Phases Of Langmuir Monolayers Research Articles

Grazing incidence diffraction studies of the interactions between ursane-type antimicrobial triterpenes and bacterial anionic phospholipids

α-Amyrin (AMalf) and ursolic acid (Urs) are ursane-type pentacyclic triterpenes which exhibit wide spectrum of antibacterial activity. These surface active compounds can be incorporated into bacterial membranes and alter their structure and function; however, the exact mechanism of their action still needs to be elucidated. Thus, we decided to study the interactions of these terpenes with specific anionic phospholipids:cardiolipins and phosphatidylglycerols extracted from Escherichia coli in the model environment of Langmuir monolayers. To characterize the ordering of the terpene molecules in one-component films as well as to study their interactions with the bacterial phospholipids in binary monolayers we applied grazing incidence X-ray diffraction (GIXD). It turned out that amyrins and ursolic acid molecules form crystalline hexagonal phases in Langmuir monolayers, in which the molecules are oriented uprightly. Regarding the mixtures, it was found that in the monolayers with Urs crystalline domains are present till moderate or even low Urs proportion. In contrast, in the mixtures with AMalf crystalline domains were observed only at the highest terpene concentration. In the interpretation of our results we underlined the significance of the interactions between the cyclopropane ring present in the hydrophobic part of the bacterial phospholipids and the terminal ring of the terpene structure. We proposed that the significant differences between the systems with AMalf and Urs are connected with the formation of hydrogen bonds between the Urs hydrophobic moieties. It can be inferred from the results that Urs is a more membrane-active agent than AMalf.

Gel-to-fluid phase transformations in solid-supported phospholipid bilayers assembled by the Langmuir-Blodgett technique: effect of the Langmuir monolayer phase state and molecular density.

Planar-supported phospholipid bilayers are increasingly used as synthetic membranes for scientific and practical applications. The thermotropic phase properties of supported bilayers are important for recreating biologically relevant situations. Unlike free-standing lipid membranes that undergo one gel-to-fluid or main phase transition, mica-supported single bilayers have been found to undergo two separate leaflet transitions. Although the distinctive nature of the main transition in mica-supported bilayers has been attributed to different effects, determining their relevance has been problematic because vesicle fusion, the technique most widely used to prepare solid-supported bilayer membranes, does not allow one to readily control the lipid surface coverage and molecular density. To circumvent the limitations of the vesicle fusion method and systematically investigate the effects on the individual leaflet transitions of the lipid phase state and molecular density before deposition on the substrate, mica-supported single bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were prepared using the Langmuir-Blodgett technique. The gel-to-fluid transitions of the bilayer leaflets were tracked by controlled-temperature atomic force microscopy to determine the relative fractions of the gel and fluid phases as a function of temperature. The fraction of solid versus temperature data was fit to the van't Hoff equation to determine the leaflet melting temperatures and transition enthalpies. The phase state and molecular density of the Langmuir monolayer precursor at the transfer pressure of 35 mN m(-1) was found to have a greater effect on the main transition temperature and width of the distal (upper) leaflet than that of the proximal (lower) one. The contributions of substrate-mediated condensation, asymmetric lipid densities, and surface area available for thermal expansion of the bilayer are addressed. This work demonstrates the potential of the Langmuir-Blodgett technique as a tool for identifying and manipulating the factors that govern the phase transition properties of surface-confined lipid bilayers.

A Systematic Investigation of the Phase Behavior of Langmuir Monolayers Containing Polyunsaturated Fatty Acids

Langmuir monolayers present an ideal model system to investigate the phase behavior of raft forming lipid compositions. Here, we present the results of a systematic study of Langmuir monolayer phase behavior for ternary mixtures containing polyunsaturated acids (PUFAs). PUFAs have been implicated in modifying membrane organization. In this study, four different mixed acyl phospholipid species with varying degrees of unsaturation in the acyl chain (1, 2, 4, 6) were mixed with sphingomyelin and cholesterol. The investigated compositions include ratios of 1:2, 1:1, and 2:1 PUFA to sphingomyelin mixtures with cholesterol composition varying from 10 to 40%. Fluorescence microscopy images provide domain size analysis, area fraction and miscibility transition pressures. We observe an increase in miscibility transition pressure with increased unsaturation or decreased cholesterol composition. Finally, traditional Langmuir film balance techniques measured pressure-area isotherms as well as area condensation.

Thermodynamic distribution functions associated to the isothermal phase transition in Langmuir monolayers

The theoretical basis for statistical mechanical deconvolution of an isothermal two-dimensional, surface pressure-induced surfactant phase transition is presented. It is demonstrated that all the thermodynamic quantities characterizing a Langmuir monolayer phase transition can be obtained from the variation of the average molecular area of the system, as a function of the surface pressure, without the need for any assumption of the particular model or mechanism. The cooperativity of the phase changes becomes clearly evident and it is possible to discriminate the behaviour of each coexisting phase state in terms of the average size and statistical distribution functions of thermodynamically correlated molecules undergoing the liquid-expanded to condensed transition in the film.

Characterizing the phases of Langmuir monolayers of cylindrical brushes with x-rayreflectivity and diffraction

Langmuir monolayers of polymacromonomers (cylindricalbrushes) with poly(vinyl)pyridine side chains of different length,polyPV P20.8 and polyPV P46.7, are studied at the air–water interface by means of x-ray diffraction and reflectivity. Theadvantages of measurements on an aqueous subphase, which contains NaI, aredemonstrated. This subphase does not affect the structure of an uncharged monolayer,although NaI is incorporated into the side chains, it provides an enhanced contrast for thex-ray investigations. A structural transition from aligned single molecules to ahomogeneous monolayer is found, which is attributed to intra- and intermolecularinteractions.

The Effect of Gauche Molecular Conformations on the Phase Diagram of a Langmuir Monolayer

Experimental and simulation studies have shown that the gauche conformational degrees of freedom of long-chain amphiphile molecules assembled in a dense Langmuir monolayer play an important role in determining the structures of the several phases that the monolayer supports. Nevertheless, for simplicity the extant theoretical analyses of the Langmuir monolayer phase diagram ignore gauche molecular conformations, thereby treating the amphiphile molecules as rigid rods. We propose a description of the influence of the conformational degrees of freedom of a long-chain amphiphile molecule on the phase diagram of a Langmuir monolayer. Our analysis extends the Landau-type theory of the Langmuir monolayer diagram formulated by Kaganer and Loginov (Phys. Rev. E 1995, 51, 2237-2249). The gauche defects are represented by a secondary order parameter coupled to the Kaganer-Loginov primary order parameters (the tilt vector of the molecules and the density waves describing herringbone ordering along and normal to the line of centers between molecules). The effect of the gauche conformations is to modify the coefficients of the primary order parameters in the free energy expansion and thereby to change the location of the transition lines in the phase diagram. For transitions that are induced by a change in the surface pressure, the tilting transitions, we obtain a shift of the transition lines to lower surface pressure. For transitions that are induced by lowering the temperature, the crystallization transitions, we suggest, given some restrictions on the magnitude of the coefficients of the coupling terms, that the transition lines shift to lower temperature.

Stability Criteria for Two-Dimensional Wetting in Monolayers

Two-dimensional pendant liquid expanded droplets partially wet the liquid condensed/gas-phase boundaries in methyl octadecanoate Langmuir monolayers. Their shape is described by the Young−Laplace equation including long-range electrostatic interactions on a scale Δ. It is invariant under shape-invariant scale transformations. We show that the local stability at the three-phase intersection point is described by Young's equation for the contact angle. The contact angle is not invariant under shape-invariant scale transformations but is a materials constant at a fixed scale parameter Δ. By comparison of numerically simulated droplets with experimental droplets observed with a fluorescence microscope, we determine the spreading coefficient of wetting Langmuir monolayer phases as well as a lower limit for Δ. We find 0.12 μm < Δ and suggest that the scale parameter shall be interpreted as a dipolar correlation length, not as a molecular cutoff length.

Two-Dimensional Pendant Droplet Tensiometry in a Langmuir Monolayer

The line tension and relative surface potentials of Langmuir monolayer phases are determined in the three-phase coexisting region gas/liquid expanded /liquid condensed of methyl octadecanoate using the pendant droplet technique. The Young−Laplace equation for two-dimensional systems with long-range dipolar interactions is applied to fluorescence images of liquid expanded droplets wetting liquid condensed surfaces in thermodynamic equilibrium. The method yields the line tension of the liquid expanded/gas borderline and the ratio of the surface potentials.

Model for Phase Transitions in the Solid Phases of Langmuir Monolayers

A quasi two-dimensional (2D) model for the solid phases (CS, S, and LS) of Langmuir monolayers is proposed. The model can reproduce the CS-S and S-LS transitions and many other experimental results observed in the monolayers of fatty acids using parameters that can be estimated from experiments. It is demonstrated that, although being ignored in many theories, electrostatic interactions indeed play a crucial role in the polymorphism of the monolayer.

Local Surface Potentials in the Three-Phase Coexistence Region of a Langmuir Monolayer

The relative surface potentials of Langmuir monolayer phases at the air/water interface have been determined in the three-phase coexistence region gas/liquid expanded/liquid condensed of methyloctadecanoate. The monolayer is assumed to be in hydrostatic equilibrium. A force balance applied to the morphology of the monolayer observed with fluorescence microscopy directly yields the relative surface potentials. This enables the determination of surface potential variations on a micron scale.

1C04 ラングミュア膜のTilted Smectic相

1C04 ラングミュア膜のTilted Smectic相

What X-rays tell us about the ordering of molecular backbones in Langmuir monolayers

Fatty acid Langmuir monolayer phases are commonly described in terms of the lattice, molecular tilt angle and tilt direction. However, the molecules are not cylinders with circular cross-sections, and so the orientation of the molecular ‘backbones’ about their long axes is also relevant. This article surveys some of the currently available experimental evidence regarding backbone ordering and order–disorder transitions in fatty acid monolayers.

Heterochirality in Langmuir monolayers and antiferromagnetic Blume–Emery–Griffiths model

Chirality is quite a common property in organic molecules. Chiral molecules exist in two forms (called enantiomers) which cannot be superimposed by rotations and translations and may have very different biochemical properties. Experiments on Langmuir monolayers made up of chiral amphiphiles have shown that, in most cases, heterochiral interactions dominate, implying the formation of a racemic compound. It has been shown that a monolayer mixture of two enantiomers can be simply described by a two-dimensional (2-D) spin-1 lattice gas [Blume–Emery–Griffiths (BEG) model], where the heterochiral preference is represented by an effective antiferromagnetic coupling. By now just mean field calculations have been performed on this model. Here we present a revisitation of the tripod amphiphile model, proposed by Andelman and de Gennes [D. Andelman and P.-G. de Gennes, Compt. Rend. Acad. Sci. (Paris) 307, 233 (1988)], together with a rigorous proof of the heterochiral preference shown by the model in the hypothesis of van der Waals interactions. Moreover, a cluster variation analysis of the antiferromagnetic BEG model on a triangular lattice is performed and possible interpretations in terms of surface pressure–concentration phase diagrams for monolayer mixtures of enantiomers are discussed. The choice of a triangular lattice has been suggested by the triangularlike structure of condensed phases of Langmuir monolayers, shown by x-ray diffraction experiments.

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 shich 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 result 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 moleucles 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 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 Π-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 Π( 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 Π( t) penetration kinetics curves with BAM and GIXD measurements. The GIXD results on the penetration of β-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.

Line tension of Langmuir monolayer phase boundaries determined with optical tweezers

The line tension λ of the liquid expanded (LE)/gas (G)-phase boundary of a methyl octadecanoate Langmuir monolayer at the air/water interface is measured using fluorescence microscopy combined with optical tweezers. Silica spheres, immersed into the monolayer and manipulated by the tweezers, deform the phase boundary. After switching off the tweezers, the relaxation of the deformed region is dominated by the competition between line tension and hydrodynamic resistance while dipole–dipole forces between the molecules can be neglected. A linear relation between the deformation length and time is found, from which a line tension of λ=7.5 pN is deduced. The linearity gives an upper bound for the surface potential differences of both phases. It is shown that viscous forces from the two-dimensional LE surroundings dominate the subphase friction. The optical tweezers enable one to observe relaxations on a shorter time scale, extending the range of measurement of previous techniques toward higher line tension or lower viscosities of the monolayer and of the subphase.

Theory of monolayers with boundaries: exact results and perturbative analysis.

Domains and bubbles in tilted phases of Langmuir monolayers contain a class of textures known as boojums. The boundaries of such domains and bubbles may display either cusplike features or indentations. We derive analytic expressions for the textures within domains and surrounding bubbles, and for the shapes of the boundaries of these regions. The derivation is perturbative in the deviation of the bounding curve from a circle. This method is not expected to be accurate when the boundary suffers large distortions, but it does provide important clues with regard to the influence of various energetic terms on the order-parameter texture and the shape of the domain or bubble bounding curve. We also look into the effects of thermal fluctuations, which include a sample-size-dependent effective line tension.

Packing of Hydrocarbon Chains and Symmetry of Condensed Phases in Langmuir Monolayers

We compare the packing characteristics of alkyl chains in Langmuir monolayers of nonchiral and racemic compounds as determined from available grazing incidence X-ray diffraction data. The analysis demonstrates a gradual change of the projected unit cell dimensions from those of a hexagonal packing of hydrocarbon chains, characteristic of high-temperature monolayer phases, to one of two more dense rectangular packing modes with the projected unit cell dimensions 5.0 × 7.5 A2 and 4.4 × 8.7 A2, characteristic of low-temperature phases. The 5.0 × 7.5 A2 unit cell incorporates the well-known herringbone arrangement, with an ideally 90° dihedral angle between the planes of carbon backbone chains. The 4.4 × 8.7 A2 cell, almost never observed in 3D structures, is characterized by a 40° dihedral angle. We characterize the packing modes by lattice energy calculations. The distribution of the projected unit cell dimensions for the various Langmuir monolayers reveals no discontinuity in the local molecular order betw...

Texture Change Separate from the Transition between Two Tilted Phases in Langmuir Monolayers

Palmityl acetate monolayers are studied by grazing-incidence X-ray diffraction (GID). The monolayers are found to undergo a transition from a phase with alkyl chains tilted toward nearest neighbors (NN) to a phase with chains tilted toward next-nearest neighbors (NNN) upon monolayer compression. This transition gives rise to a sudden reorientation of the molecules that is determined by Brewster angle microscopy (BAM). At T > 15 °C, palmityl acetate monolayers undergo a first-order transition from a fluid phase to a condensed phase indicated by a plateau region in the π−A isotherm. The transition pressure increases with increasing temperature. At low temperatures a phase with alkyl chains tilted toward nearest neighbors is formed from the fluid phase. The critical pressure for the transition between the tilted phases changes only slightly with temperature. Consequently, a phase with chains tilted toward next-nearest neighbors is formed directly from the fluid phase at high temperatures. Then no reorientati...

Ordered Phases in Langmuir Monolayers of an Azobenzene Derivative

We have performed grazing incidence X-ray diffraction studies of Langmuir monolayers of an azobenzene derivative. There are only two phases over a range of temperatures and pressures in which at least seven phases exist in monolayers of simpler molecules (0−30 °C, 0−40 dyn/cm). The low-pressure phase has tilt toward a nearest neighbor, while the high-pressure phase has tilt toward a next-nearest neighbor. Within each phase the tilt angle does not change significantly with pressure. This behavior is very different from that of monolayers with saturated hydrocarbon tails and implies that the azobenzene groups ‘lock' and prevent the molecules from sliding against each other. However, the lattice parameters change with pressure; this suggests that the molecular conformation changes. The structure of the low-pressure phase is similar to one observed in azo-derivative bilayer membranes and crystals, while the high-pressure phase has a structure not seen before in such systems.

Influence of the head group size on the direction of tilt in Langmuir monolayers

A model of rods with heads of variable size, which are confined to a planar surface, is used to study the influence of the head group size on tilted phases in Langmuir monolayers. Simple free energy considerations as well as exact zero temperature calculations indicate that molecules with small head groups tilt towards next nearest neighbors, and molecules with larger head groups towards nearest neighbors. This provides a possible explanation for recent experimental results, and for details of the generic phase diagram for fatty acid monolayers.