Effects of an Amphipathic α-Helical Peptide on Lateral Pressure and Water Penetration in Phosphatidylcholine and Monoolein Mixed Membranes

Determination of Elastic Parameters of Lipid Membranes with Molecular Dynamics: A Review of Approaches and Theoretical Aspects

Lipid membranes are abundant in living organisms, where they constitute a surrounding shell for cells and their organelles. There are many circumstances in which the deformations of lipid membranes are involved in living cells: fusion and fission, membrane-mediated interaction between membrane inclusions, lipid-protein interaction, formation of pores, etc. In all of these cases, elastic parameters of lipid membranes are important for the description of membrane deformations, as these parameters determine energy barriers and characteristic times of membrane-involved phenomena. Since the development of molecular dynamics (MD), a variety of in silico methods have been proposed for the determination of elastic parameters of simulated lipid membranes. These MD methods allow for the consideration of details unattainable in experimental techniques and represent a distinct scientific field, which is rapidly developing. This work provides a review of these MD approaches with a focus on theoretical aspects. Two main challenges are identified: (i) the ambiguity in the transition from the continuum description of elastic theories to the discrete representation of MD simulations, and (ii) the determination of intrinsic elastic parameters of lipid mixtures, which is complicated due to the composition-curvature coupling effect.

Membrane Lipids Assist Catalysis by CTP: Phosphocholine Cytidylyltransferase

While most of the articles in this issue review the workings of integral membrane enzymes, in this review, we describe the catalytic mechanism of an enzyme that contains a soluble catalytic domain but appears to catalyze its reaction on the membrane surface, anchored and assisted by a separate regulatory amphipathic helical domain and inter-domain linker. Membrane partitioning of CTP: phosphocholine cytidylyltransferase (CCT), a key regulatory enzyme of phosphatidylcholine metabolism, is regulated chiefly by changes in membrane phospholipid composition, and boosts the enzyme's catalytic efficiency >200-fold. Catalytic enhancement by membrane binding involves the displacement of an auto-inhibitory helix from the active site entrance-way and promotion of a new conformational ensemble for the inter-domain, allosteric linker that has an active role in the catalytic cycle. We describe the evidence for close contact between membrane lipid, a compact allosteric linker, and the CCT active site, and discuss potential ways that this interaction enhances catalysis.

Vancomycin Loaded Glycerol Monooleate Liquid Crystalline Phases Modified with Surfactants

The influence of two tuning agents, polyglycerol ester (PE) and triblock copolymer (TC), on the properties of glycerol monooleate (MO) liquid crystalline phase (LCP) was investigated to achieve the therapeutic concentration of vancomycin hydrochloride (VHCl) into the eye, topically during 60 min (1 h) and intravitreally during 2880 min (48 h). Different techniques were used to elucidate the impact of surfactants on the structure of the LCP: polarized light microscopy (PLM), small-angle X-ray scattering (SAXS), and in vitro release tests I and II (simulating local and intravitreal application in the eye). The structure analysis by SAXS depicts that the inclusion of PE into the MO LCP provided partial transition of a hexagonal phase into a lamellar phase, and TC induced a partial transition of a hexagonal phase into an LCP which identification was difficult. The LCP modulated with PE and TC demonstrated different VHCl's release patterns and were evaluated by comparing our release data with the literature data. The comparison indicated that the LCP modulated with 30% w/w PE could be a promising VHCl delivery system intravitreally during 2880 min.

Effects of lipid composition on membrane permeation

Passive permeation through lipid membranes is an essential process in biology. In vivo membranes typically consist of mixtures of lamellar and nonlamellar lipids. Lamellar lipids are characterized by their tendency to form lamellar sheet-like structures, which are predominant in nature. Nonlamellar lipids, when isolated, instead form more geometrically complex nonlamellar phases. While mixed lamellar/nonlamellar lipid membranes tend to adopt the ubiquitous lamellar bilayer structure, the presence of nonlamellar lipids is known to have profound effects on key membrane properties, such as internal distributions of stress and elastic properties, which in turn may alter related biological processes. This work focuses on one such process, i.e., permeation, by utilising atomistic molecular dynamics simulations in order to obtain transfer free energy profiles, diffusion profiles and permeation coefficients for a series of thirteen small molecules and drugs. Each permeant is tested on two bilayer membranes of different lipid composition, i.e., purely lamellar and mixed lamellar/nonlamellar. Our results indicate that the presence of nonlamellar lipids reduces permeation for smaller molecules (molecular weight < 100) but facilitates it for the largest ones (molecular weight > 100). This work represents an advancement towards the development of more realistic in silico permeability assays, which may have a substantial future impact in the area of rational drug design.

Small Cationic Peptides: Influence of Charge on Their Antimicrobial Activity

The first stage of the action mechanism of small cationic peptides with antimicrobial activity is ruled by electrostatic interactions between the peptide and the pathogen cell membrane. Thus, an increase in its activity could be expected with an increase in the positive charge on the peptide. By contrast, the opposite behavior has been observed when the charge increases to reach a critical value, beyond which the activity falls. This work studies the perturbation effects in a cell membrane model for two small cationic peptides with similar length and morphology but with different cationic charges. The synthesis and antibacterial activity of the two peptides used in this study are described. The thermodynamic study associated with the insertion of these peptides into the membrane and the perturbing effects on the bilayer structure provide valuable insights into the molecular action mechanism associated with the charge of these small cationic peptides.

Lyotropic liquid crystal engineering moving beyond binary compositional space - ordered nanostructured amphiphile self-assembly materials by design

Ordered amphiphile self-assembly materials with a tunable three-dimensional (3D) nanostructure are of fundamental interest, and crucial for progressing several biological and biomedical applications, including in meso membrane protein crystallization, as drug and medical contrast agent delivery vehicles, and as biosensors and biofuel cells. In binary systems consisting of an amphiphile and a solvent, the ability to tune the 3D cubic phase nanostructure, lipid bilayer properties and the lipid mesophase is limited. A move beyond the binary compositional space is therefore required for efficient engineering of the required material properties. In this critical review, the phase transitions upon encapsulation of more than 130 amphiphilic and soluble additives into the bicontinuous lipidic cubic phase under excess hydration are summarized. The data are interpreted using geometric considerations, interfacial curvature, electrostatic interactions, partition coefficients and miscibility of the alkyl chains. The obtained lyotropic liquid crystal engineering design rules can be used to enhance the formulation of self-assembly materials and provides a large library of these materials for use in biomedical applications (242 references).

Effects of High Pressure on Phospholipid Bilayers

The response of lipid membranes to changes in external pressure is important for many biological processes, and it can also be exploited for technological applications. In this work, we employ all-atom molecular dynamics simulations to characterize the changes in the physical properties of phospholipid bilayers brought about by high pressure (1000 bar). In particular, we study how the response differs, in relation to different chain unsaturation levels, by comparing monounsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and biunsaturated dioleoyl-phosphatidylcholine (DOPC) bilayers. Various structural, mechanical, and dynamical features are found to be altered by the pressure increase in both bilayers. Notably, for most properties, including bilayer area and thickness, lipid order parameters, lateral pressure profile, and curvature frustration energy, we observe significantly more pronounced effects for monounsaturated POPC than biunsaturated DOPC. Possible biological implications of the results obtained are discussed, especially in relation to how different lipids can control the structure and function of membrane proteins.

Membrane stress profiles from self-consistent field theory

Using self-consistent field theory (SCFT), we develop an accurate, local expression for the stress profiles in membranes and soft matter interfaces, in general. The bond stresses are expressed in terms of pre-computed chain propagators, which are used to describe the statistical weight of the molecules and therefore require minimal additional calculations. In addition, we overcome the resolution limit of the molecular bond length by including the Irving and Kirkwood bond assignment and recover a constant normal stress profile across an interface. Using this theory, we find that the membrane lateral stress profile contains repulsive (positive) stresses in the regions of the head and tail groups, and attractive (negative) stresses near the hydrophobic/hydrophilic interface. We also verify that the zeroth and first moments of the stress profile correspond to the thermodynamic tension and product of the bending modulus and the spontaneous curvature, respectively.

Impact of monoolein on aquaporin1-based supported lipid bilayer membranes

Aquaporin (AQP) based biomimetic membranes have attracted considerable attention for their potential water purification applications. In this paper, AQP1 incorporated biomimetic membranes were prepared and characterized. The morphology and structure of the biomimetic membranes were characterized by in situ atomic force microscopy (AFM), infrared absorption spectroscopy, fluorescence microscopy, and contact angle measurements. The nanofiltration performance of the AQP1 incorporated membranes was investigated at 4 bar by using 2 g l^-1 NaCl as feed solution. Lipid mobility plays an important role in the performance of the AQP1 incorporated supported lipid bilayer (SLB) membranes. We demonstrated that the lipid mobility is successfully tuned by the addition of monoolein (MO). Through in situ AFM and fluorescence recovery after photo-bleaching (FRAP) measurements, the membrane morphology and the molecular mobility were studied. The lipid mobility increased in the sequence DPPC < DPPC/MO (R_MO = 5/5) < DOPC/MO (R_MO = 5/5) < DOPC, which is consistent with the flux increment and salt rejection. This study may provide some useful insights for improving the water purification performance of biomimetic membranes.

Effects on lipid bilayer and nitrogen distribution induced by lateral pressure

The lateral pressure exerted on cell membrane is of great importance to signal transduction. Here, we perform molecular dynamics simulation to explore how lateral pressure affects the biophysical properties of lipid bilayer as well as nitrogen distribution in the membrane. Our results show that both physical properties of cell membrane and nitrogen distribution are highly sensitive to the lateral pressure. With the increasing lateral pressure, area per lipid drops and thickness of membrane increases obviously, while nitrogen molecules are more congested in the center of lipid bilayer than those under lower lateral pressure. These results suggest that the mechanism of nitrogen narcosis may be related to the lateral pressure.

Hydrogen bonding of cholesterol in the lipidic cubic phase

The addition of cholesterol to the monoolein-based lipidic cubic phase (LCP) has been instrumental in obtaining high-resolution crystal structures of several G protein-coupled receptors. Here, we report the use of high-resolution magic angle spinning NMR spectroscopy to record and assign the isotropic (13)C chemical shifts of cholesterol in lipidic lamellar and cubic phases at different hydration levels with monoolein and chain-deuterated DMPC as host lipids. The hydrogen-bonding patterns of cholesterol in these phases were determined from the NMR data by quantum chemical calculations. The results are consistent with the normal orientation of cholesterol in lipid bilayers and with the cholesterol hydroxyl group located at the hydrophobic/hydrophilic interface. The (13)C chemical shifts of cholesterol are mostly affected by the host lipid identity with little or no dependency on the hydration (20% vs 40%) or the phase identity (lamellar vs LCP). In chain-deuterated DMPC bilayers, the hydroxyl group of cholesterol forms most of its hydrogen bonds with water, while in monoolein bilayers it predominately interacts with monoolein. Such differences in the hydrogen-bonding network of cholesterol may have implications for the design of experiments in monoolein-based LCP.

Differential effects of the hydrophobic surfactant proteins on the formation of inverse bicontinuous cubic phases

Prior studies have shown that the biological mixture of the two hydrophobic surfactant proteins, SP-B and SP-C, produces faster adsorption of the surfactant lipids to an air/water interface, and that they induce 1-palmitoyl-2-oleoyl phosphatidylethanolamine (POPE) to form inverse bicontinuous cubic phases. Previous studies have shown that SP-B has a much greater effect than SP-C on adsorption. If the two proteins induce faster adsorption and formation of the bicontinuous structures by similar mechanisms, then they should also have different abilities to form the cubic phases. To test this hypothesis, we measured small-angle X-ray scattering on the individual proteins combined with POPE. SP-B replicated the dose-related ability of the combined proteins to induce the cubic phases at temperatures more than 25 °C below the point at which POPE alone forms the curved inverse-hexagonal phase. With SP-C, diffraction from cubic structures was either absent or present at very low intensities only with larger amounts of protein. The correlation between the structural effects of inducing curved structures and the functional effects on the rate of adsorption fits with the model in which SP-B promotes adsorption by facilitating formation of an inversely curved, rate-limiting structure.

Small-Angle X-ray Scattering Investigations of Biomolecular Confinement, Loading, and Release from Liquid-Crystalline Nanochannel Assemblies

This Perspective explores the recent progress made by means of small-angle scattering methods in structural studies of phase transitions in amphiphilic liquid-crystalline systems with nanochannel architectures and outlines some future directions in the area of hierarchically organized and stimuli-responsive nanochanneled assemblies involving biomolecules. Time-resolved small-angle X-ray scattering investigations using synchrotron radiation enable monitoring of the structural dynamics, the modulation of the nanochannel hydration, as well as the key changes in the soft matter liquid-crystalline organization upon stimuli-induced phase transitions. They permit establishing of the inner nanostructure transformation kinetics and determination of the precise sizes of the hydrophobic membraneous compartments and the aqueous channel diameters in self-assembled network architectures. Time-resolved structural studies accelerate novel biomedical, pharmaceutical, and nanotechnology applications of nanochannel soft materials by providing better control of DNA, peptide and protein nanoconfinement, and release from diverse stimuli-responsive nanocarrier systems.

Water gradient in the membrane-water interface: a time-resolved study of the series of n-(9-anthroyloxy) stearic acids incorporated in AOT/water/iso-octane reverse micelles

The water radial distribution in AOT/iso-octane/water reverse micelles (RM), used to mimic the membrane-water interface, was examined by excited-state lifetime and transient spectral measurements of the series of n-(9-anthroyloxy) stearic acids (n-AS), with n = 2, 3, 6, 7, 9, 10, and 12. A water gradient in the RM extended from the polar head group region up to the middle of the surfactant carbon chains. A fast intramolecular excited-state relaxation, involving the rotation of the carboxylic group of the ester bond with respect to the anthracene ring, gave rise to a nanosecond time-dependent fluorescence Stokes shifts (TDFSS). In water-filled RMs, we only observed a water-induced TDFSS occurring over subnano- and nanosecond time scales with decreasing amplitudes and rates as a function of depth, according to the decreasing water gradient and the slowing down of the anthroyloxy moiety rotational motion. This water-induced TDFSS is most likely the result of both H-bond formation and general dipolar relaxation, as indirectly showed by measurements with DMF (a nonprotic polar solvent) instead of water in RMs.

Dual-resolution molecular dynamics simulation of antimicrobials in biomembranes

Triclocarban and triclosan, two potent antibacterial molecules present in many consumer products, have been subject to growing debate on a number of issues, particularly in relation to their possible role in causing microbial resistance. In this computational study, we present molecular-level insights into the interaction between these antimicrobial agents and hydrated phospholipid bilayers (taken as a simple model for the cell membrane). Simulations are conducted by a novel 'dual-resolution' molecular dynamics approach which combines accuracy with efficiency: the antimicrobials, modelled atomistically, are mixed with simplified (coarse-grain) models of lipids and water. A first set of calculations is run to study the antimicrobials' transfer free energies and orientations as a function of depth inside the membrane. Both molecules are predicted to preferentially accumulate in the lipid headgroup-glycerol region; this finding, which reproduces corresponding experimental data, is also discussed in terms of a general relation between solute partitioning and the intramembrane distribution of pressure. A second set of runs involves membranes incorporated with different molar concentrations of antimicrobial molecules (up to one antimicrobial per two lipids). We study the effects induced on fundamental membrane properties, such as the electron density, lateral pressure and electrical potential profiles. In particular, the analysis of the spontaneous curvature indicates that increasing antimicrobial concentrations promote a 'destabilizing' tendency towards non-bilayer phases, as observed experimentally. The antimicrobials' influence on the self-assembly process is also investigated. The significance of our results in the context of current theories of antimicrobial action is discussed.

New insights into the molecular mechanisms of general anaesthetics

This paper provides new insights of how general anaesthetic research should be carried out in the future by an analysis of what we know, what we do not know and what we would like to know. I describe previous hypotheses on the mechanism of action of general anaesthetics (GAs) involving membranes and protein receptors. I provide the reasons why the GABA type A receptor, the NMDA receptor and the glycine receptor are strong candidates for the sites of action of GAs. I follow with a review on attempts to provide a mechanism of action, and how future research should be conducted with the help of physical and chemical methods.

Effects of pressure and temperature on the self-assembled fully hydrated nanostructures of monoolein-oil systems

Synchrotron small-angle X-ray scattering (SAXS) was applied for studying the effects of hydrostatic pressure and temperature on the structural behavior of fully hydrated tetradecane (TC)-loaded monoolein (MO) systems. Our main attention focused on investigating the impact of isobaric and isothermal changes on the stability of the inverted type discontinuous Fd3m cubic phase as compared to the inverted type hexagonal (H(2)) liquid crystalline phase. The present results show that compressing the TC-loaded Fd3m phase under isothermal conditions induces a significant increase of its lattice parameter: it approximately increases by 1 A per 75 bar. Further, the Fd3m phase is more pressure-sensitive as compared to the Pn3m and the H(2) phases. At ambient temperatures, we observed the following structural transitions as pressure increases: Fd3m --> H(2) --> Pn3m. Our findings under isobaric conditions reveal more complicated structural transitions. At high pressures, we recorded the interesting temperature-induced structural transition of (Pn3m + L(alpha)) --> (Pn3m + L(alpha) + H(2)) --> (L(alpha) + H(2)) --> H(2) --> Fd3m --> traces of Fd3m coexisting with L(2). At high pressures and low temperatures, the TC molecules partially crystallize as indicated by the appearance of an additional diffraction peak at q = 3.46 nm(-1). This crystallite disappears at high temperatures and also as the system gets decompressed. The appearance of the Pn3m and the L(alpha) phases during compressing the fully hydrated MO/TC samples at high pressures and low temperatures is generally related to a growing hydrocarbon chain condensation, which leads to membrane leaflets with less negative interfacial curvatures (decreasing the spontaneous curvatures |H(0)|). Both the effects of pressure and temperature are discussed in detail for all nonlamellar phases on the basis of molecular shape and packing concepts.

[Elucidation of lipid complex formation mechanisms by static/dynamic structural evaluation]

Biomembrane is a place for signal transduction, where protein-membrane interactions are controlled by the membrane environment. This environment could be modified by lipid dynamics, such as interbilayer transport and transbilayer movement, which are governed by lipid transfer proteins and translocase enzymes, respectively. Thus, static and dynamic structural evaluations of the membranes are important to understand the links among function, structure, and dynamics of lipid membranes. This review describes our recent studies for 1) the production of lipid nanoparticles with nonlamellar liquid crystalline phases, 2) membrane-protein interaction that relates to the biogenesis of high-density lipoproteins, and 3) the characterization of lipid transfer dynamics by small-angle neutron scattering (SANS). It was demonstrated that different phospholipids have individual effect on membrane-apolipoprotein A-I (apoA-I) interactions that bring about a discoidal lipid-protein complex formation: Phosphatidylethanolamine, possessing the negative spontaneous curvature, increases both the degree of hydration at the membrane interface and the acyl chain order, and enhances the binding of amphipathic helices. A gel phase-forming lipid, sphingomyelin, forms heterogeneous interface of the mixed membranes and facilitates the discoidal particle formation with apoA-I. Phosphatidylserine partly contributes to a reduction in pH at membrane surface, which induces the conformational change of apoA-I and accelerates the discoidal complex formation. It was also demonstrated that SANS is available as a novel method to determine the dynamics of membrane lipids. This technique is perfectly suited to evaluate the activity of proteins relevant to lipid migrations.

3D pressure field in lipid membranes and membrane-protein complexes

We calculate full 3D pressure fields for inhomogeneous nanoscale systems using molecular dynamics simulation data. The fields represent systems with increasing level of complexity, ranging from semivesicles and vesicles to membranes characterized by coexistence of two phases, including also a protein-membrane complex. We show that the 3D pressure field is distinctly different for curved and planar bilayers, the pressure field depends strongly on the phase of the membrane, and that an integral protein modulates the tension and elastic properties of the membrane.

Adsorption behaviors of DPPC/MO aggregates on SiO2 surfaces

The adsorption kinetics of extruded 1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine (DPPC)/1-(cis-9-octadecenoyl)- rac-glycerol (monoolein, MO) aggregates on SiO 2 surface at 25 degrees C is investigated in real time, using the dissipative quartz crystal microbalance (QCM) technique. Four adsorption pathways have been identified depending on the molar fraction of MO in the DPPC/MO system: (I) intact vesicle adsorption, (II) vesicle reorganization on a SiO 2 surface, (III) supported lipid bilayer (SLB) formation, and (IV) cubosome adsorption. The results can be understood by the fact that DPPC is a lamellar phase-forming lipid, whereas MO prefers the cubic phase. Therefore, the incorporation of MO in DPPC increases the packing parameter. Equally important, MO also increases the mobility of lipid molecules and lateral pressure in the bilayers as a result of the presence of a unique cis- double bond. Before extrusion, the vesicles size increases with the MO content when X MO <or= 0.7 and cubosomes are formed for X MO >or= 0.8. The extruded DPPC/MO suspensions consist of reformed vesicles for X MO <or= 0.7 and filtered cubosomes for X MO >or= 0.8, all with a uniform diameter of approximately 100 nm. Differential scanning calorimetry (DSC) further indicates that the addition of MO lowers the main phase transition temperature of DPPC and thus makes the hydrophobic interior more fluid.

The influence of 1-alkanols and external pressure on the lateral pressure profiles of lipid bilayers

The suggestion by Robert Cantor, that drug-induced pressure changes in lipid bilayers can change the conformational equilibrium between open and closed states of membrane proteins and thereby cause anesthesia, attracted much attention lately. Here, we studied the effect of both large external pressure and of 1-alkanols of different chain lengths--some of them anesthetics, others not--on the lateral pressure profiles across dimyristoylphosphatidylcholine (DMPC) bilayers by molecular dynamics simulations. For a pure DMPC bilayer, high pressure both reduced and broadened the tension at the interface hydrophobic/hydrophilic and diminished the repulsion between the phospholipid headgroups. Whereas the effect of ethanol on the lateral pressure profile was similar to the effect of a large external pressure on a DMPC bilayer, long-chain 1-alkanols significantly amplified local maxima and minima in the lateral pressure profile. For most 1-alkanols, external pressure had moderate effects and did not reverse the changes 1-alkanols exerted on the pressure profile. Nevertheless, assuming the bent helix model as a simple geometric model for the transmembrane region of a membrane protein, protein conformational equilibria were shifted in opposite directions by addition of 1-alkanols and additional application of external pressure.

[Disk-like complex formation by apolipoprotein-lipid membrane interaction and relevance to high-density lipoprotein formation]

The interaction of apolipoprotein A-I (apoA-I) with transmembrane ATP-binding cassette transporter A1 (ABCA1) is a crucial step for high-density lipoprotein (HDL) formation, however, its molecular mechanism is less well understood. Here, we used apoA-I and its model peptide, Ac-18A-NH(2), to investigate their interaction with mixed membranes of phosphatidylcholine (PC) with phosphatidylethanolamine (PE) or sphingomyelin (SM). It was shown that PE, possessing the negative spontaneous curvature, increased both the degree of hydration at the membrane interface and the acyl chain order, and that Ac-18A-NH(2) had opposite effects to PE, since the alpha-helix formation at the membrane surface induced positive curvature strain. In addition, increased PE in PC/PE large unilamellar vesicles (LUVs) enhanced the peptide's binding. Although SM significantly lowered the peptide binding capacity, the peptide's binding to PC/SM LUV led to membrane disruption. The interaction with PC/SM LUVs was then investigated using ApoA-I. The spontaneous rHDL formation from pure PC LUV proceeded very slowly at 37 degrees C, but SM-rich PC/SM LUVs, which are in a gel/liquid-disordered (L(d)) phase at this temperature, were rapidly solubilized to form rHDL by apoA-I. The addition of cholesterol decreased the rate of the rHDL formation and induced the selective extraction of lipids by apoA-I, which preferably extracted lipids of L(d) phase rather than lipids of liquid-ordered (L(o)) phase. These results suggest that heterogeneous interface of the mixed membranes facilitates the insertion of apoA-I and induces L(d) phase-selective lipid extraction to form rHDL, and are compatible with recent cell works on the apoA-I-dependent HDL generation.

A quantitative coarse-grain model for lipid bilayers

A simplified particle-based computer model for hydrated phospholipid bilayers has been developed and applied to quantitatively predict the major physical features of fluid-phase biomembranes. Compared with available coarse-grain methods, three novel aspects are introduced. First, the main electrostatic features of the system are incorporated explicitly via charges and dipoles. Second, water is accurately (yet efficiently) described, on an individual level, by the soft sticky dipole model. Third, hydrocarbon tails are modeled using the anisotropic Gay-Berne potential. Simulations are conducted by rigid-body molecular dynamics. Our technique proves 2 orders of magnitude less demanding of computational resources than traditional atomic-level methodology. Self-assembled bilayers quantitatively reproduce experimental observables such as electron density, compressibility moduli, dipole potential, lipid diffusion, and water permeability. The lateral pressure profile has been calculated, along with the elastic curvature constants of the Helfrich expression for the membrane bending energy; results are consistent with experimental estimates and atomic-level simulation data. Several of the results presented have been obtained for the first time using a coarse-grain method. Our model is also directly compatible with atomic-level force fields, allowing mixed systems to be simulated in a multiscale fashion.

Interaction of an amphipathic peptide with phosphatidycholine/phosphatidylethanolamine mixed membranes

The effect of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) in mixed membranes with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) on interaction with a class A amphipathic peptide, Ac-DWLKAFYDKVAEKLKEAF-NH(2) (Ac-18A-NH(2)), was investigated. The fluorescence lifetime of 2-(9-anthroyloxy)stearic acid and (2)H NMR spectra were used to evaluate the penetration of water molecules into the membrane interface and the order of lipid acyl chains, respectively. The results demonstrated that DOPE in the mixed membranes decreased the fluorescence lifetime and increased the acyl-chain order, and that Ac-18A-NH(2) affected them more for membranes with higher DOPE fractions. The partition coefficient (K(p)) of the peptide to the mixed membranes was increased with the increase in the DOPE mole fractions. From the temperature dependence of the K(p) values, the binding of Ac-18A-NH(2) to POPC/DOPE mixed membranes was found to be entropy-driven. The formation of an alpha-helix at the membrane's surface is supposed to induce positive curvature strain, which decreases the headgroup hydration and acyl-chain order of lipids. Thus, the binding of Ac-18A-NH(2) to membranes is entropically more favorable at higher DOPE fractions since the peptide's insertion into the membrane can decrease the order parameter and unfavorable headgroup hydration, which explains the enhanced peptide binding.

Tuning curvature and stability of monoolein bilayers by designer lipid-like peptide surfactants

This study reports the effect of loading four different charged designer lipid-like short anionic and cationic peptide surfactants on the fully hydrated monoolein (MO)-based Pn3m phase (Q(224)). The studied peptide surfactants comprise seven amino acid residues, namely A(6)D, DA(6), A(6)K, and KA(6). D (aspartic acid) bears two negative charges, K (lysine) bears one positive charge, and A (alanine) constitutes the hydrophobic tail. To elucidate the impact of these peptide surfactants, the ternary MO/peptide/water system has been investigated using small-angle X-ray scattering (SAXS), within a certain range of peptide concentrations (R Collapse

Key Words Collapse

MESH Headings

• Anions
• Lipid Bilayers
• Lipids/chemistry
• Models, Molecular
• Peptides/chemistry
• Scattering, Radiation
• Surface-Active Agents/chemistry
• Temperature

Collapse

Grants Collapse

Affiliation(s)

Anan Yaghmur Institute of Biophysics and Nanosystems Research (IBN), Austrian Academy of Sciences, Graz, Austria.
Peter Laggner
Shuguang Zhang
Michael Rappolt

Collapse