Dimyristoylphosphatidylcholine Bilayers Research Articles

Cholesterol Incorporation in Membranes Attenuates the Disruption Ability of Antimicrobial Peptide Protegrin-1

The ability of antimicrobial peptides (AMPs) to target and lyse the harmful microbial membrane over that of a host's is a unique characteristic making these innate immune effectors promising candidates to fill the growing therapeutic void resulting from antibiotic drug resistance. This discriminatory behavior is believed to strongly depend on the chemical and structural properties of the lipids that comprise the cell membrane. For instance, the selectivity of AMPs can be based on the electrostatic attraction of these predominately cationic peptides for the bacterial membrane surface heavily populated with negatively charged lipid components. We have previously shown that zwitterionic dimyristoylphosphatidylcholine (DMPC) bilayers display concentration-dependent structural transformations induced by protegrin-1 (PG-1) that progress from fingerlike instabilities at bilayer edges, to the formation of pores, and finally to a network of wormlike micelles. The increasing degree of membrane structural disruption in charge-neutral membranes demonstrates that a more complex interaction than that suggested by a simple electrostatic argument is needed to explain AMP selectivity. We propose that in addition to an electrostatic consideration, specific membrane compositional differences between host and pathogen tunes AMP activity to selectively disrupt microbial membranes rather than those of the host. We have tailored our investigations to utilize membrane components which eukaryotes and prokaryotes contain in drastically different proportions, specifically the presence and absence of cholesterol respectively. In these results we have employed a variety of biophysical techniques to elucidate how increasing cholesterol content in solid-supported DMPC bilayers retards the ability of PG-1 to induce membrane disruption. Atomic force microscopy was used to assess the propensity for pore formation, while neutron reflectivity and oriented circular dichroism studies were advantageous in providing molecular level detail on the location and orientation of PG-1 with respect to the membrane.

X-ray structure, thermodynamics, elastic properties and MD simulations of cardiolipin/dimyristoylphosphatidylcholine mixed membranes

Cardiolipins (CLs) are important biologically for their unique role in biomembranes that couple phosphorylation and electron transport like bacterial plasma membranes, chromatophores, chloroplasts and mitochondria. CLs are often tightly coupled to proteins involved in oxidative phosphorylation. The first step in understanding the interaction of CL with proteins is to obtain the pure CL structure, and the structure of mixtures of CL with other lipids. In this work we use a variety of techniques to characterize the fluid phase structure, material properties and thermodynamics of mixtures of dimyristoylphosphatidylcholine (DMPC) with tetramyristoylcardiolipin (TMCL), both with 14-carbon chains, at several mole percentages. X-ray diffuse scattering was used to determine structure, including bilayer thickness and area/lipid, the bending modulus, KC, and SXray, a measure of chain orientational order. Our results reveal that TMCL thickens DMPC bilayers at all mole percentages, with a total increase of ∼6Å in pure TMCL, and increases AL from 64Å2 (DMPC at 35°C) to 109Å2 (TMCL at 50°C). KC increases by ∼50%, indicating that TMCL stiffens DMPC membranes. TMCL also orders DMPC chains by a factor of ∼2 for pure TMCL. Coarse grain molecular dynamics simulations confirm the experimental thickening of 2Å for 20mol% TMCL and locate the TMCL headgroups near the glycerol-carbonyl region of DMPC; i.e., they are sequestered below the DMPC phosphocholine headgroup. Our results suggest that TMCL plays a role similar to cholesterol in that it thickens and stiffens DMPC membranes, orders chains, and is positioned under the umbrella of the PC headgroup. CL may be necessary for hydrophobic matching to inner mitochondrial membrane proteins. Differential scanning calorimetry, SXray and CGMD simulations all suggest that TMCL does not form domains within the DMPC bilayers. We also determined the gel phase structure of TMCL, which surprisingly displays diffuse X-ray scattering, like a fluid phase lipid. AL=40.8Å2 for the ½TMCL gel phase, smaller than the DMPC gel phase with AL=47.2Å2, but similar to AL of DLPE=41Å2, consistent with untilted chains in gel phase TMCL.

Dimyristoyl Phosphatidylcholine: A Remarkable Exception to α-Tocopherol’s Membrane Presence

Using data obtained from different physical techniques (i.e., neutron diffraction, NMR and UV spectroscopy), we present evidence which explains some of the conflicting and inexplicable data found in the literature regarding α-tocopherol's (aToc's) behavior in dimyristoyl phosphatidylcholine (di-14:0PC) bilayers. Without exception, the data point to aToc's active chromanol moiety residing deep in the hydrophobic core of di-14:0PC bilayers, a location that is in stark contrast to aToc's location in other PC bilayers. Our result is a clear example of the importance of lipid species diversity in biological membranes and importantly, it suggests that measurements of aToc's oxidation kinetics, and its associated byproducts observed in di-14:0PC bilayers, should be reexamined, this time taking into account its noncanonical location in this bilayer.

The effect of furazolidone on the physico-chemical properties of dimyristoylphosphatidylcholine bilayers: Relevance to anti-leishmanial therapy

In this study, the influence of furazolidone, an anti-leishmanial drug, on dimyristoylphosphatidylcholine (DMPC) liposome hydration degree, mobility and thermodynamics was investigated by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). FTIR results showed that furazolidone was responsible for an increase in the hydrogen bound number and mobility of the lipid phosphate group. Furazolidone also affected the lipid choline group by increasing its motional freedom, as shown by FTIR and 1H NMR spin–lattice relaxation time measurements. At the DMPC interfacial region, FTIR results showed a drug-induced reduction of the carbonyl hydration and order degrees. Very weak interaction among furazolidone and the hydrophobic lipid chains was also observed. However, no furazolidone-induced changes on thermodynamical parameters, such as phase transition temperature (Tm) and enthalpy variation (ΔH), were detected by the DSC technique. Thus, furazolidone seems to interact preferentially with lipid polar and interfacial regions, enhancing the freedom for gauche-trans isomerization of the first methylene groups of DMPC acyl chains. Responses described in this paper may explain the improved activity of furazolidone-encapsulated liposomes by comparison with the effect of the free drug, described in literature. The findings can also improve further strategies for the potential therapeutic application of liposomal furazolidone as a drug delivery system and minimize the risk of drug resistance and collateral effects related to high toxicity.

Computational analysis of local membrane properties

In the field of biomolecular simulations, dynamics of phospholipid membranes is of special interest. A number of proteins, including channels, transporters, receptors and short peptides are embedded in lipid bilayers and tightly interact with phospholipids. While the experimental measurements report on the spatial and/or temporal average membrane properties, simulation results are not restricted to the average properties. In the current study, we present a collection of methods for an efficient local membrane property calculation, comprising bilayer thickness, area per lipid, deuterium order parameters, Gaussian and mean curvature. The local membrane property calculation allows for a direct mapping of the membrane features, which subsequently can be used for further analysis and visualization of the processes of interest. The main features of the described methods are highlighted in a number of membrane systems, namely: a pure dimyristoyl-phosphatidyl-choline (DMPC) bilayer, a fusion peptide interacting with a membrane, voltage-dependent anion channel protein embedded in a DMPC bilayer, cholesterol enriched bilayer and a coarse grained simulation of a curved palmitoyl-oleoyl-phosphatidyl-choline lipid membrane. The local membrane property analysis proves to provide an intuitive and detailed view on the observables that are otherwise interpreted as averaged bilayer properties.Electronic supplementary materialThe online version of this article (doi:10.1007/s10822-013-9684-0) contains supplementary material, which is available to authorized users.

Acetylsalicylic acid (aspirin) and salicylic acid interaction with the human erythrocyte membrane bilayer induce in vitro changes in the morphology of erythrocytes

Despite the well-documented information, there are insufficient reports concerning the effects of salicylate compounds on the structure and functions of cell membranes, particularly those of human erythrocytes. With the aim to better understand the molecular mechanisms of the interaction of acetylsalicylic acid (ASA) and salicylic acid (SA) with cell membranes, human erythrocyte membranes and molecular models were utilized. These consisted of bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), representative of phospholipid classes located in the outer and inner monolayers of the human erythrocyte membrane, respectively. The capacity of ASA and SA to perturb the multibilayer structures of DMPC and DMPE was evaluated by X-ray diffraction while DMPC unilamellar vesicles (LUV) were studied by fluorescence spectroscopy. Moreover, we took advantage of the capability of differential scanning calorimetry (DSC) to detect the changes in the thermotropic phase behavior of lipid bilayers resulting from ASA and SA interaction with PC and PE molecules. In an attempt to further elucidate their effects on cell membranes, the present work also examined their influence on the morphology of intact human erythrocytes by means of defocusing and scanning electron microscopy, while isolated unsealed human erythrocyte membranes (IUM) were studied by fluorescence spectroscopy. Results indicated that both salicylates interact with human erythrocytes and their molecular models in a concentration-dependent manner perturbing their bilayer structures.

A guiding potential method for evaluating the bending rigidity of tensionless lipid membranes from molecular simulation

A new method is proposed to estimate the bending rigidity of lipid membranes from molecular dynamics simulations. An external cylindrical guiding potential is used to impose a sinusoidal deformation to a planar membrane. The bending rigidity is obtained from the mean force acting on the cylinder by calibrating against a discretized Helfrich model that accounts for thermal fluctuations of the membrane surface. The method has been successfully applied to a dimyristoyl phosphatidylcholine bilayer simulated with a coarse-grained model. A well-converged bending rigidity was obtained for the tension-free membrane and showed reasonable agreement with that obtained from the height fluctuation spectrum.

Formation of Cholesterol Bilayer Domains Precedes Formation of Cholesterol Crystals in Cholesterol/Dimyristoylphosphatidylcholine Membranes: EPR and DSC Studies

Saturation-recovery EPR along with DSC were used to determine the cholesterol content at which pure cholesterol bilayer domains (CBDs) and cholesterol crystals begin to form in dimyristoylphosphatidylcholine (DMPC) membranes. To preserve compositional homogeneity throughout the membrane suspension, lipid multilamellar dispersions were prepared using a rapid solvent exchange method. The cholesterol content increased from 0 to 75 mol %. With spin-labeled cholesterol analogues, it was shown that the CBDs begin to form at ~50 mol % cholesterol. It was confirmed by DSC that the cholesterol solubility threshold for DMPC membranes is detected at ~66 mol % cholesterol. At levels above this cholesterol content, monohydrate cholesterol crystals start to form. The major finding is that the formation of CBDs precedes formation of cholesterol crystals. The region of the phase diagram for cholesterol contents between 50 and 66 mol % is described as a structured one-phase region in which CBDs have to be supported by the surrounding DMPC bilayer saturated with cholesterol. Thus, the phase boundary located at 66 mol % cholesterol separates the structured one-phase region (liquid-ordered phase of DMPC with CBDs) from the two-phase region where the structured liquid-ordered phase of DMPC coexists with cholesterol crystals. It is likely that CBDs are precursors of monohydrate cholesterol crystals.

Electrochemical SERS study of a biomimetic membrane supported at a nanocavity patterned Ag electrode

Lipid bilayers in which two leaflets were made of dimyristoyl-phosphatidylcholine (DMPC) and hybrid bilayers with one leaflet composed of hydrogenated lipid and another leaflet of deuterium substituted molecules (d63-DMPC) were deposited on highly ordered nanocavity patterned Ag electrodes using LB–LS and vesicle fusion techniques. In situ electrochemical surface enhanced Raman scattering (EC-SERS) was then used to study potential driven changes in these model biological membranes. The nanocavity structures provided a SERS active substrate with uniformly distributed surface enhancement. To ensure that the bilayer was separated from the metal surface by a hydrophilic space layer, the electrodes were chemically modified with a self-assembled monolayer (SAM) of β-thioglucose (TG) molecules. The monolayer of TG ensured that the solid substrate surface was hydrophilic. The electrochemical properties of the bilayer were monitored by recording differential capacitance curves. EC-SERS indicated that at the silver surface modified by the monolayer of TG the lower leaflet (in contact with the support) is more ordered than the top leaflet that is contact with solution. However, both leaflets remained in the liquid crystalline (LC) state for the entire range of investigated potentials. The results of this study show that the DMPC bilayer at the nanocavity patterned Ag surface may be used as a good biomimetic membrane model in future SERS studies of membrane proteins. The information concerning the effect of the electrode potential on membrane stability may be useful for the development of biosensors.

The New Fluorescent Membrane Probe Ahba: A Comparative Study with the Largely Used Laurdan

Lipid bilayers have been largely used as model systems for biological membranes. Hence, their structures, and alterations caused on them by biological active molecules, have been the subject of many studies. Accordingly, fluorescent probes incorporated into lipid bilayers have been extensively used for characterizing lipid bilayer fluidity and/or polarity. However, for the proper analysis of the alterations undergone by a membrane, a comprehensive knowledge of the fluorescent properties of the probe is fundamental. Therefore, the present work compares fluorescent properties of a relative new fluorescent membrane probe, 2-amino-N-hexadecyl-benzamide (Ahba), with the largely used probe 6-dodecanoyl-N,N-dimethyl-2-naphthylamine (Laurdan), using both static and time resolved fluorescence. Both Ahba and Laurdan have the fluorescent moiety close to the bilayer surface; Ahba has a rather small fluorescent moiety, which was shown to be very sensitive to the bilayer surface pH. The main goal was to point out the fluorescent properties of each probe that are most sensitive to structural alterations on a lipid bilayer. The two probes were incorporated into bilayers of the well-studied zwitterionic lipid dimyristoyl phosphatidylcholine (DMPC), which exhibits a gel-fluid transition around 23 °C. The system was monitored between 5 and 50 °C, hence allowing the study of the two different lipid structures, the gel and fluid bilayer phases, and the transition between them. As it is known, the fluorescent emission spectrum of Laurdan is highly sensitive to the bilayer gel-fluid transition, whereas the Ahba fluorescence spectrum was found to be insensitive to changes in bilayer structure and polarity, which are known to happen at the gel-fluid transition. However, both probes monitor the bilayer gel-fluid transition through fluorescence anisotropy measurements. With time-resolved fluorescence, it was possible to show that bilayer structural variations can be monitored by Laurdan excited state lifetimes changes, whereas Ahba lifetimes were found to be insensitive to bilayer structural modifications. Through anisotropy time decay measurements, both probes could monitor structural bilayer changes, but the limiting anisotropy was found to be a better parameter than the rotational correlation time. It is interesting to have in mind that the relatively small fluorophore of Ahba (o-Abz) could possibly be bound to a phospholipid hydrocarbon chain, not disturbing much the bilayer packing and being a sensitive probe for the bilayer core.

Using Two-Dimensional Crystals of Aquaporin-0 to Investigate Lipid-Protein Interactions

Membrane proteins play crucial roles in many cellular processes such as signaling, nutrient uptake and cell adhesion. Although the lipid bilayer influences many aspects of membrane protein function, our understanding of lipid-protein interactions is limited. Aquaporin-0 (AQP0) is a water channel in the lens membrane, but it also assembles into orthogonal arrays that form membrane junctions between lens fiber cells. In vitro reconstitution of AQP0 with the lipid dimyristoyl phosphatidylcholine (DMPC) yielded large and well ordered double-layered two-dimensional (2D) crystals that allowed electron crystallographic structure determination of AQP0 and its surrounding DMPC bilayer at 1.9A resolution. Since AQP0 forms high-quality 2D crystals not only with DMPC but also with various other lipids, AQP0 2D crystals are an ideal model system to investigate lipid-protein interactions. By studying AQP0 2D crystals formed with different lipids, we can begin to address very basic questions in membrane biology, such as the driving forces that define lipid-protein interactions, the effects of hydrophobic mismatch, and the molecular basis of raft formation.

Determining the Mode of Action Involved in the Antimicrobial Activity of Synthetic Peptides: A Solid-State NMR and FTIR Study

We have previously shown that leucine to lysine substitution(s) in neutral synthetic crown ether containing 14-mer peptide affect the peptide structure and its ability to permeabilize bilayers. Depending on the substitution position, the peptides adopt mainly either a α-helical structure able to permeabilize dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) vesicles (nonselective peptides) or an intermolecular β-sheet structure only able to permeabilize DMPG vesicles (selective peptides). In this study, we have used a combination of solid-state NMR and Fourier transform infrared spectroscopy to investigate the effects of nonselective α-helical and selective intermolecular β-sheet peptides on both types of bilayers. 31P NMR results indicate that both types of peptides interact with the headgroups of DMPC and DMPG bilayers. 2H NMR and Fourier transform infrared results reveal an ordering of the hydrophobic core of bilayers when leakage is noted, i.e., for DMPG vesicles in the presence of both types of peptides and DMPC vesicles in the presence of nonselective peptides. However, selective peptides have no significant effect on the ordering of DMPC acyl chains. The ability of these 14-mer peptides to permeabilize lipid vesicles therefore appears to be related to their ability to increase the order of the bilayer hydrophobic core.

Peroxidase immobilized on phospholipid bilayers supported on au (111) by DTT self‐assembled monolayers: Application to dopamine determination

In this work, horseradish peroxidase (HRP) was immobilized on dimyristoylphosphatidylcholine (DMPC) bilayers supported on Au (111) by dithiotreitol (DTT) self-assembled monolayers and used as a nanostructured electrochemical biosensor to dopamine determination. The morphology of the phospholipid bilayers and the immobilization of HRP to these layers were characterized by atomic force microscopy (AFM). Square-wave voltammetry (SWV) experiments were done to investigate the performance of the HRP-modified electrode. The AFM images indicate that the enzyme is adsorbed at the external layer of the lipid bilayer and, although the electrical charges on the surface were not measured, the enzyme and phospholipids surface interaction occurs probably by electrostatic forces due to the pH used in the experiments. Interestingly, the present system can be used as one-shot sensor for the rapid detection of dopamine. The analytical performance of this system was linear for dopamine concentrations from 3.3 × 10⁻⁵ to 1.3 × 10⁻³ mol L⁻¹ (r = 0.9997) with a detection limit of 2.0 × 10⁻⁶ mol L⁻¹. Our results indicate that the use of HRP-DMPC bilayer system may be useful not only in developing new nanostructured materials for technological purposes, but could be very useful in fundamental studies to investigate the interactions between different micro-and macromolecules, even with soluble proteins, and lipid membranes.

Conformational analysis of two novel cytotoxic C2-substituted pyrrolo[2,3-f]quinolines in aqueous media, organic solvents, membrane bilayers and at the putative active site

We have performed: (i) conformational analysis of two novel cytotoxic C2-substituted pyrrolo[2,3-f]quinolines 5e and 5g in deuterated dimethylsulfoxide (DMSO-d6) utilizing NOE results from NMR spectroscopy; (ii) molecular dynamics (MD) calculations in water, DMSO and dimyristoyl phosphatidylcholine bilayers and (iii) molecular docking and MD calculations on DNA nucleotide sequences. The obtained results for the two similar in structure molecules showed differences in: (i) their conformational properties in silico and in media that reasonably simulate the biological environment; (ii) the way they are incorporated into the lipid bilayers and therefore their diffusion ability and (iii) molecular docking capacity as it is depicted from their different binding scores.

A Molecular Dynamics Study of DMPC Lipid Bilayers Interacting with Dimethylsulfoxide–Water Mixtures

The diffusion process of dimethylsulfoxide (DMSO) through zwitterionic dimyristoylphosphatidylcholine (DMPC) lipid bilayer was studied by means of molecular dynamics (MD) simulations. To account for the cryoprotectant concentration difference between the inside and the outside of the cell, dual DMPC lipid bilayers which separate two aqueous reservoirs with and without DMSO were modeled. The initial configuration of the simulation model had DMSO molecules present in one of the aqueous phases (outside the cell) at two different concentrations of ~3 and ~6mol%. MD simulations were performed on the systems for 50ns at 323K and 1bar. Although the simulation time considered in the study was insufficient for the DMSO molecules to reach the other aqueous phase and equilibrium, early stages of the diffusion process indicated that DMSO molecules had a tendency to diffuse towards the other aqueous phase. The effects of DMSO on bilayer structural characteristics during the diffusion process were investigated. Simulations were analyzed to correlate the following properties of lipid bilayers in the presence of two different aqueous phases: area per lipid, lipid thickness, mass density profiles, lipid tail order parameter and water dipole orientation. Area per lipid calculated for the leaflet facing the aqueous DMSO-water mixture did not show any significant difference compared to area per lipid for the DMSO-free pure DMPC bilayer. Mass density profiles revealed that DMSO molecules had a strong tendency to diffuse toward the aqueous phase with pure water. The lipid tail order parameter calculated for the sn-1 tail of the leaflet facing the aqueous DMSO-water mixture showed that the ordering of lipid tails decreased compared to the leaflet exposed to pure water. However, the ordering of lipid tails in a system where a single bilayer is hydrated by an aqueous DMSO-water mixture is far lower.

Molecular dynamics simulation of HIV-1 fusion domain-membrane complexes: Insight into the N-terminal gp41 fusion mechanism

To understand how viral proteins fuse with the cell membrane, a process that is important for the initial stages of infection, the conformation of the membrane-bound viral fusion peptides (FPs) must be identified. Here, molecular dynamics (MD) simulations were performed to investigate the conformation of the FP-16 and FP-23 of human immunodeficiency virus (HIV) bound to a dimyristoyl phosphatidylcholine (DMPC) bilayer. All FPs were found to penetrate into the bilayer despite the different initial orientations. In addition, the inclusion of residues 17 to 23 was found to play significant role in the fusion ability. Each of the FPs adopts at least two distinct conformations (predominantly α-helical and β-structures) while association with membrane. The peptide seriously affects structural and dynamical parameter of the contacted lipids. The previously experimental data together with our simulation data reveal that fusion ability depends on the membrane-associated conformation and alignment of the peptide.

The effect of pressure on halothane binding to hydrated DMPC bilayers

Halothane binding to hydrated dimyristoylphosphatidylcholine (DMPC) bilayer membranes has been examined over a wide range of pressures from 105 to 4 × 108 Pa. We show that the solvation of halothane by the membrane and bulk water are both pressure dependent, with an increased pressure driving halothane into the membrane. Analysis of these results shows that this pressure dependence is not the cause of pressure reversal, the process whereby general anaesthetics lose their efficacy at pressures of about 8 × 106 to about 2.5 × 107 Pa.

Human erythrocytes and neuroblastoma cells are in vitro affected by sodium orthovanadate

Research on biological influence of vanadium has gained major importance because it exerts potent toxic, mutagenic, and genotoxic effects on a wide variety of biological systems. However, hematological toxicity is one of the less studied effects. The lack of information on this issue prompted us to study the structural effects induced on the human erythrocyte membrane by vanadium (V). Sodium orthovanadate was incubated with intact erythrocytes, isolated unsealed human erythrocyte membranes (IUM) and molecular models of the erythrocyte membrane. The latter consisted of bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), phospholipid classes located in the outer and inner monolayers of the human erythrocyte membrane, respectively. This report presents evidence in order that orthovanadate interacted with red cell membranes as follows: a) in scanning electron microscopy (SEM) studies it was observed that morphological changes on human erythrocytes were induced; b) fluorescence spectroscopy experiments in isolated unsealed human erythrocyte membranes (IUM) showed that an increase in the molecular dynamics and/or water content at the shallow depth of the lipids glycerol backbone at concentrations as low as 50μM was produced; c) X-ray diffraction studies showed that orthovanadate 0.25–1mM range induced increasing structural perturbation to DMPE; d) somewhat similar effects were observed by differential scanning calorimetry (DSC) with the exception of the fact that DMPC pretransition was shown to be affected; and e) fluorescence spectroscopy experiments performed in DMPC large unilamellar vesicles (LUV) showed that at very low concentrations induced changes in DPH fluorescence anisotropy at 18°C. Additional experiments were performed in mice cholinergic neuroblastoma SN56 cells; a statistically significant decrease of cell viability was observed on orthovanadate in low or moderate concentrations.

Influence of Hydrophobic Mismatch on Structures and Dynamics of Gramicidin A and Lipid Bilayers

Gramicidin A (gA) is a 15-amino-acid antibiotic peptide with an alternating L-D sequence, which forms (dimeric) bilayer-spanning, monovalent cation channels in biological membranes and synthetic bilayers. We performed molecular dynamics simulations of gA dimers and monomers in all-atom, explicit dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dioleoylphosphatidylcholine (DOPC), and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers. The variation in acyl chain length among these different phospholipids provides a way to alter gA-bilayer interactions by varying the bilayer hydrophobic thickness, and to determine the influence of hydrophobic mismatch on the structure and dynamics of both gA channels (and monomeric subunits) and the host bilayers. The simulations show that the channel structure varied little with changes in hydrophobic mismatch, and that the lipid bilayer adapts to the bilayer-spanning channel to minimize the exposure of hydrophobic residues. The bilayer thickness, however, did not vary monotonically as a function of radial distance from the channel. In all simulations, there was an initial decrease in thickness within 4–5 Å from the channel, which was followed by an increase in DOPC and POPC or a further decrease in DLPC and DMPC bilayers. The bilayer thickness varied little in the monomer simulations—except one of three independent simulations for DMPC and all three DLPC simulations, where the bilayer thinned to allow a single subunit to form a bilayer-spanning water-permeable pore. The radial dependence of local lipid area and bilayer compressibility is also nonmonotonic in the first shell around gA dimers due to gA-phospholipid interactions and the hydrophobic mismatch. Order parameters, acyl chain dynamics, and diffusion constants also differ between the lipids in the first shell and the bulk. The lipid behaviors in the first shell around gA dimers are more complex than predicted from a simple mismatch model, which has implications for understanding the energetics of membrane protein-lipid interactions.

Structural effects of the antimicrobial peptide maculatin 1.1 on supported lipid bilayers

The interactions of the antimicrobial peptide maculatin 1.1 (GLFGVLAKVAAHVVPAIAEHF-NH(2)) with model phospholipid membranes were studied by use of dual polarisation interferometry and neutron reflectometry and dimyristoylphosphatidylcholine (DMPC) and mixed DMPC-dimyristoylphosphatidylglycerol (DMPG)-supported lipid bilayers chosen to mimic eukaryotic and prokaryotic membranes, respectively. In DMPC bilayers concentration-dependent binding and increasing perturbation of bilayer order by maculatin were observed. By contrast, in mixed DMPC-DMPG bilayers, maculatin interacted more strongly and in a concentration-dependent manner with retention of bilayer lipid order and structure, consistent with pore formation. These results emphasise the importance of membrane charge in mediating antimicrobial peptide activity and emphasise the importance of using complementary methods of analysis in probing the mode of action of antimicrobial peptides.