Increase In Bilayer Thickness Research Articles

Growth and Structural Study of Cadmium Arachidate Multilayers Deposited by LB Technique

Langmuir–Blodgett (LB) multilayer films of cadmium arachidate acid (CdAA) have been studied by X-ray diffraction and X-ray reflectivity techniques, which reveal uniform ordered packing of CdAA molecules with a small tilt angle of long alkyl chain equal to 6.8º. Electron density profiles reveal that the coverage of films remains almost constant with increase in bilayer thickness, which indicates very little presence of pinhole defects. FTIR studies confirm orthorhombic packing in CdAA LB films and the formation of unidentate bridging metal-carboxylate coordination type headgroups consistent with one cadmium metal ion between two carboxylic (COO) groups in each headgroup.

Effects of cholesterol concentration on the interaction of cytarabine with lipid membranes: a molecular dynamics simulation study

Liposomal cytarabine, DepoCyt, is a chemotherapy agent which is used in cancer treatment. This form of cytarabine has more efficacy and fewer side effects relative to the other forms. Since DepoCyt contains the cytarabine encapsulated within phosphatidylcholine and the sterol molecules, we modeled dioleoylphosphatidylcholine (DOPC)/cholesterol bilayer membrane as a carrier for cytarabine to study drug–bilayer interactions. For this purpose, we performed a series of united-atom molecular dynamics (MD) simulations for 25 ns to investigate the interactions between cytarabine and cholesterol-containing DOPC lipid bilayers. Only the uncharged form of cytarabine molecule was investigated. In this study, different levels of the cholesterol content (0, 20, and 40%) were used. MD simulations allowed us to determine dynamical and structural properties of the bilayer membrane and to estimate the preferred location and orientation of the cytarabine molecule inside the bilayer membrane. Properties such as membrane thickness, area per lipid, diffusion coefficient, mass density, bilayer packing, order parameters, and intermolecular interactions were examined. The results show that by increasing the cholesterol concentration in the lipid bilayers, the bilayer thickness increases and area per lipid decreases. Moreover, in accordance with the experiments, our calculations show that cholesterol molecules have ordering effect on the hydrocarbon acyl chains. Furthermore, the cytarabine molecule preferentially occupies the polar region of the lipid head groups to form specific interactions (hydrogen bonds). Our results fully support the experimental data. Our finding about drug–bilayer interaction is crucial for the liposomal drug design.

Structural morphology study of Cd2+ induced Langmuir Blodgett multilayer films of arachidic acid

The organization and headgroup co-ordination of Cadmium Arachidate (CdAA) molecule in Langmuir–Blodgett (LB) multilayer films deposited on hydrophilic Glass (SiO2) and Silicon (100) substrate at normal subphase pH (6.8) are studied. X-ray diffraction (XRD) and X-ray reflectivity (XRR) study reveals ordered layer by layer organization with uniform packing of CdAA molecules, and with a small tilt angle of alkyl chain of CdAA molecule equal to 6.8°±1.75°. Electron density profiles (EDPs) shows that the coverage of films remains almost constant with increase in bilayer thickness which indicate very little presence of pinhole defects. AFM study for 25 ML shows that coverage of the film remain intact upto 22nd ML and then decreases sharply due to presence of pinhole defects. Fourier transform infrared spectroscopy (FTIR) study is also consistent with XRD and XRR study of ordered deposition of CdAA molecule. FTIR and X-ray photoelectron spectroscopy (XPS) study indicates the formation of unidentate bridging metal-carboxylate coordination type headgroups consistent with one cadmium metal ion between two carboxylate (COO) groups in each headgroup structure.

50. Relevance of cell biophysical behaviour and membrane fluidity for explaining freezing resistance of lactic acid bacteria

The cell membrane is the primary site for freezing injury. During cooling, ice forms outside the cell resulting in an increase of the solutes concentration in the extracellular medium. This cryo-concentration of solutes induces water transport from the intracellular medium of cell to the extracellular medium and thus cell dehydration and cell injury. Cell membrane permeability to water is influenced by its fluidity property which is governed by the composition and the structure of the lipid bilayer. By changing the composition of the medium, the composition and organisation of the lipid bilayer of the cell membrane will be modified resulting in increasing or decreasing freezing resistance. Furthermore, another important phenomenon occurs during cooling of cell suspensions leading to the modification of membrane fluidity: the lipid phase transition from a liquid crystalline phase to a gel phase. The liquid crystalline state is characterized by increased lipid head group spacing, increased disorder in the lipid acyl chains, and a small bilayer thickness. In the gel phase the lipid head groups are very tightly packed, the lipid acyl chains become straighter and ordered, and the bilayer thickness increases. Our objective is to gain a better understanding of the relationships between membrane fluidity, lipid phase transition, membrane water permeability and freezing resistance of lactic acid bacteria. Two suspensions of Lactobacillus bulgaricus CFL1 were produced using either MRS broth or mild whey based culture media. The freezing resistance of both bacterial populations protected with sucrose was evaluated by measuring the cultivability and acidification activity of bacteria before and after freezing at −80 °C. The membrane lipid phase behaviour was studied in situ using Fourier transform infrared spectroscopy during freezing and thawing. Membrane fluidity was evaluated with fluorescence polarization using a lipophilic fluorescent probe embedded in the membrane, namely the DPH. The fluorescence measurements were performed on a flow cytometer at various temperatures. Bacterial cell dehydration was investigated by submitting cells to osmotic stress by increasing the concentration on a non penetrating solute (sucrose). The changes in cell volume with time were measured by Coulter techniques and the biophysical parameters of water transport were estimated. Bacterial population grown in MRS broth exhibited a better resistance to freezing than bacteria grown in mild whey. This better resistance can be ascribed to a lower lipid phase transition temperature and higher membrane fluidity. The detailed analysis of the results is in progress and will enable clarification of the relationships between freezing resistance, membrane fluidity and water permeability.

Cholesterol Enhances or Reduces Metarhodopsin II Formation Depending on Bilayer Thickness

Cholesterol is one of the most efficient modulators of G Protein-Coupled Receptor (GPCR) function. We monitored the effect of cholesterol on rhodopsin function for a set of bilayers with different hydrophobic thickness. Surprisingly, cholesterol shifts the Metarhodopsin-I (MI)/Metarhodopsin-II (MII) equilibrium toward MII for bilayers thinner than the average length of hydrophobic transmembrane helices (2.7 nm), and to MI for thicker bilayers. In previous work conducted on rod outer segment disks and model membranes, increasing cholesterol concentration always shifted the equilibrium towards MI. It was proposed that the cholesterol effect is primarily related to a tighter packing of lipid hydrocarbon chains which generates a less permissive environment for the formation of MII. To gain deeper insights into mechanisms, we followed changes in lipid-rhodopsin interaction by 2H NMR using deuterated lipids. It was reported by us and the Brown laboratory that an increase of bilayer hydrophobic thickness in the absence of cholesterol favors MII with a turnover to MI for bilayers that are very thick. Indeed, the cholesterol-induced shifts towards MII for thinner membranes correlated nicely with the cholesterol-induced increase of bilayer hydrophobic thickness measured by NMR suggesting that the increase in bilayer thickness by cholesterol plays a major role in controlling the energetics of the MI-MII equilibrium. Furthermore, changes in average lipid order parameters due to the presence of rhodopsin were much larger in cholesterol-containing membranes than in cholesterol-free membranes suggesting strongly that the perturbation in the lipid matrix from protein insertion reaches much further away from the protein. This is expected for membranes that are stiffer due to the presence of cholesterol. The consequences of these findings for lipid mediated shifts in rhodopsin function and rhodopsin-rhodopsin interactions will be discussed.

Membrane Area Deformation under Osmotic Stress: Deuterium NMR Approach

We address the hypothesis that the sensitivity of lipid bilayers to pressure, temperature, and osmotic stress represents influences of non-specific lipid-protein interactions on functions of cellular membranes [1,2]. Measurements of membrane structural parameters such as bilayer thickness and area per lipid employ a mean-torque analysis [3] of 2H solid-state NMR order parameters (SCD). NMR lipid order parameters are very sensitive to changes in cross-sectional area per molecule. We observed striking (≈20%) changes in structural properties (decrease in area per lipid and increase in bilayer thickness) when ≈200 atmospheres of pressure (dehydration pressure or osmotic pressure) are applied to lipid (DMPC) bilayers in the liquid-crystalline state. We show the equivalence of osmotic and dehydration pressures by NMR experimental measurements as well as thermodynamically [2]. The elastic area compressibility modulus (Ka) of bilayers is determined by employing different pressure techniques in combination with NMR and vapor pressure osmometry methods. Our findings agree well with the reported Ka value for DMPC [4] obtained with a much smaller range of osmotic pressures. However, we observe an additional variation in Ka determined at higher osmotic pressures, where the role of complex dynamics in the bilayer structural changes becomes more evident. We propose that the hierarchy of forces and motions is perturbed by membrane dehydration (osmotic pressure) due to the alteration of interlamellar spacings, with corresponding changes in elastic area compressibility moduli. Our findings have significant implications for the applicability of solid-state 2H NMR spectroscopy together with membrane stress techniques for understanding the mechanisms of action of pressure-sensitive proteins.[1] A.V. Botelho et al. (2006) BJ 91, 4464-4477.[2] K.J. Mallikarjunaiah et al. (2011) BJ 100, 98-107.[3] H.I. Petrache et al. (2000) BJ 79, 3172-3192.[4] H.I. Petrache et al. (1998) CPL 95, 83-94.

Cholesterol Promotes the Interaction of Alzheimer β-Amyloid Monomer with Lipid Bilayer

Cholesterol in the plasma membrane plays an important role in the pathogenesis of Alzheimer's disease, but the exact function of cholesterol in the regulation of amyloid-β (Aβ) generation, aggregation, and toxicity remains elusive. To gain insight into the bioactivity of cholesterol, we investigate the effect of cholesterol levels on the interaction of Aβ1–42 monomer with the zwitterionic 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) bilayer containing different mole fractions of cholesterol from χ=0, 0.2, to 0.4 using all-atom molecular dynamics simulations. Simulation results show that an increased cholesterol level alters the structure, dynamics, and surface chemistry of the lipid bilayer, leading to increased bilayer thickness, hydrophobic chain order, surface hydrophobicity, and decreased lipid mobility. All these effects significantly promote the binding of Aβ to the lipid bilayer. Mechanistically, the adsorption of Aβ on the bilayer is a cooperative process. First, charged residues act as anchors to establish the initial binding of Aβ to phosphate headgroups of the bilayer driven by electrostatic interactions, which further facilitates hydrophobic residues to reside on the bilayer. Once hydrophobic residues especially from C-terminus are locked on the bilayer, the interactions among charged residues, lipid bilayer, and calcium ions are optimized to provide additional attractive forces to stabilize Aβ adsorbed on or inserted into the lipid bilayer. Inclusion of cholesterol makes this binding process more energetically favorable. Upon adsorption on the bilayer, Aβ appears to preferentially adopt α-helical or unstructured conformation. This work supports that cholesterol acts as a promoter for Aβ––membrane interactions, which would facilitate Aβ aggregation and membrane insertion.

Atomic Force Microscopy Studies of a Floating-Bilayer Lipid Membrane on a Au(111) Surface Modified with a Hydrophilic Monolayer

The surface of a gold electrode was functionalized with a hydrophilic monolayer of 1-thio-β-D-glucose formed by spontaneous self-assembly. The Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) method was then used to assemble a bilayer onto the modified Au(111) surface. The bilayer lipid membrane (BLM) was separated from the Au(111) electrode surface by incorporating the monosialoganglioside GM1 into the inner leaflet of a bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol. To make the inner leaflet, monolayers of GM1/DMPC/cholesterol with mole ratios of 1:6:3, 2:5:3, and 3:4:3 were used. The outer leaflet was composed of a 7:3 mole ratio of DMPC/cholesterol. Because of the amphiphilic properties of GM1, the hydrophobic acyl chains were incorporated into the BLM, whereas the large hydrophilic carbohydrate headgroups were physically adsorbed to the Au(111) electrode surface, creating a "floating" BLM (fBLM). This model contained a water-rich reservoir between the BLM and the gold surface. In addition, because of the bilayer being physically adsorbed onto the support, the fluidity of the BLM was maintained. The compression isotherms were measured at the air/water interface to determine the phase behavior and optimal transfer conditions. The images acquired using atomic force microscopy (AFM) and the force-distance measurements showed that the structure of the fBLM evolved with increasing GM1 content from 10 to 30 mol %, undergoing a transition from a corrugated to a homogeneous phase. This change was associated with a significant increase in bilayer thickness (from ∼5.3 to 7.3 nm). The highest-quality fBLM was produced with 30 mol % GM1.

Scattering Studies of Hydrophobic Monomers in Liposomal Bilayers: An Expanding Shell Model of Monomer Distribution

Hydrophobic monomers partially phase separate from saturated lipids when loaded into lipid bilayers in amounts exceeding a 1:1 monomer/lipid molar ratio. This conclusion is based on the agreement between two independent methods of examining the structure of monomer-loaded bilayers. Complete phase separation of monomers from lipids would result in an increase in bilayer thickness and a slight increase in the diameter of liposomes. A homogeneous distribution of monomers within the bilayer would not change the bilayer thickness and would lead to an increase in the liposome diameter. The increase in bilayer thickness, measured by the combination of small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS), was approximately half of what was predicted for complete phase separation. The increase in liposome diameter, measured by dynamic light scattering (DLS), was intermediate between values predicted for a homogeneous distribution and complete phase separation. Combined SANS, SAXS, and DLS data suggest that at a 1.2 monomer/lipid ratio approximately half of the monomers are located in an interstitial layer sandwiched between lipid sheets. These results expand our understanding of using self-assembled bilayers as scaffolds for the directed covalent assembly of organic nanomaterials. In particular, the partial phase separation of monomers from lipids corroborates the successful creation of nanothin polymer materials with uniform imprinted nanopores. Pore-forming templates do not need to span the lipid bilayer to create a pore in the bilayer-templated films.

Effects of cholesterol on thermal stability of discoidal high density lipoproteins

Reverse cholesterol transport in plasma involves variations in HDL cholesterol concentration. To understand physicochemical and functional implications of such variations, we analyzed stability of reconstituted HDL containing human apolipoproteins (apoA-I, apoA-II, or apoC-I), phosphatidylcholines varying in chain length (12-18 carbons) and unsaturation (0 or 1), and 0-35 mol% cholesterol. Lipoprotein heat denaturation was monitored by circular dichroism for protein unfolding/dissociation and by light scattering for particle fusion. We found that cholesterol stabilizes relatively unstable complexes; for example, incorporation of 10-30 mol% cholesterol in apoC-I:dimyristoyl phosphatidylcholine complexes increased their kinetic stability by deltaDeltaG* congruent with 1 kcal/mol. In more stable complexes containing larger proteins and/or longer-chain lipids, incorporation of 10% cholesterol did not significantly alter the disk stability; however, 15% or more cholesterol destabilized the apoA-I-containing complexes and led to vesicle formation. Thus, cholesterol tends to stabilize less stable lipoproteins, apparently by enhancing favorable packing interactions, but in more stable lipoproteins, where such interactions are already highly optimized, the stabilizing effect of cholesterol decreases and, eventually, becomes destabilizing. These results help uncouple the functional roles of particle stability and chain fluidity and suggest that structural disorder in HDL surface, rather than chain fluidity, is an important physicochemical determinant of HDL function.

Asymmetric dipping of bacteriophage M13 coat protein with increasing lipid bilayer thickness

Knowledge about the vertical movement of a protein with respect to the lipid bilayer plane is important to understand protein functionality in the biological membrane. In this work, the vertical displacement of bacteriophage M13 major coat protein in a lipid bilayer is used as a model system to study the molecular details of its anchoring mechanism in a homologue series of lipids with the same polar head group but different hydrophobic chain length. The major coat proteins were reconstituted into 14:1PC, 16:1PC, 18:1PC, 20:1PC, and 22:1PC bilayers, and the fluorescence spectra were measured of the intrinsic tryptophan at position 26 and BADAN attached to an introduced cysteine at position 46, located at the opposite ends of the transmembrane helix. The fluorescence maximum of tryptophan shifted for 700 cm -1 on going from 14:1PC to 22:1PC, the corresponding shift of the fluorescence maximum of BADAN at position 46 was approximately 10 times less (∼ 70 cm -1). Quenching of fluorescence with the spin label CAT 1 indicates that the tryptophan is becoming progressively inaccessible for the quencher with increasing bilayer thickness, whereas quenching of BADAN attached to the T46C mutant remained approximately unchanged. This supports the idea that the BADAN probe at position 46 remains at the same depth in the bilayer irrespective of its thickness and clearly indicates an asymmetrical nature of the protein dipping in the lipid bilayer. The anchoring strength at the C-terminal domain of the protein (provided by two phenylalanine residues together with four lysine residues) was estimated to be roughly 5 times larger than the anchoring strength of the N-terminal domain.

Cholesterol Regulation of Membrane Protein Function and Sorting By Changes in Lipid Bilayer Elastic Properties

Cholesterol regulates the function of numerous membrane proteins. This regulation may depend on specific cholesterol-protein interactions, as well as on cholesterol-induced changes in the collective properties of the cell membrane lipid bilayer. We have studied the role of cholesterol-induced changes in the bilayer elastic properties on the function of: voltage dependent sodium channels, N-type calcium channels and GABAA receptors. For all three channel types, the effects of cholesterol, which increases lipid stiffness (measured using gramicidin channels as molecular force transducers), are opposite those induced by amphiphiles that decrease bilayer stiffness. Similar correlations exist for a number of other membrane proteins. These findings strongly suggest that cholesterol can regulate membrane protein function by altering the bilayer elastic properties.Cholesterol, similarly, has been proposed to regulate membrane protein sorting between different cellular membranes, by altering the energetic cost of the bilayer deformation associated with a mismatch between the length of a protein trans-membrane segment and the thickness of the bilayer hydrophobic core. We have investigated the energetic feasibility of such a sorting mechanism, using the continuum theory of liquid crystal deformations. The relative importance of a cholesterol-induced increase in bilayer thickness vs. an increase in the bilayer elastic moduli was evaluated. We find that cholesterol-induced sorting of membrane proteins indeed should be feasible - primarily due to the increase in the bilayer elastic moduli. Subsequent experimental studies support this conclusion.

Characterization of Symmetric and Asymmetric Lipid Bilayers Composed of Varying Concentrations of Ganglioside GM1 and DPPC

Gangliosides are a group of structurally diverse, sialic acid containing glycosphingolipids embedded into the membrane via their hydrophobic ceramide moiety. To gain atomic level insights into the structural perturbations caused by Galbeta3GalNAcbeta4(NeuAcalpha3)Galbeta4Glc1Cer (GM1), molecular dynamics (MD) simulations of a 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) bilayer containing GM1 at five different concentrations have been performed. Biological membranes contain GM1 only on the exoplasmic leaflet. However, vesicles prepared in the laboratory contain GM1 in both the leaflets albeit unequally. Hence, simulations were performed with GM1 present in only one (asymmetric bilayers) or in both of the leaflets (symmetric bilayers) of the bilayer. In symmetric bilayers, there is a decrease in surface area, an increase in deuterium order parameter, and an increase in peak-to-peak distance of DPPC with increasing concentration of GM1. Thus, the overall area of the lipid bilayer decreases (condensation effect) and the thickness increases with increasing concentrations of GM1. Even in asymmetric systems, decrease in surface area and increase in deuterium order parameter of hydrocarbon chains of DPPC are observed. However, the decrease in bilayer area and the increase in bilayer thickness are not as much as in the symmetric bilayer.

Influence of Chain Length and Unsaturation on Sphingomyelin Bilayers

Sphingomyelins (SMs) are among the most common phospholipid components of plasma membranes, usually constituting a mixture of several molecular species with various fatty acyl chain moieties. In this work, we utilize atomistic molecular dynamics simulations to study the differences in structural and dynamical properties of bilayers comprised of the most common natural SM species. Keeping the sphingosine moiety unchanged, we vary the amide bonded acyl chain from 16 to 24 carbons in length and examine the effect of unsaturation by comparing lipids with saturated and monounsaturated chains. As for structural properties, we find a slight decrease in average area per lipid and a clear linear increase in bilayer thickness with increasing acyl chain length both in saturated and unsaturated systems. Increasing the acyl chain length is found to further the interdigitation across the bilayer center. This is related to the dynamics of SM molecules, as the lateral diffusion rates decrease slightly for an increasing acyl chain length. Interdigitation also plays a role in interleaflet friction, which is stronger for unsaturated chains. The effect of the cis double bond is most significant on the local order parameters and rotation rates of the chains, though unsaturation shows global effects on overall lipid packing and dynamics as well. Regarding hydrogen bonding or properties related to the lipid/water interface region, no significant effects were observed due to varying chain length or unsaturation. The significance of the findings presented is discussed.

Plasmon-waveguide Resonance Studies of Lateral Segregation of Lipids and Proteins into Microdomains (Rafts) in Solid-supported Bilayers

Plasmon-waveguide resonance (PWR) spectroscopy has been used to examine solid-supported lipid bilayers consisting of dioleoylphosphatidylcholine (DOPC), palmitoyloleoylphosphatidylcholine (POPC), sphingomyelin (SM), and phosphatidylcholine/SM binary mixtures. Spectral simulation of the resonance curves demonstrated an increase in bilayer thickness, long-range order, and molecular packing density in going from DOPC to POPC to SM single component bilayers, as expected based on the decreasing level of unsaturation in the fatty acyl chains. DOPC/SM and POPC/SM binary mixtures yielded PWR spectra that can be ascribed to a superposition of two resonances corresponding to microdomains (rafts) consisting of phosphatidylcholine- and SM-rich phases coexisting within a single bilayer. These were formed spontaneously over time as a consequence of lateral phase separation. Each microdomain contained a small proportion (<20%) of the other lipid component, which increased their kinetic and thermodynamic stability. Incorporation of a glycosylphosphatidylinositol-linked protein (placental alkaline phosphatase) occurred within each of the single component bilayers, although the insertion was less efficient into the DOPC bilayer. Incorporation of placental alkaline phosphatase into a DOPC/SM binary bilayer occurred with preferential insertion into the SM-rich phase, although the protein incorporated into both phases at higher concentrations. These results demonstrate the utility of PWR spectroscopy to provide insights into raft formation and protein sorting in model lipid membranes.

Composition of Supported Model Membranes Determined by Neutron Reflection

We have investigated the formation of supported bilayers by coadsorption of dipalmitoyl phosphatidylcholine (DPPC) with the nonionic surfactant beta-D-dodecyl maltoside. The adsorption of mixed phospholipid-surfactant micelles on hydrophilic silica surfaces at 25 degrees C was followed as a function of bulk concentration by neutron reflection. Using chain-deuterated d(25)-beta-D-dodecyl maltoside and d(62)-DPPC, we demonstrate that it is possible to determine the composition of the bilayers at each stage of a sequential dilution process, which enriches the adsorbed layer in phospholipid and leads to complete elimination of the surfactant. The final supported bilayers have thicknesses of 51 +/- 3 A and are stable to heating to 37 degrees C once all surfactant has been removed, and the structures agree well with other published data on DPPC supported bilayers. The coadsorption of cholesterol in a DPPC-surfactant mixture was also achieved, and the location and volume fraction of cholesterol in the DPPC bilayer was determined. Cholesterol is located in a 18 +/- 1 A thick layer below the lipid headgroup region and leads to an increased bilayer thickness of 58 +/- 2 A at 26 mol % of cholesterol.

Effect of cholesterol and phospholipid on the behavior of dialkyl polyoxyethylene ether surfactant (2C18E12) monolayers and bilayers.

Surface pressure-area isotherm, neutron specular reflection, and small-angle neutron scattering studies have been carried out to determine the effects of added cholesterol and distearoylphosphatidylcholine (DSPC), on the molecular structures of monolayers and vesicles containing the dialkyl polyoxyethylene ether surfactant, 1,2-di-O-octadecyl-rac-glyceryl-3-(alpha-dodecaethylene glycol) (2C18E12). Previous neutron reflectivity studies on 2C18E12 monolayers at the air/water interface have shown them to possess a thickness of approximately 24 angstoms and highly disordered structure with significant intermixing of the polymer headgroups and alkyl chains. SANS studies of 2C18E12 vesicles gave a bilayer thickness of approximately 51 angstroms. Addition of cholesterol to 2C18E12 monolayers (1:1 molar ratio), produced a marked condensing effect coupled with an increased the layer thickness of approximately 7 angstroms, and in vesicles, increased bilayer thickness by approximately 16 angstroms. Monolayers consisting of 2C18E12:DSPC:cholesterol (1:1:2 molar ratio), showed a layer thickness of approximately 31 angstroms, whereas in vesicles, three-component bilayer was found to be only approximately 9 angstroms thicker than those possessed by vesicles composed solely of 2C18E12. Mixing between the molecules in three-component monolayers was shown to be ideal through analysis of the neutron reflectivity data. These findings are discussed in relation to increased ordering and decreased headgroup/hydrophobe intermixing within both monolayers and vesicle bilayers containing 2C18E12. The inferred increase in molecular order within vesicles composed of 2C18E12 with additional cholesterol and phospholipid is used as a model for explaining theoretical differences in bilayer permeability.

Structure and fluctuations of phosphatidylcholines in the vicinity of the main phase transition.

We have determined the structural properties and bending fluctuations of fully hydrated phosphatidylcholine multibilayers in the fluid (Lalpha) phase, as well as the structure of the ripple (Pbeta') phase near the main phase transition temperature (TM) by x-ray diffraction. The number of carbons, nHC, per acyl chain of the studied disaturated lipids varied from 14 to 22. All lipids exhibit a nonlinear increase of the lamellar repeat distance d in the Lalpha phase upon approaching TM, known as "anomalous swelling." The nonlinear increase reduces with chain length, but levels off at a constant value of about 0.5 A for lipids with more than 18 hydrocarbons per chain. A detailed analysis shows that anomalous swelling has two components. One is due to an expansion of the water layer, which decreases with chain length and finally vanishes for nHC >18. The second component is due to a bilayer thickness increase, which remains unchanged in its temperature dependence, including a nonlinear component of about 0.5 A in the vicinity of TM. Thus, anomalous swelling above 18 hydrocarbons per chain is due to the pretransitional effects on the membrane only. These results are supported by a bending fluctuation analysis revealing increased undulations close to TM only for the short chain lipids. We have further calculated the electron density maps in the ripple phase and find no coupling of the magnitude of the ripple amplitude to the chain length effects observed in the Lalpha phase. Hence, in agreement with an earlier report by Mason et al. [Phys. Rev. E 63, 030902 (2001)] there is no connection between the formation of the ripple phase and anomalous swelling.

Effect of the Pore Region of a Transmembrane Ion Channel on the Physical Properties of a Simple Membrane

The effect of membrane proteins and peptides on their surrounding lipids is crucial for the structure, dynamics, and function of complex biological membranes as well as the interplay between membrane proteins and their environment. Here, we present a study of the influence of the transmembrane pore region of an ion channel on the physical properties of a phospholipid bilayer. We performed multinanosecond molecular dynamics simulations of the pore-forming aggregate of α-helical transmembrane peptides, which constitutes a model for the channel region of the nicotinic acetylcholine receptor, inserted in a simple lipid bilayer at conditions similar to those of the recent NMR experiments [Opella, S. J.; et al. Nature Struct. Biol. 1999, 6, 374]. The results obtained are compared with simulations of the pure lipid bilayer membrane. In particular, we consider here the simplest possible model membrane: a fully hydrated dimyristoyl-sn-glycerophosphocholine lipid bilayer, which is in the biologically relevant fluid lamellar phase at room temperature. Our analysis reveals that the peptides affect the properties of the lipid bilayer in several ways: (1) the bilayer thickness increases, (2) the number of gauche defects of the hydrocarbon chains decreases, (3) the orientational order of the hydrocarbon chains increases, (4) the orientational probability distribution of the lipid headgroup dipole moments becomes broader (more disordered), and (5) the lipid headgroup dipole moments are on average more oriented toward the water phase. Some of these effects depend on the specific properties of the case studied, such as the hydrophobic length of the bundle and the charges at the interface. Interestingly, the pore does not affect the two different sides of the membrane in the same way.

Small-angle neutron scattering study of the lipid bilayer thickness in unilamellar dioleoylphosphatidylcholine vesicles prepared by the cholate dilution method: n-decane effect

Previous X-ray diffraction studies on fully hydrated fluid lamellar egg phosphatidylcholine phases indicated a ∼10 Å increase of bilayer thickness in the presence of excess n-decane [Biochim. Biophys. Acta 597 (1980) 455], while the small-angle neutron scattering (SANS) on unilamellar extruded dioleoylphosphatidylcholine (DOPC) vesicles detected substantially smaller 2.4±1.3 Å bilayer thickness increase at n-decane/DOPC molar ratio of 1.2 [Biophys. Chem. 88 (2000) 165]. The purpose of the present study is to investigate the n-decane effect on the bilayer thickness in unilamellar DOPC vesicles prepared by the sodium cholate (NaChol) dilution method. Mixed DOPC+NaChol micelles at DOPC and NaChol concentrations of 0.1 mol/l were prepared in 2H 2O containing 0.135 mol/l NaCl. This micellar solution was diluted in 0.135 mol/l NaCl in 2H 2O to reach the final DOPC and NaChol concentrations of 0.008 mol/l. Thirty microliters of n-decane solution in methanol was added to 1 ml of this dispersion. After methanol evaporation, SANS was conducted on the dispersions. From the Kratky–Porod plot ln[ I( Q) Q 2] vs. Q 2 of SANS intensity I( Q) in the range of scattering vector values Q corresponding to interval 0.001 Å −2≤ Q 2≤0.006 Å −2, the bilayer radius of gyration R g and the bilayer thickness parameter d g=12 0.5 R g were obtained. The values of d g indicate that the bilayer thickness increases by 2.4±0.9 Å up to n-decane/DOPC molar ratio of 1.6. This result confirms the previous SANS findings obtained with extruded vesicles.