Bilayer structure in membranes

Membrane Mixer: A Toolkit for Efficient Shuffling of Lipids in Heterogeneous Biological Membranes

Molecular dynamics (MD) simulations of biological membranes have achieved such levels of sophistication that are commonly used to predict unresolved structures and various properties of lipids and to substantiate experimental data. While achieving sufficient sampling of lipid dynamics remains a major challenge, a commonly used method to improve lipid sampling, e.g., in terms of specific interactions with membrane-associated proteins, is to randomize the initial arrangement of lipid constituents in multiple replicas of simulations, without changing the overall lipid composition of the membrane of interest. Here, we introduce a method that can rapidly generate multiple replicas of lipid bilayers with different spatial and conformational configurations for any given lipid composition. The underlying algorithm, which allows one to shuffle lipids at any desired level, relies on the application of an external potential, here referred to as the "carving potential", that removes clashes/entanglements before lipid positions are exchanged (shuffled), thereby minimizing the energy penalty due to abrupt lipid repositioning. The method is implemented as "Membrane Mixer Plugin (MMP) 1.0" in VMD, with a convenient graphical user interface that guides the user in setting various options and parameters. The plugin is fully automated and generates new membrane replicas more rapidly and conveniently than other analogous tools. The plugin and its capabilities introduced here can be extended to include additional features in future versions.

Enzyme-Responsive Aqueous Two-Phase Systems in a Cationic-Anionic Surfactant Mixture

Enzyme-instructed self-assembly is an increasingly attractive topic owing to its broad applications in biomaterials and biomedicine. In this work, we report an approach to construct enzyme-responsive aqueous surfactant two-phase (ASTP) systems serving as enzyme substrates by using a cationic surfactant (myristoylcholine chloride) and a series of anionic surfactants. Driven by the hydrophobic interaction and electrostatic attraction, self-assemblies of cationic-anionic surfactant mixtures result in biphasic systems containing condensed lamellar structures and coexisting dilute solutions, which turn into homogeneous aqueous phases in the presence of hydrolase (cholinesterase). The enzyme-sensitive ASTP systems reported in this work highlight potential applications in the active control of biomolecular enrichment/release and visual detection of cholinesterase.

Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1

The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl^-/HCO₃^- exchange, one of the steps in CO₂ excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl^- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO₃^-, Cl^-, O₂, CO₂, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.

How We Came to Understand the "Tumultuous Chemical Heterogeneity" of the Lipid Bilayer Membrane

The path to our modern understanding of the structure of the lipid bilayer membrane is a long one that can be traced from today perhaps as far back as Benjamin Franklin in the eighteenth century. Here, I provide a personal account of one of the important steps in that path, the description of the "Complete Structure" of a hydrated, fluid phase dioleoyl phosphatidylcholine bilayer by the joint refinement of neutron and X-ray diffraction data by Stephen White and his colleagues.

Photo-responsive liposomes composed of spiropyran-containing triazole-phosphatidylcholine: investigation of merocyanine-stacking effects on liposome-fiber assembly-transition

A spiropyran-containing triazole-phosphatidylcholine (SPTPC) was synthesized through a copper-catalyzed azide alkyne cyclo-addition (CuAAC) reaction. In water, SPTPCs self-assembled and a spontaneous spiropyran-to-merocyanine (SP-to-MC) isomerization occurred, resulting in coexistence of liposomes and fibers, and switching from the spiropyran (SP) to the merocyanine (MC) isomeric structure induced a reversible transition between these molecular assemblies. Study of the self-assembly of SPTPCs and photo-induced liposome-fiber assembly-transition revealed that the presence of MC enabled additional inter-membrane interaction during self-assembly and that the MC-stacking effect was the driving force for the assembly-transition. Exposure to UV light induced switching from SP to MC, where the planar structure of MC and the confinement of MC led to enhanced MC-stacking. The effect of MC-stacking was both advantageous and disadvantageous: MC-stacking perturbed the hydrophobic phase in the bilayer membrane and facilitated the liposome-to-fiber transition, otherwise the MC-stacking retarded switching of MC to SP, and caused an incomplete recovery of MC to SP during fiber-to-liposome recovery, thus a fatigue of SP was induced by MC-stacking during the liposome-to-fiber transition cycle. To decrease the intermolecular interactions and suppress MC-stacking, photo-inert triazole-phosphatidylcholine (TPC) was incorporated to prepare two-component TPC/SPTPC-liposomes, which exhibited better recovery kinetics. The photo-adaptive behavior of TPC/SPTPC-liposomes confirmed the disturbance of bilayer membranes by inter-membrane MC-stacking and the formation of MCTPC-enriched phases in the bilayer membrane.

The lipid bilayer membrane and its protein constituents

Robertson recounts the historical experiments that gave rise to our current understanding of cell membranes.

In 1918, the year the Journal of General Physiology was founded, there was little understanding of the structure of the cell membrane. It was evident that cells had invisible barriers separating the cytoplasm from the external solution. However, it would take decades before lipid bilayers were identified as the essential constituent of membranes. It would take even longer before it was accepted that there existed hydrophobic proteins that were embedded within the membrane and that these proteins were responsible for selective permeability in cells. With a combination of intuitive experiments and quantitative thinking, the last century of cell membrane research has led us to a molecular understanding of the structure of the membrane, as well as many of the proteins embedded within. Now, research is turning toward a physical understanding of the reactions of membrane proteins and lipids in this unique and incredibly complex solvent environment.

Biophysical characterization of asolectin-squalene liposomes

Liposomes are shell nanoparticles able to embed hydrophobic molecules into their lipid layers to be released to cells. In pharmaceutical sciences, liposomes remain the delivery system with the highest biocompatibility, stability, loading characteristics, tunable physicochemical properties. Squalene is a natural, water insoluble, lipid, abundant in olive oil and shark liver. Studies in vitro and in animal models suggest protective and inhibitory effects of squalene against cancer. To study its effect on cells, and to overcome its insolubility in water, we have designed and produced large unilamellar liposomes containing different quantities of this terpene (0%, 2.8%, 5% w/w). Liposomes have been characterized by different biophysical techniques. Size-exclusion and affinity chromatography showed a unimodal size distribution and confirmed the squalene loaded dose. Laurdan fluorescence evidenced the changes in the hydration of the external layer of liposomes as a function of squalene concentration. Dynamic light scattering and small angle X-ray scattering revealed squalene induced structural differences in the hydrodynamic radius distribution and in the bilayer thickness respectively. Finally, preliminary experiments on the effects of liposome-delivered squalene on tumor and non-tumor cell lines showed time- and dose-dependent cytotoxic effects on LAN5 tumor cells and no effect on NIH-3T3 normal cells.

Pickering Membranes Stabilized by Saturn Particles

We report on a novel method to synthesize particles-called by us Saturn particles-having two hydrophobic caps that oppose each other and are separated from each other by a hydrophilic belt that encircles the particle. Mixtures of these particles with water and air, without the usage of low molar mass surfactants, easily form Pickering foams and Pickering membranes that are stable for days. These Pickering membranes are composed of a thin film of water into which the particles are embedded in such a way that the belt is surrounded by the water and the caps protrude out of the water into the air at the top and bottom side of the water film. As expected for a liquid membrane, these Pickering membranes are permeable for gases, with the permeance being proportional to the solubility and diffusion coefficient of the gas considered. Experimentally obtained permeance values agree reasonably well with theoretical calculations.

The in vivo structure of biological membranes and evidence for lipid domains

Examining the fundamental structure and processes of living cells at the nanoscale poses a unique analytical challenge, as cells are dynamic, chemically diverse, and fragile. A case in point is the cell membrane, which is too small to be seen directly with optical microscopy and provides little observational contrast for other methods. As a consequence, nanoscale characterization of the membrane has been performed ex vivo or in the presence of exogenous labels used to enhance contrast and impart specificity. Here, we introduce an isotopic labeling strategy in the gram-positive bacterium Bacillus subtilis to investigate the nanoscale structure and organization of its plasma membrane in vivo. Through genetic and chemical manipulation of the organism, we labeled the cell and its membrane independently with specific amounts of hydrogen (H) and deuterium (D). These isotopes have different neutron scattering properties without altering the chemical composition of the cells. From neutron scattering spectra, we confirmed that the B. subtilis cell membrane is lamellar and determined that its average hydrophobic thickness is 24.3 ± 0.9 Ångstroms (Å). Furthermore, by creating neutron contrast within the plane of the membrane using a mixture of H- and D-fatty acids, we detected lateral features smaller than 40 nm that are consistent with the notion of lipid rafts. These experiments-performed under biologically relevant conditions-answer long-standing questions in membrane biology and illustrate a fundamentally new approach for systematic in vivo investigations of cell membrane structure.

Principal Component Analysis of Lipid Molecule Conformational Changes in Molecular Dynamics Simulations

Molecular dynamics simulations of lipid bilayers are ubiquitous nowadays. Usually, either global properties of the bilayer or some particular characteristics of each lipid molecule are evaluated in such simulations, but the structural properties of the molecules as a whole are rarely studied. Here, we show how a comprehensive quantitative description of conformational space and dynamics of a single lipid molecule can be achieved via the principal component analysis (PCA). We illustrate the approach by analyzing and comparing simulations of DOPC bilayers obtained using eight different force fields: all-atom generalized AMBER, CHARMM27, CHARMM36, Lipid14, and Slipids and united-atom Berger, GROMOS43A1-S3, and GROMOS54A7. Similarly to proteins, most of the structural variance of a lipid molecule can be described by only a few principal components. These major components are similar in different simulations, although there are notable distinctions between the older and newer force fields and between the all-atom and united-atom force fields. The DOPC molecules in the simulations generally equilibrate on the time scales of tens to hundreds of nanoseconds. The equilibration is the slowest in the GAFF simulation and the fastest in the Slipids simulation. Somewhat unexpectedly, the equilibration in the united-atom force fields is generally slower than in the all-atom force fields. Overall, there is a clear separation between the more variable previous generation force fields and significantly more similar new generation force fields (CHARMM36, Lipid14, Slipids). We expect that the presented approaches will be useful for quantitative analysis of conformations and dynamics of individual lipid molecules in other simulations of lipid bilayers.

Cell membrane causes the lipid bilayers to behave as variable capacitors: A resonance with self-induction of helical proteins

Cell membrane has a unique feature of storing biological energies in a physiologically relevant environment. This study illustrates a capacitor model of biological cell membrane including DPPC structures. The electron density profile models, electron localization function (ELF) and local information entropy have been applied to study the interaction of proteins with lipid bilayers in the cell membrane. The quantum and coulomb blockade effects of different thicknesses in the membrane have also been specifically investigated. It has been exhibited the quantum effects can appear in a small region of the free space within the membrane thickness due to the number and type of phospholipid layers. In addition, from the viewpoint of quantum effects by Heisenberg rule, it is shown the quantum tunneling is allowed in some micro positions while it is forbidden in other forms of membrane capacitor systems. Due to the dynamical behavior of the cell membrane, its capacitance is not fixed which results a variable capacitor. In presence of the external fields through protein trance membrane or ions, charges exert forces that can influence the state of the cell membrane. This causes to appear the charge capacitive susceptibility that can resonate with self-induction of helical coils; the resonance of which is the main reason for various biological pulses.

Structural Significance of Lipid Diversity as Studied by Small Angle Neutron and X-ray Scattering

We review recent developments in the rapidly growing field of membrane biophysics, with a focus on the structural properties of single lipid bilayers determined by different scattering techniques, namely neutron and X-ray scattering. The need for accurate lipid structural properties is emphasized by the sometimes conflicting results found in the literature, even in the case of the most studied lipid bilayers. Increasingly, accurate and detailed structural models require more experimental data, such as those from contrast varied neutron scattering and X-ray scattering experiments that are jointly refined with molecular dynamics simulations. This experimental and computational approach produces robust bilayer structural parameters that enable insights, for example, into the interplay between collective membrane properties and its components (e.g., hydrocarbon chain length and unsaturation, and lipid headgroup composition). From model studies such as these, one is better able to appreciate how a real biological membrane can be tuned by balancing the contributions from the lipid's different moieties (e.g., acyl chains, headgroups, backbones, etc.).

Studying antibiotic-membrane interactions via X-ray diffraction and fluorescence microscopy

Antibiotic drug resistance is a serious issue for the treatment of bacterial infection. Understanding the resistance to antibiotics is a key issue for developing new drugs. We used penicillin and sulbactam as model antibiotics to study their interaction with model membranes. Cholesterol was used to target the membrane for comparison with the well-known insertion model. Lamellar X-ray diffraction (LXD) was used to determine membrane thickness using successive drug-to-lipid molar ratios. The aspiration method for a single giant unilamellar vesicle (GUV) was used to monitor the kinetic binding process of antibiotic-membrane interactions in an aqueous solution. Both penicillin and sulbactam are found positioned outside the model membrane, while cholesterol inserts perpendicularly into the hydrophobic region of the membrane in aqueous solution. This result provides structural insights for understanding the antibiotic-membrane interaction and the mechanism of antibiotics.

Organization and dynamics of SNARE proteins in the presynaptic membrane

Our view of the lateral organization of lipids and proteins in the plasma membrane has evolved substantially in the last few decades. It is widely accepted that many, if not all, plasma membrane proteins and lipids are organized in specific domains. These domains vary widely in size, composition, and stability, and they represent platforms governing diverse cell functions. The presynaptic plasma membrane is a well-studied example of a membrane which undergoes rearrangements, especially during exo- and endocytosis. Many proteins and lipids involved in presynaptic function are known, and major efforts have been made to understand their spatial organization and dynamics. Here, we focus on the mechanisms underlying the organization of SNAREs, the key proteins of the fusion machinery, in distinct domains, and we discuss the functional significance of these clusters.

Model-based approaches for the determination of lipid bilayer structure from small-angle neutron and X-ray scattering data

Some of our recent work has resulted in the detailed structures of fully hydrated, fluid phase phosphatidylcholine (PC) and phosphatidylglycerol (PG) bilayers. These structures were obtained from the joint refinement of small-angle neutron and X-ray data using the scattering density profile (SDP) models developed by Kučerka et al. (Biophys J 95:2356-2367, 2008; J Phys Chem B 116:232-239, 2012). In this review, we first discuss models for the standalone analysis of neutron or X-ray scattering data from bilayers, and assess the strengths and weaknesses inherent to these models. In particular, it is recognized that standalone data do not contain enough information to fully resolve the structure of naturally disordered fluid bilayers, and therefore may not provide a robust determination of bilayer structure parameters, including the much-sought-after area per lipid. We then discuss the development of matter density-based models (including the SDP model) that allow for the joint refinement of different contrast neutron and X-ray data, as well as the implementation of local volume conservation within the unit cell (i.e., ideal packing). Such models provide natural definitions of bilayer thicknesses (most importantly the hydrophobic and Luzzati thicknesses) in terms of Gibbs dividing surfaces, and thus allow for the robust determination of lipid areas through equivalent slab relationships between bilayer thickness and lipid volume. In the final section of this review, we discuss some of the significant findings/features pertaining to structures of PC and PG bilayers as determined from SDP model analyses.

Thermal, dynamic and structural properties of drug AT1 antagonist olmesartan in lipid bilayers

It is proposed that AT1 antagonists (ARBs) exert their biological action by inserting into the lipid membrane and then diffuse to the active site of AT1 receptor. Thus, lipid bilayers are expected to be actively involved and play a critical role in drug action. For this reason, the thermal, dynamic and structural effects of olmesartan alone and together with cholesterol were studied using differential scanning calorimetry (DSC), 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR), cross-polarization (CP) MAS NMR, and Raman spectroscopy as well as small- and wide angle X-ray scattering (SAXS and WAXS) on dipalmitoyl-phosphatidylcholine (DPPC) multilamellar vesicles. 13C CP/MAS spectra provided direct evidence for the incorporation of olmesartan and cholesterol in lipid bilayers. Raman and X-ray data revealed how both molecules modify the bilayer's properties. Olmesartan locates itself at the head-group region and upper segment of the lipid bilayers as 13C CP/MAS spectra show that its presence causes significant chemical shift changes mainly in the A ring of the steroidal part of cholesterol. The influence of olmesartan on DPPC/cholesterol bilayers is less pronounced. Although, olmesartan and cholesterol are residing at the same region of the lipid bilayers, due to their different sizes, display distinct impacts on the bilayer's properties. Cholesterol broadens significantly the main transition, abolishes the pre-transition, and decreases the membrane fluidity above the main transition. Olmesartan is the only so far studied ARB that increases the gauche:trans ratio in the liquid crystalline phase. These significant differences of olmesartan may in part explain its distinct pharmacological profile.

Unprecedented observation of days-long remnant orientation of phospholipid bicelles: a small-angle X-ray scattering and theoretical study

Nanometric bilayer-based self-assembled micelles commonly named as bicelles, formed with a mixture of long and short chains phosphatidylcholine lipids (PC), are known to orient spontaneously in a magnetic field. This field-induced orientational order strongly depends on the molecular structure of the phospholipids. Using small-angle X-ray scattering (SAXS), we performed detailed structural studies of bicelles and investigated the orientation/relaxation kinetics in three different systems: saturated-chain lipid bicelles made of DMPC (dimyristoyl PC)/DCPC (1,2-dicaproyl PC) with and without the added paramagnetic lanthanide ions Eu(3+), as well as bicelles of TBBPC (1-tetradecanoyl-2-(4-(4-biphenyl)butanoyl)-sn-glycero-3-PC)/DCPC. The structural study confirmed the previous NMR studies, which showed that DMPC bicelles orient with the membrane normal perpendicular (defined here as "nematic" orientation) to the magnetic field, whereas they orient parallel (defined here as "smectic" orientation) to the magnetic field in the presence of Eu(3+). The TBBPC bicelles also show smectic orientation. Surprisingly, the orientational order induced in the magnetic field remains even after the magnetic field is removed, which allowed us to investigate the orientation and relaxation kinetics of different bicelle structures. We demonstrate that this kinetics is very different for all three types of bicelles at the same lipid concentration; DMPC bicelles (~40 nm diameter) with and without Eu(3+) orient faster than TBBPC bicelles (~80 nm diameter). However, for the relaxation, DMPC bicelles (nematic) lose their macroscopic orientation only after one hour, whereas both DMPC bicelles with Eu(3+) and TBBPC bicelles (smectic) remarkably stay oriented for up to several days! These results indicate that the orientation mechanism of these nanometric disks in the magnetic field is governed by their size, with smaller bicelles orienting faster than the larger bicelles. Their relaxation mechanism outside the magnetic field, however, is governed by the degree of ordering. Indeed, the angular distribution of oriented bicelles is much narrower for the bicelles with smectic orientation, and, consequently, they keep aligned for much longer time (days) than those with nematic ordering (hours) outside the magnetic field. The understanding of the orientation/relaxation kinetics, as well as the morphologies of these "molecular goniometers" at molecular and supramolecular levels, allows controlling such an unprecedented long-range and long-lived smectic ordering of nanodisks and opens a wide field of applications for structural biology or material sciences.

Neutron diffraction studies on the location of water in lecithin bilayer model membranes

Lamellar neutron diffraction from oriented multilayers of hydrated dipalmitoyl lecithin was phased by isomorphous H(2)O-D(2)O exchange and swelling techniques. Bound water sites were located in the polar head group region of the bilayer profile. A 6-A resolution structure based on the neutron scattering density profile is proposed for the bilayer. It is consistent with the electron density profile from x-ray diffraction.

Measurement of the membrane curvature preference of phospholipids reveals only weak coupling between lipid shape and leaflet curvature

In biological processes, such as fission, fusion and trafficking, it has been shown that lipids of different shapes are sorted into regions with different membrane curvatures. This lipid sorting has been hypothesized to be due to the coupling between the membrane curvature and the lipid's spontaneous curvature, which is related to the lipid's molecular shape. On the other hand, theoretical predictions and simulations suggest that the curvature preference of lipids, due to shape alone, is weaker than that observed in biological processes. To distinguish between these different views, we have directly measured the curvature preferences of several lipids by using a fluorescence-based method. We prepared small unilamellar vesicles of different sizes with a mixture of egg-PC and a small mole fraction of N-nitrobenzoxadiazole (NBD)-labeled phospholipids or lysophospholipids of different chain lengths and saturation, and measured the NBD equilibrium distribution across the bilayer. We observed that the transverse lipid distributions depended linearly on membrane curvature, allowing us to measure the curvature coupling coefficient. Our measurements are in quantitative agreement with predictions based on earlier measurements of the spontaneous curvatures of the corresponding nonfluorescent lipids using X-ray diffraction. We show that, though some lipids have high spontaneous curvatures, they nevertheless showed weak curvature preferences because of the low values of the lipid molecular areas. The weak curvature preference implies that the asymmetric lipid distributions found in biological membranes are not likely to be driven by the spontaneous curvature of the lipids, nor are lipids discriminating sensors of membrane curvature.

Structure of chlorosomes from the green filamentous bacterium Chloroflexus aurantiacus

The green filamentous bacterium Chloroflexus aurantiacus employs chlorosomes as photosynthetic antennae. Chlorosomes contain bacteriochlorophyll aggregates and are attached to the inner side of a plasma membrane via a protein baseplate. The structure of chlorosomes from C. aurantiacus was investigated by using a combination of cryo-electron microscopy and X-ray diffraction and compared with that of Chlorobi species. Cryo-electron tomography revealed thin chlorosomes for which a distinct crystalline baseplate lattice was visualized in high-resolution projections. The baseplate is present only on one side of the chlorosome, and the lattice dimensions suggest that a dimer of the CsmA protein is the building block. The bacteriochlorophyll aggregates inside the chlorosome are arranged in lamellae, but the spacing is much greater than that in Chlorobi species. A comparison of chlorosomes from different species suggested that the lamellar spacing is proportional to the chain length of the esterifying alcohols. C. aurantiacus chlorosomes accumulate larger quantities of carotenoids under high-light conditions, presumably to provide photoprotection. The wider lamellae allow accommodation of the additional carotenoids and lead to increased disorder within the lamellae.

Examination of the structure of the bovine milk-fat-globule membrane and of cheese by X-ray crystallography

Low-angle X-ray diffraction techniques have been applied to the study of the structure of the fat-globule membrane of milk and of Cheddar cheese. Membrane was prepared by freeze-thawing, churning and ether extraction, the endproduct of each method being a pellet produced by centrifuging. High-angle and low-angle reflexions from membrane could be related to triglyceride spacings, and low-angle reflexions due to spacings at about 5·5 nm to phospholipid bilayers. Spacings due to triglycerides were observed in mature Cheddar together with spacings that agreed well with values for some of the amino acids.

Simulation-based methods for interpreting x-ray data from lipid bilayers

The fully hydrated liquid crystalline phase of the dimyristoylphosphatidycholine lipid bilayer at 30 degrees C was simulated using molecular dynamics with the CHARMM potential for five surface areas per lipid (A) in the range 55-65 A(2) that brackets the previously determined experimental area 60.6 A(2). The results of these simulations are used to develop a new hybrid zero-baseline structural model, denoted H2, for the electron density profile, rho(z), for the purpose of interpreting x-ray diffraction data. H2 and also the older hybrid baseline model were tested by fitting to partial information from the simulation and various constraints, both of which correspond to those available experimentally. The A, rho(z), and F(q) obtained from the models agree with those calculated directly from simulation at each of the five areas, thereby validating this use of the models. The new H2 was then applied to experimental dimyristoylphosphatidycholine data; it yields A = 60.6 +/- 0.5 A(2), in agreement with the earlier estimate obtained using the hybrid baseline model. The electron density profiles also compare well, despite considerable differences in the functional forms of the two models. Overall, the simulated rho(z) at A = 60.7 A(2) agrees well with experiment, demonstrating the accuracy of the CHARMM lipid force field; small discrepancies indicate targets for improvements. Lastly, a simulation-based model-free approach for obtaining A is proposed. It is based on interpolating the area that minimizes the difference between the experimental F(q) and simulated F(q) evaluated for a range of surface areas. This approach is independent of structural models and could be used to determine structural properties of bilayers with different lipids, cholesterol, and peptides.

Structure and thermotropic phase behaviour of detergent-resistant membrane raft fractions isolated from human and ruminant erythrocytes

Detergent-resistant membrane raft fractions have been prepared from human, goat, and sheep erythrocyte ghosts using Triton X-100. The structure and thermotropic phase behaviour of the fractions have been examined by freeze-fracture electron microscopy and synchrotron X-ray diffraction methods. The raft fractions are found to consist of vesicles and multilamellar structures indicating considerable rearrangement of the original ghost membrane. Few membrane-associated particles typical of freeze-fracture replicas of intact erythrocyte membranes are observed in the fracture planes. Synchrotron X-ray diffraction studies during heating and cooling scans showed that multilamellar structures formed by stacks of raft membranes from all three species have d-spacings of about 6.5 nm. These structures can be distinguished from peaks corresponding to d-spacings of about 5.5 nm, which were assigned to scattering from single bilayer vesicles on the basis of the temperature dependence of their d-spacings compared with the multilamellar arrangements. The spacings obtained from multilamellar stacks and vesicular suspensions of raft membranes were, on average, more than 0.5 nm greater than corresponding arrangements of erythrocyte ghost membranes from which they were derived. The trypsinization of human erythrocyte ghosts results in a small decrease in lamellar d-spacing, but rafts prepared from trypsinized ghosts exhibit an additional lamellar repeat 0.4 nm less than a lamellar repeat coinciding with rafts prepared from untreated ghosts. The trypsinization of sheep erythrocyte ghosts results in the phase separation of two lamellar repeat structures (d=6.00; 5.77 nm), but rafts from trypsinized ghosts produce a diffraction band almost identical to rafts from untreated ghosts. An examination of the structure and thermotropic phase behaviour of the dispersions of total polar lipid extracts of sheep detergent-resistant membrane preparations showed that a reversible phase separation of an inverted hexagonal structure from coexisting lamellar phase takes place upon heating above about 30 degrees C. Non-lamellar phases are not observed in erythrocytes or detergent-resistant membrane preparations heated up to 55 degrees C, suggesting that the lamellar arrangement is imposed on these membrane lipids by interaction with non-lipid components of rafts and/or that the topology of lipids in the erythrocyte membrane survives detergent treatment.

Structure of fully hydrated fluid phase DMPC and DLPC lipid bilayers using X-ray scattering from oriented multilamellar arrays and from unilamellar vesicles

Quantitative structures of the fully hydrated fluid phases of dimyristoylphosphatidylcholine (DMPC) and dilauroylphosphatidylcholine (DLPC) were obtained at 30 degrees C. Data for the relative form factors F(q(z)) for DMPC were obtained using a combination of four methods. 1), Volumetric data provided F(0). 2), Diffuse x-ray scattering from oriented stacks of bilayers provided relative form factors |F(q(z))| for high q(z), 0.22 < q(z) < 0.8 A(-1). 3), X-ray scattering from extruded unilamellar vesicles with diameter 600 A provided |F(q(z))| for low q(z), 0.1 < q(z) < 0.3 A(-1). 4), Previous measurements using a liquid crystallographic x-ray method provided |F(2 pi h/D)| for h = 1 and 2 for a range of nearly fully hydrated D-spacings. The data from method 4 overlap and validate the new unilamellar vesicles data for DMPC, so method 4 is not required for DLPC or future studies. We used hybrid electron density models to obtain structural results from these form factors. Comparison of the model electron density profiles with that of gel phase DMPC provides areas per lipid A, 60.6 +/- 0.5 A(2) for DMPC and 63.2 +/- 0.5 A(2) for DLPC. Constraints on the model provided by volume measurements and component volumes obtained from simulations put the electron density profiles rho(z) and the corresponding form factors F(q(z)) on absolute scales. Various thicknesses, such as the hydrophobic thickness and the steric thickness, are obtained and compared to literature values.

Patterning adjacent supported lipid bilayers of desired composition to investigate receptor-ligand binding under shear flow

To achieve efficient targeting, carriers containing either drugs or imaging agents must have surface properties that promote binding to targets yet at the same time block rapid immune system clearance. Here we describe a versatile technique that allows simultaneous comparison of the effects of carrier surface composition on binding properties under identical flow conditions. Parallel lanes of supported lipid bilayers that mimic the surface of liposomal delivery vehicles are formed using the vesicle fusion method in microfluidic channels created via standard soft lithography techniques. Vesicle stock solutions are premixed and injected into lanes formed by a poly(dimethylsiloxane) (PDMS) stamp reversibly sealed to a glass slide to create adjacent lanes of distinct composition. After removing the stamp, an adsorbed layer of bovine serum albumin (BSA) is used to prevent bilayer spreading before assembling the patterned substrate into a flow chamber for binding studies. Advantages of this method include easy and rapid preparation of bilayers with desired compositions from an unlimited number of lipid types, choice of feature size, time-stable features, and low nonspecific binding. Feature sizes on the order of tens of microns allow multiple compositions to be analyzed in one field of view, thereby reducing the number of experiments, ensuring identical flow conditions, and enabling simultaneous incorporation of controls. We show that the presence of a long poly(ethylene glycol) (PEG) tether (MW 2000) between the lipid and ligand results in higher detachment resistances as compared to a short six-carbon spacer.

Modulation of the bilayer thickness of exocytic pathway membranes by membrane proteins rather than cholesterol

A biological membrane is conceptualized as a system in which membrane proteins are naturally matched to the equilibrium thickness of the lipid bilayer. Cholesterol, in addition to lipid composition, has been suggested to be a major regulator of bilayer thickness in vivo because measurements in vitro have shown that cholesterol can increase the thickness of simple phospholipid/cholesterol bilayers. Using solution x-ray scattering, we have directly measured the average bilayer thickness of exocytic pathway membranes, which contain increasing amounts of cholesterol. The bilayer thickness of membranes of the endoplasmic reticulum, the Golgi, and the basolateral and apical plasma membranes, purified from rat hepatocytes, were determined to be 37.5 +/- 0.4 A, 39.5 +/- 0.4 A, 35.6 +/- 0.6 A, and 42.5 +/- 0.3 A, respectively. After cholesterol depletion using cyclodextrins, Golgi and apical plasma membranes retained their respective bilayer thicknesses whereas the bilayer thickness of the endoplasmic reticulum and the basolateral plasma membrane decreased by 1.0 A. Because cholesterol was shown to have a marginal effect on the thickness of these membranes, we measured whether membrane proteins could modulate thickness. Protein-depleted membranes demonstrated changes in thickness of up to 5 A, suggesting that (i) membrane proteins rather than cholesterol modulate the average bilayer thickness of eukaryotic cell membranes, and (ii) proteins and lipids are not naturally hydrophobically matched in some biological membranes. A marked effect of membrane proteins on the thickness of Escherichia coli cytoplasmic membranes, which do not contain cholesterol, was also observed, emphasizing the generality of our findings.

Bilayer thickness and lipid interface area in unilamellar extruded 1,2-diacylphosphatidylcholine liposomes: a small-angle neutron scattering study

Small-angle neutron scattering (SANS) experiments have been performed on large unilamellar liposomes prepared from 1,2-dilauroylphosphatidylcholine (DLPC), 1,2-dimyristoyl-phosphatidylcholine (DMPC) and 1,2-distearoylphosphatidylcholine (DSPC) in heavy water by extrusion through polycarbonate filters with 500 A pores. The neutron scattering intensity I(Q) in the region of scattering vectors Q corresponding to 0.0015 A(-2) < or = Q(2) < or = 0.0115 A(-2) was fitted using a step function model of bilayer neutron scattering length density and supposing that the liposomes are spherical and have a Gaussian distribution of radii. Using the lipid volumetric data, and supposing that the thickness of bilayer polar region equals to d(H) = 9+/-1 A and the water molecular volume intercalated in the bilayer polar region is the same as in the aqueous bulk aqueous phase, the steric bilayer thickness d(L), the lipid surface area A(L) and the number of water molecules per lipid molecule N intercalated in the bilayer polar region were obtained: d(L) = 41.58+/-1.93 A, A(L) = 57.18+/-1.00 A(2) and N = 6.53+/-1.93 in DLPC at 20 degrees C, d(L) = 44.26+/-1.42 A, A(L) = 60.01+/-0.75 A(2) and N = 7.37+/-1.94 in DMPC at 36 degrees C, and d(L) = 49.77+/-1.52 A, A(L) = 64.78+/-0.46 A(2) and N = 8.67+/-1.97 in DSPC at 60 degrees C. After correcting for area thermal expansivity alpha approximately 0.00417 K(-1), the lipid surface area shows a decrease with the lipid acyl chain length at 60 degrees C: A(L) = 67.56+/-1.18 A(2) in DLPC, A(L) = 66.33+/-0.83 A(2) in DMPC and A(L) = 64.78+/-0.46 A(2) in DSPC. It is also shown that a joint evaluation of SANS and small-angle X-ray scattering on unilamellar liposomes can be used to obtain the value of d(H) and the distance of the lipid phosphate group from the bilayer hydrocarbon region d(H1).

Physical and biological properties of cationic triesters of phosphatidylcholine

The properties of a new class of phospholipids, alkyl phosphocholine triesters, are described. These compounds were prepared from phosphatidylcholines through substitution of the phosphate oxygen by reaction with alkyl trifluoromethylsulfonates. Their unusual behavior is ascribed to their net positive charge and absence of intermolecular hydrogen bonding. The O-ethyl, unsaturated derivatives hydrated to generate large, unilamellar liposomes. The phase transition temperature of the saturated derivatives is very similar to that of the precursor phosphatidylcholine and quite insensitive to ionic strength. The dissociation of single molecules from bilayers is unusually facile, as revealed by the surface activity of aqueous liposome dispersions. Vesicles of cationic phospholipids fused with vesicles of anionic lipids. Liquid crystalline cationic phospholipids such as 1, 2-dioleoyl-sn-glycero-3-ethylphosphocholine triflate formed normal lipid bilayers in aqueous phases that interacted with short, linear DNA and supercoiled plasmid DNA to form a sandwich-structured complex in which bilayers were separated by strands of DNA. DNA in a 1:1 (mol) complex with cationic lipid was shielded from the aqueous phase, but was released by neutralizing the cationic charge with anionic lipid. DNA-lipid complexes transfected DNA into cells very effectively. Transfection efficiency depended upon the form of the lipid dispersion used to generate DNA-lipid complexes; in the case of the O-ethyl derivative described here, large vesicle preparations in the liquid crystalline phase were most effective.

Mapping the mAb 383C epitope to alpha 2(187-199) of the Torpedo acetylcholine receptor on the three-dimensional model

Monoclonal antibody 383C is an anti-acetylcholine receptor antibody whose binding to the receptor is blocked by alpha-bungarotoxin and by carbamylcholine. Monoclonal antibody 383C binds to the alpha subunit of the Torpedo acetylcholine (ACh) receptor as well as to its V8-protease 20 kDa fragment that possesses the affinity alkylatable Cys192/193. In an epitope scanning experiment spanning the N-terminal 211 amino acid residues of the alpha subunit, 383C binds uniquely to three overlapping peptides; alpha(184-196), alpha(187-199) and alpha(190-202). These peptides span a cluster of amino acid residues implicated in the binding of acetylcholine, including Cys192/193. To map the location of these residues on the three-dimensional model of the ACh receptor, we have employed a combination of X-ray diffraction from oriented complexes of 383C with ACh receptor-enriched membrane vesicles and electron microscopy of negatively stained tubular arrays of 383C/receptor complexes. The X-ray diffraction study finds extra electron density in the presence of 383C centered 35 A above the synaptic side phosphate head groups. The electron micrographic images display extra stain exclusion from the antibody at a site adjacent to the alpha2 subunit on the periphery of the rosette clockwise to the alpha2 vertex. This mapping localizes several residues of the ACh receptor alpha subunit involved in the binding of acetylcholine. Despite these residues being present in both alpha subunits, only the alpha2 subunit is decorated with this monoclonal antibody.

X-ray structure of bacteriorhodopsin at 2.5 angstroms from microcrystals grown in lipidic cubic phases

Lipidic cubic phases provide a continuous three-dimensional bilayer matrix that facilitates nucleation and growth of bacteriorhodopsin microcrystals. The crystals diffract x-rays isotropically to 2.0 angstroms. The structure of this light-driven proton pump was solved at a resolution of 2.5 angstroms by molecular replacement, using previous results from electron crystallographic studies as a model. The earlier structure was generally confirmed, but several differences were found, including loop conformations and side chain residues. Eight water molecules are now identified experimentally in the proton pathway. These findings reveal the constituents of the proton translocation pathway in the ground state.

Small angle x-ray scattering studies of magnetically oriented lipid bilayers

Magnetically oriented lipid/detergent bilayers are potentially useful for studies of membrane-associated molecules and complexes using x-ray scattering and nuclear magnetic resonance (NMR). To establish whether the system is a reasonable model of a phospholipid bilayer, we have studied the system using x-ray solution scattering to determine the bilayer thickness, interparticle spacing, and orientational parameters for magnetically oriented lipid bilayers. The magnetically orientable samples contain the phospholipid L-alpha-dilauroylphosphatidylcholine (DLPC) and the bile salt analog 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxy-1-propanesulfonate (CHAPSO) in a 3:1 molar ratio in 70% water (w/v) and are similar to magnetically orientable samples used as NMR media for structural studies of membrane-associated molecules. A bilayer thickness of 30 A was determined for the DLPC/CHAPSO particles, which is the same as the bilayer thickness of pure DLPC vesicles, suggesting that the CHAPSO is not greatly perturbing the lipid bilayer. These data, as well as NMR data on molecules incorporated in the oriented lipid particles, are consistent with the sample consisting of reasonably homogeneous and well dispersed lipid particles. Finally, the orientational energy of the sample suggests that the size of the cooperatively orienting unit in the samples is 2 x 10(7) phospholipid molecules.

Small angle X-ray scattering: possibilities and limitations in characterization of vesicles

The use of small angle X-ray scattering (SAXS) for characterization of lipid vesicle dispersions is described. The effect of curvature of the membrane, the presence of proteins in the core and on the surface of the membrane, variations in membrane thickness and distribution in the number of bilayers of the vesicles in the dispersion on the scattering curve is discussed. Concerning unilamellar vesicles, either the membrane curvature of vesicles smaller than 50 nm or variations in membrane thickness result in a disappearance of the first node in the scattering curve, even if the bilayer is symmetric with respect to the electron density distribution. In the case of dispersion in which unilamellar as well as multilamellar vesicles are present it is shown that a small fraction of multilamellar liposomes changes the scattering curve dramatically. Liposomes were prepared from various compositions of dipalmitoylphosphatidylcholine (DPPC) and cholesterol hemisuccinate (CHEMS) by the film method. The electron density profile of the bilayers and distribution in the number of bilayers of the liposome dispersions were determined. The average number of bilayers increased as a function of the decrease in CHEMS content. Liposomes with higher CHEMS content than 10 mol% were unilamellar. It seems that increase in charge intercalated in the bilayers resulted in unilamellar vesicles.

Rat cerebral cortical synaptoneurosomal membranes. Structure and interactions with imidazobenzodiazepine and 1,4-dihydropyridine calcium channel drugs

Small angle x-ray scattering has been used to investigate the structure of synaptoneurosomal (SNM) membranes from rat cerebral cortex. Electron micrographs of the preparation showed SNM with classical synaptic appositions intact, other vesicles, occasional mitochondria, and some myelin. An immunoassay for myelin basic protein placed the myelin content of normal rat SNM at less than 2% by weight of the total membrane present. X-Ray diffraction patterns showed five diffraction orders with a unit cell repeat for the membrane of 71 to 78 A at higher hydration states. At lower hydration, 11 orders appeared; the unit cell repeat was 130 A, indicating that the unit cell contained two membranes. Electron density profiles for the 130-A unit cell were determined; they clearly showed the two opposed asymmetrical membranes of the SNM vesicles. SNM membrane/buffer partition coefficients (Kp) of imidazobenzodiazepine and 1,4-dihydropyridine (DHP) calcium channel drugs were measured; Kp's for DHP drugs were approximately five times higher in rabbit light sarcoplasmic reticulum than in SNM. Ro 15-1788 and the DHP BAY K 8644 bind primarily to the outer monolayer of vesicles of intact SNM membranes. Nonspecific equilibrium binding of Ro 15-1788 occurs mainly in the upper acyl chain of the bilayer in lipid extracts of SNM membrane.

Interactions between charged, uncharged, and zwitterionic bilayers containing phosphatidylglycerol

Pressure vs. distance relationships have been obtained for phosphatidylglycerol bilayers, in both charged and uncharged states. Water was removed from the lipid multilayers by the application of osmotic pressures in the range of 0-2.7 x 10(9) dyn/cm2, and the distance between adjacent bilayers was obtained from Fourier analysis of lamellar x-ray diffraction data. For phosphatidylglycerol bilayers made electrically neutral either by lowering the pH or by adding equimolar concentrations of the positively charged lipid stearylamine, the pressure-distance data could be fit with a single exponential. The measured decay lengths were 1.1 A at low pH and 1.5 A with stearylamine, which are similar to decay lengths of the hydration pressure found for gel phases of other neutral bilayers. In addition, the magnitude of this repulsive pressure was proportional to the square of the Volta potential (equivalent to the dipole potential for electrically neutral bilayers) measured in monolayers in equilibrium with bilayers, in agreement with results previously found for the hydration pressure between phosphatidylcholine bilayers. For charged phosphatidylglycerol bilayers, the pressure-distance relation had two distinct regions. For bilayer separations greater than 10 A, the pressure-distance data had an exponential decay length (11 A) and a magnitude consistent with that expected for electrostatic repulsion from double-layer theory. For bilayer separations of 2-10 A, the pressure decayed much more rapidly with increasing bilayer separation (decay length less than 1 A). We interpret these data at low bilayer separations in terms of a combination of hydration repulsion and steric hindrance between the lipid head groups and the sodium ions trapped between apposing bilayers.

X-ray diffraction analysis of myelin lipid/proteolipid protein multilayers

To examine the proposal that myelin proteolipid protein underlies the adhesion of neighboring membranes in central nervous system (CNS) myelin, we carried out X-ray diffraction studies on the structure and interactions of model bilayers composed of total myelin lipids plus proteolipid apoprotein (PLP). Total myelin lipids were used because their heterogeneity was expected to provide an appropriate environment for the integral membrane protein to achieve its native conformation and establish appropriate contacts with the apposed bilayer. We found that incorporation of PLP into myelin lipid bilayers, whether organized into multilamellar vesicles or oriented multilayers, did not appreciably affect the lamellar period, which ranged from 65-71 A. In oriented multilayers, the wide-angle spacing at approximately 4.8 A, which arises from the lateral packing of lipid chains and is perpendicular to the lamellar diffraction, was less oriented and weaker in bilayers containing the protein. These results indicate that PLP was incorporated into the bilayers and had a disordering effect on the hydrocarbon chains but did not extend into the spaces between bilayers. Bilayer profiles calculated from the lamellar diffraction to about 15 A spacing did not show any major changes in the distribution of electron density, suggesting that to moderate resolution, the protein was distributed uniformly across the width of the lipid bilayer. Periodicities measured from osmotically stressed multilamellar vesicles did not depend on the presence of PLP, indicating that the protein did not form stabilizing contacts between bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)