Two-component Bilayers Research Articles

Synthesis and characterization of naphthalene derivatives for two-component heterojunction-based ambipolar field-effect transistors complemented with copper hexadecafluorophthalocyanine (F16CuPc).

In order to develop organic semiconductor materials with good performance, herein, a series of naphthalene derivatives were designed and synthesized by a “building-blocks approach” connected through α-bond, double bond, and triple bond, respectively. Thin-film transistors were fabricated in single-component and two-component modes based on these naphthalene derivatives by combining the F16CuPc as the n-type material. The ambipolar performance was investigated by adjusting the device preparation procedure with the hole and electron mobility of up to 10−2 cm2 V−1 s−1. Furthermore, the electrical performance was also improved to 0.73 cm2 V−1 s−1 using the two-component bilayer configuration.

Gramicidin A Channel Formation Induces Local Lipid Redistribution I: Experiment and Simulation

Integral membrane protein function can be modulated by the host bilayer. Because biological membranes are diverse and nonuniform, we explore the consequences of lipid diversity using gramicidin A channels embedded in phosphatidylcholine (PC) bilayers composed of equimolar mixtures of di-oleoyl-PC and di-erucoyl-PC (dC18:1+dC22:1, respectively), di-palmitoleoyl-PC and di-nervonoyl-PC (dC16:1+dC24:1, respectively), and di-eicosenoyl-PC (pure dC20:1), all of which have the same average bilayer chain length. Single-channel lifetime experiments, molecular dynamics simulations, and a simple lipid compression model are used in tandem to gain insight into lipid redistribution around the channel, which partially alleviates the bilayer deformation energy associated with channel formation. The average single-channel lifetimes in the two-component bilayers (95 ± 10 ms for dC18:1+dC22:1 and 195 ± 20 ms for dC16:1+dC24:1) were increased relative to the single-component dC20:1 control bilayer (65 ± 10 ms), implying lipid redistribution. Using a theoretical treatment of thickness-dependent changes in channel lifetimes, the effective local enrichment of lipids around the channel was estimated to be 58 ± 4% dC18:1 and 66 ± 2% dC16:1 in the dC18:1+dC22:1 and dC16:1+dC24:1 bilayers, respectively. 3.5-μs molecular dynamics simulations show 66 ± 2% dC16:1 in the first lipid shell around the channel in the dC16:1+dC24:1 bilayer, but no significant redistribution (50 ± 4% dC18:1) in the dC18:1+dC22:1 bilayer; these simulated values are within the 95% confidence intervals of the experimental averages. The strong preference for the better matching lipid (dC16:1) near the channel in the dC16:1+dC24:1 mixture and lesser redistribution in the dC18:1+dC22:1 mixture can be explained by the energetic cost associated with compressing the lipids to match the channel’s hydrophobic length.

Structural Properties of Phospholipid-based Bilayers with Long-Chain Alcohol Molecules in the Gel Phase.

The structural properties of two-component gel-phase bilayers of distearylphosphatidylcholine (DSPC) and alcohol molecules with different compositions and chain lengths (12-24 carbons long) are studied via molecular dynamics simulations. Several bilayer properties, including area per lipid, tilt angle, chain interdigitation, and headgroup offset, are studied for each system and compared, revealing important structural implications depending upon headgroup size and chain length. While tail tilt is the primary mechanism for single-component bilayers to balance tail attraction and headgroup repulsion, our results demonstrate that the lipid mixtures studied adjust this balance via an offset between the depths of the different molecular species in the bilayer; this behavior is found to depend both on composition and on the length of alcohol molecules relative to the length of DSPC tails. It is shown that the structural properties of bilayers with asymmetric tail lengths depend strongly on the bilayer composition, while the composition has less influence on mixed-component bilayers with nearly symmetric tail lengths. These findings are explained on the basis of the interdigitation between bilayer leaflets and how interdigitation is related to other structural properties.

(TFPP)Eu[Pc(OPh)8]Eu[Pc(OPh)8]/CuPc two-component bilayer heterojunction-based organic transistors with high ambipolar performance.

Organic thin film transistor (OTFT) devices fabricated by the solution-based QLS technique from a mixed (phthalocyaninato)(porphyrinato) europium complex (TFPP)Eu[Pc(OPh)8]Eu[Pc(OPh)8] exhibit air-stable ambipolar performance with mobilities of 6.0 × 10(-5) cm(2) V(-1) s(-)1 for holes and 1.4 × 10(-4) cm(2) V(-1) s(-1) for electrons, respectively. In good contrast, the two-component bilayer heterojunction thin film devices constructed by directly growing (TFPP)Eu[Pc(OPh)8]Eu[Pc(OPh)8] on vacuum deposited (VCD) CuPc film using solution based QLS method were revealed to show unprecedented ambipolar performance with carrier mobilities of 0.16 cm(2) V(-1) s(-1) for holes and 0.30 cm(2) V(-1) s(-1) for electrons. In addition to the intrinsic role of p-type organic semiconductor, the VCD CuPc film on the substrate also acts as a good template that induces significant improvement over the molecular ordering of triple-decker compound in the film. In particular, it results in the change in the aggregation mode of (TFPP)Eu[Pc(OPh)8]Eu[Pc(OPh)8] from J-type in the single-layer film to H-type in the bilayer film according to the UV-vis, XRD, and AFM observations.

Microscopic Model and Analytic Derivation of Area Per Molecule for DPPC-Cholesterol Bilayers

An area per molecule dependence on components concentration for DPPC-Cholesterol bilayers is calculated analytically using a microscopic model in a biologically relevant concentration range. DPPC lipid is modeled as flexible string with finite bending rigidity [1,2]. Cholesterol molecule is modeled as rigid rod of finite thickness [3]. Surface tension at hydrophobic interface is linear combination of “partial tensions” of bilayer components. The model's three important parameters are: surface tension at hydrophobic interface for pure DPPC membrane, bending rigidity of DPPC lipid, extrapolated surface tension at hydrophobic interface for cholesterol membrane. These parameters are chosen by nearly perfect fitting agreement of our theoretical curve with molecular dynamics simulations data [4] for these two-component bilayers. The molecular chains order parameter dependence on coordinate for 40% cholesterol - 60% DPPC membrane, measurable in NMR experiments, is calculated analytically and compared with molecular dynamics simulation data [5]. The order parameter calculation allows for DPPC tilt angle. The model parameters found by fitting the MD data are used further to calculate lateral pressure distribution and coefficient of thermal area expansion. The microscopic model allows one to study other thermodynamic coefficients and diffusion phenomena in multi-component bilayers. 1.Mukhin S.I., S. Baoukina, Analytical derivation of thermodynamic characteristics of lipid bilayer from a flexible string model. Phys. Rev. E. 71: 061918 (2005). 2.Mukhin S.I., B.B. Kheyfets, Analytical approach to thermodynamics of bolalipid membranes. Phys. Rev. E. 82: 051901 (2010). 3.Mukhin S.I., B.B. Kheyfets, Pore formation phase diagrams for lipid membranes. JETP Lett. 99: 358-362 (2014). 4. Edholm O., J.F. Nagle. Areas of Molecules in Membranes Consisting of Mixtures. Biophys. J. 89: 1827-1832 (2005). 5. Hofsass C., Lindahl E., and Edholm O. Molecular Dynamics Simulations of Phospholipid Bilayers with Cholesterol. Biophys. J. 84: 2192-2206 (2003).

Aggregation of nanoparticles on one and two-component bilayer membranes.

Using dissipative particle dynamics simulations the aggregation of nanoparticles on single and two-component bilayers is investigated. For a uniform bilayer the aggregation of nanoparticles depends strongly on the location of the particles in the bilayer; particles residing on the bilayer exterior cluster strongly under the influence of bilayer-mediated interactions, whereas the interaction between the particles in the bilayer interior is significantly weaker leading to more loosely bound, dynamic aggregates. The aggregation of nanoparticles on two-component bilayers composed of immiscible components changes due to competition between nanoparticle clustering and their adsorption on the boundary between the bilayer components. This reduces the size of the nanoparticle clusters formed on the bilayer exterior, with the clusters adhering onto the boundary between the bilayer components. Due to their weaker attraction nanoparticles in the interior of a mixed bilayer no longer aggregate and instead form strings along the boundary between the two bilayer components. Nanoparticles with an affinity to one bilayer component nucleate small domains of their favoured component around themselves. For asymmetric mixtures this leads to a notable change in the aggregation behaviour of the nanoparticles.

Singular perturbation models in phase transitions for second-order materials

A variational model proposed in the physics literature to describe the onset of pattern formation in two-component bilayer membranes and amphiphilic monolayers leads to the analysis of a Ginzburg-Landau type energy, precisely, u 7→ Z Ω » W (u)− q |∇u| + ∇u 2 – dx. When the stiffness coefficient −q is negative, one expects curvature instabilities of the membrane and, in turn, these instabilities generate a pattern of domains that differ both in composition and in local curvature. Scaling arguments motivate the study of the family of singular perturbed energies u 7→ Fe(u, Ω) := Z Ω » 1 e W (u)− qe|∇u| + e|∇u| – dx. Here, the asymptotic behavior of {Fe} is studied using Γ-convergence techniques. In particular, compactness results and an integral representation of the limit energy are obtained.

Building Self-Assembled Molecular Layers with Axially Substituted Titanium Phthalocyanines

Adsorption on graphite (HOPG) by titanium phthalocyanine axially bonded to a catechol ligand (TiPcat), titanylphthalocyanine (TiOPc), and 1:1 mixtures of these are studied at the HOPG-octylbenzene interface. The surface structures of a two-component bilayer, and of the individual monolayers of TiOPc and TiPcat, were determined by scanning tunneling microscopy (STM). TiPcat self-segregates onto a monolayer of TiOPc when an equal molar mixture is used. The preferential formation of a TiOPc monolayer from a solution containing both molecules is attributed to the difference in adsorption energies between TiPcat and TiOPc on graphite. The transformation of the hexagonal lattice of the pure TiOPc monolayer into a pseudo-square lattice was induced by the adsorption of TiPcat molecules. DFT calculations of the catechol orientation are presented.

Influence of cholesterol on the collective dynamics of the phospholipid acyl chains in model membranes

We have studied the packing and collective dynamics of the phospholipid acyl chains in a model membrane composed of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and cholesterol in varied phase state. After a structural characterization of this two-component model bilayer using X-ray reflectivity, we have carried out coherent inelastic neutron scattering to investigate the chain dynamics. Both DMPC/cholesterol membranes exhibited much sharper and more pronounced low-energy inelastic excitations than a pure DMPC membrane. In the high-energy regime above 10 meV, the insertion of cholesterol into the membrane was found to shift the position of the inelastic excitation towards values otherwise found in the pure lipids gel phase. Thus, the dissipative collective short-range dynamics of the acyl chains is strongly influenced by the presence of cholesterol.

On the Inner Structure and Topology of Clusters in Two-Component Lipid Bilayers. Comparison of Monomer and Dimer Ising Models

It has been shown on model and biological systems that membrane clusters can affect in-plane membrane reactions and can control biochemical reaction cascades. Clusters of two-component phospholipid bilayers have been simulated by two Ising-type lattice models: the monomer and the dimer model. In each model the plane of one layer of the bilayer is represented by a triangular lattice, each site of which is occupied by an acyl chain of either a component 1 or a component 2 lipid molecule. The dimer model assumes that pairs of acyl chains (lipid molecules) are permanently connected, forming dimers on the lattice, while in the case of the monomer model this covalent connection between acyl chains is ignored. Phase diagrams of two-component phospholipid bilayers were successfully calculated by both models. In this work, we use Monte Carlo techniques to calculate thermodynamic averages of global and local characteristics of the largest component 2 cluster (such as outer/inner perimeter, percolation, minimal linear size, and local density) and compare the results obtained by the two models. A new method is developed to characterize the inner structure of the clusters. Each point of a cluster is classified based on its shortest distance (or depth) from the cluster's outer perimeter. Then local cluster properties, such as density, are calculated as a function of the depth. The depth analysis reveals that toward the cluster interior the average density usually decreases in midsize clusters and remains constant in very large clusters. On the basis of the simulations the following typical cluster topologies are identified at different cluster sizes and cooperativity parameter values: (i) branch-like, (ii) circular, (iii) band-like, and (iv) planar.We did not find qualitative differences between the cluster structures in the dimer and monomer model. However, at the same cluster size and cooperativity parameter value the cluster of the dimer model is more compact. The cluster properties of the dimer model are different from that of the monomer model because of the lower mixing entropy and higher formation energy of an elementary inner island.

Molecular Dynamics Studies of the Molecular Structure and Interactions of Cholesterol Superlattices and Random Domains in an Unsaturated Phosphatidylcholine Bilayer Membrane

The effect of the molecular organization of lipid components on the properties of the bilayer membrane has been a topic of increasing interest. Several experimental and theoretical studies have suggested that cholesterol is not randomly distributed in the fluid-state lipid bilayer but forms nanoscale domains. Several cholesterol-enriched nanodomain structures have been proposed, including rafts, regular or maze arrays, complexes, and superlattices. At present, the molecular mechanisms by which lipid composition influences the formation and stability of lipid nanodomains remain unclear. In this study, we have used molecular dynamics (MD) simulations to investigate the effects of the molecular organization of cholesterol--superlattice versus random--on the structure of and interactions between lipids and water in lipid bilayers of cholesterol and 1-palmitoyl-2-oleoylphosphatidylcholine (cholesterol/POPC) at a fixed cholesterol mole fraction of 0.40. On the basis of four independent replicates of 200-ns MD simulations for a superlattice or random bilayer, statistically significant differences were observed in the lipid structural parameters, area per lipid, density profile, and acyl chain order profile, as well as the hydrogen bonding between various pairs (POPC and water, cholesterol and water, and POPC and cholesterol). The time evolution of the radial distribution of the cholesterol hydroxy oxygen suggests that the lateral distribution of cholesterol in the superlattice bilayer is more stable than that in the random bilayer. Furthermore, the results indicate that a relatively long simulation time, more than 100 ns, is required for these two-component bilayers to reach equilibrium and that this time is influenced by the initial lateral distribution of lipid components.

Shape variation of bilayer membrane daughter vesicles induced by anisotropic membrane inclusions

A theoretical model of a two-component bilayer membrane was used in order to describe the influence of anisotropic membrane inclusions on shapes of membrane daughter micro and nano vesicles. It was shown that for weakly anisotropic inclusions the stable vesicle shapes are only slightly out-of-round. In contrast, for strongly anisotropic inclusions the stable vesicle shapes may significantly differ from spheres, i.e. they have a flattened oblate shape at small numbers of inclusions in the membrane, and an elongated cigar-like prolate shape at high numbers of inclusions in the vesicle membrane.

Lateral Diffusion of Molecules in Two-Component Lipid Bilayer: A Monte Carlo Simulation Study

Lateral diffusion of membrane components makes possible any in-plane membrane reaction and has a key role in signaling in cell membranes. In this report the equilibrium lateral diffusion of intrinsic molecules in an equimolar DMPC/DSPC mixture is simulated using a thoroughly tested two-state model of two-component phospholipid bilayers. The model has been successful in calculating the excess heat capacity function, the most frequent center-to-center distances between DSPC clusters, and the fractal dimensions of gel clusters (Sugar, I. P., Thompson, T. E., Biltonen, R. L. Biophys. J. 1999, 76, 2099-2110). In the gel/fluid mixed phase region, a diffusing intrinsic molecule may change its state from fluid to gel (or from gel to fluid) at any time. A common characterization of the diffusion of intrinsic molecules is given by the simulated average first-passage time curves. We find that these curves can be described as power functions containing two parameters, alpha and beta, except near the percolation threshold of gel/fluid or compositional clusters. We find also that the intrinsic molecules are involved in approximately normal diffusion, i.e., beta approximately 2 in the extreme gel and fluid phase regions, while in the gel/fluid and gel/gel mixed phase regions the diffusion is anomalous, i.e., beta not equal 2. In the mixed phase regions, when the initial local state of the diffusing molecule is not specified, each component is involved in sub-diffusion (beta > 2). In the gel/fluid mixed phase region molecules situated initially inside a fluid cluster are involved in sub-diffusion, but DMPC molecules situated initially inside a gel cluster are involved in super-diffusion (beta < 2). The possibility of anomalous diffusion in membranes apparently arises because the diffusing molecule visits a variety of different environments characterized by its relative proximity to various membrane components. The diffusion is actually anomalous when the components of the bilayer are nonrandomly distributed. The deviation from random distribution is strongly correlated with beta. Similar to the results of the NMR experiments, the calculated relative diffusion coefficient continuously decreases in the gel/fluid mixed phase region with decreasing temperature. In apparent contradiction, diffusion measured by fluorescence recovery after photobleaching (FRAP) demonstrates the existence of a threshold temperature, below which long-range diffusion of FRAP probe molecules is essentially blocked. This threshold temperature is highly correlated with the percolation temperature of gel clusters.

Curvature elasticity of mixed amphiphilic bilayers.

Elastic bending constants of mixed amphiphilic bilayers are calculated using a molecular approach. The free energy is expanded up to quadratic order in curvatures and compositions, choosing a flat symmetrical bilayer as the reference state. Bending constants are then calculated from the derivatives of the free energy evaluated at this reference state. Two-component bilayers are considered. As a novelty, the local compositions are allowed to fully relax upon bending so that the 2 monolayers are at chemical equilibrium with each other at every curvature. The compositional degree of freedom is shown to affect the bending constant k, but not the saddle-splay constant k. The influence on the membrane elastic properties of various chain structural features, such as length, volume, and stiffness, is investigated. This may prove useful to model mixed bilayers composed of hydrocarbon/hydrocarbon and hydrocarbon/fluorocarbon chains.

Lateral and transverse diffusion in two-component bilayer membranes

We study the lateral and transverse diffusion of amphiphiles in two-component bilayer membranes, using a coarse-grained model for amphiphilic molecules and combined Monte Carlo-Molecular Dynamics simulations. Membrane structural properties, such as the mean thickness, are also measured. The dependence of such properties on membrane composition, inter-molecular interactions, and amphiphile stiffness is determined. In particular, we show that addition of shorter amphiphiles drives the model membrane towards a more fluid state, with increased amphiphile lateral diffusion rates. These results can be understood in the framework of a simple free-volume model. Furthermore, we observe an increase in the trans-membrane diffusion when the interaction energy of amphiphiles with their neighboring molecules is decreased.

Tunneling conductivity of one- and two-component alkanethiol bilayers in Hg–Hg junctions

Electron tunneling (ET) through alkanethiol bilayers trapped between two small mercury drops (Hg–Hg tunneling junction, geometric area: 8×10 −4 cm 2) was investigated. Self-assembled monolayers were formed on Hg drops using one- or two-component solutions of n-alkanethiols (ranging from nonanethiol to hexadecanethiol) in hexadecane. Mercury drops covered by monolayers were brought into contact using micromanipulators. Current–voltage cyclic curves were used to measure the capacitance and tunneling current for the alkanethiol bilayer. The experimental current–voltage curves were compared with the following theoretical models: classical Simmons theory, Simmons theory modified by including effective electron mass (Lindsay's model), and a model for off-resonance tunneling through a molecular bridge (Ratner's model). The classical Simmons theory does not fit our tunneling data while Lindsay's and Ratner's models agree reasonably well with the tunneling characteristics of Hg–Hg junctions. The electrical properties of two-component bilayers, containing a mixture of hexadecanethiol and nonanethiol deposited on each Hg drop, were studied as a function of a monolayer composition. The thickness of the two-component monolayer on each Hg drop depends linearly on the mole-fraction of nonanethiol. ET through a two-component system is less efficient than ET through single-component bilayers. This result is rationalized in terms of diminished electronic coupling through van der Waals contacts.

Two-component fluid membranes near repulsive walls: Linearized hydrodynamics of equilibrium and nonequilibrium states.

We study the linearized hydrodynamics of a two-component fluid membrane near a repulsive wall, using a model that incorporates curvature-concentration coupling as well as hydrodynamic interactions. This model is a simplified version of a recently proposed one [J.-B. Manneville et al., Phys. Rev. E 64, 021908 (2001)] for nonequilibrium force centers embedded in fluid membranes, such as light-activated bacteriorhodopsin pumps incorporated in phospholipid egg phosphatidyl choline (EPC) bilayers. The pump-membrane system is modeled as an impermeable, two-component bilayer fluid membrane in the presence of an ambient solvent, in which one component, representing active pumps, is described in terms of force dipoles displaced with respect to the bilayer midpoint. We first discuss the case in which such pumps are rendered inactive, computing the mode structure in the bulk as well as the modification of hydrodynamic properties by the presence of a nearby wall. These results should apply, more generally, to equilibrium fluid membranes comprised of two components, in which the effects of curvature-concentration coupling are significant, above the threshold for phase separation. We then discuss the fluctuations and mode structure in the steady state of active two-component membranes near a repulsive wall. We find that proximity to the wall smoothens membrane height fluctuations in the stable regime, resulting in a logarithmic scaling of the roughness even for initially tensionless membranes. This explicitly nonequilibrium result is a consequence of the incorporation of curvature-concentration coupling in our hydrodynamic treatment. This result also indicates that earlier scaling arguments which obtained an increase in the roughness of active membranes near repulsive walls upon neglecting the role played by such couplings may need to be reevaluated.

In Situ Atomic Force Microscope Imaging of Supported Lipid Bilayers

In situ AFM images of phospholipase A (PLA2) hydrolysis of mica-supported one- and two-component lipid bilayers are presented. For one-component DPPC bilayers an enhanced enzymatic activity is observed towards preexisting defects in the bilayer. Phase separation is observed in two-component DMPC-DSPC bilayers and a remarkable enhanced hydrolytic activity of the PLA2-enzyme for the DMPC-rich phase is seen. Furthermore, in a supported double bilayer system a characteristic ripple structure, most likely related to the formation of the Pβ′-ripple phase is observed.

In Situ Atomic Force Microscope Imaging of Supported Lipid Bilayers

In situ AFM images of phospholipase A (PLA2) hydrolysis of mica-supported one- and two-component lipid bilayers are presented. For one-component DPPC bilayers an enhanced enzymatic activity is observed towards preexisting defects in the bilayer. Phase separation is observed in two-component DMPC-DSPC bilayers and a remarkable enhanced hydrolytic activity of the PLA2-enzyme for the DMPC-rich phase is seen. Furthermore, in a supported double bilayer system a characteristic ripple structure, most likely related to the formation of the Pβ′-ripple phase is observed.

Structure-function relationships in two-component phospholipid bilayers: Monte Carlo simulation approach using a two-state model

This chapter discusses that the functions of cell membranes such as permeability of small water-soluble molecules and signaling through in-plane chemical reactions are strongly related to and affected by the lateral organization of the multicomponent lipid matrix. The functional significance of membrane lateral heterogeneity is emphasized by discoveries in membrane cell biology. Glycosphingolipid clusters, also called detergent-resistant, glycosphingolipid-enriched complexes or “glycolipid rafts,” serve as target sites for the fusion/budding of envelope viruses with eukaryotic cells. Most of what is currently postulated about glycosphingolipid organization in cell membranes originates from studies of model membrane systems. It reviews that computer simulations of lipid bilayers makes it possible to study the relationships between the microscopic level membrane configurations and the phenomenological level membrane functions. The chapter describes a two-state, two-component minimal model of dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) mixtures. It also makes a comparison between the calculated average size of gel clusters and the average linear size of the gel clusters observed by atomic force microscopy. Finally, based on the simulation results, a comparison is made between the predictions of the two existing theories of excess membrane permeability for small molecules.