Properties Of Lipid Monolayers Research Articles

Effect of hydrophobic and hydrophilic surfaces on the morphological and physical properties of supported lipid membranes.

The characteristics of supported lipid bilayers (SLBs) have long been studied as SLBs are commonly accepted as basic model systems of cell membranes. Atomic force microscopy (AFM) has proved to be effective in characterising physical and thermodynamic properties of SLBs, owing to its high resolution and the ability of environmental control. Through study of these properties, information such as lipid phase transitions can be investigated. Lipid phase transitions, from solid-ordered to liquid-disordered states, are well known to vary depending upon temperature, pH and ionic strength etc., and play a substantial role into events such as membrane protein mobility. Although previously studied on hydrophilic substrates, knowledge on the impact of hydrophobic substrates, such as van der Waals (vdW) crystals, on lipid phase transition is still lacking. This is particularly important for developing new bio- and electrochemical sensing technologies, where conductive vdW materials are used as sensing electrodes. It is expected that the thermodynamic properties of SLBs affect their conductive and dielectric polarisation and in turn molecular charge distribution and dielectric properties are known to strongly influence hydration layers occurring at the interface of lipids. Here, we present the study of adsorption of lipid membranes on various substrates, where morphological and physical (i.e. thermodynamic, electrical, and interfacial) properties of the membrane were investigated using advanced AFM modes as a function of temperature. We find that the transition temperature of the lipid monolayers greatly differs depending upon the chosen substrate. We then probe interfacial properties of these electrically charged interfaces. Finally, we demonstrate that substrates with different chemical-physical properties used as a lipids support have a substantial effect on the physical properties of lipid monolayers.

Composition, functionality and structural correlates of mixed lipid monolayers at air-water interface

Physicochemical properties of lipid monolayer depend on its composition where a blend of lipids exhibit superior behaviour than the individual component. Surface pressure ( π ) – area ( A ) isotherms of mixed monolayer (PC mix ) formed by dipalmitoylphosphatidylcholine (DPPC), palmitoleylphosphatidylcholine (POPC), soyphosphatidylcholine (SPC) and hydrogenated soy phosphatidylcholine (HSPC) in combination with 30 mole % cholesterol (Chol) were obtained by using Langmuir trough. Effects of dipalmitoylphosphatidyethanolamine (DPPE) on its mutual miscibility at the air-water interface with DPPC ​+ ​Chol, POPC ​+ ​Chol, HSPC ​+ ​Chol and SPC ​+ ​Chol were also investigated separately, followed by studying the effects of DPPE and dipalmitoylphosphatidylglycerol (DPPG) to PC mix ​+ ​Chol and PC mix ​+ ​DPPE ​+ ​Chol respectively. Lift-off area, minimum molecular area, excess molecular area, collapse pressure, Gibbs free energy of interfacial mixing, compressibility modulii values were evaluated by analyzing the isotherms. Deviations from the ideal mixing behaviours were dependent on the composition of the lipid blends. Surface dilatational rheology studies could assess monolayer elasticity, whereas the film morphologies were analysed by Brewster angle microscopic (BAM) studies. DPPC induced formation of condensed monolayer, whereas film rigidity was increased with the incorporation of DPPE and DPPG into mixed systems. Interactions among the lipid components of the investigated mixed systems were thoroughly discussed from the point of view of polar head and acyl chain saturation and obtained results follow the sequence: PC mix ​+ ​DPPE ​+ ​DPPG ​+ ​Chol ​> ​PC mix ​+ ​DPPE ​+ ​Chol ​> ​individual PC ​+ ​DPPE ​+ ​Chol which could be translated into the bilayer studies. Such investigation of mixed lipid is important for class of its own composition and combined results are expected to contribute in better understanding the interaction of selected lipid in proposed blend.

Interaction of cyclotide Kalata B1 protein with model cellular membranes of varied electrostatics

A uni-molecular layer of lipids at air-water interface mimicking one of the leaflets of the cellular membrane provides a simple model to understand the interaction of any foreign molecules with the membrane. Here, the interactions of protein Kalata B1 (KB1) of cyclotide family with the phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DPPG), and 1,2-distearoyl-sn-glycero-3-ethylphosphocholine chloride salt (DSEPC) have been investigated. The addition of KB1 induces a change in pressure of the lipid monolayers. The characteristic time of the change in pressure is found to be dependent on the electrostatic nature of the lipid. Even though the protein is weakly surface active, it is capable of modifying the phase behavior and elastic properties of lipid monolayers with differences in their strength and nature making the layers more floppy. The KB1-lipid interaction has been quantified by calculating the excess Gibb's free energy of interaction and the 1-anilino-8-naphthalenesulfonate (ANS) binding studies. The interaction with zwitterionic DPPC and negatively charged DPPG lipids are found to be thermodynamically favorable whereas the protein shows a weaker response to positively charged DSEPC lipid. Therefore, the long ranged electrostatic is the initial driving force for the KB1 to recognize and subsequently attach to a cellular membrane. Thereafter, the hydrophobic region of the protein may penetrate into the hydrophobic core of the membrane via specific amino acid residues.

Neutron reflectometry study of the interface between two immiscible electrolyte solutions: Effects of electrolyte concentration, applied electric field, and lipid adsorption.

The properties of lipid monolayers at the interface between two immiscible electrolyte solutions (ITIES) have attracted much attention over the last 30 years. This is mainly because of the biological relevance of lipids and the possibility of controlling ion and electron transfer across one leaflet of a cellular membrane. In the last decade, the electrochemical characterization of phosphatidylcholine (PC) adsorbed monolayers at ITIES suggested that the transfer of aqueous cations across the interface is facilitated by the complexation of aqueous cations with the PC zwitterionic head groups, followed by the depletion of lipids from the interface. In this work, we present a study on the effects of applied electric fields and electrolyte concentration on the interfacial structure of the ITIES by combining neutron reflectometry (NR) and electrochemical characterization techniques including the effects of an adsorbed lipid layer. Our results confirm that lipid depletion occurs as cations are transferred. However, we found that the presence of lipids favors the intermixing of the two-liquid phases on a length scale of a few tens of nanometers. To our knowledge, this has been the first NR-electrochemistry study of the ITIES. We believe that our findings could open new possibilities for coupling bioelectrochemical characterization and scattering based techniques at the liquid-liquid interface.

Real-time monitoring of the effect of carbon nanoparticles on the surface behavior of DPPC/DPPG Langmuir monolayer

Air pollution has become increasingly serious. Fine particulate matter (PM2.5) is the most well-known air pollutant, which leads to some common respiratory diseases when inhaled into the lungs to certain concentration. However, there is a lack of research on the process of dynamically monitoring the real-time effect of nanoparticles on the pulmonary surfactant monolayer. In this study, the DPPC/DPPG monolayer is prepared by the Langmuir method to simulate the lung surfactant monolayer during respiration and the carbon nanoparticles are introduced to the monolayer under different surface pressures to simulate the real dynamic process of inhaling nanoparticles during breathing. The effect of carbon nanoparticles on the surface behavior of DPPC/DPPG monolayer in real-time was examined in details by a combination of surface pressure (π)-area (A) isotherms, compressibility modulus (Cs-1)-surface pressure (π) isotherms and the Brewster angle microscopy (BAM). The results have shown that the introduction of carbon nanoparticles under different surface pressures affects the properties of lipid monolayers. The added carbon nanoparticles under lower surface pressure are easy to penetrate the lipid molecules to inhibit monolayer phase transition. When the carbon nanoparticles are introduced to the monolayer under higher surface pressure, they tend to self-aggregate to reduce the monolayer stability rather than interact with lipid tail chains. This work not only confirms the exotic hydrophobic carbon nanoparticles retain in the DPPC/DPPG monolayer irreversibly and affect the surface behavior of monolayer during respiration, but also opens a new idea for real-time monitoring of the effects of PM2.5 on lung health.

Mechanical Properties of DPPC-POPE Mixed Langmuir Monolayers.

The mechanical properties of lipid monolayers and their responses to shear and compression stresses play an important role in processes such as breathing and eye blinking. We studied the mechanical properties of Langmuir monolayers of a model mixture, composed of an unsaturated lipid, 1-palmitoyl-2-oleoyl-sn-glycero-phosphoethanolamine (POPE), and a saturated lipid, 1,2-dipalmitoyl-sn-glycero-phosphocholine (DPPC). We performed isothermal compressions and sinusoidal shear deformations of these mixed monolayers. Also, the different phases were observed with Brewster angle microscopy. We found that the mechanical behavior is affected by the miscibility of both lipids. In the two-phase region, the compression elastic modulus increases with the amount of the LC phase but does not follow the predictions of a simple effective medium model. The discrepancies arise from the fact that, upon compression, the domains grow at a rate faster than the compression rate but not fast enough to reach thermodynamic equilibrium. Before reaching the LC phase, domain percolation is observed and compression and shear moduli become equal to those of the pure LC phase. Most of the monolayers behave as viscoelastic fluids at the frequencies investigated. A minimum in the compression modulus and shear viscosity was observed for mixtures close to equimolar composition, with the minimum being accompanied by a change in domain shapes.

Microdomain Structure and Mechanical Properties of Lipid Monolayers Mimicking Red Blood Cell Membranes Under Oxidative Stress

Microdomain Structure and Mechanical Properties of Lipid Monolayers Mimicking Red Blood Cell Membranes Under Oxidative Stress

Interactions of Engineered Silica Nanoparticles with Lipid Monolayers and Bilayers

The widespread use of engineered nanomaterials has led to significant interest in studying their interactions with the cell plasma membrane. Mechanistic studies of nanoparticle-membrane interactions have generally used lipid monolayers and lipid vesicles as membrane models. However, it is unclear whether differences in membrane models might affect their interactions with nanoparticles. The current study aimed to elucidate the interactions of engineered silica nanoparticles (105 nm) with lipid monolayers and vesicles in order to provide information on the potential differences between these commonly used models in their interactions with nanomaterials. Lipid monolayers and vesicles, both comprised of equimolar concentrations of sphingomyelin, cholesterol, and dioleoylphosphatidylcholine, were used as membrane models. Silica nanoparticles, surface-modified with hydroxyl, amine, and polyethylene glycol (PEG) with molecular weights of 2K, 5K, and 20K Da, were used as particle model. A Langmuir-Wilhelmy apparatus was used to study nanoparticle effects on lipid monolayer interfacial properties. Atomic force microscopy (AFM) was used to examine nanoparticle effects on lipid topography. Nanoparticle effects on vesicle integrity were monitored using a vesicle leakage assay with carboxylfluorescein (CF) as the fluorescent probe. Nanoparticle effects on lipid monolayers were dependent on particle surface properties. Hydroxyl- and amine-modified nanoparticles did not affect the interfacial properties of lipid monolayers while PEGylated nanoparticles induced a reduction in surface tension. AFM experiments demonstrated that only PEGylated nanoparticles penetrated into the lipid monolayer. Nanoparticles effects on lipid bilayers were different from monolayers. Hydroxyl-, amine-, and PEG 20K-modified nanoparticles disrupted the vesicles, causing significant CF leakage, while PEG 2K- and PEG 5K-modified nanoparticles did not cause vesicle disruption. These results demonstrate that lipid monolayers and vesicles interact differently with engineered nanoparticles, suggesting that it might not be possible to translate the results from one model to another.

Improved prediction of bilayer and monolayer properties using a refined BMW-MARTINI force field

Coarse-grained (CG) models allow enlarging the size and time scales that are reachable by atomistic molecular dynamics simulations. A CG force field (FF) for lipids and amino acids that possesses a polarizable water model has been developed following the MARTINI parametrization strategy, the BMW-MARTINI [1]. We tested the BMW-MARTINI FF capability to describe some structural and thermodynamical properties of lipid monolayers and bilayers. We found that, since the surface tension values of oil/water interfaces calculated with the model are not correct, compression isotherms of lipid monolayers present artifacts. Also, this FF predicts DPPC and DAPC bilayers to remain in the Lα phase at temperatures as low as 283K, contrary to the expected from their experimental Tm values. Finally, simulations at constant temperature of bilayers of saturated lipids belonging to PC homologous, showed an increase in the mean molecular area (Mma) upon increasing the chain length, inversely to the experimental observation.We refined BMW-MARTINI FF by modifying as few parameters as possible in order to bring simulated and experimental measurements closer. We have also modified structural parameters of the lipid geometry that do not have direct influence in global properties of the bilayer membranes or monolayers, but serve to approach the obtained CG geometry to atomistic reference values. The refined FF is able to better reproduce phase transition temperatures and Mma for saturated PC bilayers than BMW-MARTINI and MARTINI FF. Finally, the simulated surface pressure-Mma isotherms of PC monolayers resemble the experimental ones and eliminate serious artifacts of previous models.

The influence of environmentally friendly pesticide ⿿ Eucalyptol ⿿ alone and in combination with terpinen-4-ol ⿿ on model bacterial membranes

Eucalyptus oil is known from antimicrobial properties and due to natural origin and low toxicity it can be applied as a green pesticide. This plant extract is a mixture of compounds among which Eucalyptol prevails strongly over the number of minor components including terpinen-4-ol. However, antimicrobial effect of total extract is stronger than the effect of its major component. The mechanism of action of essential oils involves their incorporation into membrane of pathogenic cell. To investigate the effect of eucalyptus oil components on bacterial membrane in this work the influence of Eucalyptol on model lipid systems of various compositions was studied and then compared with the effect induced by the mixture of Eucalyptol and terpinen-4-ol. The analysis of the lipid monolayer properties modified by the presence of the essential oils evidenced that Eucalyptol reduces the lipids packing and decreases membrane rigidity, however, its influence is determined both by the concentration of this terpene and by the composition of lipid film. As indicated the results of calculation of the surface excess values Eucalyptol molecules tend to adsorb at the interface, however, at higher concentration in the subphase, the molecules undergo desorption at membrane-related surface pressure. By mixing of Eucalyptol and terpinen-4-ol stronger monolayer fluidization and destabilization can be achieved as well as stronger adsorption, even at higher surface pressures, occurs. However, these effects are also determined by the composition of model system. The collected data suggest that these compounds may act by penetrating bacterial membrane. Moreover, it can be proposed that although Eucalyptol is a major component of the Eucalyptus oil the antimicrobial effect of total extract is generated also by a minor component. This may explain stronger antimicrobial effect of Eucalyptus oil as compared to sole Eucalyptol confirmed in biological studies.

Computer simulations of lung surfactant

Lung surfactant lines the gas-exchange interface in the lungs and reduces the surface tension, which is necessary for breathing. Lung surfactant consists mainly of lipids with a small amount of proteins and forms a monolayer at the air–water interface connected to bilayer reservoirs. Lung surfactant function involves transfer of material between the monolayer and bilayers during the breathing cycle. Lipids and proteins are organized laterally in the monolayer; selected species are possibly preferentially transferred to bilayers. The complex 3D structure of lung surfactant and the exact roles of lipid organization and proteins remain important goals for research. We review recent simulation studies on the properties of lipid monolayers, monolayers with phase coexistence, monolayer–bilayer transformations, lipid–protein interactions, and effects of nanoparticles on lung surfactant. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Molecular Packing in Langmuir Monolayers Composed of a Phosphatidylcholine and a Pyrene Lipid.

Pyrene lipids are useful tools to investigate membrane organization and intracellular lipid trafficking. The molecular interactions controlling the organization of lipid monolayers composed of a cationic amphiphile tagged with a pyrene residue and a saturated or unsaturated phospholipid, namely, 1,2-dimyristoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine, were investigated by Langmuir trough isotherms to understand how the molecular structure of the components and their relative amount affect the physicochemical properties of lipid monolayers. The obtained results show that the cationic headgroups and unsaturation of hydrophobic chains strongly affect the organization of the lipid monolayer as a function of the amount of components. On the other hand, the presence of the pyrene moiety does not seem to have a marked influence on the interaction within lipid assembly.

The Rheological Properties of Lipid Monolayers Modulate the Incorporation of l-Ascorbic Acid Alkyl Esters.

In this work, we tested the hypothesis that the incorporation of amphiphilic drugs into lipid membranes may be regulated by their rheological properties. For this purpose, two members of the l-ascorbic acid alkyl esters family (ASCn) were selected, ASC16 and ASC14, which have different rheological properties when organized at the air/water interface. They are lipophilic forms of vitamin C used in topical pharmacological preparations. The effect of the phase state of the host lipid membranes on ASCn incorporation was explored using Langmuir monolayers. Films of pure lipids with known phase states have been selected, showing liquid-expanded, liquid-condensed, and solid phases as well as pure cholesterol films in liquid-ordered state. We also tested ternary and quaternary mixed films that mimic the properties of cholesterol containing membranes and of the stratum corneum. The compressibility and shear properties of those monolayers were assessed in order to define its phase character. We found that the length of the acyl chain of the ASCn compounds induces differential changes in the rheological properties of the host membrane and subtly regulates the kinetics and extent of the penetration process. The capacity for ASCn uptake was found to depend on the phase state of the host film. The increase in surface pressure resultant after amphiphile incorporation appears to be a function of the capacity of the host membrane to incorporate such amphiphile as well as the rheological response of the film. Hence, monolayers that show a solid phase state responded with a larger surface pressure increase to the incorporation of a comparable amount of amphiphile than liquid-expanded ones. The cholesterol-containing films, including the mixture that mimics stratum corneum, allowed a very scarce ASCn uptake independently of the membrane diffusional properties. This suggests an important contribution of Cho on the maintenance of the barrier function of stratum corneum.

Role of Confined Water on the Compressibility Modulus of Lipid Monolayers

The studies on lipid monolayer properties have been done by compressing the lipid spread in a large water surface up to its collapse. In this procedure, a surface pressure vs area per molecule isotherm can be obtained. In this methodology, it is assumed that the lipids spread at huge areas are considered as being in a gas phase and when they are compressed at a temperature below the critical one (the transition temperature) a condensed phase appears. The thermodynamic treatment is compared with a real gas in which the condensation is analyzed as a consequence of the manifestation of the intermolecular forces between the lipids. In other words, lipid interacts between them as gas particles in vacuum.A more realistic approach to account on monolayer behavior is that lipids, even at large areas, are in contact with the water phase. Upon compression, the energy input is not merely used to make work of compression but to overcome the friction of hydrated lipid molecules with the water (stationary) phase. Thus, the lipids drag water during its compression, force its reorganization and/or a displacement work can occur. With this picture, the whole thermodynamic of lipid membrane is reevaluated. For this purpose, we analyzed the compression of the monolayer from a state identified as a free-force monolayer state up to the collapse for lipids with different hydration degrees (mainly PCs and PEs). The comparison of these compression curves with the branch of the isotherm obtained by compressing the lipids from the gaseous state allow to conclude that the water arrangements, mainly confined water and hydration water, determines the hysteresis and synergistic effects of membrane response.

The physical properties of lipid monolayers and bilayers containing calixarenes sensitive to cytochrome c

We studied physical properties of the monolayers and bilayer lipid membranes (BLM) formed by calix[6]arene carboxylic acid derivative (CX) and its mixtures with diphytanoylphosphatidylcholine (DPhPC) by means of measurement surface pressure, surface dipole potential and electrostriction. CX forms stable monolayers at an air-water interface and complexes in mixed monolayers contained DPhPC. Calixarenes increase the elastic moduli of lipid monolayers as well as BLM. Cytochrome c (cyt c) specifically binds to CX by incorporation of the amino groups of lysine residues at the protein surface. This binding affected the physical properties of CX monolayers depending on their initial surface pressure. Addition of cyt c into the water subphase induced increase of surface pressure of CX monolayers at relatively low initial pressure (15 mN/m) when monolayer was in liquid expanded state (LE). This may be due interaction of positively charged cyt c with negatively charged carboxylic groups of CX and also by its penetration into the air-water interface. However, much subtle changes were observed for higher initial surface pressure (20 and 35 mN/m) when monolayer is in liquid condensed (LC) and solid (S) state, respectively. Lysine induced substantially lower changes in surface pressure in comparison with that of cyt c.

Molecular View of Phase Coexistence in Lipid Monolayers

We used computer simulations to study the effect of phase separation on the properties of lipid monolayers. This is important for understanding the lipid-lipid interactions underlying lateral heterogeneity (rafts) in biological membranes and the role of domains in the regulation of surface tension by lung surfactant. Molecular dynamics simulations with the coarse-grained MARTINI force field were employed to model large length (~80 nm in lateral dimension) and time (tens of microseconds) scales. Lipid mixtures containing saturated and unsaturated lipids and cholesterol were investigated under varying surface tension and temperature. We reproduced compositional lipid demixing and the coexistence of liquid-expanded and liquid-condensed phases as well as liquid-ordered and liquid-disordered phases. Formation of the more ordered phase was induced by lowering the surface tension or temperature. Phase transformations occurred via either nucleation or spinodal decomposition. In nucleation, multiple domains formed initially and subsequently merged. Using cluster analysis combined with Voronoi tessellation, we characterized the partial areas of the lipids in each phase, the phase composition, the boundary length, and the line tension under varying surface tension. We calculated the growth exponents for nucleation and spinodal decomposition using a dynamical scaling hypothesis. At low surface tensions, liquid-ordered domains manifest spontaneous curvature. Lateral diffusion of lipids is significantly slower in the more ordered phase, as expected. The presence of domains increased the monolayer surface viscosity, in particular as a result of domain reorganization under shear.

Simulation Study of Domains in Lipid Monolayers

Using computer simulations we investigated the effect of coexisting phases on the properties of lipid monolayers. This is important for understanding the role of domains in surface activity of monolayers in general, and specifically in regulation of surface tension by lung surfactant. Molecular dynamics simulations with the coarse-grained force field MARTINI were employed to study monolayers approaching 100 nm in lateral dimension on the time scale of tens of microseconds. Lipid mixtures containing saturated and unsaturated lipids and cholesterol were studied under varying surface tension (0-40 mN/m) and temperature (270-323 K). Compositional lipid de-mixing and coexistence of liquid-expanded and liquid-condensed phases as well as liquid-ordered and liquid-disordered phases was reproduced. Formation of the more ordered phase induced by lowering the surface tension or temperature occurred via either nucleation and growth or spinodal decomposition. Using cluster analysis combined with Voronoi tessellation we characterized in detail the properties of the phases and kinetics of domain growth. Area fraction and lipid composition of each phase, and boundary length were obtained as a function of temperature and surface tension. We also simulated lipid monolayers connected to bilayer reservoirs in water, which are relevant for the function of lung surfactant. The distribution of phases between the monolayers and bilayers, and the effect of domains on monolayer stability were determined.

Curvature Elastic Properties of Phosphatidylethanolamines with Mono-and Polyunsaturated Hydrocarbon Chains - An NMR and X-Ray Diffraction Study

We report bending elastic coefficients and spontaneous radii of lipid monolayer curvature for a series of phosphatidylethanolamines (PE) with mono- and polyunsaturated hydrocarbon chains. Monolayer curvature in the inverse hexagonal (HII) phase was altered by osmotic stress exerted by polyethylene glycol solutions. Water content was measured by 1H-MAS-NMR and monolayer radii measured by x-ray diffraction. Interestingly, curvature elastic properties of lipid monolayers composed of PE with two oleic acid chains (DOPE) are very close to properties of mixed-chain PEs with a saturated stearoyl chain at sn-1 and a polyunsaturated docosahexaenoyl (22:6n-3) or docosapentaenoyl (22:5n-6) chain at sn-2.

Surface Properties and Behavior of Lipid Extracts from Plasma Membranes of Cells Cultured as Monolayer and in Tissue-Like Conditions

The differences in the surface active properties of native lipids extracted from plasma membranes of cells cultured as a monolayer and in three-dimensional (3D) matrix were investigated. This experimental model was chosen because most of the current knowledge on cellular physiological processes is based on studies performed with conventional monolayer two-dimensional (2D) cell cultures, where cells are forced to adjust to unnaturally rigid surfaces that differ significantly from the natural matrix surrounding cells in living organisms. Differences between monolayer and 3D cells were observed in the lipid composition of plasma membranes and especially in the level of the two major microdomain-forming lipids--sphingomyelin (SM) and cholesterol, which were significantly elevated in 3D cells. The obtained results showed that culturing of cells in in vivo-like environment affected the surface active properties of plasma membrane lipids at interfaces which might influence certain membrane-associated interface processes. The detected differences in the lipid levels in 2D and 3D cell extracts affected significantly the behavior of the model lipid monolayers at the air-water interface (Langmuir monolayers) which resulted in different values of the monolayer equilibrium (gamma(eq)) and dynamic (gamma(max), gamma(min)) surface tension and surface potential. Compensation of the SM content in extracts of 2D cell cultures up to a level close to the one measured in 3D cells approximated the monolayer properties to the values observed for 3D cells. These results implied that the interactions between the cells and the surrounding medium affected the level of plasma membrane SM and other lipids, which had a strong impact on the surface properties of lipid monolayers, such as gamma(eq), gamma(max), and gamma(min), the compression/decompression curve shape, the hysteresis area during cycling of the monolayers, etc. We suggest that the elevated content of SM observed in plasma membranes of 3D fibroblasts could be responsible for an increased rigidity and possibly reduced permeability of cells cultured in 3D environment. The current results provide useful information that should be taken into account in the interpretation of the membrane physico-chemical properties of cells cultured under different conditions.

Molecular dynamics study of the effect of cholesterol on the properties of lipid monolayers at low surface tensions

We have investigated the effect of cholesterol concentration on the properties of lipid monolayers at air/water interfaces at low surface tensions. This is of interest for understanding the properties and function of lung surfactant monolayers. Lung surfactant lines the gas exchange interface in the lungs and dramatically reduces the surface tension, thereby preventing lung collapse and decreasing the work associated with breathing. Changes in the lipid composition of lung surfactant, particularly an increase in cholesterol concentration, can result in inhibition of its function, as in the case of acute respiratory distress syndrome. We have used molecular dynamics simulations with both atomistic and coarse-grained force fields to study lipid monolayers containing DPPC, POPG and cholesterol in molecular ratios of 8:2:1 and 4:1:4 at surface tensions of 40, 20 and 0 mN m(-1) at 310 K. These mixtures model the lipid component of lung surfactant at normal (approximately 9%) and elevated (approximately 44%) cholesterol concentration. We have characterised the structural and dynamic properties of these monolayers and calculated the free energy for transfer of each lipid from its equilibrium position in the monolayer into water and into air (vacuum). The results show that at low surface tensions an increase in cholesterol concentrations leads to formation of a liquid-condensed phase with low area compressibility, which is in agreement with experimental findings.