The equilibrium of phosphatidylcholine–cholesterol in monolayers at the air/water interface

The Effect of Dextran Sulfate-as Model Glycosaminoglycan Analogue-on Membrane Lipids: DPPC, Cholesterol, and DPPC-Cholesterol Mixture. The Monolayer Study

Glycosaminoglycans (GAGs) are essential components of the extracellular matrices (ECMs) located on the outer surface of cellular membranes. They belong to the group of polysaccharides involved in diverse biological processes acting on the surface and across natural lipid membranes. Recently, particular attention has been focused on possible role of GAGs in the amyloid deposits. The amyloid formation is related to a disorder in protein folding, causing that soluble-in normal conditions-peptides become deposited extracellularly as insoluble fibrils, impairing tissue structure and its function. One of the hypothesis holds that GAGs may inhibit amyloid formation by interacting with the lipid membrane by blocking the accumulation of protein aggregates on the membrane surface. Although the biophysical properties of GAGs are described rather well, little is known about the nature of association between these polysaccharides and components of natural cell membranes. Therefore, a study of GAGs influence on membrane lipids is of particular importance. The aim of the present work is to get insight into the effect of hydrophilic dextran sulfate (DS)-that can be considered as GAG analogue-on membrane lipids organization. This study was based on examining interactions between DS sodium salt of molecular weight equal to about 40 kDa (DS40), dissolved in water subphase, and a model membrane, mimicked as Langmuir monolayer, formed by representative natural membrane lipids: cholesterol and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as well as their mixtures. Due to the fact that calcium ions in excess may accumulate in the lipid membrane, attracting high molecular weight molecules to their surface, the influence of calcium ions present in the subphase on the DS40 activity has also been examined. It has been found that negatively charged DS, forming a sublayer underneath the monolayer, barely interacts with membrane lipids; however, in the presence of calcium ions the electrostatic interactions between DS40 and lipid membrane are significantly enhanced, leading to the formation of network-like crystalline structures at the surface of model membrane, which can prevent incorporation and interaction with other extracellular molecules, e.g., proteins.

Cholesterol and Cardiolipin Importance in Local Anesthetics-Membrane Interactions: The Langmuir Monolayer Study

Local anesthetics (LAs) are known to act on membrane level; however, the molecular mechanism of their activity is still not fully understood. One hypothesis holds that these drugs can incorporate into lipid membrane of nerve cells and in this way change conformation of channel proteins responsible for transport of sodium ions. However, the action of anesthetics is not limited to nerve cells. These drugs also affect other types of cells and organelles, causing severe side effects. In this paper, we applied Langmuir monolayers—as model of cellular membranes—and investigated interactions between selected amide-type local anesthetics (lidocaine prilocaine, mepivacaine and ropivacaine, in the form of hydrochlorides) and lipid components of natural membranes: cholesterol, POPC and cardiolipin (CL) and their mixtures (POPC/cholesterol and POPC/CL/cholesterol), which can serve as simplified models of nerve cell membranes, erythrocytes, and mitochondria. The influence of the drug was monitored by registering the surface pressure (π) as a function of surface area per molecule (A) in a monolayer in the presence of the drug in the subphase. The structure of lipid monolayers on subphases containing and devoid of the studied drugs were visualized with Brewster angle microscopy (BAM). Langmuir monolayer studies complemented with surface visualization technique reveal the expansion and fluidization of lipid monolayers, with the most pronounced effect observed for cardiolipin. In mixed systems, the effect of LAs was found to depend on cholesterol proportion. The observed fluidization of membranes by local anesthetics may negatively affect cells functioning and therefore can explain side effects of these drugs both on the cardiovascular and nervous systems.

Lipogels for Encapsulation of Hydrophilic Proteins and Hydrophobic Small Molecules

Lipid-polymer hybrid materials have the potential to exhibit enhanced stability and loading capabilities in comparison to parent liposome or polymer materials. However, complexities lie in formulating and characterizing such complex nanomaterials. Here we describe a lipid-coated polymer gel (lipogel) formulated using a single-pot methodology, where self-assembling liposomes template a UV-curable polymer gel core. Using fluorescently labeled lipids, protein, and hydrophobic molecules, we characterized their formation, purification, stability, and encapsulation efficiency via common instrumentation methods such as dynamic light scattering (DLS), matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS), UV-vis spectroscopy, fluorescence spectroscopy, and single-particle total internal reflection fluorescence (TIRF) microscopy. In addition, we confirmed that these dual-guest-loaded lipogels are stable in solution for several months. The simplicity of this complete aqueous formation and noncovalent dual-guest encapsulation holds potential as a tunable nanomaterial scaffold.

Assessing gastric toxicity of xanthone derivatives of anti-inflammatory activity using simulation and experimental approaches

Xanthones are tricyclic compounds of natural or synthetic origin exhibiting a broad spectrum of therapeutic activities. Three synthetic xanthone derivatives (KS1, KS2, and KS3) with properties typical for nonsteroidal anti-inflammatory drugs (NSAID) were objects of the presented model study. NSAIDs are in common use however; several of them exhibit gastric toxicity predominantly resulting from their direct interactions with the outermost lipid layer of the gastric mucosa that impair its hydrophobic barrier property. Among the studied xanthones, gastric toxicity of only KS2 has been determined in previous pharmacological studies, and it is low. In this study, carried out using X-ray diffraction and computer simulation, a palmitoyloleoylphosphatidylcholine-cholesterol bilayer (POPC-Chol) was used as a model of a hydrophobic layer of lipids protecting gastric mucosa as POPC and Chol are the main lipids in human mucus. X-ray diffraction data were used to validate the computer model. The aim of the study was to assess potential gastric toxicity of the xanthones by analysing their atomic level interactions with lipids, ions, and water in the lipid bilayer and their effect on the bilayer physicochemical properties. The results show that xanthones have small effect on the bilayer properties except for its rigidity whereas their interactions with water, ions, and lipids depend on their protonation state and for a given state, are similar for all the xanthones. As gastric toxicity of KS2 is low, based on MD simulations one can predict that toxicity of KS1 and KS3 is also low.

The influence of an antitumor lipid - erucylphosphocholine - on artificial lipid raft system modeled as Langmuir monolayer

Outer layer of cellular membrane contains ordered domains enriched in cholesterol and sphingolipids, called 'lipid rafts', which play various biological roles, i.e., are involved in the induction of cell death by apoptosis. Recent studies have shown that these domains may constitute binding sites for selected drugs. For example alkylphosphocholines (APCs), which are new-generation antitumor agents characterized by high selectivity and broad spectrum of activity, are known to have their molecular targets located at cellular membrane and their selective accumulation in tumor cells has been hypothesized to be linked with the alternation of biophysical properties of lipid rafts. To get a deeper insight into this issue, interactions between representative APC: erucylphosphocholine, and artificial lipid raft system, modeled as Langmuir monolayer (composed of cholesterol and sphingomyelin mixed in 1:2 proportion) were investigated. The Langmuir monolayer experiments, based on recording surface pressure-area isotherms, were complemented with Brewster angle microscopy results, which enabled direct visualization of the monolayers structure. In addition, the investigated monolayers were transferred onto solid supports and studied with AFM. The interactions between model raft system and erucylphosphocholine were analyzed qualitatively (with mean molecular area values) as well as quantitatively (with ΔG(exc) function). The obtained results indicate that erucylphosphocholine introduced to raft-mimicking model membrane causes fluidizing effect and weakens the interactions between cholesterol and sphingomyelin, which results in phase separation at high surface pressures. This leads to the redistribution of cholesterol molecules in model raft, which confirms the results observed in biological studies.

Cyclosporin A in Membrane Lipids Environment: Implications for Antimalarial Activity of the Drug--The Langmuir Monolayer Studies

Cyclosporin A (CsA), a hydrophobic cyclic peptide produced by the fungus Tolypocladium inflatum, is well known for its high efficiency as an immunosuppressor for transplanted organs and anti-inflammatory properties; however, it is also active as antiparasitic (antimalarial) drug. Antimalarial mechanism of CsA action lacks a detailed understanding at molecular level. Due to a high lipophilicity of CsA, it is able to interact with lipids of cellular membrane; however, molecular targets of this drug are still unknown. To get a deeper insight into the mode of antimalarial activity of CsA, it is of utmost importance to examine its interactions with membrane components. To reach this goal, the Langmuir monolayer technique, which serves as a very useful, easy to handle and controllable model of biomembranes, has been employed. In this work, the interactions between CsA and main membrane lipids, i.e., cholesterol (Chol), 2-oleoyl-1-palmitoyl-3-phosphocholine (POPC), and sphingomyelin (SM), have been investigated. Attractive interactions are observed only for CsA mixtures with SM, while repulsive forces occur in systems containing remaining membrane lipids. Taking into consideration mutual interactions between membrane lipids (Chol-SM; Chol-POPC and SM-POPC), the behavior of CsA in model erythrocyte membrane of normal and infected cells has been analyzed. Our results prove strong affinity of CsA to SM in membrane environment. Since normal and parasitized erythrocytes differ significantly in the level of SM, this phospholipid may be considered as a molecular target for antimalarial activity of CsA.

The influence of pH on phosphatidylethanolamine monolayer at the air/aqueous solution interface

The dependence of the interfacial tension of a phosphatidylethanolamine (PE) monolayer on the pH of the aqueous solution has been studied. A theoretical equation is derived to describe this dependence. A simple model of the influence of pH on the phosphatidylethanolamine monolayer at the air/hydrophobic chains of PE is presented. The contributions of additive phosphatidylethanolamine forms (both interfacial tension values and molecular area values) depend on pH. The interfacial tension values and the molecular area values for PEH⁺ and PEOH⁻ forms of phosphatidylethanolamine were calculated. The assumed model was verified experimentally. The experimental results agreed with those derived from the theoretical equation in a whole range of pH values.

Comparison between cucurbiturils and β-cyclodextrin interactions with cholesterol molecules present in Langmuir monolayers used as a biomembrane model

Specific surface techniques can probe the interaction of cholesterol (Chol) with substances that are able to host and/or sequester this biomolecule, provided that the additives are properly assembled at the interface. Reports on inclusion complexes of Chol with β-cyclodextrins exist in the literature. Here we compare the interaction of β-cyclodextrin and cucurbiturils with Chol present in Langmuir phospholipid (dipalmitoylphosphatidylcholine, DPPC) monolayers, used as a biomembrane model. Cucurbiturils, CB[n], comprise macrocyclic host molecules consisting of n glycoluril units. Classic surface pressure curves, dilatational surface viscoelasticity measurements, and fluorescence emission spectra and images obtained by time-resolved fluorescence of the corresponding Langmuir-Blodgett films have shown that homologues with 5 and 6 glycoluril units, CB[5] and CB[6], do not form inclusion complexes. Higher-order homologues, such as CB[7], are likely to complex with Chol with changes in the minimum molecular areas recorded for DPPC/Chol monolayers, the fluorescence decay lifetimes, and the dilatational surface viscosities of the monolayers generated in the presence of these molecules. Moreover, we proof the removal of cholesterol from the biomimetic interface in the presence of CB[7] by means of fluorescence spectra from the subphase support of monolayers containing fluorescent-labeled Chol.

Interactions of alkylphosphocholines with model membranes-the Langmuir monolayer study

Alkylphosphocholines (APCs) belong to a class of synthetic antitumor lipids, which are new-generation anticancer agents. In contrast to traditional antitumor drugs, they do not attack the cell nucleus but, rather, the cellular membrane; however, their mechanism of action is not fully understood. This work compared the interactions of selected APCs [namely, hexadecylphosphocholine (miltefosine), octadecylphosphocholine and erucylphosphocholine] with the most important membrane lipids [cholesterol, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)] and examined their influence on a model membrane of tumor and normal cells. As a simple model of membranes, Langmuir monolayers prepared by mixing cholesterol either with a saturated phosphatidylcholine (DPPC), for a normal cell membrane, or with an unsaturated one (POPC), for a tumor cell membrane, have been applied. The APC–lipid interactions, based on experimental surface pressure (π) versus mean molecular area (A) isotherms, were analyzed qualitatively (with mean molecular area values) as well as quantitatively (with the ΔG^exc function). Strong attractive interactions were observed for mixtures of APCs with cholesterol, contrary to the investigated phosphatidylcholines, for which the interactions were found to be weak with a tendency to separation of film components. In ternary monolayers it has been found that the investigated model systems (cholesterol/DPPC/APC vs cholesterol/POPC/APC) differ significantly as regards the interactions between film-forming molecules. The results demonstrate stronger interactions between the components of cholesterol/POPC/APC monolayers compared to cholesterol/POPC film, mimicking tumor cell membranes. In contrast, the interactions in cholesterol/DPPC/APC films were found to be weaker than those in the cholesterol/DPPC system, serving as a model of healthy cell membranes, thus proving that the incorporation of APCs is, from a thermodynamic point of view, unfavorable for binary cholesterol/DPPC monolayers. It can be concluded that the composition of healthy cell membranes is a natural barrier preventing the incorporation of APCs into normal cells.

The equilibria between monovalent ions and phosphatidylcholine monolayer at the air/water interface

The effect of monovalent ion (Li⁺, Na⁺, Cs⁺) interaction with monolayers of phosphatidylcholine (lecithin, PC) was investigated at the air/water interface. We present surface tension measurements of lipid monolayers obtained using a Langmuir method as a function of monovalent ion concentration. Measurements were carried out at 22 °C using a Teflon trough and a Nima 9000 tensiometer. Interactions between lecithin and monovalent ions result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants and area occupied by one molecule of PC–monovalent ion complexes (PC⁻Me⁺).

Phosphatidylcholine — Mg2+ equilibria in a monolayer at the air/water interface

Abstract The interaction between Mg2+ and a phosphatidylcholine (lecithin, L) monolayer at the air/water interface was investigated. Surface tension measurements (Langmuir method) of phosphatidylcholine monolayers as a function of Mg2+ concentration were carried out at 22°C using a Teflon trough and a Nima 9000 tensiometer. Interactions between phosphatidylcholine and Mg2+ result in significant deviations from additivity. An equilibrium theory was developed to obtain the stability constants and areas occupied by one molecule of LMg+ and L2Mg. The stability constants were K 1 = 9.95×102 m2 mol−1 and K 2 = 3.87×104 m2 mol−1. The area occupied by LMg+ is 77 Å2 molecule−1, while that occupied by L2Mg is 109 Å2 molecule−1. The Gibbs free energies of complexation for LMg+ and L2Mg are −16.91 ± 0.51 and −25.88 ± 0.76 kJ mol−1. Graphical abstract

The equilibria of lipid-k(+) ions in monolayer at the air/water interface

The effect of K⁺ ion interaction with monolayers of phosphatidylcholine (lecithin, PC) or cholesterol (Ch) was investigated at the air/water interface. We present surface tension measurements of lipid monolayers obtained using a Langmuir method as a function of K⁺ ion concentration. Measurements were carried out at 22°C using a Teflon trough and a Nima 9000 tensiometer. Interactions between lecithin and K⁺ ions or Ch and K⁺ ions result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants and area occupied by one molecule of lipid–K⁺ ion complex (LK⁺). The stability constants for lecithin–K⁺ ion (PCK⁺) complex, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ K_{{{\text{PCK}}^{ + } }} = { 3}. 2 6\times 10^{ 2} {\text{dm}}^{ 3} \,{\text{mol}}^{ - 1} $$\end{document}, and for cholesterol–K⁺ ion (ChK⁺) complex, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ K_{{{\text{ChK}}^{ + } }} = { 1}.00 \times 10^{ 3} {\text{dm}}^{ 3} \,{\text{mol}}^{ - 1} $$\end{document}, were calculated by inserting the experimental data. The value of area occupied by one PCK⁺ complex is 60 Å² molecule⁻¹, while the area occupied by one ChK⁺ complex is 40.9 Å² molecule⁻¹. The complex formation energy (Gibbs free energy) values for the PCK⁺ and ChK⁺ complexes are −14.18 ± 0.71 and −16.92 ± 0.85 kJ mol⁻¹, respectively.

Comparative model studies of gastric toxicity of nonsteroidal anti-inflammatory drugs

A high percentage of people treated with a long-term nonsteroidal anti-inflammatory drug (NSAID) therapy suffer NSAID-induced gastrointestinal-tract-related side effects. A current hypothesis states that the side effects are related to the topical action of NSAID molecules on gastric mucus that lowers its resistance to luminal acid. The main lipids in human mucus are palmitoyloleoylphosphatidylcholine (POPC) and cholesterol (Chol). In this study, both X-ray diffraction and molecular dynamics (MD) simulation methods were employed to investigate the effects of selected NSAIDs in protonated and deprotonated states on the structural parameters of a POPC-Chol bilayer. The drugs were three commonly used NSAIDs with apparently different gastric toxicity: ketoprofen (KET), aspirin (ASP), and piroxicam (PXM). Both methods revealed that the effects of the NSAIDs on the POPC-Chol bilayer parameters were moderate and only slightly differentiated among the drugs. Much larger differences among the drugs were noticed in their interactions with interfacial water and Na(+) as well as with the polar groups of POPC and Chol, mainly via H-bonds. Of the three NSAIDs, KET interacted with POPC and water the most extensively, whereas ASP interacted with Chol and Na(+) more than did the other two. Interactions of PXM with POPC and Chol polar groups as well as with water and Na(+) were limited.

Monolayers of mixture of alkylaminomethyl rutin and lecithin at the air/water interface

A compound of flavonol-based biosurfactant, as C(8)-substituted alkylaminomethyl rutin (DAMR) for a potential pharmaceutical or agrochemical use, was prepared experimentally. The surface behavior of DAMR and its mixture with lecithin from soybean (SL) had been studied. DAMR, regarded as a pseudo-amphoteric character, exhibits both liquid-condensed (LC) and liquid-expanded (LE) phases while SL is in the form of the LE phase only. The phase parameters of DAMR (including A(limt), pi(coll)) are observed to strongly depend on both the subphase temperature and the pH, which regulate the degree of ionization. In addition, the observed positive deviation calculated from excess Gibbs free energies of the DAMR-SL system suggests a repulsive interaction between DAMR and SL at all X(DAMR) values. Also the interaction parameter is found to increase linearly with surface pressure, regardless of composition. Notably, the relationship of logarithmic activity coefficient vs. X(DAMR)(2) reveals that the molecular interaction of DAMR-SL can be adequately simulated using a simple regular mixture model. Importantly, lower C(S)(-1) values of this mixture than those with pure DAMR and SL denote weak elasticity of mixed monolayers with values of X(DAMR) of 0.2-0.8, indicating that the direct addition of DAMR may exert a somewhat adverse influence on SL membranes.

Effect of cholesterol on the structure of a phospholipid bilayer

Cholesterol plays an important role in regulating the properties of phospholipid membranes. To obtain a detailed understanding of the lipid-cholesterol interactions, we have developed a mesoscopic water-lipid-cholesterol model. In this model, we take into account the hydrophobic-hydrophilic interactions and the structure of the molecules. We compute the phase diagram of dimyristoylphosphatidylcholine-cholesterol by using dissipative particle dynamics and show that our model predicts many of the different phases that have been observed experimentally. In quantitative agreement with experimental data our model also shows the condensation effect; upon the addition of cholesterol, the area per lipid decreases more than one would expect from ideal mixing. Our calculations show that this effect is maximal close to the main-phase transition temperature, the lowest temperature for which the membrane is in the liquid phase, and is directly related to the increase of this main-phase transition temperature upon addition of cholesterol. We demonstrate that no condensation is observed if we slightly change the structure of the cholesterol molecule by adding an extra hydrophilic head group or if we decrease the size of the hydrophobic part of cholesterol.

What happens if cholesterol is made smoother: importance of methyl substituents in cholesterol ring structure on phosphatidylcholine-sterol interaction

Although sterols constitute one of the most important molecular species in cells, the reasons for their structure-function relationships in lipid membranes are not well understood. The main objective of this work is to elucidate the recently suggested possibility that the ordering and condensing effects of sterols on phospholipid membranes are related to the smoothness of a sterol. We focus on cholesterol, which has two methyl groups attached to its beta-face, and compare its properties to those of demethylated cholesterol (Dchol), from which the two methyl groups have been removed. Atomic-scale molecular dynamics simulations of lipid membranes comprised of saturated lipids and sterols, either cholesterol or Dchol, provide compelling evidence that despite its smoother structure, the ordering and condensing effects of Dchol are less effective than those of cholesterol. The ordering capability of both cholesterol and Dchol is highly asymmetric with respect to their ring structure, but whereas cholesterol favors the alpha-face, Dchol favors the beta-face. The origin and implications of this difference are analyzed in detail. The picture that emerges from this study supports a view that the two methyl groups at the steroid ring system of cholesterol play an important role in cholesterol-lipid interactions by reducing sterol tilt in the bilayer and hence allowing for an optimal orientation for cholesterol.

Interactions between dialkyldimethylammonium bromides (DXDAB) and sterols--a monolayer study

Langmuir monolayers of cholesterol/ergosterol and dialkyldimethylammonium bromides (DXDABs) differing in alkyl chain length-14 (DTDAB), 16 (DHDAB), and 18 (DODAB)-spread at the air/water interface are examined. All the systems investigated are found to be nonideal and miscible. Negative values of the total free energy of mixing, proving film stability in the whole range of compositions and surface pressures, are observed for all the studied mixtures except for DTDAB/cholesterol. The strength of interactions, quantified with DeltaG(Exc) values, was found to be of the same order for mixtures of cholesterol/ergosterol and DHDAB or DODAB. Differences occurring for the mixtures of DTDAB with sterols indicate the affinity of DTDAB to ergosterol in contrast to cholesterol.

Cholesterol effects on a mixed-chain phosphatidylcholine bilayer: a molecular dynamics simulation study

A molecular dynamics simulation of a mono-cis-unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine bilayer containing approximately 22 mol% of cholesterol (POPC-Chol) was carried out for 15 ns. An 8-ns trajectory was analysed to determine the effects of Chol on the membrane properties and compare it with that on the fully saturated 1,2-dimyristoyl-phosphatidylcholine bilayer containing approximately 22 mol% of Chol (DMPC-Chol). The study suggests that the experimentally observed weaker effect of Chol on the POPC than DMPC bilayer might result from a different vertical localisation of the Chol hydroxyl group (OH-Chol) in both bilayers: in the POPC-Chol bilayer, OH-Chol is placed approximately 3 A higher in the bilayer interface than in the DMPC-Chol bilayer. Because of the rigid cis double bond in the beta-chain of POPC, Chol fits worse to the POPC-Chol membrane environment and is pushed up, in effect all Chol ring atoms are, on average, located above the double bond. Both in mono-cis-unsaturated and fully saturated PC bilayers, Chol induces stronger van der Waals interactions among the chains, whereas its interactions with the chains are weak. In contrast to DMPC, the smooth alpha-face of the Chol ring lowers the order of POPC chains, whereas the rough beta-face increases the order.

Impedance analysis of lipid domains in phosphatidylcholine bilayer membranes containing ergosterol

The effect of ergosterol on the electrochemical features of the phosphatidylcholine bilayer membrane was investigated by impedance spectroscopy. The experimental impedance values obtained in the presence of different amounts of ergosterol showed evidence of domain structures within the bilayer containing < 0.06 molar fraction of ergosterol. Based on derived mathematical equations, the surface area of phospholipid/ergosterol domain was calculated; it amounts to 262 A(2). This value is consistent, taking into consideration the presented measurements as well as ordering and condensation effects of ergosterol, with a stoichiometry of such a domain equal to 3:1. The result of the investigation is the proposal of a new and simple method for the determination of the surface area and description stoichiometry of domains formed in any two-component system.

Interactions between phosphatidylcholines and cholesterol in monolayers at the air/water interface

Mixtures of cholesterol and synthetic phospholipids, differing in saturation of phosphatidylcholine (PC) acyl chains, such as distearoyl phosphatidylcholine (DSPC), stearoyl-oleoyl phosphatidylcholine (SOPC) and dioleoyl phosphatidylcholine (DOPC) have been studied as floating Langmuir monolayers at the air/water interface. In order to examine the influence of a polar group, distearoyl phosphatidylethanolamine (DSPE) was chosen. The films were spread at room temperature on aqueous subphases and characterized by the surface pressure-area (pi-A) isotherms and compression modulus (C(s)(-1)) values. The interactions were examined by analyzing the mean molecular areas and quantified by the excess free energy of mixing values. The obtained results indicate that the affinity of cholesterol to saturated/unsaturated phosphatidylcholines does not differ significantly, and revealed strong influence of the kind of a polar group on the cholesterol-phospholipid interactions. On the other hand, the apolar group structure was found to modify the stoichiometry of sterol-PC complexes.

Effects of epicholesterol on the phosphatidylcholine bilayer: a molecular simulation study

Epicholesterol (Echol) is an epimeric form of cholesterol (Chol). A molecular dynamics simulation of the fully hydrated dimyristoylphosphatidylcholine-Echol (DMPC-Echol) bilayer membrane containing approximately 22 mol % of Echol was carried out for 5 ns. A 3-ns trajectory generated between 2 and 5 ns of molecular dynamics simulation was used for analyses to determine the effects of Echol on the membrane properties. As reference systems, pure DMPC and mixed DMPC-Chol bilayers were used. The study shows that Echol, like Chol, changes the organization of the bilayer/water interface and increases membrane order and condensation, but to a lesser degree. Effects of both sterols are based on the same atomic level mechanisms; their different strength arises from different vertical localizations of Echol and Chol hydroxyl groups in the membrane/water interface.

Interfacial tension of phosphatidylcholine-cholesterol system in monolayers at the air/water interface

Interfacial tension of an egg lecithin-cholesterol system was measured across the whole concentration range. Surface pressure-area isotherm measurements were carried out in a Langmuir trough at the air/water interface at room temperature (22 degrees C). The interfacial tension of the air/water interface was divided into contributions of components. The interfacial tension of a 1:1 complex between phosphatidylcholine and cholesterol was calculated. Its value equals 18 mN/m. The difference between the stability constant of 1:1 complex in the bilayer and the monolayer at the air/water interface is discussed.