Binary Lipid Membranes Research Articles

Hydrophobic Matching Dictates over the Linear Rule of Mixtures in Binary Lipid Membranes.

Biological membranes feature heterogeneous mixtures of lipids with different head and tail characteristics. Their biophysical properties are dictated by the intimate interaction among different constituent lipids. Previous studies suggest that the membrane area-per-lipid (APL) deviates from the linear rule of mixtures (LRM) for binary lipid membranes, but the underlying mechanism remains elusive. Our molecular dynamics (MD) simulations of binary lipid membranes consisting of lipids with different tail characteristics reveal a competitive mechanism whereby lipids tend to deform each other to minimize the hydrophobic mismatch between their tails. Depending on the relative tail lengths and saturation levels, this may result in an either positive or negative deviation of APL from the LRM. As lipid packing plays an essential role in membrane fusion and peptide-membrane binding, our findings may help guide the selection of lipids for the effective rational design of nanoliposomes and membrane-targeting peptides.

Demixing of homogeneous binary lipid membranes induced by protein inclusions.

We study a model of a lipid bilayer membrane described by two order parameters: the chemical composition described using the Gaussian model and the spatial configuration described with the elastic deformation model of a membrane with a finite thickness or, equivalently, for an adherent membrane. We assume and explain on physical grounds the linear coupling between the two order parameters. Using the exact solution, we calculate the correlation functions and order parameter profiles. We also study the domains that form around inclusions on the membrane. We propose and compare six distinct ways to quantify the size of such domains. Despite its simplicity, the model has many interesting features like the Fisher-Widom line and two distinct critical regions.

Interdomain exchange and the flip-flop of cholesterol in ternary component lipid membranes and their effects on heterogeneous cholesterol diffusion.

Cell membranes are heterogeneous with a variety of lipids, cholesterol, and proteins and are composed of domains of different compositions. Such heterogeneous environments make the transport of cholesterol complicated: cholesterol not only diffuses within a particular domain but also travels between domains. Cholesterol also flip-flops between upper and lower leaflets such that cholesterol may reside both within leaflets and in the central region between two leaflets. How the presence of multiple domains and the interdomain exchange of cholesterol would affect the cholesterol transport, however, remains elusive. In this study, therefore, we perform molecular dynamics simulations up to 100μs for ternary component lipid membranes, which consist of saturated lipids (dipalmitoylphosphatidylcholine, DPPC), unsaturated lipids (dilinoleylphosphatidylcholine, DIPC), and cholesterol. The ternary component membranes in our simulations form two domains readily: DPPC and DIPC domains. We find that the diffusion of cholesterol molecules is much more heterogeneous and non-Gaussian than expected for binary component lipid membranes of lipids and cholesterol. The non-Gaussian parameter of the cholesterol molecules is about four times larger in the ternary component lipid membranes than in the binary component lipid membranes. Such non-Gaussian and heterogeneous transport of cholesterol arises from the interplay among the interdomain kinetics, the different diffusivity of cholesterol in different domains, and the flip-flop of cholesterol. This suggests that in cell membranes that consist of various domains and proteins, the cholesterol transport can be very heterogeneous. We also find that the mechanism of the interdomain exchange differs for different domains: cholesterol tends to exit the DIPC domain along the central region of the membrane for the DIPC-to-DPPC transition, while the cholesterol is likely to exit the DPPC domain within the membrane leaflet for the DPPC-to-DIPC transition. Also, the interdomain exchange kinetics of cholesterol for the DPPC-to-DIPC transition is up to 7.9 times slower than the DIPC-to-DPPC transition.

Three-Phase Coexistence in Binary Charged Lipid Membranes in a Hypotonic Solution

We investigated the phase separation of dioleoylphosphatidylserine (DOPS) and dipalmitoylphosphatidylcholine (DPPC) in giant unilamellar vesicles in hypotonic solution using fluorescence and confocal laser scanning microscopy. Although phase separation in charged lipid membranes is generally suppressed by the electrostatic repulsion between the charged headgroups, osmotic stress can promote the formation of charged lipid domains. Interestingly, we observed three-phase coexistence even in DOPS/DPPC binary lipid mixtures. The three phases were DPPC-rich, dissociated DOPS-rich, and nondissociated DOPS-rich phases. The two forms of DOPS were found to coexist owing to the ionization of the DOPS headgroup, such that the system could be regarded as quasi-ternary. The three formed phases with differently ionized DOPS domains were successfully identified experimentally by monitoring the adsorption of positively charged particles. In addition, coarse-grained molecular dynamics simulations confirmed the stability of the three-phase coexistence. Attraction mediated by hydrogen bonding between protonated DOPS molecules and reduction of the electrostatic interactions at the domain boundaries stabilized the three-phase coexistence.

The effects of the lipid type on the spatial arrangement and dynamics of cholesterol in binary component lipid membranes.

Intermolecular interactions between cholesterol and lipids in cell membranes, which play critical roles in cellular processes such as the formation of nano-domains, depend on the molecular structure of the lipids. The diffusion and the spatial arrangement of cholesterol within the lipid membranes also change with the type of lipids. For example, the flip-flop, an important transport mechanism for cholesterol in the membranes, can be facilitated significantly by the presence of unsaturated lipids. However, how the structure of lipids affects the spatial arrangement and the dynamics of cholesterol remains elusive at a molecular level. In this study, we investigate the effects of lipid-cholesterol interactions on the spatial arrangement and the dynamics of cholesterol. We perform molecular dynamics simulations for the binary component membranes of lipids and cholesterol. We employ seven different kinds of lipids by changing either the degree of a saturation level or the length of lipid tails. We find from our simulations that the rate of cholesterol flip-flop is enhanced as the lipids are either less saturated or shorter, which is consistent with previous studies. Interestingly, when the lipid tails are fully saturated and sufficiently long, the center in between two leaflets becomes metastable for cholesterol to stay at. Because the cholesterol at the membrane center diffuses faster than that within leaflets, regardless of the lipid type, such an emergence of the metastable state (in terms of the cholesterol position) complicates the cholesterol diffusion significantly.

Melting-Temperature-Dependent Interactions of Ergosterol with Unsaturated and Saturated Lipids in Model Membranes.

Sterols such as cholesterol (Chol) and ergosterol (Erg) are known to regulate membrane properties in higher eukaryotes and in lower eukaryotes, respectively. To better understand the modulation of membrane properties by Erg, binary lipid membranes composed of Erg and diacylglycerophosphocholine (PC) were studied in Langmuir monolayer and bilayer vesicle systems. From the excess area measured by pressure-area isotherms, attractive interactions between Erg and saturated PC were significant above the melting temperature (Tm) of PC. Conversely, repulsive interactions were observed at temperatures below Tm. From the analyses of membrane fluidity and polarity using fluorescence probes, similar trends were observed for bilayer systems where Erg had an ordering effect on saturated PC vesicles in the fluid state. However, Chol had a stronger ordering effect than Erg. In unsaturated PC systems, Erg did not alter membrane ordering. These findings demonstrate that the interaction of Erg with the fluid-state PC lipids will maintain lower-eukaryote membranes in a more ordered state, similar to the effect of cholesterol in higher eukaryotes.

Quantitative spontaneous Raman scattering spectroscopy in artificial binary lipid membranes

The cholesterol concentration in lipid membranes is an important parameter that influences signaling processes as well as the structure of membranes. We show that the cholesterol content in giant unilamellar vesicles from binary lipid mixtures of cholesterol and 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine can be quantitatively assessed with spontaneous Raman scattering spectroscopy. An analytical model employing correlation analysis is introduced, which can be applied for the determination of the actual cholesterol content in unilamellar lipid membranes of lipid compositions that are unknown or, because of external effects, deviant from the weighed‐in values. Copyright © 2017 John Wiley & Sons, Ltd.

Coarse-grained molecular dynamics simulation of binary charged lipid membranes: Phase separation and morphological dynamics.

Biomembranes, which are mainly composed of neutral and charged lipids, exhibit a large variety of functional structures and dynamics. Here, we report a coarse-grained molecular dynamics (MD) simulation of the phase separation and morphological dynamics in charged lipid bilayer vesicles. The screened long-range electrostatic repulsion among charged head groups delays or inhibits the lateral phase separation in charged vesicles compared with neutral vesicles, suggesting the transition of the phase-separation mechanism from spinodal decomposition to nucleation or homogeneous dispersion. Moreover, the electrostatic repulsion causes morphological changes, such as pore formation, and further transformations into disk, string, and bicelle structures, which are spatiotemporally coupled to the lateral segregation of charged lipids. Based on our coarse-grained MD simulation, we propose a plausible mechanism of pore formation at the molecular level. The pore formation in a charged-lipid-rich domain is initiated by the prior disturbance of the local molecular orientation in the domain.

Structure of Cholesterol in Lipid Rafts.

Rafts, or functional domains, are transient nano-or mesoscopic structures in the plasma membrane and are thought to be essential for many cellular processes such as signal transduction, adhesion, trafficking, and lipid or protein sorting. Observations of these membrane heterogeneities have proven challenging, as they are thought to be both small and short lived. With a combination of coarse-grained molecular dynamics simulations and neutron diffraction using deuterium labeled cholesterol molecules, we observe raftlike structures and determine the ordering of the cholesterol molecules in binary cholesterol-containing lipid membranes. From coarse-grained computer simulations, heterogenous membranes structures were observed and characterized as small, ordered domains. Neutron diffraction was used to study the lateral structure of the cholesterol molecules. We find pairs of strongly bound cholesterol molecules in the liquid-disordered phase, in accordance with the umbrella model. Bragg peaks corresponding to ordering of the cholesterol molecules in the raftlike structures were observed and indexed by two different structures: a monoclinic structure of ordered cholesterol pairs of alternating direction in equilibrium with cholesterol plaques, i.e., triclinic cholesterol bilayers.

Measuring the composition-curvature coupling in binary lipid membranes by computer simulations.

The coupling between local composition fluctuations in binary lipid membranes and curvature affects the lateral membrane structure. We propose an efficient method to compute the composition-curvature coupling in molecular simulations and apply it to two coarse-grained membrane models-a minimal, implicit-solvent model and the MARTINI model. Both the weak-curvature behavior that is typical for thermal fluctuations of planar bilayer membranes as well as the strong-curvature regime corresponding to narrow cylindrical membrane tubes are studied by molecular dynamics simulation. The simulation results are analyzed by using a phenomenological model of the thermodynamics of curved, mixed bilayer membranes that accounts for the change of the monolayer area upon bending. Additionally the role of thermodynamic characteristics such as the incompatibility between the two lipid species and asymmetry of composition are investigated.

Xenon and Other Volatile Anesthetics Change Domain Structure in Model Lipid Raft Membranes

Inhalation anesthetics have been in clinical use for over 160 years, but the molecular mechanisms of action continue to be investigated. Direct interactions with ion channels received much attention after it was found that anesthetics do not change the structure of homogeneous model membranes. However, it was recently found that halothane, a prototypical anesthetic, changes domain structure of a binary lipid membrane. The noble gas xenon is an excellent anesthetic and provides a pivotal test of the generality of this finding, extended to ternary lipid raft mixtures. We report that xenon and conventional anesthetics change the domain equilibrium in two canonical ternary lipid raft mixtures. These findings demonstrate a membrane-mediated mechanism whereby inhalation anesthetics can affect the lipid environment of transmembrane proteins.

Composite contact of binary lipid membranes

Upon adhesion, binary lipid membranes may retain their structural identity but they may also undergo partial fusion promoted by microscopic phase separation of the components. In membranes composed of cylindrical and inverted conical lipids, a partially fused contact zone may consist of inverted micelles sandwiched between two monolayers. We theoretically analyze the elastic properties of such a composite contact zone. We calculate its bending moduli and we show that they may be considerably larger than the moduli of a phase-nonseparated double bilayer. The main mechanism responsible for the enhanced rigidity of the composite contact structure is the increased separation of the two monolayers caused by the presence of water in the inverted micelles.

Inhalation Anesthetics Change the Domain Structure of Model Ternary Lipid Raft Membranes

The mechanism of action for volatile anesthetics remains obscure despite clinical use for over 150 years. No single ion channel or protein receptor appears necessary and sufficient to account for anesthetic action, and the physical effects of volatile anesthetics on homogeneous model membranes appear too small to produce anesthesia. We recently reported that halothane changes the domain structure of a binary lipid mixture1, increasing the ratio of disordered phase to ordered phase.We have now studied two ternary model lipid raft mixtures with X-ray diffraction: Dioleoylphosphatidylcholine (DOPC)/dipalmitylphosphatidylcholine (DPPC)/cholesterol, and Dioleoylphosphatidylcholine (DOPC)/sphingomyelin (porcine)/cholesterol. Multi-layers were prepared upon glass slides, hydrated overnight at 98% relative humidity, and maintained at 27.0 ± 0.1 C on a Peltier-controlled stage in a sealed X-ray chamber. Volatile anesthetics were introduced as solutions in hexadecane. For both raft mixtures, two series of lamellar diffraction peaks are observed, with d-spacings differing by about 10%. These correspond to the liquid ordered phase and the liquid disordered phase. The relative intensities of diffraction for these phases change with increasing temperature and anesthetic concentration, both favoring the liquid disordered phase.A variety of different volatile anesthetics--halothane, isoflurane, chloroform, and hexane_all produce significant increases in the ratios of liquid disordered to liquid ordered lipid phases in these mixtures. These shifts occur at clinically relevant concentrations and are reversible upon withdrawal of anesthetic. There were no consistent effects of the anesthetics on the d-spacings of the lipid layers.These findings suggest that some effects of volatile anesthetics may be mediated through physical changes in membrane domain structures that interact with membrane proteins.1. Weinrich, M., Nanda, H., et al., Halothane changes the domain structure of a binary lipid membrane. Langmuir, 2012. 28(10): p. 4723-8.

GPU-Based Massive Parallel Kawasaki Kinetics in the Dynamic Monte Carlo Simulations of Lipid Nanodomains.

Multicomponent lipid membranes in the liquid phase exhibit dynamic lateral heterogeneities which play an important role in specific cell membrane functions. A GPU-based parallel algorithm for two-dimensional lattice Dynamic Monte Carlo simulations of nanodomain formation in binary lipid membranes was developed and tested. Speedups of up to 50-times over CPU-based calculations were achieved, and simulations employing lattices of up to 1800 × 1800 sites were performed. The existence of large nonregular lipid domains of sizes up to 160 nm was demonstrated. This reveals the necessity to employ lattices of at least ∼900 × 900 sites to study the lateral lipid organization in complex lipid membranes.

Halothane Changes the Domain Structure of a Binary Lipid Membrane

X-ray and neutron diffraction studies of a binary lipid membrane demonstrate that halothane at physiological concentrations produces a pronounced redistribution of lipids between domains of different lipid types identified by different lamellar d-spacings and isotope composition. In contrast, dichlorohexafluorocyclobutane (F6), a halogenated nonanesthetic, does not produce such significant effects. These findings demonstrate a specific effect of inhalational anesthetics on mixing phase equilibria of a lipid mixture.

SERS imaging for label-free detection of the phospholipids distribution in hybrid lipid membrane

A label-free and low-cost mapping method based on SERS imaging was reported for illustrating the distribution of phospholipids with similar structures in binary lipid membranes on Ag nanoparticles (Ag NPs) films. The Ag NPs films exhibited strong SERS activity and good reproducibility which were investigated with p-aminothiophenol (p-ATP) as probe molecules. Atomic force microscope (AFM) measurement proved that compact lipid membranes formed on the Ag NPs films. Basing on the Ag NPs films, the SERS spectra of phospholipids in the mixed lipid membranes were achieved and the inherent vibration of 1,2-dimyristoyl-sn-lycero-3-phosphoglycerol, sodium salt (DMPG), 1482 cm−1, was used to distinguish between DMPG and dimyristoylphosphatidylcholine (DMPC). The proportions of phospholipids in the mixed lipid membranes were represented by the intensity ratio of peaks at 1482 cm−1 and 1650 cm−1 (R 1482/1650) simultaneously: increasing R 1482/1650 indicated higher proportion of DMPG and lower proportion of DMPC. SERS imaging of the lipid membranes was constructed as a combination of spacial information and the semiquantitative detection of phospholipids according to R 1482/1650, which showed that the charged phospholipids, DMPG, aggregated in the hybrid lipid membranes. The presented mapping strategy based on SERS imaging carried out on Ag NPs films supplied a facile, label-free and inexpensive way for potential applications in the research on the structure of the lipid membrane, such as lipid domains and rafts.

Texture of Membrane Gel Domains

In this work1 we investigate the texture of gel (g) domains in supported binary lipid membranes. Lateral organization of lipid bilayer membranes is a topic of fundamental and biological importance. Whereas questions related to the size and composition of fluid domains are well studied, the possibility of texture in condensed solid/gel domains has received limited attention. Gel domains are expected to be prominent in skin membranes and in ceramide domains during apoptosis. When using polarized light for two-photon excitation of the lipid probe Laurdan, the emission intensity is highly sensitive to the angle between the polarization and the tilt orientation of lipid acyl chains. By imaging the intensity variations as a function of the polarization angle, we map the lateral variations of the lipid tilt within domains. Results reveal that gel domains are composed of distinct subdomains with different lipid tilt directions. Vortex structures centered at the domain core can be observed. Texture patterns of the same type have historically been associated with the presence of hexatic order in monolayers. The hexatic phase is an intermediate phase between the crystal and fluid states, having short range positional order and long range orientation order. The present results provide some support for the notion that hexatic order may persist in bilayers. Using the generalized polarization (GP) function of Laurdan, we demonstrate that although gel domains have heterogeneous texture, the membrane phase state is uniform within domains.[1] U. Bernchou et. al. ‘Texture of Lipid Bilayer Domains’. J. Am. Chem. Soc., 2009. DOI: 10.1021/ja903375m

Texture of Lipid Bilayer Domains

We investigate the texture of gel (g) domains in binary lipid membranes composed of the phospholipids DPPC and DOPC. Lateral organization of lipid bilayer membranes is a topic of fundamental and biological importance. Whereas questions related to size and composition of fluid membrane domain are well studied, the possibility of texture in gel domains has so far not been examined. When using polarized light for two-photon excitation of the fluorescent lipid probe Laurdan, the emission intensity is highly sensitive to the angle between the polarization and the tilt orientation of lipid acyl chains. By imaging the intensity variations as a function of the polarization angle, we map the lateral variations of the lipid tilt within domains. Results reveal that gel domains are composed of subdomains with different lipid tilt directions. We have applied a Fourier decomposition method as a convenient way to analyze the angular intensity variations. Texture patterns of the same type have been associated with the presence of hexatic order in monolayers. The present results provide some support for the notion that hexatic order may persist in bilayers. Laurdan exhibits an emission spectral shift which correlates with the phase state of the membrane. This is quantified by the generalized polarization (GP) function, and we demonstrate that a GP analysis can be performed on supported membranes. The results show that although the gel domains have heterogeneous texture, the membrane phase state does not show spatial variation within each domain.

Numerical study of the phase separation in binary lipid membrane containing protein inclusions under stationary shear flow

The phase separation of lipids is believed to be responsible for the formation of lipid rafts in biological cell membrane. In the present work, a continuum model and a particle model are constructed to study the phase separation in binary lipid membrane containing inclusions under stationary shear flow. In each model, employing the cell dynamical system (CDS) approach, the kinetic equations of the confusion-advection process are numerically solved. Snapshot figures of the phase morphology are performed to intuitively display such phase evolving process. Considering the effects from both the inclusions and the shear flow, the time growth law of the characteristic domain size is discussed.

Membrane lipid segregation in endocytosis

We explore the equilibrium mechanics of a binary lipid membrane that wraps around a spherical or cylindrical particle. One of the lipid membrane components induces a positive spontaneous curvature, while the other induces a negative local curvature. Using a Hamiltonian approach, we derive the equations governing the membrane shape and lipid concentrations near the wrapped object. Asymptotic expressions and numerical solutions for membrane shapes are presented. We determine the regimes of bending rigidity, surface tension, intrinsic lipid curvature, and effective receptor binding energies that lead to efficient wrapping and endocytosis. Our model is directly applicable to the study of invagination of clathrin-coated pits and receptor-induced wrapping of colloids such as spherical virus particles.