Pore nucleation in mechanically stretched bilayer membranes

We report a computer-simulation study of the free-energy barrier for the nucleation of pores in the bilayer membrane under constant stretching lateral pressure. We find that incipient pores are hydrophobic but as the lateral size of the pore nucleus becomes comparable with the molecular length, the pore becomes hydrophilic. In agreement with previous investigations, we find that the dynamical process of growth and closure of hydrophilic pores is controlled by the competition between the surface tension of the membrane and the line tension associated with the rim of the pore. We estimate the line tension of a hydrophilic pore from the shape of the computed free-energy barriers. The line tension thus computed is in a good agreement with available experimental data. We also estimate the line tension of hydrophobic pores at both macroscopic and microscopic levels. The comparison of line tensions at these two different levels indicates that the "microscopic" line tension should be carefully distinguished from the "macroscopic" effective line tension used in the theoretical analysis of pore nucleation. The overall shape of the free-energy barrier for pore nucleation shows no indication for the existence of a metastable intermediate during pore nucleation.     

Photoswitching in azobenzene self-assembled monolayers capped on zinc oxide: nanodots vs nanorods

We report the synthesis and spectroscopic characterization of nanohybrid structures consisting of an azobenzene compound grafted on the surface of zinc oxide nanoparticles. Characteristic bathochromic shifts indicate that the azobenzene photochromic molecules self-assemble onto the surface of the nanocrystals. The extent of packing is dependent on the shape of the nanoparticle. ZnO nanorods, with flat facets, enable a tighter organization of the molecules in the self-assembled monolayer than in the case of nanodots that display a more curvated shape. Consistently, the efficiency of photochromic switching of the self-assembled monolayer on ZnO nanoparticles is also shown to be strongly affected by nanoparticle shape.     

Formation of silver nanoparticles at the air-water interface mediated by a monolayer of functionalized hyperbranched molecules

Nanofibrillar micellar structures formed by the amphiphilic hyperbranched molecules within a Langmuir monolayer were utilized as matter for silver nanoparticle formation from the ion-containing water subphase. We observed that silver nanoparticles were formed within the multifunctional amphiphilic hyperbranched molecules. The diameter of nanoparticles varied from 2-4 nm and was controlled by the core dimensions and the interfibrillar free surface area. Furthermore, upon addition of potassium nitrate to the subphase, the Langmuir monolayer templated the nanoparticles' formation along the nanofibrillar structures. The suggested mechanism of nanoparticle formation involves the oxidation of primary amino groups by silver catalysis facilitated by "caging" of silver ions within surface areas dominated by multibranched cores. This system provides an example of a one-step process in which hyperbranched molecules with outer alkyl tails and compressed amine-hydroxyl cores mediated the formation of stable nanoparticles placed along/among/beneath the nanofibrillar micelles.     

Activating mechanosensitive channels embedded in droplet interface bilayers using membrane asymmetry

Droplet microcompartments linked by lipid bilayers show great promise in the construction of synthetic minimal tissues. Central to controlling the flow of information in these systems are membrane proteins, which can gate in response to specific stimuli in order to control the molecular flux between membrane separated compartments. This has been demonstrated with droplet interface bilayers (DIBs) using several different membrane proteins combined with electrical, mechanical, and/or chemical activators. Here we report the activation of the bacterial mechanosensitive channel of large conductance (MscL) in a dioleoylphosphatidylcholine:dioleoylphosphatidylglycerol DIB by controlling membrane asymmetry. We show using electrical measurements that the incorporation of lysophosphatidylcholine (LPC) into one of the bilayer leaflets triggers MscL gating in a concentration-dependent manner, with partial and full activation observed at 10 and 15 mol% LPC respectively. Our findings could inspire the design of new minimal tissues where flux pathways are dynamically defined by lipid composition.

Electrophysiology shows asymmetric lysophosphatidylcholine-containing DIBs trigger mechanosensitive channel gating, enabling user-designed, autonomous flux pathways in droplet networks.     

Roles of gamma-Globulin in the Dynamic Interfacial Behavior of Mixed Dipalmitoyl Phosphatidylcholine/gamma-Globulin Monolayers at Air/Liquid Interfaces

This study investigated the roles of gamma-globulin in the dynamic interfacial behavior of dipalmitoyl phosphatidylcholine (DPPC)/gamma-globulin monolayers at air/liquid interfaces at 25 degrees C. The surface tension behavior demonstrated that gamma-globulin had a large adsorption time scale. Moreover, the surface pressure-area hysteresis behavior of adsorbed gamma-globulin monolayers suggested that no significant desorption occurred during the compression stage, and the respreading of gamma-globulin molecules at the interface during the expansion stage was slow. From the hysteresis behavior of adsorbed gamma-globulin monolayers with spread DPPC molecules, it was found that gamma-globulin molecules were expelled from the interface as DPPC molecules were in a condensed state. The squeeze-out of gamma-globulin molecules seemed to induce the loss of DPPC molecules at the interface with the extent depending on the initial gamma-globulin surface concentration. Furthermore, the expelled gamma-globulin molecules re-entered the monolayer and participated in the surface pressure increase during the following expansion stage. The exclusion of gamma-globulin associated with the removal of DPPC during monolayer compression and the re-entry of gamma-globulin during subsequent monolayer expansion represented a mechanism for DPPC depletion and gamma-globulin enrichment at the interface, which may explain the inhibitory effect of certain proteins on the surface activity of DPPC. Copyright 2000 Academic Press.     

Thermodynamic Analysis of the Adsorption and Micellization Activity of the Mixtures of Rhamnolipid and Surfactin with Triton X-165

The surface tension of aqueous solutions of Triton X-165 with rhamnolipid or surfactin mixtures was measured. The obtained results were applied for the determination of the concentration and composition of the Triton X-165 and biosurfactants mixture at the water–air interface as well as the contribution of the particular component of the mixtures to water surface tension reduction and the mutual influence of these components on the critical micelle concentration. The determination of these quantities was based on both the commonly used concepts and a new one proposed by us, which assumes that the composition of the mixed monolayer at the water–air interface depends directly on the pressure of the monolayer of the single mixture component and allows us to determine the surface concentration of each mixture component independently of surface tension isotherms shape. Taking into account the composition of the mixed monolayer at the water–air interface, the standard Gibbs adsorption free energy was considered. The obtained results allow us to state that the concentration of both mixture components corresponding to their saturated monolayer and the surface tension of their aqueous solution can be predicted using the surfactants’ single monolayer pressure and their mole fraction in the mixed monolayer determined in the proposed way.     

Surfactant-assisted spreading of a liquid drop on a smooth solid surface

Axisymmetric spreading of a liquid drop covered with an insoluble surfactant monolayer on a smooth solid substrate is numerically investigated. As the drop spreads, the adsorbed surfactant molecules are constantly redistributed along the air-liquid interface by convection and diffusion, leading to nonuniformities in surface tension along the interface. The resulting Marangoni stresses affect the spreading rate by altering the surface flow and the drop shape. In addition, surfactant accumulation in the vicinity of the moving contact line affects the spreading rate by altering the balance of line forces. Two different models for the constitutive relation at the moving contact line are used, in conjunction with a surface equation of state based on the Frumkin adsorption framework, to probe the surfactant influence. The coupled evolution equations for the drop shape and monolayer concentration profile are integrated using a pseudospectral method to determine the rate of surfactant-assisted spreading over a wide range of the dimensionless parameters governing the spreading process. The insoluble monolayer enhances spreading through two mechanisms; a reduction in the equilibrium contact angle, and an increase in the magnitude of the radial pressure gradient within the drop due to the formation of positive surface curvature near the moving contact line. Both mechanisms are driven by the accumulation of surfactant at the contact line due to surface convection. Although the Marangoni stresses induced at the air-liquid interface reduce the rate of spreading during the initial stages of spreading, their retarding effect is overwhelmed by the favorable effects of the aforementioned mechanisms to lead to an overall enhancement in the rate of spreading in most cases. The spreading rate is found to be higher for bulkier surfactants with stronger repulsive interactions. With the exception of monolayers with strong cohesive interactions which tend to retard the spreading process, the overall effect of an insoluble monolayer is to increase the rate of drop spreading. Simulation results for small Bond numbers indicate the existence of a power-law region for the time-dependence of the basal radius of the drop, consistent with experimental measurements.     

Mixed alkaline phosphatase/sphingomyelin monolayer at the air-buffer interface: phase behavior and morphology

The glycosylphosphatidyl inositol(GPI)-anchored proteins are localized on the outer of the plasma membrane and clustered in membrane microdomain known as lipid rafts. Among them, mammalian alkaline phosphatase(AP) is an enzyme widely distributed. So, it has important biological significance to study the combination of AP with lipid monolayer. In our work, the interaction between AP and sphingomyelin has been studied at the air-buffer interface as a biomimetic membrane system by the Langmuir film technique and atomic force microscopy. The surface pressure-area isotherm for the mixed alkaline phosphatase/sphingomyelin monolayer shown the presence of a transition from a liquid-expanded phase to the liquid-expanded/liquidcondensed coexist phase. And the surface compressional modulus suggested the mixed alkaline phosphatase/sphingomyelin monolayer has larger compressibility compared with the pure sphingomyelin monolayer. Besides, according to the micrographs, we inferred when combined with lipid monolayer at the air-buffer interface, the AP molecules formed polymer not multilayer or micelle. And, according to the limiting molecules area of AP, we inferred that 12 AP molecules formed a hexagon polymer unit.     

Molecular properties of acetone and some of its chloroderivatives adsorbed at the surface of water

The relations between the electric surface potential (V) and the surface tension () of aqueous solutions of acetone, chloroacetone, 1,3-dichloroacetone, and their concentration were investigated. The vertical components of dipole moments of the above mentioned compounds were determined using the Helmholtz equation. The calculations were carried out on the basis of surface excess values, which were obtained from surface tension measurements and surface potential changes. Once the vertical component of dipole moments were found and the orientation of adsorbed molecules was assumed, the local dielectric permittivities of the surface monolayer were estimated.     

Experimental evidence to support a theory of lipid membrane fusion

Membrane fusion between two lipid membranes with different curvatures was measured by using a fluorescence fusion assay for lipid vesicle systems and was also obtained by measuring lipid monolayer surface tension upon the fusion of vesicles to monolayer membranes. For such membrane systems, it was found that when lysolipid was incorporated only in the membrane with a greater curvature, membrane fusion was more suppressed than those for the case where the same amount (molar ratio of lysolipid to non-lysolipids) of lysolipid was incorporated only in the membrane with a lower curvature. When lysolipid was incorporated only in a flat membrane (e.g., monolayer) and the fusion of small vesicles (SUV) to the monolayer was measured, suppression of membrane fusion by lysolipid was minimal. It is known that lysolipid lowers the surface energy of curved membranes, which stabilizes energetically such membrane surfaces, and thus suppresses membrane fusion. Our results support our theory of lipid membrane fusion where the membrane fusion occurs through the most curved membrane region at the contact area of two interacting membranes.     

Nanocrystal induced organization of a langmuir phospholipid monolayer

The influence of crystal surface charge on the thermodynamic and structural behavior of phospholipid monolayers has been investigated. We present how charged nanocrystals in the vicinity of an inherently nonordered lipid membrane provoke strong effects on the molecular arrangement within the monolayer. Apart from the induction of phase shifts and nucleation processes, the molecules were forced to adopt an ordered phase. A very recently developed X-ray scattering method is used for the first time to replace time-consuming specular reflectivity measurements. We conclude on the potential effects of crystal surface charge on cellular membranes.     

嵌入花生酸镉单层中的卟啉-紫精分子聚集行为的研究

本文制备了卟啉-紫精与花生酸镉混合LB膜, 用紫外可见光谱研究了膜中卟啉基团的聚集及取向, X射线衍射说明混合LB膜具有层状有序的周期结构。扫描电镜结果表明: 通过调节膜的表面压可使聚集成"微畴"的卟啉-紫精均匀分布在花生酸镉单分子层中, 随膜表面压的增大, 小的"微畴"相互连接形成更大的"微畴"。电子衍射说明混合膜中两组份分相存在, 且都为六角对称的有序结构。     

ADSORPTION AND ASSOCIATION IN BINARY MIXTURES OF CATIONIC SURFACTANTS

The adsorption and solution properties of cationic surfactants dodecylammonium chloride, tetradecylammonium chloride, and hexadecylammonium chloride, as well as hexadecyltrimtehylammonium bromide in pure and mixed states, were studied by surface tension and conductance measurements. The surfactants mixed non-ideally in both mixed monolayer and in the mixed micelles. The regular solution theory was used for evaluating the non-ideal interactions between molecules in adsorbed and micellar states. Similar values of the mixed monolayer and mixed micelle molecular interaction parameters in a mixture imply the molecules at the interface and in the mixed micelle have similar interactions. No synergism in surface tension effectiveness was observed. The adsorbed film at the air/water interface and mixed micelles were richer in more surface-active component.     

Kinetics for the collapse of trilayer liquid-crystalline disks from a monolayer at an air-water interface

Unlike surfactants considered in previous studies, when phosphatidylcholine (PC) monolayers collapse at constant surface tension to form a 3D bulk phase, surface area decreases at rates that slow. The different kinetics could result from collapse by a distinct mechanism. Rather than the transfer of molecules all along the interface between the monolayer and bulk phase, PC films can collapse by the folding and subsequent sliding of a bilayer over the monolayer. By this mechanism, molecules can transfer to collapsed trilayers through a locus of constant size. In this article, we use the theory of nucleation and growth to show analytically that during collapse, the area can decrease at rates that decelerate when each individual structure grows through a region of fixed dimensions. We also show that binary films of 30% dihydrocholesterol (dchol) and dipalmitoyl phosphatidylcholine (DPPC), which have previously been shown to form a homogeneous monolayer from which trilayer disks grow through a point, collapse with rates of area decay that slow, in agreement with our analytical expressions.     

Anomalous phase behavior in Langmuir monolayers of monomyristoyl-rac-glycerol at the air-water interface

The effect of temperature on the surface phase behavior in Langmuir monolayers of monomyristoyl-rac-glycerol (MMG) at the air-water interface has been studied by film balance and Brewster angle microscopy (BAM). It is observed that the domains of the MMG monolayers formed in the coexistence region between the liquid expanded (LE) and liquid condensed (LC) phases retain their circular shape over the studied temperature range, showing a sharp contrast to the temperature-dependent monolayer morphologies of amphiphilic systems where the shape of condensed domains changes either from compact circular to fingering or from irregular or spiral to compact patterns with increasing temperature. It is concluded that the system is capable of tuning the line tension of the interface by the effect of the increase in the hydrophobic character because of dehydration of the headgroup, which imparts to the molecules the properties of similar molecules but with less hydrophilic headgroups. As a result, the domains can retain their circular shape even up to the maximum possible temperature of the phase transition.     

Ultrathin monolayer of rubrene on Au(111) induced by charge transfer

An ultrathin self‐assembly monolayer of rubrene on Au(111) has been fabricated and studied by scanning tunneling microscopy. The apparent thickness of such monolayer is 0.08 nm and close to the radius of a carbon atom. Moreover, the rubrene molecules within the second layer prefer adsorbing on to the positions corresponding to the herringbone structure underneath the Au(111)–() while the Au surface is fully covered by a monolayer of rubrene. With the assistant with theoretical simulations, we reveal that small apparent height of such monolayer is due to the coupling between the molecular orbitals and the gold surface. About 0.237 electron per rubrene molecule is transferred to the surface, and as a consequence, an interfacial dipole is formed on the rubrene/Au interface. The formation of such interfacial dipole induced by charge transfer from molecules to surfaces is believed to be applied in organic molecules adsorbed on metal surfaces. Copyright © 2011 John Wiley & Sons, Ltd.     

Electrical manipulation of supported lipid membranes by embedded electrodes

Alkanethiol modified gold electrodes patterned over a silica surface provided a dual hydrophobic/hydrophilic surface suitable for phospholipid monolayer and bilayer formation over the alkylated gold and glass surfaces, respectively. The phospholipid monolayer and bilayer were connected, allowing free diffusion of lipids within both leaflets of the glass-supported bilayer over the alkanethiol/gold-to-glass interface. Application of large alternating current fields to these electrodes irreversibly switched the gold electrodes to diffusion barriers. Enclosure of the electrode devices within protein barriers revealed a resting state surface potential driven reorganization of the charged fluorescent probes. Application of lower magnitude direct current fields resulted in electrophoretic redistribution of the membrane probes and electro-osmotic reorganization of membrane associated proteins.     

DOPC/DPPC单层膜中分子间的相互作用及形态特征

利用Langmuir-Blodgett(LB)技术制备了不同表面压力下的1,2-二油酸-甘油-3-磷脂酰胆碱(DOPC)/1,2-二棕榈酸甘油-3-磷脂酰胆碱(DPPC)(摩尔比为1:1)和DOPC/DPPC/Chol(摩尔比为2:2:1)单层膜, 对单层膜内分子间的相互作用进行了热力学分析, 并用荧光显微镜和原子力显微镜对其形态进行了观测.热力学分析表明, DOPC与DPPC分子在单层膜结构中相互作用为排斥力, 诱导单层膜出现相变; DOPC, DPPC与胆固醇(Chol)间的相互作用均为吸引力, 当表面压力(π)大于18 mN/m时, DPPC与胆固醇的作用力大于DOPC.荧光显微镜观测表明, DOPC/DPPC单层膜出现明显相分离现象, 富含DPPC微区成“花形”结构, 且随着表面压力的升高微区逐渐增大, “花瓣”增多; 当胆固醇加入到DOPC/DPPC体系时, 单层膜相态由液相与凝胶相共存转变为液态无序相与液态有序相共存结构, 富含DPPC的微区形状从“花形”转变成“圆形”.原子力显微镜对单层膜的表征验证了荧光显微镜的观测结果, 表明胆固醇加入到DOPC/DPPC体系中对单层膜排列具有明显的影响, 压力和溶液状态等是影响脂膜结构的重要因素.     

Lateral reorganization of myelin lipid domains by myelin basic protein studied at the air-water interface

It has been speculated that adsorption of myelin basic protein (MBP) to the myelin lipid membrane leads to lateral reorganization of the lipid molecules within the myelin membrane. This hypothesis was tested in this study by surface pressure measurement and fluorescent imaging of a monolayer composed of a myelin lipid mixture. The properties of the lipid monolayer before and after addition of MBP into the subphase were monitored. Upon addition of MBP to the monolayer subphase, the surface pressure rose and significant rearrangement of the lipid domains was observed. These results suggest that binding and partial insertion of MBP into the lipid monolayer led to dramatic rearrangement and morphological changes of the lipid domains. A model of adsorption of MBP to the lipid domains and subsequent domain fusion promoted by minimization of electrostatic repulsion between the domains was proposed to account for the experimental observations. The significance of these results in light of the role of MBP in maintaining the myelin structural integrity is discussed.     

Surface tension in bilayer membranes with fixed projected area

We study the elastic response of bilayer membranes with fixed projected area to both the stretching and shape deformations. A surface tension is associated to each of these deformations. By using model amphiphilic membranes and computer simulations, we are able to observe both the types of deformation, and thus, both the surface tensions, related to each type of deformation, are measured for the same system. These surface tensions are found to assume different values in the same bilayer membrane, in particular, they vanish for different values of the projected area. We introduce a simple theory which relates the two quantities and successfully apply it to the data obtained with computer simulations.