRRAS). The lipid bilayers were formed on SiO2|Au and Au surfaces using the Langmuir-Blodgett and Langmuir-Schaeffer techniques. As we showed in part 1 (Zawisza, I.; Wittstock, G.; Boukherroub, R.; Szunertis, S. Langmuir 2007, 23, 9303-9309), SiO2 layers of 7 nm thickness, synthesized by plasma-enhanced chemical vapor deposition on 200 nm thick gold covered glass slides, allow PM IRRAS investigations. Only minor changes in the order and structure of the lipid bilayer are observed when deposited on SiO2|Au and Au surfaces. The choline moiety in the leaflet directed toward the SiO2 surface exists in trans conformation and shows a tilt of 28 degrees with the surface normal of the CN bond. On the silica surface in the second leaflet directed toward air and in two layers deposited on the Au surface, trans and gauche isomers of the choline moiety are present and the tilt of the CN bond increases to 55 degrees with respect to the surface normal. The order and molecular orientation in the DMPC bilayers on SiO2 and Au surfaces are not affected by time. The analysis of the phosphate stretching mode on the Au surface shows slight dehydration of this group and reorientation of the phosphate moiety.     

Structure and electrochemistry of self-organized fullerene-lipid bilayer films

The unique properties of C(60)-bearing artificial lipids with three C(16) (lipid 1), C(14) (lipid 2), or C(12) (lipid 3) alkyl chains have been characterized by a variety of techniques, including (13)C NMR, UV/Vis, and FT-IR spectroscopies, differential scanning calorimetry (DSC), X-ray diffraction, and electrochemistry. The (13)C NMR and UV/Vis spectra show that the lipids 1-3 have a closed aziridine structure at a 6/6-ring junction of C(60). The DSC data reveal that cast films of 1 exhibit two endothermic peaks at temperature ranges of 35-40 degrees C (main transition) and 47-49 degrees C (subtransition) in air, water, and 0.5 M aqueous tetraethylammonium chloride solution, while cast films of 2 and 3 each display one endothermic peak at 50-57 degrees C. The results of temperature-dependent FT-IR and UV/Vis studies of cast films of 1-3, together with the above data, reveal that the main peak in the DSC thermogram of a film of 1 can be attributed to a typical phase transition as seen in lipid bilayer membranes, while the sub-endothermic peak seen with 1 and the peaks for 2 and 3 stem from a change in the orientation of the C(60) moieties. X-ray diffraction patterns of each of the cast films of 1-3 show a diffraction peak corresponding to the (001) plane, suggesting the formation of molecular bilayer membrane structures. Cyclic voltammograms and Osteryoung square-wave voltammograms obtained from cast films of 1-3 on basal plane pyrolytic graphite (BPG) electrodes show strong temperature dependences. Finally, the thermodynamics of the binding of nine different alkylammonium ions and two alkylphosphonium ions to the electrogenerated radical monoanions and dianions of 1-3 cast on electrodes is described.     

Interactions of lipid bilayers with supports: a coarse-grained molecular simulation study

The study of lipid structure and phase behavior at the nanoscale is of utmost importance due to implications in understanding the role of the lipids in biochemical membrane processes. Supported lipid bilayers play a key role in understanding real biological systems, but they are vastly underrepresented in computational studies. In this paper, we discuss molecular dynamics simulations of supported lipid bilayers using a coarse-grained model. We first focus on the technical implications of modeling solid supports for biomembrane simulations. We then describe noticeable influences of the support on the systems. We are able to demonstrate that the bilayer system behavior changes when supported by a hydrophilic surface. We find that the thickness of the water layer between the support and the bilayer (the inner-water region in the latter part of this paper) adapts through water permeation on the microsecond time scale. Additionally, we discuss how different surface topologies affect the bilayer. Finally, we point out the differences between the two leaflets induced by the support.     

Comparative molecular dynamics study of ether- and ester-linked phospholipid bilayers

The lipid membranes found in archaea have high bilayer stability and low permeability. The molecular structure of their constituent lipids is characterized by ether-linked, branched hydrophobic chains, whereas the conventional lipids obtained from eukaryotic or eubacterial sources have ester linked straight chains. In order to elucidate the influence of the ether linkage, instead of an ester one, on the physical properties of the lipid bilayers, we have carried out comparative 10 ns molecular dynamics simulations of diphytanyl phosphatidylcholine (ether-DPhPC) and diphytanoyl phosphatidylcholine (ester-DPhPC) bilayers in water, respectively. We analyze bilayer structures, hydration of the lipids, membrane dipole potentials, and free energy profiles of water and oxygen across the bilayers. We observe that the membrane dipole potential for the ether-DPhPC bilayer, which arises mainly from the ether linkage, is about half of that of the ester-DPhPC. The calculated free energy barrier for a water molecule in the ether-DPhPC bilayer system is slightly higher than that in the ester-DPhPC counterpart, which is in accord with experimental data.