Scanning electrochemical microscopy as a probe of Ag+ binding kinetics at Langmuir phospholipid monolayers

A new method has been developed for measuring local adsorption rates of metal ions at interfaces based on scanning electrochemical microscopy (SECM). The technique is illustrated with the example of Ag+ binding at Langmuir phospholipid monolayers formed at the water/air interface. Specifically, an inverted 25 microm diameter silver disc ultramicroelectrode (UME) was positioned in the subphase of a Langmuir trough, close to a dipalmitoyl phosphatidic acid (DPPA) monolayer, and used to generate Ag+ via Ag electro-oxidation. The method involved measuring the transient current-time response at the UME when the electrode was switched to a potential to electrogenerate Ag+. Since the Ag+/Ag couple is reversible, the response is highly sensitive to local mass transfer of Ag+ away from the electrode, which, in turn, is governed by the interaction of Ag+ with the monolayer. The methodology has been used to determine the influence of surface pressure on the adsorption of Ag+ ions at a phospholipid (dipalmitoyl phosphatidic acid) Langmuir monolayer. It is shown that the capacity for metal ion adsorption at the monolayer increased as the density of surface adsorption sites increased (by increasing the surface pressure). A model for mass transport and adsorption in this geometry has been developed to explain and characterise the adsorption process.     

Development of a new experimental system for monitoring biomembrane reactions: combination of laser spectroscopic techniques and biomembrane models formed at an oil/water interface.

We present a new experimental system to observe reactions in biomembranes by combining laser spectroscopic techniques with phospholipid monolayers formed at oil/water interfaces. The system can monitor reactions through changes in interfacial tension at oil/water interfaces induced by the reactions under non-destructive and non-contact conditions. In addition, oil/water interfaces with defined areas can define the composition of different kinds of phospholipids. Furthermore, the system allows using, as an oil phase, alkanes whose number of carbon atoms is close to the number of the alkyl chains of phospholipids in biomembranes (C > or = 16). We demonstrated the hydrolysis reaction in DPPC (dipalmitoyl phosphatidylcholine)/DPPS (dipalmitoyl phosphatidylserine)-mixed monolayers by phospholipase A2 by using the system.     

Action mechanism of water soluble ethanol on phospholipid monolayers using a quartz crystal oscillator

Interaction between phospholipid monolayers (dihexadecyl phosphate: DHP, dipalmitoyl phosphatidyl choline: DPPC) and water soluble ethanol has been studied using quartz crystal microbalance (QCM) method and quartz crystal impedance (QCI) method. The quartz crystal oscillator was attached horizontally on the DHP and DPPC monolayers that were formed on the water surface. At low concentration, increased ethanol concentration decreased the frequency for QCM and increased the resistance for QCI. Both frequency and resistance approached asymptotically to a saturation value. A further increase in ethanol concentration induced a sudden and discontinuous linear change (a decrease in frequency and an increase in resistance). Based on these results, we propose the following action mechanism of ethanol on phospholipid monolayers: at low concentration, the ethanol hydrates adsorb into the monolayer/water interface and saturate on the interface. The monolayer viscosity also increases with the adsorption of hydrates. A further increase in concentration causes multilayer formation of hydrates and/or penetration of hydrates into the monolayer core. The viscosity of the interfacial layer (monolayer and interfacial structured water) changes dramatically according to the action of ethanol hydrates.     

Reorganization and caging of DPPC, DPPE, DPPG, and DPPS monolayers caused by dimethylsulfoxide observed using Brewster angle microscopy

The interaction between dimethylsulfoxide (DMSO) and phospholipid monolayers with different polar headgroups was studied using "in situ" Brewster angle microscopy (BAM) coupled to a Langmuir trough. For a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayer, DMSO was shown to significantly impact the structure of the liquid expanded (LE) and gaseous phases. The domains reorganized to much larger domain structures. Domains in the liquid condensed (LC) phase were formed on the DMSO-containing subphase at the mean molecular area where only gaseous and LE phases were previously observed on the pure water subphase. These results clearly demonstrate the condensing and caging effect of DMSO molecules on the DPPC monolayer. Similar effects were found on dipalmitoyl phosphatidyl ethanolamine, glycerol, and serine phospholipids, indicating that the condensing and caging effect is not dependent upon the phospholipid headgroup structure. The DMSO-induced condensing and caging effect is the molecular mechanism that may account for the enhanced permeability of membranes upon exposure to DMSO.     

The interaction of an antiparasitic peptide active against African Sleeping Sickness with cell membrane models

Zwitterionic peptides with trypanocidal activity are promising lead compounds for the treatment of African Sleeping Sickness, and have motivated research into the design of compounds capable of disrupting the protozoan membrane. In this study, we use the Langmuir monolayer technique to investigate the surface properties of an antiparasitic peptide, namely S-(2,4-dinitrophenyl)glutathione di-2-propyl ester, and its interaction with a model membrane comprising a phospholipid monolayer. The drug formed stable Langmuir monolayers, whose main feature was a phase transition accompanied by a negative surface elasticity. This was attributed to aggregation upon compression due to intermolecular bond associations of the molecules, inferred from surface pressure and surface potential isotherms, Brewster angle microscopy (BAM) images, infrared spectroscopy and dynamic elasticity measurements. When co-spread with dipalmitoyl phosphatidyl choline (DPPC), the drug affected both the surface pressure and the monolayer morphology, even at high surface pressures and with low amounts of the drug. The results were interpreted by assuming a repulsive, cooperative interaction between the drug and DPPC molecules. Such repulsive interaction and the large changes in fluidity arising from drug aggregation may be related to the disruption of the membrane, which is key for the parasite killing property.     

Chitosan as a removing agent of beta-lactoglobulin from membrane models

Many chitosan biological activities depend on the interaction with biomembranes, but so far it has not been possible to obtain molecular-level evidence of chitosan action. In this article, we employ Langmuir phospholipid monolayers as cell membrane models and show that chitosan is able to remove beta-lactoglobulin (BLG) from negatively charged dimyristoyl phosphatidic acid (DMPA) and dipalmitoyl phosphatidyl glycerol (DPPG). This was shown with surface pressure isotherms and elasticity and PM-IRRAS measurements in the Langmuir monolayers, in addition to quartz crystal microbalance and fluorescence spectroscopy measurements for Langmuir-Blodgett (LB) films transferred onto solid substrates. Some specificity was noted in the removal action because chitosan was unable to remove BLG incorporated into neutral dipalmitoyl phosphatidyl choline (DPPC) and cholesterol monolayers and had no effect on horseradish peroxidase and urease interacting with DMPA. An obvious biological implication of these findings is to offer reasons that chitosan can remove BLG from lipophilic environments, as reported in the recent literature.     

Approach to knowledge of the interaction between the constituents of contact lenses and ocular tears: mixed monolayers of poly(methyl methacrylate) and dipalmitoyl phosphatidyl choline

Mixed monolayers of poly(methyl methacrylate) (PMMA), the main component of hard contact lenses, and dipalmitoyl phosphatidyl choline (DPPC), a characteristic phospholipidic constituent of ocular tear films, were selected as an in vitro model in order to observe the behavior of contact lenses on the eye. Using Langmuir monolayer and Brewster angle microscopy (BAM) techniques, the interaction between both components was analyzed from the data of surface pressure-area isotherms, compressional modulus-surface pressure, and relative film thickness versus time elapsed from the beginning of compression, together with BAM images. Regardless of the surface pressure at which the molecular/monomer areas (A(m)) were recorded, the A(m) mole fractions of PMMA (X(PMMA)) plots show that the experimental results match the theoretical values calculated from additivity rule A(m) = X(PMMA)A(PMMA) + X(DPPC)A(DPPC). The application of the Crisp phase rule to the phase diagram of the PMMA-DPPC system can explain the existence of a mixed monolayer made up of miscible components with ideal behavior at surface pressures below 25 mN/m. However, at very high surface pressures, when collapse is reached (at 60 mN/m), the single collapsed components are segregated into two independent phases. These results allows us to argue that PMMA hard contact lenses in the eye do not alter the structural characteristics of the phospholipid (DPPC) in tears.     

Stability of binary model membranes--prediction of the liposome stability by the Langmuir monolayer study

In this paper, usefulness of the Langmuir monolayer study is demonstrated for predictions of the stability of liposomes composed of dipalmitoyl phosphatidylcholine (DPPC) and cholesterol (Chol). Thermodynamic analysis of the surface pressure (π)-area (A) isotherms of the DPPC/Chol systems was performed, which allowed for concluding on miscibility of the components, their molecular packing, and the interactions between molecules. It was found that the most stable system, in which the strongest interactions between molecules occured, was DPPC/Chol at x(Chol)=0.25. The stability of liposomes of the same composition as that in the Langmuir monolayers was analyzed by determining the size distribution of vesicles and the polydispersity as a function of time. The changes of these parameters confirmed that the system of the greatest stability is that with low Chol content.     

Roles of gamma-Globulin in the Dynamic Interfacial Behavior of Mixed Dipalmitoyl Phosphatidylcholine/gamma-Globulin Monolayers at Air/Liquid Interfaces

This study investigated the roles of gamma-globulin in the dynamic interfacial behavior of dipalmitoyl phosphatidylcholine (DPPC)/gamma-globulin monolayers at air/liquid interfaces at 25 degrees C. The surface tension behavior demonstrated that gamma-globulin had a large adsorption time scale. Moreover, the surface pressure-area hysteresis behavior of adsorbed gamma-globulin monolayers suggested that no significant desorption occurred during the compression stage, and the respreading of gamma-globulin molecules at the interface during the expansion stage was slow. From the hysteresis behavior of adsorbed gamma-globulin monolayers with spread DPPC molecules, it was found that gamma-globulin molecules were expelled from the interface as DPPC molecules were in a condensed state. The squeeze-out of gamma-globulin molecules seemed to induce the loss of DPPC molecules at the interface with the extent depending on the initial gamma-globulin surface concentration. Furthermore, the expelled gamma-globulin molecules re-entered the monolayer and participated in the surface pressure increase during the following expansion stage. The exclusion of gamma-globulin associated with the removal of DPPC during monolayer compression and the re-entry of gamma-globulin during subsequent monolayer expansion represented a mechanism for DPPC depletion and gamma-globulin enrichment at the interface, which may explain the inhibitory effect of certain proteins on the surface activity of DPPC. Copyright 2000 Academic Press.     

Properties of lipophilic nucleoside monolayers at the air–water interface

The capability of self-assembly and molecular recognition of biomolecules is essential for many nanotechnological applications, as in the use of alkyl-modified nucleosides and oligonucleotides to increase the cellular uptake of DNA and RNA. In this study, we show that a lipophilic nucleoside, which is an isomer mixture of 2′-palmitoyluridin und 3′-palmitoyluridin, forms Langmuir monolayers and Langmuir–Blodgett films as a typical amphiphile, though with a smaller elasticity. The nucleoside may be incorporated into dipalmitoyl phosphatidyl choline (DPPC) monolayers that serve as a simplified cell membrane model. The molecular-level interactions between the nucleoside and DPPC led to a remarkable condensation of the mixed monolayer, which affected both surface pressure and surface potential isotherms. The morphology of the mixed monolayers was dominated by the small domains of the nucleoside. The mixed monolayers could be deposited onto solid substrates as a one-layer Langmuir Blodgett film that displayed UV–vis absorption spectra typical of aggregated nucleosides owing to the interaction between the nucleoside and DPPC. The formation of solid films with DNA building blocks in the polar heads may open the way for devices and sensors be produced to exploit their molecular recognition properties.     

Structural studies of the monolayers and bilayers formed by a novel cholesterol-phospholipid chimera

Langmuir isotherm, neutron reflectivity, and small angle neutron scattering studies have been conducted to characterize the monolayers and vesicular bilayers formed by a novel chimeric phospholipid, ChemPPC, that incorporates a cholesteryl moeity and a C-16 aliphatic chain, each covalently linked via a glycerol backbone to phosphatidylcholine. The structures of the ChemPPC monolayers and bilayers are compared against those formed from pure dipalmitoylphoshatidylcholine (DPPC) and those formed from a 60:40 mol % mixture of DPPC and cholesterol. In accord with previous findings showing that very similar macroscopic properties were exhibited by ChemPPC and 60:40 mol % DPPC/cholesterol vesicles, it is found here that the chimeric lipid and lipid/sterol mixture have very similar monolayer structures (each having a monolayer thickness of ～26 ?), and they also form vesicles with similar lamellar structure, each having a bilayer thickness of ～50 ? and exhibiting a repeat spacing of ～65 ?. The interfacial area of ChemPPC, however, is around 10 ?(2) greater than that of the combined DPPC/cholesterol unit in the mixed lipid monolayer (viz., 57 ± 1 vs 46 ± 1 ?(2), at 35 mN·m(-1)), and this difference in area is attributed to the succinyl linkage which joins the ChemPPC steroid and glyceryl moieties. The larger area of the ChemPPC is reflected in a slightly thicker monolayer solvent distribution width (9.5 vs 9 ? for the DPPC/cholesterol system) and by a marginal increase in the level of lipid headgroup hydration (16 vs 13 H(2)O per lipid, at 35 mN·m(-1)).     

Thermodynamic characterization of the prevailing molecular interactions in mixed floating monolayers of phospholipids and usnic acid

The investigation of the characteristics of mixed floating monolayers of phospholipids and usnic acid (UA), an active metabolite from lichens, can provide valuable information on how to prepare stable liposomes that could serve as carriers of UA for therapeutic proposes. The present paper is concerned with the thermodynamic analysis of the behavior of Langmuir monolayers formed by mixing different phospholipids (dibehenoylphosphatidylcholine, DBPC, dipalmitoylphosphatidylcholine, DPPC, and dioleoylphosphatidylcholine, DOPC) and UA at varied molar fractions. Relevant thermodynamic parameters such as excess areas, excess free energies and free energy of mixing were derived from the surface pressure data obtained from compression measurements performed in a Langmuir trough. For the largest lateral pressure examined (25 mN/m), negative values of the excess free energy were found only for the DOPC/UA monolayer, which should be the most stable of them. Based on the calculated values of the free energy of mixing, we note that the DBPC/UA and DPPC/UA systems present the best mixed character at low pressures and when the molar fraction of the UA is 0.5; at that relative concentration and at low values of the external pressure, the UA molecules can better mix and interact with the phospholipid molecules. The compression isotherms for mixed monolayers show no visible transitions, exhibiting a more organized phase that corresponds to a negative free energy of mixing. We have established that the most stable monolayers were those corresponding to DOPC/UA mixtures with a UA molar fraction of 0.75.     

Interaction of Dipalmitoyl Phosphatidylcholine with n‐Hexadecanol in Monolayer and Liposome

The monolayer collapse behavior of n‐hexadecanol/dipalmitoyl phosphatidylcholine (DPPC) was investigated in this study at the air/water interface at 37 °C. Surface pressure variations with time for the mixed monolayers of DPPC with 20 mol% and 50 mol% n‐hexadecanol at corresponding collapse points were recorded by a Langmuir trough system. In addition, the interaction of n‐hexadecanol with a pure DPPC monolayer was identified by fluorescence microscopy (FM). The results demonstrated distinct differences between these systems; according to our observation, the higher the ratio of n‐hexadecanol to DPPC, the more nucleation domains can be induced. The FM images demonstrated that pronounced domain formation was associated with a longer relaxation time of the collapsed DPPC and DPPC/n‐hexadecanol monolayers, and the presence of n‐hexadecanol appeared to enhance the relaxation processes. The liposome was prepared by the thin‐film hydration method. The average diameter of DPPC and DPPC/n‐hexadecanol liposomes was investigated by dynamic light scattering. It is shown that the diameter of DPPC liposome with n‐hexadecanol is smaller than pure DPPC liposome at the initial state. After 24 hours, DPPC/n‐hexadecanol liposome became larger than pure DPPC liposome and lasted for the next four days. The effects of a greater ratio of n‐hexadecanol did not play an important role in DPPC liposome formation based on our dynamic light scattering analysis. Our result demonstrated that n‐hexadecanol might affect the DPPC liposome stability. The increased ratio of n‐hexadecanol in DPPC liposomes could only a play a minor role in DPPC liposome fusion.     

Penetration of dissolved amphiphiles into two-dimensional aggregating lipid monolayers

The review demonstrates the recent theoretical and experimental progress in the understanding of penetration systems at the air-water interface in which a dissolved amphiphile (surfactant, protein) penetrates into a Langmuir monolayer. The critical review of the existing theoretical models which describe the thermodynamics of the penetration are critically reviewed. Although a rigorous thermodynamic analysis of penetration systems is unavailable due to their complexity, some model assumptions, e.g. the invariability of the activity coefficient of the insoluble component of the monolayer during the penetration of the soluble component results in reasonable solutions. New theoretical models describing the equilibrium behaviour of the insoluble monolayers which undergo the 2D aggregation in the monolayer, and the equations of state and adsorption isotherms which assume the existence of multiple states (conformations) of a protein molecule within the monolayer and the non-ideality of the adsorbed monolayers are now available. The theories which describe the penetration of a soluble surfactant into the main phases of Langmuir monolayers were presented first for the case of the mixture of the molecules possessing equal partial molar surfaces (the mixture of homologues), with further extension of the models to include the interesting process of the protein penetration into the monolayer of 2D aggregating phospholipid. This extension was based on a concept which subdivides the protein molecules into independent fragments with areas equal to those of the phospholipid molecule. Various mechanisms for the effect of the soluble surfactant on the aggregation of the insoluble component were considered in the theoretical models: (i) no effect on the aggregate formation process; (ii) formation of mixed aggregates; and (iii) the influence on the aggregating process via the change of aggregation constant, but without any formation of mixed aggregates. Accordingly depending on the mechanism, different forms of the equations of state of the monolayer and of the adsorption isotherms of soluble surfactant are predicted. Based on the shape of the experimental pi-A isotherms, interesting conclusions can be drawn on the real mechanism. First experimental evidence has been provided that the penetration of different proteins and surfactants into a DPPC monolayer in a fluid-like state induces a first order main phase transition of pure DPPC. The phase transition is indicated by a break point in the pi(t) penetration kinetics curves and the domain formation by BAM. Mixed aggregates of protein with phospholipid are not formed. These results agree satisfactorily with the predictions of the theoretical models. New information on phase transition and phase properties of Langmuir monolayers penetrated by soluble amphiphiles are obtained by coupling of the pi(t) penetration kinetics curves with BAM and GIXD measurements. The GIXD results on the penetration of beta-lactoglobulin into DPPC monolayers have shown that protein penetration occurs without any specific interactions with the DPPC molecules and the condensed phase consists only of DPPC.     

Penetration of surfactin into phospholipid monolayers: nanoscale interfacial organization

Atomic force microscopy (AFM) combined with surface pressure-area isotherms were used to probe the interfacial behavior of phospholipid monolayers following penetration of surfactin, a cyclic lipopeptide produced by Bacillus subtilis strains. Prior to penetration experiments, interfacial behavior of different surfactin molecules (cyclic surfactins with three different aliphatic chain lengths--S13, S14, and S15--and a linear surfactin obtained by chemical cleavage of the cycle of the surfactin S15) has been investigated. A more hydrophobic aliphatic chain induces greater surface-active properties of the lipopeptide. The opening of the peptide ring reduces the surface activity. The effect of phospholipid acyl chain length (dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine- (DPPC), and distearoylphosphatidylcholine) and phospholipid polar head (DPPC, dipalmitoylphosphatidylethanolamine and dipalmitoylphosphatidylserine) on monolayer penetration properties of the surfactin S15 has been explored. Results showed that while the lipid monolayer thickness and the presence of electrostatic repulsions from the interfacial film do not significantly influence surfactin insertion, these parameters strongly modulate the ability of the surfactin to alter the nanoscale organization of the lipid films. We also probed the effect of surfactin structure (influence of the aliphatic chain length and of the cyclic structure of the peptide ring) on the behavior of DPPC monolayers. AFM images and isotherms showed that surfactin penetration is promoted by longer lipopeptide chain length and a cyclic polar head. This indicates that hydrophobic interactions are of main importance for the penetration power of surfactin molecules.     

The use of dielectric spectroscopy for the characterisation of polymer-induced flocculation of core-shell particles

This work is aimed at investigating the influence of a plant stanol (β-sitostanol) on Langmuir monolayers from various phospholipids and comparing the effect of phytostanol versus its unsaturated analog--phytosterol (β-sitosterol). The studied phospholipids differed in the structure of polar head (phosphatidylcholine--PC, phosphatidylethanolamine--PE, phosphatidylserine--PS) as well as in the number of monounsaturated chains in PC molecule. It was found that the introduction of stanol into PC monolayers is thermodynamically favorable, contrary to its effect on PE and PS films. The magnitude of condensing and ordering effect of stanol depends both on the number of monounsaturated chains in PC molecule and on the composition of stanol-PC mixture. The analysis of BAM images evidenced phase separation of immiscible components in stanol/DPPS systems. The results of Langmuir monolayer studies for stanol/phospholipids mixtures compared with those for corresponding sterol/phospholipids systems proved quite a similar effect of both compounds on the investigated phospholipid monolayers, despite differences in the structure of tetracyclic ring skeleton.     

Cratylia mollis lectin at the air–aqueous solution interface: adsorption and lectin–lipid interactions

The interfacial behaviour of Cratylia mollis (Cra) at the air/water interface and its penetrant ability into spread phospholipid monolayers (Lipoid E80 and Epicuron 200) has been monitored by surface tension measurements. The first-order rate constants defining adsorption and rearrangement obtained from surface tension kinetics data reveal that Cra is a rather stable protein which exhibits characteristic protein adsorption patterns in which the breaking points separating diffusion–penetration and rearrangement profiles could have been easily distinguished. The penetration of Cra into Lipoid E80 and Epicuron 200 phospholipid monolayers has been inferred in terms of penetration pressure increments (ΔΠ) versus time relationships. The data clearly showed that penetrant ability of the lectin was, to a large extent, dependent on monolayer compressibilities. Thus, for Lipoid E80, which contained a rather high percentage of phosphatidylethanolamine (DPPE) in the mixture with phosphatidylcholine (DPPC), penetration of Cra at the high monolayer compression (20 mN m⁻¹) was lower than that observed for Epicuron 200, which did not contain DPPE. Indeed, in the middle of the Π-A isotherm, DPPE was markedly less compressible than DPPC. However, at the low monolayer surface coverage (3 mN m⁻¹), the rates of Cra penetration into both Lipoid E80 and Epicuron 200, although much higher for the latter at the beginning of adsorption, yielded similar limiting values of ΔΠ. This has been attributed to the occurrence of a hydrophobic interaction between the lectin and hydrophobic phospholipid chains that have the same length for both Lipoid E80 and Epicuron 200.     

Interfacial properties of a novel pyrimidine derivative and poly(ethylene glycol)-grafted phospholipid floating monolayers

4-amino-2-phenyl, 6(p-fluor-phenyl)-5-carbonitrile-pyrimidine (APCP) is a new derivative of pyrimidine with low solubility in water and anti-inflammatory properties. We compared the interfacial behaviors of spread films of poly(ethylene glycol)-grafted phospholipid (DSPE-PEG₂₀₀₀), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and APCP and a mixture of these molecules. The surface pressure–area (Π–A) isotherm showed that APCP and DSPE-PEG₂₀₀₀ molecules were stable at the air/water interface and could be evenly inserted into a DPPC floating monolayer. The introduction of APCP into the DPPC/(DSPE-PEG₂₀₀₀) binary monolayer generally causes an overall increase in surface potential. Analyses of distance variation between the grafted sites are associated with a change of mushroom to brush conformation and this behavior is observed for the DPPC/(DSPE-PEG₂₀₀₀) and DPPC/(DSPE-PEG₂₀₀₀)/APCP monolayers. Langmuir–Blodgett (LB) films of molecules of biological interest were transferred onto mica in order to investigate their interaction. AFM images do not show any regular shape or size and are randomly distributed.     

Preparation of microscopic and planar oil-water interfaces that are decorated with prescribed densities of insoluble amphiphiles

Langmuir monolayers (monolayers of insoluble molecules formed at the surface of water), and associated Langmuir-Blodgett/Schaefer monolayers prepared by transfer of Langmuir films to the surfaces of solids, are widely used in studies aimed at understanding the physicochemical properties of biological and synthetic molecules at interfaces. In this article, we report a general and facile procedure that permits transfer of Langmuir monolayers from the surface of water onto microscopic and planar interfaces between oil and aqueous phases. In these experiments, a metallic grid supported on a hydrophobic solid is used to form oil films with thicknesses of 20 mum and interfacial areas of 280 mum x 280 mum. Passage of the supported oil films through a Langmuir monolayer is shown to lead to quantitative transfer of insoluble amphiphiles onto the oil-water interfaces. The amphiphile-decorated oil-water interfaces hosted within the metallic grids (i) are approximately planar, (ii) are sufficiently robust mechanically so as to permit further characterization of the interfaces outside of the Langmuir trough, (iii) can be prepared with prescribed and well-defined densities of amphiphiles, and (iv) require only approximately 200 nL of oil to prepare. The utility of this method is illustrated for the case of the liquid crystalline oil 4-pentyl-4'-cyanobiphenyl (5CB). Transfer of monolayers of either dilauroyl- or dipalmitoylphosphatidylcholine (DLPC and DPPC, respectively) to the nematic 5CB-aqueous interface is demonstrated by epifluorescence imaging of fluorescently labeled lipid and polarized light imaging of the orientational order within the thin film of nematic 5CB. Interfaces prepared in this manner are used to reveal key differences between the density-dependent phase properties of DLPC and DPPC monolayers formed at air-water as compared to that of nematic 5CB-aqueous interfaces. The methodology described in this article should be broadly useful in advancing studies of the interfacial behavior of synthetic and biological molecules at liquid-liquid interfaces.     

Study of penetration of amphotericin B into cholesterol or ergosterol containing dipalmitoyl phosphatidylcholine Langmuir monolayers

The interactions of amphotericin B (AmB) with sterols and phospholipids have been studied by adsorption of AmB from aqueous solutions into Langmuir monolayers from dipalmitoyl phosphatidylcholine (DPPC), ergosterol, cholesterol and their mixtures. The results show that AmB exhibits stronger interaction with cholesterol than ergosterol in one-component monolayers. However, for DPPC–sterol monolayers, the effectiveness of AmB penetration depends on the proportion of both film components in the mixed film as well as on the strength of interaction between DPPC and particular sterol.