Langmuir-Blodgett film characteristics and phospholipid membrane ion conduction : Part 1. Modification by Cholesterol and Oxidized Derivatives

The energy barrier to inorganic ion conduction through bilayer lipid membranes (BLM) is investigated as a function of molecular packing and dipolar potential characteristics. Arrhenius energy barrier information is derived from temperature-dependent electrochemical experiments with phosphatidyl choline/steroid BLM. The steroids studied at 0.65 mole fraction in phospholipid were 5-cholesten-3β-ol, 5,7-cholestadien-3β-ol, 5-cholesten-3β,7α-diol, 5α-cholestan-3β,5α,6β-triol, 5α-cholestan-5α,6α-epoxy-3β-ol, 5-cholesten-3β-ol-7-one and 5α-cholestan-3-one. Correlation of the barrier magnitude with molecular packing characteristics, obtained by collecting monolayer data from a Langmuir-Blodgett trough, indicates that the BLM ion current is almost completely controlled by molecular density. The sensitivity of the energy barrier as a function of molecular packing is as great as 0.1 eV for a 0.01-nm² adjustment.     

Mechanism of unassisted ion transport across membrane bilayers

To establish how charged species move from water to the nonpolar membrane interior and to determine the energetic and structural effects accompanying this process, we performed molecular dynamics simulations of the transport of Na+ and Cl- across a lipid bilayer located between two water lamellae. The total length of molecular dynamics trajectories generated for each ion was 10 ns. Our simulations demonstrate that permeation of ions into the membrane is accompanied by the formation of deep, asymmetric thinning defects in the bilayer, whereby polar lipid head groups and water penetrate the nonpolar membrane interior. Once the ion crosses the midplane of the bilayer the deformation "switches sides"; the initial defect slowly relaxes, and a defect forms in the outgoing side of the bilayer. As a result, the ion remains well solvated during the process; the total number of oxygen atoms from water and lipid head groups in the first solvation shell remains constant. A similar membrane deformation is formed when the ion is instantaneously inserted into the interior of the bilayer. The formation of defects considerably lowers the free energy barrier to transfer of the ion across the bilayer and, consequently, increases the permeabilities of the membrane to ions, compared to the rigid, planar structure, by approximately 14 orders of magnitude. Our results have implications for drug delivery using liposomes and peptide insertion into membranes.     

Effect of chlorogenic acid on the phase transition in phospholipid and phospholipid/cholesterol membranes

Chlorogenic acid (CGA) is present in many plants, especially in green coffee, dry plums, and bilberries. It is an important bioactive polyphenol. Studies showed that CGA has an antioxidative, bacteriostatic, anticancer, antiviral, and anti-inflammatory activity. Despite great interest in this compound, its interaction with the lipid model membrane has not yet been investigated. To better understand the relationship between the biological activity of CGA and its interaction with biological membranes, the thermotropic behavior of model lipid membranes was investigated. The effect of CGA on the model lipid membrane, specifically on the lipid bilayer phase transitions, was examined by the combined methods: differential scanning calorimetry and fluorescence spectroscopy. In particular, the degree of packing order of the hydrophilic phase of the lipid bilayer was determined using the fluorimetric method with Laurdan and Prodan probes, while the fluorescence anisotropy of the hydrophobic phase with the DPH and TMA-DPH probes. The results of the study show that CGA incorporates mainly into the hydrophilic part of membrane, changing the packing order of the polar heads of lipids. No significant changes were recorded in membrane fluidity of the hydrophobic membrane region, for the fluorescence anisotropy practically did not change. One can thus infer that CGA does not penetrate deep into the hydrophobic area of the membrane.     

Rationalizing the membrane interactions of cationic amphipathic antimicrobial peptides by their molecular shape

Biophysical and structural studies of cationic amphipathic antimicrobial peptides have revealed new mechanistic details concerning their membrane interactions. In interfacial environments the peptides adopt amphipathic conformations and the resulting distribution of polar, charged and hydrophobic residues allows them to partition into the bilayer interface. For several helical peptides it was found that their long axis is oriented parallel to the membrane surface, an arrangement which results in considerable perturbations in the packing of the lipid bilayer. Within the molecular shape concept the peptides act as wedge-like structures which impose positive curvature strain on the membrane. As a consequence a wide variety of morphologies are observed of peptide–lipid mixtures which strongly depend on the detailed peptide sequence, the membrane lipid composition, buffer, temperature and other environmental parameters. Therefore, the peptide–lipid systems are best described by phase diagrams, similar to the ones of detergent–lipid mixtures, encompassing on the one extreme regions where the peptide stabilizes the bilayer and on the other extreme regions where membrane lysis occurs. The effects of peptide sequence, membrane penetration depth, lipid composition and membrane surface charge density on membrane-association, -morphology and the resulting phase boundaries are discussed.     

Molecular dynamics study of catanionic bilayers composed of ion pair amphiphile with double-tailed cationic surfactant

The physical stability of catanionic vesicles is important for the development of novel drug or DNA carriers. For investigating the mechanism by which catanionic vesicles are stabilized, molecular dynamics (MD) simulation is an attractive approach that provides microscopic structural information on the vesicular bilayer. In this study, MD simulation was applied to investigate the bilayer properties of catanionic vesicles composed of an ion pair amphiphile (IPA), hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), and a double-tailed cationic surfactant, ditetradecyldimethylammonium chloride (DTDAC). Structural information regarding membrane elasticity and the organization and conformation of surfactant molecules was obtained based on the resulting trajectory. Simulation results showed that a proper amount of DTDAC could be used to complement the asymmetric structure between HTMA and DS, resulting in an ordered hydrocarbon chain packing within the rigid membrane observed in the mixed HTMA-DS/DTDAC system. The coexistence of gel and fluid phases was also observed in the presence of excess DTDAC. MD simulation results agreed well with results obtained from experimental studies examining mixed HTMA-DS/DTDAB vesicles.     

Effect of acetone accumulation on structure and dynamics of lipid membranes studied by molecular dynamics simulations

The modulation of the properties and function of cell membranes by small volatile substances is important for many biomedical applications. Despite available experimental results, molecular mechanisms of action of inhalants and organic solvents, such as acetone, on lipid membranes remain not well understood. To gain a better understanding of how acetone interacts with membranes, we have performed a series of molecular dynamics (MD) simulations of a POPC bilayer in aqueous solution in the presence of acetone, whose concentration was varied from 2.8 to 11.2 mol%. The MD simulations of passive distribution of acetone between a bulk water phase and a lipid bilayer show that acetone favors partitioning into the water-free region of the bilayer, located near the carbonyl groups of the phospholipids and at the beginning of the hydrocarbon core of the lipid membrane. Using MD umbrella sampling, we found that the permeability barrier of ∼0.5 kcal/mol exists for acetone partitioning into the membrane. In addition, a Gibbs free energy profile of the acetone penetration across a bilayer demonstrates a favorable potential energy well of −3.6 kcal/mol, located at 15–16 Å from the bilayer center. The analysis of the structural and dynamics properties of the model membrane revealed that the POPC bilayer can tolerate the presence of acetone in the concentration range of 2.8–5.6 mol%. The accumulation of the higher acetone concentration of 11.2 mol% results, however, in drastic disordering of phospholipid packing and the increase in the membrane fluidity. The acetone molecules push the lipid heads apart and, hence, act as spacers in the headgroup region. This effect leads to the increase in the average headgroup area per molecule. In addition, the acyl tail region of the membrane also becomes less dense. We suggest, therefore, that the molecular mechanism of acetone action on the phospholipid bilayer has many common features with the effects of short chain alcohols, DMSO, and chloroform.     

Molecular aspects of the interaction between plants sterols and DPPC bilayers: an experimental and theoretical approach

The effect of sterols composition in a lipid bilayer was investigated on membranes of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and mixtures with the plant sterols β-sitosterol and stigmasterol. Differential scanning calorimetry, 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence polarization and infrared spectroscopy studies showed that both sterols changed the packing of the membrane and the hydration of the polar headgroup of the phospholipids, disordering the gel phase and, vice versa, ordering the membrane in the liquid crystalline phase. In all cases some differences among β-sitosterol and stigmasterol could be observed, being β-sitosterol slightly more efficient than stigmasterol in ordering a fluid membrane, bringing the membrane to a more packed liquid ordered phase. Molecular dynamic simulations were carried out to better characterize the distinct behavior of both sterols in a DPPC-membrane. The calculated parameters agreed quite well with the experimental results and a molecular model is proposed to explain differences in the sterols molecules and their effect on the DPPC-bilayer.     

Exciplexes of pyrene with indole and diethylaniline in liposomes and biomembranes

The formation of exciplexes of pyrene with indole and diethylaniline incorporated into the lipid bilayer was observed in phospholipid liposomes and membranes of sarcoplasmic reticulum from rabbit muscles. The intensity and lifetime of pyrene luminescence were found to decrease and the structureless emission of the exciplexes appeared. Exciplex emission in the membrane has a low quantum yield (compared with exciplexes formed in hexane solutions). This is probably due to the presence of polar groups in the membranes. It was shown that the formation of exciplexes is markedly dependent on the physical state of membranes. It is suggested also that pyrene can form an exciplex with tryptophan residues of membrane proteins.     

Effect of Headgroups on Small‐Ion Permeability across Archaea‐Inspired Tetraether Lipid Membranes

This paper examines the effects of four different polar headgroups on small‐ion membrane permeability from liposomes comprised of Archaea‐inspired glycerolmonoalkyl glycerol tetraether (GMGT) lipids. We found that the membrane‐leakage rate across GMGT lipid membranes varied by a factor of ≤1.6 as a function of headgroup structure. However, the leakage rates of small ions across membranes comprised of commercial bilayer‐forming 1‐palmitoyl‐2‐oleoyl‐sn‐glycerol (PO) lipids varied by as much as 32‐fold within the same series of headgroups. These results demonstrate that membrane leakage from GMGT lipids is less influenced by headgroup structure, making it possible to tailor the structure of the polar headgroups on GMGT lipids while retaining predictable leakage properties of membranes comprised of these tethered lipids.     

Chloride ion conduction without water coordination in the pore of ClC protein

In the present work, we have found by an atomistic molecular dynamics simulation that hydrogen atoms originating from the residues of a prokaryotic ClC protein (EcClC) stabilize the chloride ion without water molecules in the pore of ClC protein. When the chloride ion conduction is simulated by pulling a chloride ion along the pore axis, the free energy barrier for chloride ion conduction is calculated to be low (4 kcal/mol), although the chloride ion is stripped of its hydration shell as it passes through the dehydrated pore region. The calculation of the number of hydrogen atoms surrounding the chloride ion reveals that water molecules hydrating the chloride ion are replaced by polar and non‐polar hydrogen atoms protruding from the protein residues. From the analysis of the pair interaction energy between the chloride ion and these hydrogen atoms, it is realized that the hydrogen atoms from the protein residues stabilize the chloride ion at the dehydrated region instead of water molecules, by which the energetic penalty for detaching water molecules from the permeating ion is compensated. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

864 — Electrochemistry of asymmetric membranes

The ion transport behaviour of neutral carriers (ionophores) in asymmetric membrane environments has been investigated. Composite solvent-polymeric membranes consisting of two segments of different polarity exhibit two different selectivities, depending on the orientation of the membrane and on the direction of ion transport, respectively. From zero-current membrane potentials and electrodialytic ion transport numbers, the permeability selectivity Ca²⁺ > Na⁺ is found when the ions enter the polar membrane side, and the selectivity Na⁺ > Ca²⁺ for the non-polar entry. The asymmetry effects expected for ionophores in bilayer membranes are also discussed.     

Aqueous dispersions of cubic lipid–water phases

Inverse lipid–water phases such as cubic phases can form kinetically stable dispersions by fragmentation in water. Cubic lipid phases can be dispersed by polar lipids favoring lamellar phases or by block copolymers, which can close the bilayer at the surface so that the hydrocarbon chain core is not exposed to water. Monodisperse particles based on glycerol monooleate, with their bilayer curved as the P-, D- or G-minimal surface, have been prepared in this way. Their inner bilayer conformation and outer shape have been examined, mainly by X-ray diffraction and cryo transmission electron microscopy. There is also a different type of cubic lipid bilayer particles with a periodicity in the micrometer range, which have been identified in phospholipid–water dispersions and in cell membrane assemblies. The mechanism behind formation in vivo of such cubic membranes, which also follow the P-, D- and G-surfaces, is discussed. Other lipid–water dispersions with lower symmetry are finally considered; dispersions formed by the inverse hexagonal phase and the dispersed state of a tetragonal bilayer structure formed by lung surfactants.     

Phospholipid Bilayers as Molecular Models for Drug-Cell Membrana Interactions. The Case of the Antiinflamatory Drug Diclofenac

Phospholipids are amphipatic molecules with long hydrophobic acyl chains and zwitterionic polar heads which assemble into different types of molecular aggregates. The most relevant is the bilayer because of its relation with cell membranes, which are very complex entities. For this reason, simpler molecular models based on phospholipids bilayers are widely used. We have determined the bilayer structure of phospholipids located in the outer and inner monolayers of most cell membranes, and use them as molecular models to study the way different chemicals of biological interest interact with cell membranes. We present the results of our studies on the nonsteroidal anti-inflammatory drug diclofenac, from which little is known about its effects on human erythrocytes. This report presents the following evidence that diclofenac interacts with the human red cell membrane: a) X-ray diffraction and fluorescence spectroscopy of phospholipids bilayers show that diclofenac interacts with a class of lipids found in the outer moiety of the erythrocyte membrane; b) in isolated unsealed human erythrocyte membranes the drug induced a disordering effect on the acyl chains of the membrane lipid bilayer; c) in scanning electron microscopy studies on human erythrocytes it was observed that the drug induced morphological changes different from their normal biconcave shape.     

Ion current calculations based on three dimensional Poisson-Nernst-Planck theory for a cyclic peptide nanotube

Ion current calculations based on Poisson-Nernst-Planck (PNP) theory are performed for a synthetic cyclic peptide nanotube that consists of eight or ten cyclo[(-L-Trp-D-Leu-)4] embedded in a lipid bilayer membrane to investigate the ion transport properties of the nanotube. To explore systems with arbitrary geometries, three-dimensional PNP theory is implemented using a finite difference method. The influence of dipolar lipid molecules on the ion currents is also examined by turning on or off the charges of the lipid dipoles in dipalmitoylphosphatidylcholine (DPPC). Comparisons between the calculated and experimentally measured ion currents show that the PNP approach agrees well with the measurements at low ion concentrations but overestimates the currents at higher concentrations. Concentration profiles reveal the selectivity of the peptide nanotube to cations, which is attributed to the negatively charged carbonyl oxygens inside the nanotube. The dominant cation and the minimum anion concentrations inside the cyclic peptide nanotube suggest that these cyclic peptide nanotubes can be employed as ion sensors. In the case of the polar DPPC bilayer, smaller currents are obtained in the calculation. The variation of current with polarity of the lipids implies that both polar and nonpolar lipid bilayer membranes can be utilized to regulate ion currents in the peptide nanotube and other ion channels. Strengths and limitations of the PNP theory are also discussed.     

单烷基乙二胺稀水溶液头基相互作用自组装形成 双分子膜

提出普通表面活性剂(单链两亲分子)亲水头基相互作用诱导疏水尾链平行聚集形成双分子膜的新机制。设计和合成了系列单烷基取代乙二胺C~nH~2~n~+~1NHC~2H~4NH~2(n=8,12,14,16,18)。通过电镜形态,分散液凝胶/液晶相变和对应铸膜的二维双层结构,表明单链两亲分子头基相互作用和脂链引入刚性片断一样,两者形成的双分子膜具有类似的结构和性能;展示了各体系取代乙二胺双层结构和性能的密切联系。指出了广泛认同的单链两亲分子形成双分子膜必须引入刚性片断的单一成膜机制的片面性,为组装新一类功能头基表面活性剂双分子膜独辟蹊径。     

Sodium ion internalized within phospholipid membranes

Seven phospholipids, modified with ester groups in their hydrophobic chains, were synthesized and examined for their ability to promote sodium ion flux across vesicular membranes. It was found by 23Na NMR that only the phospholipids having short chain segments beyond their terminal ester groups catalyze sodium ion transfer by up to 2 orders of magnitude relative to a conventional phospholipid, POPC. The rates increase with the concentration of the ester-phospholipid admixed with POPC in the bilayer. More surprisingly, the rates increase with the time allowed for the vesicles to age. This was attributed to ester-phospholipid migrating in the bilayers to form domains that solubilize the sodium ion within the hydrocarbon interior of the membrane. Such membrane domains explain why shift reagent-modified NMR spectra display three 23Na signals representing sodium outside the vesicles, sodium within the vesicular water pools, and sodium within the membranes themselves.     

933 — Chemical modification of the bilayer lipid membrane biosensor dipolar potential

The bilayer lipid membrane (BLM) is an interesting model for a transducer, based on transmembrane ion current modulation by selective complexation of a membrane-embedded receptor with a suitable stimulant. The dipolar potential, which originates from the lipid bilayer headgroup zone, partially controls transmembrane ion current. Dipolar potentials associated with phosphatidyl choline/steroid lipid monolayers on a Langmuir-Blodgett thin-film trough have been measured with a non-contacting electrostatic voltmeter, and have been correlated with lipid chemistry. Lipid molecular interactions established by monolayer compression and BLM Arrhenius energy barriers determined from thermal dependence of ion conductivity of BLM have provided an evaluation of the dipolar potential with respect to its role in controlling ion current, surface ion adsorption and BLM structural integrity. Additionally, this work deals with the characteristics required for an optimum receptor-membrane system, which would best operate by dramatically influencing membrane fluidity/packing parameters as well as dipolar potential.     

Incorporation of alpha-hemolysin in different tethered bilayer lipid membrane architectures

Tethered bilayer lipid membranes are stable solid supported model membrane systems. They can be used to investigate the incorporation and function of membrane proteins. In order to study ion translocation mediated via incorporated proteins, insulating membranes are necessary. The architecture of the membrane can have an important effect on both the electrical properties of the lipid bilayer as well as on the possibility to functionally host proteins. Alpha-hemolysin pores have been functionally incorporated into a tethered bilayer lipid membrane coupled to a gold electrode. The protein incorporation has been monitored optically and electrically and the influence of the molecular structure of the anchor lipids on the insertion properties has been investigated.     

Impact of Strong and Weak Lipid–Protein Interactions on the Structure of a Lipid Bilayer on a Gold Electrode Surface

A lipid bilayer deposited on an electrode surface can serve as a benchmark system to investigate lipid–protein interactions in the presence of physiological electric fields. Recoverin and myelin‐associated glycoprotein (MAG) are used to study the impact of strong and weak protein–lipid interactions on the structure of model lipid bilayers, respectively. The structural changes in lipid bilayers are followed using electrochemical polarization modulation infrared reflection–absorption spectroscopy (PM IRRAS). Recoverin contains a myristoyl group that anchors in the hydrophobic part of a cell membrane. Insertion of the protein into the 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphatidylcholine (DMPC)–cholesterol lipid bilayer leads to an increase in the capacitance of the lipid film adsorbed on a gold electrode surface. The stability and kinetics of the electric‐field‐driven adsorption–desorption process are not affected by the interaction with protein. Upon interaction with recoverin, the hydrophobic hydrocarbon chains become less ordered. The polar head groups are separated from each other, which allows for recoverin association in the membrane. MAG is known to interact with glycolipids present on the surface of a cell membrane. Upon probing the interaction of the DMPC–cholesterol–glycolipid bilayer with MAG a slight decrease in the capacity of the adsorbed lipid film is observed. The stability of the lipid bilayer increases towards negative potentials. At the molecular scale this interaction results in minor changes in the structure of the lipid bilayer. MAG causes small ordering in the hydrocarbon chains region and an increase in the hydration of the polar head groups. Combining an electrochemical approach with a structure‐sensitive technique, such as PM IRRAS, is a powerful tool to follow small but significant changes in the structure of a supramolecular assembly.     

Permeability of water and polar solutes in lipid bilayers

The three commonly used formalisms to describe water and solute permeation in lipid bilayers (namely, solubility-solute properties, activated rate processes and the thermodynamics of the irreversible process theory) are analyzed in the light of experimental results. These approaches are based on the consideration of the lipid bilayer as a composite membrane containing a hydrocarbon core, an H-bonded interfacial network and a fluctuating structure in which pores can appear. The particular structure of the lipid bilayer (i.e., a hydrophobic-hydrophilic leaflet) makes the permeation process of polar solutions more complicated than that occurring in inert polymeric membranes. Thus, the permeation theories of Fick, Henry and Kedem and Katchalsky should be adapted to introduce interfacial and elastic phenomena. A critical analysis of the experimental results available in the current literature opens the possibility to formulate a broader formalism for permeation in lipid membranes.