Bilayer Model Membranes Research Articles

Bending models of lipid bilayer membranes: Spontaneous curvature and area-difference elasticity

We present a computational study of bending models for the curvature elasticity of lipid bilayer membranes that are relevant for simulations of vesicles and red blood cells. We compute bending energy and forces on triangulated meshes and evaluate and extend four well established schemes for their approximation: Kantor and Nelson (1987), Jülicher (1996), Gompper and Kroll (1996) and Meyer et al. (2003), termed A, B, C, D. We present a comparative study of these four schemes on the minimal bending model and propose extensions for schemes B, C and D. These extensions incorporate the reference state and non-local energy to account for the spontaneous curvature, bilayer coupling, and area-difference elasticity models. Our results indicate that the proposed extensions enhance the schemes to account for shape transformation including budding/vesiculation as well as for non-axisymmetric shapes. We find that the extended scheme B is superior to the rest in terms of accuracy, and robustness as well as simplicity of implementation. We demonstrate the capabilities of this scheme on several benchmark problems including the budding-vesiculating process and the reproduction of the phase diagram of vesicles.

Toward Understanding the Impact of Dimerization Interfaces in Angiotensin II Type 1 Receptor.

Angiotensin II type 1 receptor (AT1R) is a prototypical class A G protein-coupled receptor (GPCR) that has an important role in cardiovascular pathologies and blood pressure regulation as well as in the central nervous system. GPCRs may exist and function as monomers; however, they can assemble to form higher order structures, and as a result of oligomerization, their function and signaling profiles can be altered. In the case of AT1R, the classical Gαq/11 pathway is initiated with endogenous agonist angiotensin II binding. A variety of cardiovascular pathologies such as heart failure, diabetic nephropathy, atherosclerosis, and hypertension are associated with this pathway. Recent findings reveal that AT1R can form homodimers and activate the noncanonical (β-arrestin-mediated) pathway. Nevertheless, the exact dimerization interface and atomic details of AT1R homodimerization have not been still elucidated. Here, six different symmetrical dimer interfaces of AT1R are considered, and homodimers were constructed using other published GPCR crystal dimer interfaces as template structures. These AT1R homodimers were then inserted into the model membrane bilayers and subjected to all-atom molecular dynamics simulations. Our simulation results along with the principal component analysis and water pathway analysis suggest four different interfaces as the most plausible: symmetrical transmembrane (TM)1,2,8; TM5; TM4; and TM4,5 AT1R dimer interfaces that consist of one inactive and one active protomer. Moreover, we identified ILE2386.33 as a hub residue in the stabilization of the inactive state of AT1R.

Molecular Dynamics Simulations of Kir2.2 and Cholesterol Reveal State- and Concentration-Dependent Binding Sites

Cholesterol is a major component of cell membranes and is an important regulator of multiple types of ion channels. However, the dynamics of its interactions and the specific mechanisms through which it mediates ion channel function are not well understood. One family of ion channels, inwardly rectifying potassium (Kir) channels, was shown to be sensitive to cholesterol through direct interactions with “cholesterol-sensitive” regions on the channel protein. It is unclear though whether these regions correspond to a well-localized binding site or are representative of a broad and shallow thermodynamic landscape. To investigate the interactions between cholesterol and Kir channels, we developed Martini coarse-grained simulations of Kir2.2 embedded in a POPC model membrane bilayer with varying concentrations of cholesterol. This approach allowed us to simulate the unbiased diffusion of cholesterol from the bilayer environment to both the surface and recessed pockets of the channel. We found that, unlike small drug-like molecules, which tightly bind to a single site, cholesterol interacts with the channel on a range of different time scales, with multiple cholesterol molecules in contact with the channel simultaneously. Following this, we utilized principles from network theory to identify four discrete cholesterol binding sites on the channel, which exist depending on the conformational state of the channel. Furthermore, we found that a twofold decrease in the bilayer concentration of cholesterol, which was shown previously to increase Kir2 channel activity, leads to a decrease in occupancy in only a subset of identified binding sites. These results allow us to discriminate between functionally relevant and functionally non-relevant cholesterol interaction sites, and hypothesize a mechanism of cholesterol regulation.

Molecular-Level "Observations" of the Behavior of Gold Nanoparticles in Aqueous Solution and Interacting with a Lipid Bilayer Membrane.

We use coarse-grained molecular dynamics simulations to "observe" details of interactions between ligand-covered gold nanoparticles and a lipid bilayer model membrane. In molecular dynamics simulations, one puts the individual atoms and groups of atoms of the physical system to be "observed" into a simulation box, specifies the forms of the potential energies of interactions between them (ultimately quantum based), and lets them individually move classically according to Newton's equations of motion, based on the forces arising from the assumed potential energy forms. The atoms that are chemically bonded to each other stay chemically bonded, following known potentials (force fields) that permit internal degrees of freedom (internal rotation, torsion, vibrations), and the interactions between nonbonded atoms are simplified to Lennard-Jones forms (in our case) and coulombic (where electrical charges are present) in which the parameters are previously optimized to reproduce thermodynamic properties or are based on quantum electronic calculations. The system is started out at a reasonable set of coordinates for all atoms or groups of atoms, and then permitted to develop according to the equations of motion, one small step (usually 10 fs time step) at a time, for millions of steps until the system is at a quasi-equilibrium (usually reached after hundreds of nanoseconds). We then let the system play out its motions further for many nanoseconds to observe the behavior, periodically taking snapshots (saving all positions and energies), and post-processing the snapshots to obtain various average descriptions of the system. Alkanethiols of various lengths serve as examples of hydrophobic ligands and methyl-terminated PEG with various numbers of monomer units serve as examples of hydrophilic ligands. Spherical gold particles of various diameters as well as gold nanorods form the core to which ligands are attached. The nanoparticles are characterized at the molecular level, especially the distributions of ligand configurations and their dependence on ligand length, and surface coverage. Self-assembly of the bilayer from an isotropic solution and observation of membrane properties that correspond well to experimental values validate the simulations. The mechanism of permeation of a gold NP coated with either a hydrophobic or a hydrophilic ligand, and its dependence on surface coverage, ligand length, core diameter, and core shape, is investigated. Lipid response such as lipid flip-flops, lipid extraction, and changes in order parameter of the lipid tails are examined in detail. The mechanism of permeation of a PEGylated nanorod is shown to occur by tilting, lying down, rotating, and straightening up. The nature of the information provided by molecular dynamics simulations permits understanding of the detailed behavior of gold nanoparticles interacting with lipid membranes which in turn helps to understand why some known systems work better than others and aids the design of new particles and improvement of methods for preparing existing ones.

Evaporation-induced monolayer compression improves droplet interface bilayer formation using unsaturated lipids.

In this article, we report on a new experimental methodology to enable reliable formation of droplet interface bilayer (DIB) model membranes with two types of unsaturated lipids that have proven difficult for creating stable DIBs. Through the implementation of a simple evaporation technique to condition the spontaneously assembled lipid monolayer around each droplet, we increased the success rates of DIB formation for two distinct unsaturated lipids, namely 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), from less than 10% to near 100%. Separately, using a pendant drop tensiometer, we learned that: (a) DOPC and POPC monolayers do not spontaneously assemble into their tightest possible configurations at an oil-water interface, and (b) reducing the surface area of a water droplet coated with a partially packed monolayer leads to a more tightly packed monolayer with an interfacial tension lower than that achieved by spontaneous assembly alone. We also estimated from Langmuir compression isotherms obtained for both lipids that the brief droplet evaporation procedure prior to DIB formation resulted in a 6%-16% reduction in area per lipid for DOPC and POPC, respectively. Finally, the increased success rates of formation for DOPC and POPC DIBs enabled quantitative characterization of unsaturated lipid membrane properties including electrical resistance, rupture potential, and specific capacitance.

Formation and stabilization of pores in bilayer membranes by peptide-like amphiphilic polymers.

We study pore formation in models of lipid bilayer membranes interacting with amphiphilic copolymers mimicking anti-microbial peptides using Monte Carlo simulations and we rationalize our results by a simple brush-model for the fluid membrane. In our study a weak tension on the membrane is required to observe pore-formation induced by the adsorption of flexible amphiphilic copolymers. The copolymers enhance the pore stability by decreasing the line tension due to weak adsorption along the rim of the pore. Pore formation is enhanced with increasing length of copolymers or stronger stretching of the membrane. Both solvent and copolymer permeability increase as the pore becomes stable. Pore-formation proceeds via a meta-stable pore-state according to a discontinuous phase transition scenario which lead to finite pore-sizes at once. Our generic model of copolymer-induced pore-formation does not require high polymer concentration at the pores nor any self-organization of the copolymers to open the pore.

Molecular dynamics modeling of Pseudomonas aeruginosa outer membranes.

Pseudomonas aeruginosa is a common Gram-negative bacterium and opportunistic human pathogen. The distinctive structure of its outer membrane (OM) and outer membrane vesicles (OMVs) plays a fundamental role in bacterial virulence, colonization ability, and antibiotic resistance. To provide critical insights into OM and OMV functionality, we conducted an all-atom molecular dynamics study of asymmetric membranes that are biologically relevant to P. aeruginosa. We hybridized a GLYCAM06-based lipopolysaccharides force field with the Stockholm lipids force field (Slipids) to model bilayer membranes with Lipid A molecules in one leaflet and physiologically relevant phospholipid molecules in the other, including 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG), and 1,2-dioleoyl-sn-glycero-3-phosphoglycerol (DOPG). In particular, a membrane with phospholipid composition representing the P. aeruginosa OM was constructed and modeled by mixing the physiologically dominant components. The detailed structure of membranes was characterized by area per lipid, transmembrane mass and charge densities, radial distribution function (RDF), deuterium order parameter (SCD) of acyl chains, and inclination angles of phosphates and disaccharide in Lipid A. The membrane fluidity in equilibrium and the hydration of functional groups were probed and characterized quantitatively. The consistent properties of the Lipid A leaflets in different membranes demonstrate its compatibility with various phospholipids present in the P. aeruginosa OM. The more ordered acyl chains of Lipid A compared to the cytoplasmic cell membrane contribute to the low permeability of bacterial outer membrane. The findings of this computational investigation of P. aeruginosa OM will further the understanding of microbial pathogenesis and enable future study of OMV biogenesis.

Capacitive Detection of Low-Enthalpy, Higher-Order Phase Transitions in Synthetic and Natural Composition Lipid Membranes.

In-plane lipid organization and phase separation in natural membranes play key roles in regulating many cellular processes. Highly cooperative, first-order phase transitions in model membranes consisting of few lipid components are well understood and readily detectable via calorimetry, densitometry, and fluorescence. However, far less is known about natural membranes containing numerous lipid species and high concentrations of cholesterol, for which thermotropic transitions are undetectable by the above-mentioned techniques. We demonstrate that membrane capacitance is highly sensitive to low-enthalpy thermotropic transitions taking place in complex lipid membranes. Specifically, we measured the electrical capacitance as a function of temperature for droplet interface bilayer model membranes of increasing compositional complexity, namely, (a) a single lipid species, (b) domain-forming ternary mixtures, and (c) natural brain total lipid extract (bTLE). We observed that, for single-species lipid bilayers and some ternary compositions, capacitance exhibited an abrupt, temperature-dependent change that coincided with the transition detected by other techniques. In addition, capacitance measurements revealed transitions in mixed-lipid membranes that were not detected by the other techniques. Most notably, capacitance measurements of bTLE bilayers indicated a transition at ∼38 °C not seen with any other method. Likewise, capacitance measurements detected transitions in some well-studied ternary mixtures that, while known to yield coexisting lipid phases, are not detected with calorimetry or densitometry. These results indicate that capacitance is exquisitely sensitive to low-enthalpy membrane transitions because of its sensitivity to changes in bilayer thickness that occur when lipids and excess solvent undergo subtle rearrangements near a phase transition. Our findings also suggest that heterogeneity confers stability to natural membranes that function near transition temperatures by preventing unwanted defects and macroscopic demixing associated with high-enthalpy transitions commonly found in simpler mixtures.

Molecular dynamics simulations of lysozyme–lipid systems: probing the early steps of protein aggregation

Using the molecular dynamics simulation, the role of lipids in the lysozyme transition into the aggregation-competent conformation has been clarified. Analysis of the changes of lysozyme secondary structure upon its interactions with the model bilayer membranes composed of phosphatidylcholine and its mixtures with phosphatidylglycerol (10, 40, and 80 mol%) within the time interval of 100 ns showed that lipid-bound protein is characterized by the increased content of β-structures. Along with this, the formation of protein–lipid complexes was accompanied by the increase in the gyration radius and the decrease in RMSD of polypeptide chain. The results obtained were interpreted in terms of the partial unfolding of lysozyme molecule on the lipid matrix, with the magnitude of this effect being increased with increasing the fraction of anionic lipids. Based on the results of molecular dynamics simulation, a hypothetical model of the nucleation of lysozyme amyloid fibrils in a membrane environment was suggested.

Differential adsorption of a membrane skeletal protein, spectrin, in phospholipid membranes

The interaction of phospholipids with the peripheral membrane proteins like spectrin is important not only to understand the various physiological functions of cells, but also to gain insight into the mechanism involved in the self-assembly of polymer-like long chain molecules at the soft surfaces and interfaces. The lipid head-group specificity of adsorption of spectrin to supported phopsholipid bilayer model membranes has been investigated using the X-ray reflectivity (XRR) technique. Model lipid bilayers composed of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) head groups have been prepared on a soft polymer cushion and the XRR measurements have been carried out from the bilayers immersed in a water bath using high-energy synchrotron X-rays. Our results suggest that in PC-based membranes the spectrin chains form a uniform layer on top of the bilayer with their chains lying on the membrane surface, while in PE-based membranes with relatively smaller head groups, the spectrin chains are attached only through a few possible binding sites with the rest of the part projected out of the membrane surface. In addition, the reflectivity profiles reveal the penetration of spectrin polypeptide chains through the PE bilayer in its fluid phase. Pressure-area isotherm measurements on Langmuir monolayers also support similar observations on the adsorption of spectrin molecules to the membranes composed of PC and PE. The observed results were explained using a qualitative model based on the ion-mediated protein interaction in the PC-based membrane.

Modulation of Endothelial Membrane Biomechanics by Oxidized Phospholipids: A Combined Experimental and Computational Approach

Elevated concentrations of biologically active oxidized phospholipids (oxPCs) are known to exert various pathophysiological effects, including disruption of the endothelial barrier and induction of monocyte adhesion to endothelial cells. These effects are further implicated in a range of disorders, including certain cardiovascular, metabolic, and immunological diseases. The specific mechanisms by which oxPCs modulate membrane barrier function are poorly understood.We investigated the differential influence of two bioactive oxidized phospholipids on model bilayer properties, membrane packing, fluidity, elastic properties, and endothelial cell biomechanics both experimentally, using Laurdan multi‐photon imaging and atomic force microscopy (AFM) microindentation experiments, and computationally, using coarse‐grained molecular dynamics simulations. Specifically, we focused on two major oxidation products of 1‐palmitoyl‐2‐arachidonoyl‐sn‐glycero‐3‐phosphocholine (PAPC), which are known to exert multiple pathophysiological effects. The oxidized phospholipids 1‐palmitoyl‐2‐(5‐oxovaleroyl)‐sn‐glycero‐3‐phosphocholine (POVPC ‐ aldehyde‐terminated) and 1‐palmitoyl‐2‐glutaroyl‐sn‐glycero‐3‐phosphocholine (PGPC ‐ carboxyl‐terminated) have been found to induce inflammation, cell proliferation, and cell death.Our molecular simulations of multicomponent phospholipid bilayers predicted differential bilayer perturbation effects of the two oxidized phospholipids based on the chemical identities of their truncated tails. We showed that for the analogous oxPCs incorporated in a phosphatidylcholine bilayer containing sphingomyelin, the carboxylic analog, PGPC, experienced more significant reorientation of the truncated tail and displacement of the oxidized lipid from the plane of the bilayer. These phenomena induced greater perturbation of the structure of the lipid bilayer than the aldehyde analog, POVPC. Decreased bilayer packing and increased water permeation was observed, which was consistent with Laurdan imaging results. Spectral analysis of bilayer undulations further indicated increased elasticity of bilayer systems containing both oxPCs. Computational predictions of larger membrane perturbation by PGPC corresponded with greater stiffness observed for PGPC‐treated endothelial cells measured by cellular elastic moduli using AFM. These analyses confirmed the paradoxical relationship observed between membrane softening by oxPCs and increased overall cell stiffness in the presence of POVPC and PGPC.Support or Funding InformationNIH grants: T32 HL‐82547, HL‐073965, HL‐083298; AHA MWA Pre‐doctoral Fellowship: 14PRE20490156

Ring and Tails: Exploring the Intimacy of Cyclodextrin - Membrane Interactions

Cyclodextrins (CDs) are cyclic, ring-shaped molecules made of sugar moieties, consisting of a hydrophobic cavity and a hydrophilic outer part. Such a structure allows CDs to form inclusion complexes with hydrophobic molecules, where the guest molecule is physically entrapped in the host's (CD) cavity. This feature finds a wide range of applications in drug delivery, pharmaceutical and food industries, as inclusion complexes with CDs show enhanced bioavailability and longer circulation times.Recently, αCDs have been reported to directly interact with aliphatic tails of phospholipids from biological and model bilayer membranes. As the tails are not directly accessible for molecules from the aqueous solution, this observation suggests complex interplays and possible membrane remodeling occurring at the CD/membrane/water interface. Interestingly, despite the wide applications of CDs, the essentials of this process and its possible consequences in vivo remain poorly understood.Herein, we use a combination of complementary biophysical techniques to uncover the inner workings of interactions between CDs and model lipid membranes, focusing on CDs’ membrane-perturbing properties. Specifically, we employ Quartz Crystal Microbalance (QCMB-D) to reveal variations in viscoelastic properties of a lipid membrane induced by the presence of αCD. Furthermore, the application of confocal microscopy and CryoTEM allows us to explore the role of membrane curvature-mediated interactions between αCDs and the lipid membrane.

Interaction of elaiophylin with model bilayer membrane

Elaiophylin is a new macrodiolide antibiotic, which is produced by the Streptomyces strains [1]. It displays biological activities against Gram-positive bacteria and fungi. The mode of action of this antibiotic has been attributed to an alteration of the membrane permeability. When this antibiotic is inserted into the bilayer membranes destabilization of the membrane and formation of ion-penetrable channels is observed. The macrodiolide antibiotic forms stable cation selective ion channels in synthetic lipid bilayer membranes. The aim of this work was to study the interactions of Elaiophylin with model bilayer membranes and to get information on the mechanical properties of lipid bilayers in presence of this antibiotic. Patch-clamp technique [2] were used in the study

Molecular-Scale Biophysical Modulation of an Endothelial Membrane by Oxidized Phospholipids

The influence of two bioactive oxidized phospholipids on model bilayer properties, membrane packing, and endothelial cell biomechanics was investigated computationally and experimentally. The truncated tail phospholipids, 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC) and 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC), are two major oxidation products of the unsaturated phospholipid 1-palmitoyl-2-arachidonoyl-sn-glycero-phosphocholine. A combination of coarse-grained molecular dynamics simulations, Laurdan multiphoton imaging, and atomic force microscopy microindentation experiments was used to determine the impact of POVPC and PGPC on the structure of a multicomponent phospholipid bilayer and to assess the consequences of their incorporation on membrane packing and endothelial cell stiffness. Molecular simulations predicted differential bilayer perturbation effects of the two oxidized phospholipids based on the chemical identities of their truncated tails, including decreased bilayer packing, decreased bilayer bending modulus, and increased water penetration. Disruption of lipid order was consistent with Laurdan imaging results indicating that POVPC and PGPC decrease the lipid packing of both ordered and disordered membrane domains. Computational predictions of a larger membrane perturbation effect by PGPC correspond to greater stiffness of PGPC-treated endothelial cells observed by measuring cellular elastic moduli using atomic force microscopy. Our results suggest that disruptions in membrane structure by oxidized phospholipids play a role in the regulation of overall endothelial cell stiffness.

SPECTRAL BEHAVIOR OF INDICATOR DYES IN THE MODEL PROTEIN – LIPID SYSTEMS

The protolytic and partition equilibria of the indicator dyes in the model lipid and protein-lipid systems have been analyzed. A methodological approach has been developed allowing the partition coefficients of the protonated and deprotonated dye forms to be derived from the spectrophotometric measurements. The partitioning of the indicator dye bromothymol blue into the model bilayer membranes composed of phosphatidylcholine and cardiolipin (9:1, mol:mol) has been examined. The partition coefficient of the protonated dye species into a lipid phase has been found to be 5 orders of magnitude higher than that of the deprotonated dye form. This effect has been interpreted in terms of the differences in the charge distribution over the protonated and deprotonated dye ions, preventing the hydrophobic dye-lipid interactions in the latter case. The reduction of the bromothymol blue partitioning into lipid bilayer in the presence of hemoglobin has been attributed to the protein-induced changes in the structure and physicochemical characteristics of the interfacial membrane region. In the practical aspect, the obtained findings may prove of significance in the design of hemosome-based blood substitutes and elucidating the role of hemoglobin in the molecular etiology of the amyloid disorders, particularly, Alzheimer's disease.

Formation of hybrid bilayers on silanized thin-film Ti electrode

Phospholipid bilayer membranes are essential elements of living organisms as they form boundaries between the intracellular cytoplasm and the extracellular environment, as well as organelles. In this work we report on our attempts to assemble artificial phospholipid bilayer model membranes on Ti surface. To provide hydrophobic cushion for phospholipids, the surface of a thin-film Ti electrode was initially functionalized with trichloro(octadecyl)silane (OTS). Increased hydrophobicity of the solid support allowed vesicle fusion and the formation of a hybrid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayer, as probed by the electrochemical impedance spectroscopy (EIS), contact angle measurements (CA) also by the Fourier transform-infrared (FT-IR) spectroscopy, spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). Our study demonstrates the applicability of thin-film Ti electrodes for the formation of hybrid bilayer membranes. These membranes allow functional reconstitution of the pore-forming toxins and provide a bioanalytical platform for the detection of the activity of the cholesterol-dependent cytolysins.

A comparative differential scanning calorimetry study of the effects of cholesterol and various oxysterols on the thermotropic phase behavior of dipalmitoylphosphatidylcholine bilayer membranes

We have carried out a comparative differential scanning calorimetric (DSC) study of the effects of cholesterol (C) and the eight most physiologically relevant oxysterols on the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayer membranes. The structures of these oxysterols differ from that of C by the presence of additional hydroxyl, keto or epoxy groups on the steroid ring system or by the presence of a hydroxyl group in the alkyl side chain. In general, the progressive incorporation of these oxysterols reduces the temperature, cooperativity and enthalpy of the pretransition of DPPC to a greater extent than C, indicating that their presence thermally destabilizes and disorders the gel states of DPPC bilayers to a greater extent than C. Similarly, the incorporation of these oxysterols either increases the temperature of the broad component of the main phase transition to a smaller extent than C or actually decreases it. Again, this indicates that the presence of these compounds is less effective at thermally stabilizing and ordering the sterol-rich domains of DPPC bilayers than is C itself. Moreover, the incorporation of these oxysterols decrease the cooperativity and enthalpy of the main phase transition of DPPC to a smaller extent than C, indicating that they are somewhat less miscible in fluid DPPC bilayers than is C. Particularly notable in this regard is 25-hydroxycholesterol, which exhibits a markedly reduced miscibility in both gel and fluid DPPC bilayers compared to C itself. In general, the effectiveness of these oxysterols in stabilizing and ordering DPPC bilayers decreases as their rate of interbilayer exchange and the polarity of the oxysterol increases. We close by providing a tentative molecular explanation for the results of our DSC studies and of those of previous biophysical studies of the effects of various oxysterol on lipid bilayer model membranes.

Constant-pH MD Simulations of DMPA/DMPC Lipid Bilayers.

Current constant-pH molecular dynamics (CpHMD) simulations provide a proper treatment of pH effects on the structure and dynamics of soluble biomolecules like peptides and proteins. However, addressing such effects on lipid membrane assemblies has remained problematic until now, despite the important role played by lipid ionization at physiological pH in a plethora of biological processes. Modeling (de)protonation events in these systems requires a proper consideration of the physicochemical features of the membrane environment, including a sound treatment of solution ions. Here, we apply our recent CpHMD-L method to the study of pH effects on a 25% DMPA/DMPC bilayer membrane model, closely reproducing the correct lipid phases of this system, namely, gel-fluid coexistence at pH 4 and a fluid phase at pH 7. A significant transition is observed for the membrane ionization and mechanical properties at physiological pH, providing a molecular basis for the well-established role of phosphatidic acid (PA) as a key player in the regulation of many cellular events. Also, as reported experimentally, we observed pH-induced PA-PA lipid aggregation at acidic pH. By including the titration of anionic phospholipids, the current methodology makes possible to simulate lipid bilayers with increased realism. To the best of our knowledge, this is the first simulation study dealing with a continuous phospholipid bilayer with pH titration of all constituent lipids.

Gamma Convergence of a Family of Surface–Director Bending Energies with Small Tilt

We prove a Gamma-convergence result for a family of bending energies defined on smooth surfaces in $\mathbb{R}^3$ equipped with a director field. The energies strongly penalize the deviation of the director from the surface unit normal and control the derivatives of the director. Such type of energies for example arise in a model for bilayer membranes introduced by Peletier and R\"oger [Arch. Ration. Mech. Anal. 193 (2009)]. Here we prove in three space dimensions in the vanishing-tilt limit a Gamma-liminf estimate with respect to a specific curvature energy. In order to obtain appropriate compactness and lower semi-continuity properties we use tools from geometric measure theory, in particular the concept of generalized Gauss graphs and curvature varifolds.

Dynamic Structural/Amphiphilic “Portrait” of Biomembranes as their Fundamental Property Relevant to Function: Results of Atomistic Simulations

Analysis of local dynamic heterogeneities and hydrophobic surface properties of lipid bilayers is an important step toward understanding of the physical processes underlying formation of lipid domains and molecular mechanisms of the so-called “membrane response” observed upon binding of external molecules to cell membranes. Here, we used atomistic molecular dynamics simulations to address these problems and study structural/amphiphilic “portrait” of a large series of model bilayer membranes (1,2). Rearrangements of membranes upon binding of antimicrobial peptides and association of transmembrane helices were investigated as well (3,4).It was shown that the water-lipid interface of biomembranes represents a highly dynamic and “mosaic” picture, whose parameters depend on the bilayer composition. Formation of such heterogeneities is caused by various intermolecular interactions between lipids, water and ions. Because clustering of the components is determined by a thin balance of differently directed factors, even their minor changes can induce serious rearrangements on the membrane surface. These issues still resist easy experimental characterization.Probably, these phenomena are indispensable for fast mass/energy transfer near the water/membrane boundary, thus regulating diffusion of molecules in this functionally important area: e.g., approaching of ligands to membrane receptors, delivery of proteins to their sites of insertion into membrane, and so on. Putative role of local heterogeneities and collective phenomena in lipid/water systems is discussed.