Surfactant effects of chlorpromazine and imipramine on lipid bilayers containing sphingomyelin and cholesterol

The surface-active drugs chlorpromazine (CPZ) and imipramine (IP) have been tested on large unilamellar vesicles composed of phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (Ch) in different proportions. The well-characterized nonionic detergent Triton X-100 (TX) has also been used in parallel experiments. Leakage of vesicular aqueous contents and bilayer solubilization have been measured for each surfactant molecule and vesicle composition. All three surface-active molecules behave in a qualitatively similar way, irrespective of bilayer composition: they induce leakage at concentrations well below their critical micellar concentrations (cmc) and solubilization near the cmc. In these events, the potency of the three surfactants under study increases with decreasing cmc, in the order IP相似文献   

Energetics of cholesterol transfer between lipid bilayers

It is believed that natural biological membranes contain domains of lipid ordered phase enriched in cholesterol and sphingomyelin. Although the existence of these domains, called lipid rafts, is still not firmly established for natural membranes, direct microscopic observations and phase diagrams obtained from the study of three-component mixtures containing saturated phospholipids, unsaturated phospholipids, and cholesterol demonstrate the existence of lipid rafts in synthetic membranes. The presence of the domains or rafts in these membranes is often ascribed to the preferential interactions between cholesterol and saturated phospholipids, for example, between cholesterol and sphingomyelin. In this work, we calculate, using molecular dynamics computer simulation technique, the free energy of cholesterol transfer from the bilayer containing unsaturated phosphatidylcholine lipid molecules to the bilayer containing sphingomyelin molecules and find that the affinity of cholesterol to sphingomyelin is higher. Our calculations of the free-energy components, energy and entropy, show that cholesterol transfer is exothermic and promoted by the favorable change in the lipid-lipid interactions near cholesterol and not by the favorable energy of cholesterol-sphingomyelin interaction, as assumed previously.     

Controlling the charge and organization of anionic lipid bilayers: effect of monovalent and divalent ions

It is shown that the organization of lipid bilayers containing phosphatidic acid (PA) and phosphatidlycholine (PC) can be controlled by altering the monovalent and divalent ion concentrations. At high pH and/or calcium concentration, 1:1 Ca(2+)-PA(2-) complexes form; these complexes demix, and PA-rich and PC-rich regions are observable with epifluorescence microscopy. The results are compared with predictions from electrostatic theory. It is noted that the complex formation correlates in a roughly linear fashion with the monovalent/divalent ion ratio, a parameter that cells adjust.     

The perturbing effect of cholesterol on the interaction between labdanes and DPPC bilayers

Differential scanning calorimetry (DSC) was used to study the effect of cholesterol on the perturbation of DPPC bilayers induced by eight bioactive structurally related labdanes isolated from the resin ‘ladano’ of Cistus creticus subsp. creticus (Cistaceae) or semisynthesized from the mother compounds. Labdanes themselves induced profound modifications in DPPC bilayer organization and thermotropic properties that were altered when cholesterol was incorporated in equimolar amounts to the labdanes. The present work shows that, up to 10 mol% of the equimolar mixture of cholesterol and the labdanes, the modifications evoked on DPPC bilayer organization are in accordance to these induced by the labdanes themselves. When the concentration exceeded 20 mol%, cholesterol influence dominated while the effect of the labdanes was suppressed and their interaction with the bilayer was probably prevented. The degree by which cholesterol modulated the labdane interaction with the bilayer depended on their structural characteristics that determine their localization in the bilayer interior. Polar groups that force the labdanes to localize themselves at the interfacial region broadened the concentration range by which labdanes interacted with the DPPC bilayer even in the presence of high concentration of cholesterol where cholesterol-rich domains are preferentially formed. On the other hand, labdanes possessing functional groups that promote their deeper penetration in the bilayer interior compete with cholesterol in a high extent for the same localization sites resulting in their possible elimination from the bilayer when the concentration of cholesterol present exceeds the 20 mol%.     

The RIT1 C-terminus associates with lipid bilayers via charge complementarity

RIT1 is a member of the Ras superfamily of small GTPases involved in regulation of cellular signaling. Mutations to RIT1 are involved in cancer and developmental disorders. Like many Ras subfamily members, RIT1 is localized to the plasma membrane. However, RIT1 lacks the C-terminal prenylation that helps many other subfamily members adhere to cellular membranes. We used molecular dynamics simulations to examine the mechanisms by which the C-terminal peptide (CTP) of RIT1 associates with lipid bilayers. We show that the CTP is unstructured and that its membrane interactions depend on lipid composition. While a 12-residue region of the CTP binds strongly to anionic bilayers containing phosphatidylserine lipids, the CTP termini fray from the membrane allowing for accommodation of the RIT1 globular domain at the membrane-water interface.     

Detection of supported lipid bilayers using their electric charge

We describe an electronic detection method for charged lipid bilayers supported on a Si 3N 4/SiO 2/Si substrate. The flat-band voltage was used to monitor the charge of the bilayers. We show that the flat-band voltage varies with lipid adsorption depending on the polarity and mole ratio of the charged lipids, the salt concentration, and the surface coverage. Cationic and anionic bilayers produced a decrease and an increase in the flat-band voltage, respectively. The voltage change increased as the percentage of charged lipid components was elevated in the planar bilayers with full surface coverage. In addition, the voltage variation increased when the salt concentration was decreased or when the surface coverage of planar bilayer patches was increased. These results demonstrate that charged bilayers can be detected from the field effect that they exert on a solid support.     

Interaction of polyacrylic acid with lipid bilayers: effect of polymer mass

Polyanion‐coated lipid vesicles are proposed to have an appreciable potential for drug delivery because of their ability to control the permeability of lipid bilayers by environmental parameters such as pH and temperature. However, details of the interaction of this class of polymers with lipids and their mechanisms of induced permeability are still being debated. In this work, we applied ¹H NOESY to study details of the interaction of polyacrylic acid (PAA) fractions of molecular weights 5 and 240 kDa with dimyristoylphosphatidylcholine vesicles. We showed that PAA of two different molecular masses modifies lipid bilayers increasing disorder and probability of close contact between polar and hydrophobic groups. PAA molecules adsorb near the interface of lipid bilayers but do not penetrate into the hydrophobic core of the bilayer and, thus, cannot participate in formation of transbilayer channels, proposed in earlier works. Increasing the molecular mass of PAA from 5 kDa to 240 kDa does not change the effect of PAA on the bilayer, although PAA240 forms a more compact structure (either intra‐molecular or inter‐molecular) and interacts more strongly with interface lipid protons. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of lipid headgroup composition on the interaction between melittin and lipid bilayers

The effect of the lipid polar headgroup on melittin-phospholipid interaction was investigated by cryo-TEM, fluorescence spectroscopy, ellipsometry, circular dichroism, electrophoresis and photon correlation spectroscopy. In particular, focus was placed on the effect of the lipid polar headgroup on peptide adsorption to, and penetration into, the lipid bilayer, as well as on resulting colloidal stability effects for large unilamellar liposomes. The effect of phospholipid headgroup properties on melittin-bilayer interaction was addressed by comparing liposomes containing phosphatidylcholine, -acid, and -inositol at varying ionic strength. Increasing the bilayer negative charge leads to an increased liposome tolerance toward melittin which is due to an electrostatic arrest of melittin at the membrane interface. Balancing the electrostatic attraction between the melittin positive charges and the phospholipid negative charges through a hydration repulsion, caused by inositol, reduced this surface arrest and increased liposome susceptibility to the disruptive actions of melittin. Furthermore, melittin was demonstrated to induce liposome structural destabilization on a colloidal scale which coincided with leakage induction for both anionic and zwitterionic systems. The latter findings thus clearly show that coalescence, aggregation, and fragmentation contribute to melittin-induced liposome leakage, and that detailed molecular analyses of melittin pore formation are incomplete without considering also these colloidal aspects.     

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.     

Structure and thermodynamics of lipid bilayers on polyethylene glycol cushions: fact and fiction of PEG cushioned membranes

In developing well hydrated polymer cushioned membranes, structural studies are often neglected. In this work, neutron and X-ray reflectivity studies reveal that hybrid bilayer/polyethylene glycol (PEG) systems created from mixtures of phospholipids and PEG conjugated lipopolymers do not yield a hydrated cushion beneath the bilayer unless the terminal ends of the lipopolymers are functionalized with reactive end groups and can covalently bind (tether) to the underlying support surface. While reactive PEG tethered systems yielded bilayers with near complete surface coverage, a bimodal distribution of heights with sub-micrometer lateral dimensions was observed consisting of cushioned membrane domains and uncushioned regions in close proximity to the support. The membrane fraction cushioned by the hydrated polymer could be controlled by adjusting the molar ratio of lipopolymer in the bilayer. A general phase diagram based on the free energy of the various configurations is derived that qualitatively predicts the observed behavior and the resulting structure of such systems a priori. As further evidenced by ellipsometry, atomic force and fluorescence microscopy, the tethered system provides a simple means for fabricating small cushioned domains within a membrane.     

Microfluidic patterning of nanodisc lipid bilayers and multiplexed analysis of protein interaction

We report a microfluidic method for precisely patterning lipid bilayers and a multiplexed assay to examine the interaction between the lipids and protein analytes. The lipids were packaged into nanoscale lipid bilayer particles known as Nanodiscs and delivered to surfaces using microfluidic channels. Two types of lipids were used in this study: biontinylated lipids and phosphoserine lipids. The deposition of biotinylated lipids on a glass surface was confirmed by attaching streptavidin coated quantum dots to the lipids, followed by fluorescent imaging. Using this multiplexed grid assay, we examined binding of annexin to phosphoserine lipids, and compared these results to similar analysis performed by surface plasmon resonance.     

The study of the interaction of a model alpha-helical peptide with lipid bilayers and monolayers

We studied the interaction of the alpha-helical peptide acetyl-Lys(2)-Leu(24)-Lys(2)-amide (L(24)) with tethered bilayer lipid membranes (tBLM) and lipid monolayers formed at an air-water interface. The interaction of L(24) with tBLM resulted in adsorption of the peptide to the surface of the bilayer, characterized by a binding constant K(c)=2.4+/-0.6 microM(-1). The peptide L(24) an induced decrease of the elasticity modulus of the tBLM in a direction perpendicular to the membrane surface, E(radial). The decrease of E(radial) with increasing peptide concentration can be connected with a disordering effect of the peptide to the tBLM structure. The pure peptide formed a stable monolayer at the air/water interface. The pressure-area isotherms were characterized by a transition of the peptide monolayer, which probably corresponds of the partial intercalation of the alpha-helixes at higher surface pressure. Interaction of the peptide molecules with lipid monolayers resulted in an increase of the mean molecular area of phospholipids both in the gel and liquid crystalline states. With increasing peptide concentration, the temperature of the phase transition of the monolayer shifted toward lower temperatures. The analysis showed that the peptide-lipid monolayer is not an ideally miscible system and that the peptide molecules form aggregates in the monolayer.     

Selective association of cholesterol with long-chain phospholipids in liquid-ordered bilayers: support for the existence of lipid rafts

Nearest-neighbor recognition experiments, which have been carried out under fluidizing and condensing conditions, using exchangeable dimers derived from 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine, and cholesterol, have provided strong evidence that sterol-phospholipid recognition is limited to the liquid-ordered phase.     

Influence of cholesterol on the phase transition of lipid bilayers: a temperature-controlled force spectroscopy study

Cholesterol (Chol) plays the essential function of regulating the physical properties of the cell membrane by controlling the lipid organization and phase behavior and, thus, managing the membrane fluidity and its mechanical strength. Here, we explore the model system DPPC:Chol by means of temperature-controlled atomic force microscopy (AFM) imaging and AFM-based force spectroscopy (AFM-FS) to assess the influence of Chol on the membrane ordering and stability. We analyze the system in a representative range of compositions up to 50 mol % Chol studying the phase evolution upon temperature increase (from room temperature to temperatures high above the T(m) of the DPPC bilayer) and the corresponding (nano)mechanical stability. By this means, we correlate the mechanical behavior and composition with the lateral order of each phase present in the bilayers. We prove that low Chol contents lead to a phase-segregated system, whereas high contents of Chol can give a homogeneous bilayer. In both cases, Chol enhances the mechanical stability of the membrane, and an extraordinarily stable system is observed for equimolar fractions (50 mol % Chol). In addition, even when no thermal transition is detected by the traditional bulk analysis techniques for liposomes with high Chol content (40 and 50 mol %), we demonstrate that temperature-controlled AFM-FS is capable of identifying a thermal transition for the supported lipid bilayers. Finally, our results validate the AFM-FS technique as an ideal platform to differentiate phase coexistence and transitions in lipid bilayers and bridge the gap between the results obtained by traditional methods for bulk analysis, the theoretical predictions, and the behavior of these systems at the nanoscale.     

PEG-covered lipid surfaces: bilayers and monolayers

In this review paper we survey the ways in which various micropipet techniques have been used to study the mechanochemical and interactive features of lipid bilayer vesicles and monolayer-coated gas bubbles. Special emphasis will be made on characterizing the barrier properties of grafted PEG layers and how a hierarchical approach that uses a short barrier and extended ligand allows us to start to mimic nature's own solution to the problem of ubiquitous repulsion and specific attraction. The information gained from such studies not only characterizes the membrane and other lipid surfaces and their intersurface interactions from a fundamental materials science perspective, but also provides essential materials property data that are required for the successful design and deployment of lipid-based carriers and other capsules in applications involving this so-called ‘stealthy’ surface.     

Molecular dynamics simulations of bilayers containing mixtures of sphingomyelin with cholesterol and phosphatidylcholine with cholesterol

We performed six molecular dynamics simulations: three on hydrated bilayers containing pure phospholipids and three on hydrated bilayers containing mixtures of these phospholipids with cholesterol. The phospholipids in our simulations were SSM (sphingomyelin containing a saturated 18:0 acyl chain), OSM (sphingomyelin with an unsaturated 18:1 acyl chain), and POPC (palmitoyloleoylphosphatidylcholine containing one saturated and one unsaturated chain). Data from our simulations were used to study systematically the effect of cholesterol on phospholipids that differed in their headgroup and tail composition. In addition to the structural analysis, we performed an energetic analysis and observed that energies of interaction between cholesterol and neighboring SM molecules are similar to the energies of interaction between cholesterol and POPC. We also observed that the interaction energy between cholesterol and neighboring lipids cannot be used for the determination of which lipids are involved in the creation of a complex.     

Flexoelectricity of lipid bilayers

Abstract

The direct flexoeffect in single lipid bilayers in the form of black lipid membranes has been investigated experimentally by the oscillating pressure technique in the regime of voltage measurement. Black lipid membranes of various composition have been studied in order to check the effect of lipid surface charge on the curvature-electric response and its frequency dependence; these include egg yolk lecithin (low negative charge); egg yolk lecithin plus phosphatidyl serine (high negative charge); egg yolk lecithin with surface adsorbed ions of uranyl acetate (high positive charge). An increase of the response has been found by increasing the surface charge and a reversal of the sign of the flexoelectric coefficient from positive to negative has been obtained by changing the sign of the surface charge from negative to positive. These results underline the leading role of the contribution of the surface charge to the flexoelectricity of lyotropics. Their theoretical interpretation provides further insight into the molecular mechanism of this phenomenon.     

High pressure effect on phase transition behavior of lipid bilayers

Phase behavior of lipid bilayers at high pressure is critical to biological processes. Using coarse grained molecular dynamic simulations, we report critical characteristics of dipalmitoylphosphatidylcholine bilayers with applied high pressure, and also show their phase transition by cooling bilayer patches. Our results indicate that the phase transition temperature of dipalmitoylphosphatidylcholine bilayers obviously shifts with pressure increasing in the rate of 37 °C kbar(-1), which are in agreement with experimental data. Moreover, the main phase transition is revealed to be strongly dependent on lipid area. A critical lipid area of ~0.57 nm(2) is found on the main phase transition boundary. Similar structures of acyl chains lead to the same sensitivity of phase transition temperature of different lipids to the pressure. Based on the lateral density and pressure profiles, we also discuss the different effects on bilayer structure induced by high temperature and high pressure, e.g., increasing temperature induces higher degree of interdigitation of lipid tails and thinner bilayers, and increasing pressure maintains the degree of interdigitation and bilayer thickness.     

Chiral interaction and thermodynamics

In a recent series of articles a novel interaction between chiral molecules such as proteins and a polar solvent has been proposed. This interaction is criticized by Tannhauser and Kuper on the basis of thermodynamics, and the purpose of this Reply is to show that thermodynamics per se can raise no objection to this interaction. Special attention is given to time-reversal invariance, which is violated by this interaction. A temporal interplay between relaxation and stochastic times τ_R and τ can cause a non-ergodic behavior of the system.     

Formation and diffusivity characterization of supported lipid bilayers with complex lipid compositions

The moving edge of a hydrodynamically manipulated supported lipid bilayer (SLB) can be used to catalyze SLB formation of adsorbed lipid vesicles that do not undergo spontaneous SLB formation upon adsorption on SiO(2). By removing the lipid reservoir of an initially formed SLB, we show how a hydrodynamically moved SLB patch composed of POPC can be used to form isolated SLBs with compositions that to at least 95% represent that of the adsorbed lipid vesicles. The concept is used to investigate the diffusivity of lissamine rhodamine B 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (rhodamine-DHPE) in SLBs made from complex lipid compositions, revealing a decrease in diffusivity by a factor of 2 when the cholesterol content was increased from 0% to 50%. We also demonstrate how the concept can be used to induce stationary domains in SLBs containing 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and cholesterol (39:21:40 mol %, respectively). Because the method serves as a means to form SLBs with lipid compositions that hamper SLB formation via spontaneous rupture of adsorbed lipid vesicles, it opens up the possibility for new biophysical investigations of SLBs with more nativelike compositions.