Showing papers in "Advances on Planar Lipid Bilayers and Liposomes in 2011"
TL;DR: In this chapter, the application of infrared spectroscopy to studies of lipids and bilayer membrane dynamics in biological tissues, and to the research for diagnosis of human diseases is summarized.
Abstract: Functions of cell bilayer membranes are closely linked to dynamics and behavior of network of membrane components including lipids, proteins, and glycans. It is important to investigate the role of membrane components in the membrane functions without damage of the network of components. This is the reason why noninvasive and nondestructive analyses are so important for the study of the intact membranes and tissues. Vibrational spectroscopies including near- and mid-infrared absorption and resonance Raman scattering spectroscopies are useful for these purposes. In this chapter, we summarize the application of infrared spectroscopy to studies of lipids and bilayer membrane dynamics in biological tissues, and to the research for diagnosis of human diseases.
20 citations
TL;DR: In this article, the potentials of investigating the dynamics of membrane fluctuations as an experimental tool for probing the membrane material properties are presented and discussed, and the current state of theory and experiment of single vesicle dynamics in simple shear flows is reviewed.
Abstract: Deformability is a key feature of the lipid membrane, being of importance for numerous processes taking place in biological cells, as well as for the flow behavior of cells in blood circulation. In the first part of the chapter, the potentials of investigating the dynamics of membrane fluctuations as an experimental tool for probing the membrane material properties are presented and discussed. By analysing the dynamics of thermally induced shape fluctuations of nearly spherical lipid vesicles, important mechanical constants of the bilayer are possible to be extracted, namely bending elasticity modules at free and blocked exchange of molecules between the two monolayers, comprising the lipid membrane, and the intermonolayer friction coefficient of the bilayer. The second part of this contribution is dedicated to the dynamics of unconfined lipid vesicles in linear hydrodynamic fields. The current state of theory and experiment of single vesicle dynamics in simple shear flows is reviewed. Special attention is given to the relation between the overall rheological properties of vesicle suspensions and the individual vesicle dynamics in the flow.
13 citations
TL;DR: This chapter provides an overview of major computational studies of multicomponent lipid membranes with a particular focus on time-dependent Ginzburg–Landau models, dynamic triangulation Monte Carlo models, coarse-grained molecular dynamics, and dissipative particle dynamics.
Abstract: Based on indirect observations, there currently exists a consensus that the plasma membrane of mammalian cells exhibits nontrivial lateral heterogeneities in the form of nanoscale lipid domains known as lipid rafts which are rich in cholesterol and sphingolipids. Lipid rafts have been implicated in a range of biological functions, including signal transduction, endocytosis, trafficking, virus uptake, and regulation of the membrane tension. The elucidation of the finite size of lipid rafts in the plasma membrane has been a challenging problem since multicomponent lipid vesicles composed of saturated lipid, an unsaturated lipid, and cholesterol also exhibit domains, but these are much larger than the lipid rafts in the plasma membrane. Many computational studies have recently been performed to address the phase separation in multicomponent membranes and potential mechanisms leading to nanoscale phase separation in the plasma membrane. This chapter provides an overview of major computational studies of multicomponent lipid membranes with a particular focus on time-dependent Ginzburg–Landau models, dynamic triangulation Monte Carlo models, coarse-grained molecular dynamics, and dissipative particle dynamics.
12 citations
TL;DR: In this article, the electrostatic properties of a planar lipid membrane with its surrounding electrolyte in a normal DC or AC electric field are discussed. But the authors focus on the case where the membrane has selective conductivity for only one type of ion.
Abstract: For many biotechnological applications it would be useful to better understand the effects produced by electric fields on lipid membranes. This review discusses several aspects of the electrostatic properties of a planar lipid membrane with its surrounding electrolyte in a normal DC or AC electric field.
In the planar geometry, the analysis of electrokinetic equations can be carried out quite far, allowing to characterize analytically the steady state and the dynamics of the charge accumulation in the Debye layers, which results from the application of the electric field. For a conductive membrane in an applied DC electric field, we characterize the corrections to the elastic moduli, the appearance of a membrane undulation instability and the associated flows which are built up near the membrane. For a membrane in an applied AC electric field, we analytically derive the impedance from the underlying electrokinetic equations. We discuss different relevant effects due to the membrane conductivity or due to the bulk diffusion coefficients of the ions. Of particular interest is the case where the membrane has selective conductivity for only one type of ion. These results, and future extensions thereof, should be useful for the interpretation of impedance spectroscopy data used to characterize, for example, ion channels embedded in planar bilayers.
8 citations
TL;DR: In this article, the authors focus on the faceting of the interfaces of cubic mesophases, which is a special class of liquid crystalline phases, and describe the characteristic shapes seen as fingerprints of space group symmetries.
Abstract: The denomination “soft crystals” has been coined by Nozieres et al. with the purpose to mark out a special class of liquid crystalline phases—thermotropic and lyotropic cubic mesophases. Their structure, being periodic in three dimensions, is crystalline. At the same time, it is also liquid because contents of huge unit cells are partially liquid. Due to such “liquid crystalline” structures, cubic mesophases have special physical properties such as the “soft elasticity.” Here, we are dealing with another salient property of soft crystals—faceting of their interfaces. We focus on several topics such as (1) special methods allowing observation of faceted shapes, (2) description and classification of characteristic shapes seen as fingerprints of space group symmetries, (3) new phenomena occurring in faceted mesophases.
6 citations
TL;DR: The concept of lipid rafts as submicrometric cholesterol-rich lateral domains has been used to characterize the basic organizing principle of the cell membrane, but its experimental verification remains elusive as discussed by the authors.
Abstract: Spatial and temporal lipid organization in the plasmatic cell membrane is believed to be fundamental for the understanding of many cellular functions. The concept of lipid rafts as submicrometric cholesterol-rich lateral domains has been used to characterize the basic organizing principle of the cell membrane. Since such organization occurs at very small spatial scales, its experimental verification remains elusive. As a result, the raft hypothesis itself remains controversial. Meanwhile, different theoretical models are being proposed to fill this gap. Here, we survey our recent approaches to the theoretical study of the spatiotemporal organization of lipid bilayers perturbed in different ways. Transverse lipid transport and/or insertion of proteins results in domain organization that covers a wide range of submicrometric sizes and different levels of stability, suggesting plausible mechanisms for the control of nanoscale lipid organization in cell membranes. The relevance of our proposals to the understanding of lateral organization phenomena in biological cell membranes is also discussed.
6 citations
TL;DR: It is shown that proteins can affect the ordering of the lipid bilayer on substantially larger distance in comparison with that of annular lipids surrounding of the macromolecule.
Abstract: Mechanical properties of the lipid bilayers play a crucial role in maintaining the membrane stability, the shape of the cell, in functioning of the protein molecules and are responsible for protein–lipid interactions. Due to structural inhomogeneity, the membrane is characterized by anisotropy of the mechanical properties. Therefore, description of the mechanical properties of the membrane requires the study of membrane deformation in different directions. This chapter reviews current achievements in understanding the membrane mechanics and its role in explanation of the mechanisms of protein–lipid interactions. Several examples of the effect of short peptides and large integral proteins on the mechanical properties of the membrane are presented. We have shown that proteins can affect the ordering of the lipid bilayer on substantially larger distance in comparison with that of annular lipids surrounding of the macromolecule.
5 citations
TL;DR: A molecular model in which the spectrin filaments of the RBC cortical cytoskeleton control the sign and dynamics of membrane curvature depending on two types of spectrin Filaments is proposed, which can be shown to account for Ca 2+ -induced local and dynamic membrane deformations in intact RBCs.
Abstract: Human red blood cells (RBCs) lack the actin–myosin–microtubule cytoskeleton that is responsible for shape changes in other cells. Nevertheless, they can display highly dynamic local deformations in response to external perturbations, such as those that occur during the process of apical alignment preceding merozoite invasion in malaria. Moreover, after lysis in divalent cation-free media, the isolated membranes of ruptured ghosts show spontaneous inside-out curling motions at the free edges of the lytic hole, leading to inside-out vesiculation. The molecular mechanisms that drive these rapid shape changes are unknown. Here, we propose a molecular model in which the spectrin filaments of the RBC cortical cytoskeleton control the sign and dynamics of membrane curvature depending on two types of spectrin filaments. Type I spectrin filaments that are grafted at one end, or at both ends but not connected to the rest of the cytoskeleton, induce a concave spontaneous curvature. Type II spectrin filaments that are grafted at both ends to the cytoskeleton induce a local convex spontaneous curvature. Computer simulations of the model reveal that curling, as experimentally observed, can be obtained either by an overall excess of type I filaments throughout the cell, or by the flux of such filaments toward the curling edges. Divalent cations have been shown to arrest the curling process and Ca 2+ ions have also been implicated in local membrane deformations during merozoite invasion. These effects can be replicated in our model by attributing the divalent cation effects to increased filament membrane binding. This process converts the curl-inducing loose filaments into fully bound filaments that arrest curling. The same basic mechanism can be shown to account for Ca 2+ -induced local and dynamic membrane deformations in intact RBCs. The implications of these results in terms of RBC membrane dynamics under physiological, pathological, and experimental conditions are discussed.
5 citations
TL;DR: In this chapter, multiparametric fluorescence micro-spectroscopy approach will be described for quantitative and noninvasive investigation of molecular processes that trigger lateral and temporal heterogeneities in biomembranes.
Abstract: Biological membranes are heterogeneous assemblies of a variety of lipids and proteins as well as cholesterol. The dynamic nature of these biomembranes spans a wide range of spatiotemporal scales that are essential to their function in cell signaling and biomolecular trafficking. In contrast, biomimetic membranes are simple, stable, and versatile systems to study the physicochemical principles such as lipid mixing, phase separation, domain formation and intermolecular interactions, which underlie lipid bilayers assemblies under controlled conditions. In this chapter, multiparametric fluorescence micro-spectroscopy approach will be described for quantitative and noninvasive investigation of molecular processes that trigger lateral and temporal heterogeneities in biomembranes.
5 citations
TL;DR: It is indicated that membrane-bound protein complexes (PCs) may increase the membrane protrusion growth between the osteoblast and the grooves on Ti surface and thereby facilitate the attachment of osteoblasts to the Ti surface.
Abstract: Nonetheless, titanium (Ti) surface and osteoblasts are negatively charged, there is an attractive interaction between them, which we aim to explain here theoretically. It is shown that adhesion of positively charged proteins with internal charge distribution may give rise to initial attractive interactions between the Ti surface and the osteoblast membrane. A dynamic model of the osteoblast attachment is presented in order to study the impact of geometrically structured Ti surfaces on the osteoblast attachment. It is indicated that membrane-bound protein complexes (PCs) may increase the membrane protrusion growth between the osteoblast and the grooves on Ti surface and thereby facilitate the attachment of osteoblasts to the Ti surface. On the other hand, a strong local adhesion due to electrostatic forces may locally trap the osteoblast membrane and hinder the further spreading of the osteointegration boundary.
3 citations
TL;DR: In this article, the authors present a theoretical analysis of fluctuation-induced aggregation of adhesion sites in supported lipid membranes, and show that while these interactions alone are not sufficient to allow the formation of aggregation clusters, they greatly reduce the strength of the residual interactions required to facilitate cluster formation.
Abstract: Supported lipid membranes are useful and important model systems for studying cell membrane properties and membrane-mediated processes. One attractive application of supported membranes is the design of phantom cells exhibiting well-defined adhesive properties and receptor densities. Adhesion of membranes may be achieved by specific and nonspecific interactions and typically requires the clustering of many adhesion bonds into “adhesion domains.” One potential mediator of the early stages of the aggregation process is the Casimir-type forces between adhesion sites induced by the membrane thermal fluctuations. In this review, I will present a theoretical analysis of fluctuation-induced aggregation of adhesion sites in supported membranes. I will first discuss the influence of a single attachment point on the spectrum of membrane thermal fluctuations, from which the free energy cost of the attachment point will be deduced. I will then analyze the problem of a supported membrane with two adhesion points. Using scaling arguments and Monte Carlo simulations, I will demonstrate that two adhesion points attract each other via an infinitely long range effective potential that grows logarithmically with the pair distance. Finally, I will discuss the many-body nature of the fluctuation-induced interactions. I will show that while these interactions alone are not sufficient to allow the formation of aggregation clusters, they greatly reduce the strength of the residual interactions required to facilitate cluster formation. Specifically, for adhesion molecules interacting via a short-range attractive potential, the strength of the direct interactions required for aggregation is reduced by about a factor of two to below the thermal energy kBT.
TL;DR: Capability of phospholipids with positive charge to form complexes with hyaluronan in aqueous solutions, in a similar way as traditional cationic surfactants, was investigated by fluorescence probes.
Abstract: Capability of phospholipids with positive charge to form complexes with hyaluronan in aqueous solutions, in a similar way as traditional cationic surfactants, was investigated by fluorescence probes. DPPC and lecithin aggregate in aqueous solution to form micelle-like structures capable to solubilize hydrophobic molecules. Changes in aggregation behavior after adding hyaluronan were observed only in the case of lecithin. Further, nonionic biocompatible surfactant was used as additional dispersion environment in phospholipid–hyaluronan system with phospholipid molecules acting as a physical linker bonding micelles and biopolymer.
TL;DR: In this article, the surface tension and the curvature elastic constant of bilayers that form vesicles were evaluated by Monte Carlo simulations, where a molecule is formed by a hydrophilic head segment and some hydrophobic chain segments.
Abstract: Monte Carlo simulations provide some insight into self-assembled aggregates of amphiphiles in aqueous environment. A rather simple solvent-free model, where a molecule is formed by a hydrophilic head segment and some hydrophobic chain segments, is suitable for describing the formation of micelles, stable membranes, and spherical vesicles. Characteristic features of the self-assembled aggregates, such as the elastic properties of bilayers, can be obtained from simulated data. The capability of this simple approach was demonstrated for a surfactant model with three spherical segments. Analyzing vesicle fluctuations by Monte Carlo simulations, the surface tension and the curvature elastic constant of bilayers that form vesicles can be evaluated. If the vesicles contain hydrophilic solute molecules, thermal fluctuations of spherical vesicles depend on their osmotic pressure. Already at relatively low solute concentrations, the appearance of an osmotic pressure leads to a strong depression of vesicle fluctuations. The adsorption of colloidal particles on surfaces of soft materials causes elastic distortions. Biological membranes contain a large amount of embedded and adsorbed macromolecules, especially transmembrane and peripheral proteins consisting of large polypeptide chains folded in compact particles. Membrane distortions spread around each protein can superimpose and produce indirect forces between them. Similar effects are based on concentration fluctuations of the lipid components forming the membrane. Proteins as well as smaller peptides disturb the homogeneous distribution of the lipid mixture, facilitating a phase separation of the lipid components. Disturbances of the spatial lipid distribution can also be accompanied by an enhanced adsorption of water soluble peptides. This effect is amplified for nonideal mixtures, when the correlation length of concentration fluctuations is enlarged. Concentration fluctuations also produce forces between membrane proteins, which can enforce the aggregation of membrane bound proteins. Monte Carlo simulations are suitable for testing general concepts and theories on protein-membrane interactions.
TL;DR: In this paper, the authors investigated the role of β-carotene in the stability of photosynthetic membranes, using data from absorption, pyrene fluorescence, and resonance Raman spectroscopy.
Abstract: Carotenoids perform light harvesting, photoprotection, electron transfer, and structural role in photosynthetic membranes. To unravel the β-carotene contribution to the stability of membranes, liposomes with different lipid composition (resembling the photosynthetic membranes, containing mainly galactolipids with a high degree of unsaturation, and egg phosphatidylcholine) were used. The aim was to gain insight into the mechanism of β-carotene–lipid interactions with a special focus on the fluidity of the bilayer. Data from absorption, pyrene fluorescence, and resonance Raman spectroscopy revealed that the degree of lipids' unsaturation regulates the penetration of β-carotene molecules into the membrane, thus modifying the lipid–pigment interactions.
TL;DR: This chapter reviews how can such a structure attain stability and compares two models in which membrane constituents are either isotropic or anisotropic in nature and addresses the molecular nature of such a stable, release-unproductive fusion pore.
Abstract: In multicellular organisms, signaling is a necessity and an important mode of communication between cells is mediated by neurotransmitters, hormones, and other chemical messengers that are stored in secretory vesicles. In stimulated conditions secretory vesicles, which are trafficked to be docked at the plasma membrane, enter exocytosis, characterized by vesicle and plasma membrane merger. Due to repulsive forces of negatively charged membrane surfaces, it was long believed that the fusion pore is merely a short lived intermediate state leading irreversibly to a complete merger of both membranes. However, recent results show that the fusion pore is a rather stable structure, which can reversibly reopen to subnanometer diameters; dimensions too narrow to permit the exit of the cargo into the extracellular space. The aim of this chapter is to first review how can such a structure attain stability and compare two models in which membrane constituents are either isotropic or anisotropic in nature. Then we address the molecular nature of such a stable, release-unproductive fusion pore. We conclude that membrane constituents of the stable fusion pore membrane, being made of proteins and/or lipids, very likely consist of architectural elements that exhibit anisotropicity. The dynamics of fusion pore diameter is then determined by the density and architectural properties of these membrane constituents at fusion pore locales.
TL;DR: In this article, the general principles of photovoltaic solar energy conversion in biomembranes were analyzed using a BLM-based model system of artificial electron pump to develop useful concepts and to devise a null current method, which can perform meaningful signal measurements that could be analyzed in terms of conventional concepts in electricity.
Abstract: This chapter analyzes the general principles of photovoltaic solar energy conversion in biomembranes. Biological systems are inherently complex. However, unlike naturally occurring photosynthetic membranes, BLM-based photoconversion model systems are sufficiently simple to allow investigators to analyze the underlying photochemical and photophysical processes with mathematical rigor previously attainable only in physics and their progenies. We used a BLM-based model system of artificial electron pump to develop useful concepts and to devise a null current method, which can perform meaningful signal measurements that could be analyzed in terms of conventional concepts in electricity (equivalent circuit analysis). This method was then extended to the analysis of model membranes reconstituted from bacteriorhodopsin, which is perhaps the simplest photosynthetic pigment. Comparison of photosynthetic membranes with their non-biological counterparts yielded some interesting insights. Both the silicon photodiode and photosynthetic membranes start the conversion process with light-driven charge separation in an anisotropic environment. Whereas rectification is a key feature of the silicon photodiode for minimizing unwanted internal charge recombination, biological systems apparently used a different strategy to accomplish the same purpose. Illumination opens a proton conduction channel in bacteriorhodopsin, and cessation of illumination shuts off the conduction channel completely. Indirect evidence indicates that chlorophyll-based photosynthetic membranes also adopt the same strategy to prevent internal charge recombination in the dark, thus greatly enhancing the efficiency and effectiveness of photoconversion.