Properties of two-component Langmuir monolayer of single chain perfluorinated carboxylic acids with dipalmitoylphosphatidylcholine (DPPC)

Effect of Ester Moiety on Structural Properties of Binary Mixed Monolayers of Alpha-Tocopherol Derivatives with DPPC

Phospholipid membranes are ubiquitous components of cells involved in physiological processes; thus, knowledge regarding their interactions with other molecules, including tocopherol ester derivatives, is of great importance. The surface pressure–area isotherms of pure α-tocopherol (Toc) and its derivatives (oxalate (OT), malonate (MT), succinate (ST), and carbo analog (CT)) were studied in Langmuir monolayers in order to evaluate phase formation, compressibility, packing, and ordering. The isotherms and compressibility results indicate that, under pressure, the ester derivatives and CT are able to form two-dimensional liquid-condensed (LC) ordered structures with collapse pressures ranging from 27 mN/m for CT to 44 mN/m for OT. Next, the effect of length of ester moiety on the surface behavior of DPPC/Toc derivatives’ binary monolayers at air–water interface was investigated. The average molecular area, elastic modulus, compressibility, and miscibility were calculated as a function of molar fraction of derivatives. Increasing the presence of Toc derivatives in DPPC monolayer induces expansion of isotherms, increased monolayer elasticity, interrupted packing, and lowered ordering in monolayer, leading to its fluidization. Decreasing collapse pressure with increasing molar ratio of derivatives indicates on the miscibility of Toc esters in DPPC monolayer. The interactions between components were analyzed using additivity rule and thermodynamic calculations of excess and total Gibbs energy of mixing. Calculated excess area and Gibbs energy indicated repulsion between components, confirming their partial mixing. In summary, the mechanism of the observed phenomena is mainly connected with interactions of ionized carboxyl groups of ester moieties with DPPC headgroup moieties where formed conformations perturb alignment of acyl chains, resulting in increasing mean area per molecule, leading to disordering and fluidization of mixed monolayer.

Lysozyme Influence on Monolayers of Individual and Mixed Lipids

Fatty acids, cholesterol, and phospholipids are amphiphilic compounds of biological interest, which form ordered monolayers mimicking biomembranes, and can be studied with the Langmuir technique using surface pressure-area isotherms and compressibility plots. Proteins are also components of biomembranes or are present in body fluids. In this study, the influence of lysozyme on different films of a fatty acid (stearic acid or oleic acid), cholesterol, a phospholipid (dipalmitoylphosphatidylcholine, DPPC, or palmitoyloleoylphosphatidylcholine, POPC), and mixtures of them is presented using a 0.9% saline solution as subphase. Results show that the presence of lysozyme alters the lipid monolayer formation in an important way at the beginning (low surface pressures) and the middle (intermediate surface pressures) parts of the isotherm. At high surface pressures, the phospholipids DPPC and POPC and the saturated fatty acid, stearic acid, expel lysozyme from the surface, while oleic acid and cholesterol permit the presence of lysozyme on it. The mixtures of oleic acid-DPPC also expel lysozyme from the surface at high surface pressures, while mixtures of oleic acid-POPC and cholesterol-POPC permit the presence of lysozyme on it. The compressibility of the monolayer is affected in all cases, with an important reduction in the elastic modulus values and an increase in the fluidity, especially at low and intermediate surface pressures.

Headgroup-Specific Interaction of Biological Lipid Monolayer/Water Interface with Perfluorinated Persistent Organic Pollutant (f-POP): As Observed with Interface-Selective Vibrational Spectroscopy

Perfluoro compounds are widely used in various manufacturing processes, which leads to their bioaccumulation and subsequent adverse effects on human health. Using interface-selective vibrational spectroscopy (heterodyne-detected vibrational sum frequency generation (HD-VSFG)), we have elucidated the molecular mechanism of the perturbation of lipid monolayers on the water surface using a prototype perfluorinated persistent organic pollutant, perfluoroheptanoic acid (PFHA). PFHA disrupts the well-ordered all-trans conformation of a cationic lipid (1,2-dipalmitoyl-3-trimethylammonium propane (DPTAP)) monolayer and reduces the interfacial electric field at the lipid/water interface. In contrast, the hydrophobic packing of an anionic lipid (1,2-dipalmitoyl-sn-glycero-3-phospoglycerol (DPPG)) monolayer remains largely unaffected in the presence of PFHA, though the interfacial electric field is reduced. For a zwitterionic lipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC))/water interface, both alkyl chain ordering and interfacial electric field are fairly perturbed by PFHA. Lipid headgroup-specific interaction of PFHA and the repulsive interaction of oleophobic fluoroalkyl chain with the lipid alkyl chains govern these distinct perturbations of the lipid monolayers on the water surface.

Destabilizing effect of perfluorodecanoic acid on simple membrane models

Perfluoroalkyl acids (PFA) are amphiphilic surfactants widely used in industry with several commercial applications. An important feature of these compounds is their non-biodegradability and their tendency to bio-accumulate in the environment, which has led to these compounds being considered among the most persistent pollutants worldwide. Many studies have provided evidence of their toxic effect on humans and wildlife. For this reason, more and more efforts have been made to better understand the effect of these compounds on living organisms. The aim of the present study is to understand how the electrostatic interactions and film compactness of biological membrane models modulate their interaction with PFA, more specifically with perfluorodecanoic acid (PFD). Langmuir isotherms and Brewster angle microscopy (BAM) are used to evaluate the effect of PFD on lipid membrane models (air/water monolayers and vesicles), analyzing the behavior of PFD : lipid mixtures. The lipids used in this study are distearoyl phosphatidic acid (DSPA), dilauroyl phosphatidic acid (DLPA) and distearoyl phosphatidylethanolamine (DSPE). PFD induces an increase in the mean molecular area per lipid in monolayers, mainly at lower surface pressures. BAM images demonstrate that PFD mixes with DLPA, inducing a decrease in gray level, while it forms a non-miscible mixture with DSPA, segregating PFD domains. Insertion studies of PFD within monolayers and dynamic light scattering experiments demonstrate that PFD can penetrate into monolayers and bilayers above 30 mN m-1, which is the lateral pressure value accepted for a cellular bilayer.

Investigation of Surface Behavior of DPPC and Curcumin in Langmuir Monolayers at the Air-Water Interface

Langmuir monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a mixture of DPPC with curcumin (CUR) have been investigated at the air-water interface through a combination of surface pressure measurements and atomic force microscopy (AFM) observation. By analyzing the correlation data of mean molecular areas, the compressibility coefficient, and other thermodynamic parameters, we obtained that the interaction between the two components perhaps was mainly governed by the hydrogen bonding between the amino group of DPPC and the hydroxyl groups of CUR. CUR markedly affected the surface compressibility, the thermodynamic stability, and the thermodynamic phase behaviors of mixed monolayers. The interaction between CUR and DPPC was sensitive to the components and the physical states of mixed monolayers under compression. Two-dimensional phase diagrams and interaction energies indicated that DPPC and CUR molecules were miscible in mixed monolayers. AFM images results were in agreement with these analyses results of experimental data. This study will encourage us to further research the application of CUR in the biomedical field.

Mixing Behavior in Binary Anionic Gemini Surfactant-Perfluorinated Fatty Acid Langmuir Monolayers

The miscibility and film structure of mixed Langmuir monolayer films composed of an anionic gemini N,N,N',N'-dialkyl-N,N'-diacetate ethylenediamine surfactant (Ace(12)-2-Ace(12)) with perfluorotetradecanoic acid (C₁₃F₂₇COOH; PF) have been investigated using a variety of thermodynamic and structural characterization methods. The two film components were found to be miscible in monolayers at the air-water interface over a range of compositions and at all but the lowest surface pressures, with attractive interactions occurring between the two components. While pure PF monolayers formed crystalline lattices with hexagonal symmetry and with the surfactant tails oriented normal to the underlying water subphase, the pure gemini surfactant formed amorphous films with little tendency to orient at the subphase. In mixed films with mole ratios of PF:Ace(12)-2-Ace(12) < 2.5, the miscibility of the two components resulted in a nearly complete loss of crystallinity of the PF, though films at higher mole fractions of PF showed some residual crystallinity, albeit with lattice structures that were significantly different from that of pure PF. Miscibility and film structure in this mixed system are discussed in comparison with other mixed gemini surfactant systems in the literature as well as binary mixtures of phospholipids or monomeric fatty acids with PF.

Phase-separated surfactant monolayers: Exploiting immiscibility of fluorocarbons and hydrocarbons to pattern interfaces

The mutual immiscibility of hydrogenated and fluorinated surfactants at interfaces frequently leads to phase-separation, which provides a useful and flexible method for patterning air-water and solid-air interfaces. In this article, we review recent advances in the use of hydrogenated-fluorinated surfactant mixtures to achieve interfacial patterning. For even relatively simple systems comprised of binary mixed monolayers of hydrogenated and perfluorinated fatty acids, a diverse range of film morphologies can be generated at the air-water interface and successfully transferred onto solid substrates. Systematic investigations reported over the past several years have allowed for correlation between the chemical structure of the film constituents with the gross film morphology and underlying crystalline structure of the films. Early thermodynamic models based on the interplay between dipole-dipole repulsion forces between charged headgroups balanced by line tension between phases that were formulated to describe phase-behavior in simple phospholipid monolayer systems have proven highly useful to describe morphologies for the immiscible surfactant blends. Beyond simple binary fatty acid mixtures, highly-structured films have also been reported in mixed phospholipid systems, which have found important application in controlling the physical, compositional and performance properties of lung surfactant mixtures, as well as in semifluorinated alkane monolayers which form unique, hemimicellar structures at both liquid and solid interfaces. We also describe advances in using these approaches to pattern photopolymerizable, luminescent surfactants, which have found extensive use in colorimetric and fluorometric sensing devices. The long-term outlook for this field, with an emphasis on potential applications and future research directions are discussed.

Molecular-level insight into the binding of arginine to a zwitterionic Langmuir monolayer

Arginine molecules bind to a DPPC monolayer, altering the interfacial electrostatic potential and the lateral mobility of the lipids, while having little effect on the compression isotherm of the monolayer.

Overcoming inactivation of the lung surfactant by serum proteins: a potential role for fluorocarbons?

In many pulmonary conditions serum proteins interfere with the normal adsorption of components of the lung surfactant to the surface of the alveoli, resulting in lung surfactant inactivation, with potentially serious untoward consequences. Here, we review the strategies that have recently been designed in order to counteract the biophysical mechanisms of inactivation of the surfactant. One approach includes protein analogues or peptides that mimic the native proteins responsible for innate resistance to inactivation. Another perspective uses water-soluble additives, such as electrolytes and hydrophilic polymers that are prone to enhance adsorption of phospholipids. An alternative, more recent approach consists of using fluorocarbons, that is, highly hydrophobic inert compounds that were investigated for partial liquid ventilation, that modify interfacial properties and can act as carriers of exogenous lung surfactant. The latter approach that allows fluidisation of phospholipid monolayers while maintaining capacity to reach near-zero surface tension definitely warrants further investigation.

New adsorption model -- theory, phenomena and new concept

According to the conventional Gibbs adsorption model, which is a common assumption about the molecular concentration at surfaces, the adsorbed film of soluble amphiphiles is located at the air/solution interface just like Langmuir monolayer which is illustrated in many physical chemistry text books on "Colloid and Interface Science". According to many proofs of the experimental results here, the newer idea for the surface adsorption is confirmed and explained, which is quite different from the conventional Gibbs surface excess model at the air/solution interface.

Determination and comparison of how the chain number and chain length of a lipid affects its interactions with a phospholipid at an air/water interface

We determined how the number of chains in a lipid and its chain length affects its interactions with a phospholipid model membrane, and whether the number of chains or the chain length of lipids affects their interactions with the phospholipids more. This was achieved by using a Langmuir trough and a fluorescence microscope to study the interactions of mono-, di-, and triglycerides with a phospholipid monolayer at an air/water interface. The effect of the number of chains in a lipid on its interactions with phospholipids at air/water interfaces was shown by surface pressure-area per molecule isotherms and their thermodynamic analysis to worsen as the number of alkyl chains was increased to be greater than one. An increase in the packing density decreased the mixing ability of the lipids with the phospholipids, resulting in the formation of aggregates in the mixed monolayer. The aggregation was explained by the intermolecular hydrophobic and van der Waals attractions between the lipid molecules. Fluorescence microscopy revealed partial mixing without aggregation for monoglycerides, but the presence of lipid aggregation for diglycerides and triglycerides. The effect of decreasing the chain length of triglycerides from a long chain to a medium chain caused the interactions of the lipids with the phospholipid molecules at the air/water interface to significantly improve. Decreasing the chain length of monoglycerides from a long chain to a medium chain worsened their interaction with the phospholipid molecules. The effect of decreasing the triglyceride chain length on their interactions with phospholipids was much greater than the effect of decreasing the number of alkyl chains in the lipid.

Influence of film composition on the morphology, mechanical properties, and surfactant recovery of phase-separated phospholipid-perfluorinated fatty acid mixed monolayers

Monolayer surfactant films composed of a mixture of phospholipids and perfluorinated (or partially fluorinated) surfactants are of potential utility for applications in pulmonary lung surfactant-based therapies. As a simple, minimal model of such a lung surfactant system, binary mixed monolayer films composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and perfluorooctadecanoic acid (C18F) prepared on a simplified lung fluid mimic subphase (pH 7.4, 150 mM NaCl) have been characterized in terms of mixing thermodynamics and compressibility (measured through π–A compression isotherms), film morphology (via atomic force, fluorescence, and Brewster angle microscopy), as well as spreading rate and hysteresis response to repeated expansion–contraction cycles for a variety of compositions of mixed films. Under all mixing conditions, films and their components were found to be completely immiscible and phase-separated, though there were significant changes in the aforementioned film properties as a function of composition. Of particular note was the existence of a maximum in the extent of immiscibility (characterized by ΔG(ex)(π) values) and enhanced surfactant recovery during hysteresis experiments at χ(C18F) ≥ 0.30. The latter was attributed to the relatively rapid respreading rate of the perfluorinated amphiphile in comparison with DPPC alone at the air–water interface, which enhances the performance of this mixture as a potential pulmonary lung surfactant. Further, monolayer film structure could be tracked dynamically as a function of compression at the air–water interface via Brewster angle microscopy, with the C18F component being preferentially squeezed out of the film with compression, but returning rapidly upon re-expansion. In general, addition of C18F to DPPC monolayers resulted in improvements to mechanical, structural, and respreading properties of the film, indicating the potential value of these compounds as additives to pulmonary lung surfactant formulations.

Partitioning of perfluorooctanoate into phosphatidylcholine bilayers is chain length-independent

The chain length dependence of the interaction of PFOA, a persistent environmental contaminant, with dimyristoyl- (DMPC), dipalmitoyl- (DPPC) and distearoylphosphatidylcholine (DSPC) was investigated using steady-state fluorescence anisotropy spectroscopy, differential scanning calorimetry (DSC) and dynamic light scattering (DLS). PFOA caused a linear depression of the main phase transition temperature T(m) while increasing the width of the phase transition of all three phosphatidylcholines. Although PFOA's effect on T(m) and the transition width decreased in the order DMPC>DPPC>DSPC, its relative effect on the phase behavior was largely independent of the phosphatidylcholine. PFOA caused swelling of DMPC but not DPPC and DSPC liposomes at 37 degrees C in the DLS experiments, which suggests that PFOA partitions more readily into bilayers in the fluid phase. These findings suggest that PFOA's effect on the phase behavior of phosphatidylcholines depends on the cooperativity and state (i.e., gel versus liquid phase) of the membrane. DLS experiments are also consistent with partial liposome solubilization at PFOA/lipid molar ratios>1, which suggests the formation of mixed PFOA-lipid micelles.

Model and cell membrane partitioning of perfluorooctanesulfonate is independent of the lipid chain length

Perfluorooctanesulfonic acid (PFOS) is a persistent environmental pollutant that may cause adverse health effects in humans and animals by interacting with and disturbing of the normal properties of biological lipid assemblies. To gain further insights into these interactions, we investigated the effect of PFOS potassium salt on dimyristoyl- (DMPC), dipalmitoyl- (DPPC) and distearoylphosphatidylcholine (DSPC) model membranes using fluorescence anisotropy measurements and differential scanning calorimetry (DSC) and on the cell membrane of HL-60 human leukemia cells and freshly isolated rat alveolar macrophages using fluorescence anisotropy measurements. PFOS produced a concentration-dependent decrease of the main phase transition temperature (T(m)) and an increased peak width (DeltaT(w)) in both the fluorescence anisotropy and the DSC experiments, with a rank order DMPC>DPPC>DSPC. PFOS caused a fluidization of the gel phase of all phosphatidylcholines investigated, but had the opposite effect on the liquid-crystalline phase. The apparent partition coefficients of PFOS between the phosphatidylcholine bilayer and the bulk aqueous phase were largely independent of the phosphatidylcholine chain length and ranged from 4.4x10(4) to 8.8x10(4). PFOS also significantly increased the fluidity of membranes of cells. These findings suggest that PFOS readily partitions into lipid assemblies, independent of their composition, and may cause adverse biological effects by altering their fluidity in a manner that depends on the membrane cooperativity and state (e.g., gel versus liquid-crystalline phase) of the lipid assembly.

Miscibility behavior of two-component monolayers at the air-water interface: perfluorocarboxylic acids and DMPE

Surface pressure (pi)-molecular area (A) and surface potential (DeltaV)-A isotherms have been measured for two-component monolayers of four different perfluorocarboxylic acids [FCn; perfluorododecanoic acid (FC12), perfluorotetradecanoic acid (FC14), perfluorohexadecanoic acid (FC16), and perfluorooctadecanoic acid (FC18)] and dimyristoylphosphatidylethanolamine (DMPE) on 0.15M NaCl (pH 2) at 298.2K. The present study is focused on the miscibility and the interfacial behavior for the binary DMPE/FCn monolayers upon compression. From the isotherms, the miscibility has been elucidated in terms of the additivity rule, the interaction parameter, and the interaction energy. The interaction parameter (or energy) is compared with that for the previous dipalmitoylphosphatidylcholine (DPPC)/FCn systems [Colloids Surf. B 41 (2005) 285-298] to understand the effect of phospholipids' polar headgroup on the binary miscibility. Furthermore, the phase behavior of the DMPE/FCn systems has been morphologically examined using fluorescence microscopy (FM) and atomic force microscopy (AFM). These images reveal the different interaction modes among the four systems; DMPE can be miscible with FC12 and FC14 and immiscible with FC16 and FC18 in the monolayer state. These systematic examinations indicate that the miscibility of perfluorocarboxylic acids and phospholipids depends on combination of hydrocarbon and fluorocarbon chain lengths and on phospholipids' polar headgroups within a monolayer.

Langmur monolayers of cerebroside with different head groups originated from sea cucumber: binary systems with dipalmitoylphosphatidylcholine (DPPC)

Surface properties (Langmuir monolayer) of two different cerebrosides which are extracted from the sea cucumber (Bohadschia argus) were investigated. A main difference in chemical structure of cerebroside between BAC-2a and BAC-4 is their head groups (glucose and galactose, respectively). Furthermore, miscibility and interaction between dipalmitoylphosphatidylcholine (DPPC) and cerebrosides (BAC-2a and BAC-4) in the monolayer have been systematically examined. The surface pressure (pi)-area (A), the surface potential (Delta V)-A, and the dipole moment (mu(perpendicular))-A isotherms for monolayers of DPPC, cerebrosides, and their binary combinations have been measured using the Wilhelmy method and the ionizing electrode method. BAC-4 forms a stable liquid-expanded (LE) monolayer, whereas BAC-2a has a first-order phase transition from the LE phase to the liquid-condensed (LC) state on 0.15 M NaCl at 298.2 K. The fundamental properties for each cerebroside monolayer were elucidated in terms of the surface dipole moment based on the three-layer model [R.J. Demchak, T. Fort Jr., J. Colloid Interface Sci. 46 (1974) 191-202] for both cerebrosides and the apparent molar quantity change (Delta s(gamma), Delta h(gamma), and Delta u(gamma)) for BAC-2a. In addition, their miscibility with DPPC was examined by the variation of the molecular areas and the surface potentials as a function of cerebroside mole fractions, the additivity rule. The miscibility was also confirmed by constructing the two-dimensional phase diagrams. The phase diagrams for the both binary systems were of negative azeotropic type. That is, the two-component DPPC/BAC-2a and DPPC/BAC-4 monolayers are miscible. Furthermore, the Joos equation for the analysis of the collapse pressure of binary monolayers allowed calculation of the interaction parameter and the interaction energy between the DPPC and cerebroside monolayers. The miscibility in the monolayer state was also confirmed by the morphological observation with Brewster angle microscopy (BAM), fluorescence microscopy (FM), and atomic force microscopy (AFM).

Dipolar interactions and miscibility in binary Langmuir monolayers with opposite dipole moments of the hydrophilic heads

We investigate unusual binary Langmuir monolayers with the same long CH3(CH2)21 hydrocarbon chains and fluorinated -O-CH2CF3 (FEE) versus nonfluorinated -O-CH2CH3 (EE) hydrophilic heads, whose opposite dipoles assist miscibility, in contrast to the equally oriented polar head dipoles of almost all natural or synthetic amphiphiles that minister to phase separation. Although two-component bulk micelles, lipid bilayers, and monolayers with fluorinated and nonfluorinated chains, which also have opposite dipoles, often show phase separation, we find complete miscibility and nonideality of the FEE-EE mixtures demonstrated via deviation of the composition dependencies of the mean molecular area at fixed surface pressure from the additivity rule. The composition dependencies of the excess molecular areas exhibit minima and maxima which show specific structural changes at particular compositions. They originate from the dipolar and steric interactions between the polar heads, because the interactions between the same chains of FEE and EE do not vary. The pi/A isotherms and the pi/X(FEE) phase diagram reveal that mixtures with molar fractions X(FEE) > or = 0.3 exist in an upright solid phase even in uncompressed state. This result is confirmed by the compressibility values and via Brewster angle microscopy, which does not show optical anisotropy at X(FEE) > or = 0.3. Comparison of the collapse and phase-transition molecular areas with literature data suggests that the upright architecture corresponds to LS-phase or S-phase with more defects as the S-phase in the pure monolayers. The mixtures with X(FEE) < 0.3 exist in tilted L2' phase at low surface pressures. Their mean molecular areas are smaller than the corresponding values in the EE film, which manifests reduction of the tilt of the EE chains with increasing FEE content. We ascribe the chain erection to partial dehydration of the EE heads caused by dipolar attraction between the EE and FEE heads. The excess free energy of mixing deltaG(exc)pi is positive but much smaller than the negative total free energy of mixing AG mix(pi) showing a spontaneous miscibility at all compositions due to an entropy increase. The analysis of the conflict between the deltaG(mix)pi minimum at molar fraction X(FEE) = 0.5 and the minimum and negative value of the excess molecular area A(pi,exc) at X(FEE) = 0.8 shows that the A(pi,exc)/X(FEE) minimum has not an electrostatic but a short-range structural origin.

Pulmonary surfactant model systems catch the specific interaction of an amphiphilic peptide with anionic phospholipid

Interfacial behavior was studied in pulmonary surfactant model systems containing an amphiphilic alpha-helical peptide (Hel 13-5), which consists of 13 hydrophobic and five hydrophilic amino acid residues. Fully saturated phospholipids of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) were utilized to understand specific interactions between anionic DPPG and cationic Hel 13-5 for pulmonary functions. Surface pressure (pi)-molecular area (A) and surface potential (DeltaV)-A isotherms of DPPG/Hel 13-5 and DPPC/DPPG (4:1, mol/mol)/Hel 13-5 preparations were measured to obtain basic information on the phase behavior under compression and expansion processes. The interaction leads to a variation in squeeze-out surface pressures against a mole fraction of Hel 13-5, where Hel 13-5 is eliminated from the surface on compression. The phase behavior was visualized by means of Brewster angle microscopy, fluorescence microscopy, and atomic force microscopy. At low surface pressures, the formation of differently ordered domains in size and shape is induced by electrostatic interactions. The domains independently grow upon compression to high surface pressures, especially in the DPPG/Hel 13-5 system. Under the further compression process, protrusion masses are formed in AFM images in the vicinity of squeeze-out pressures. The protrusion masses, which are attributed to the squeezed-out Hel 13-5, grow larger in lateral size with increasing DPPG content in phospholipid compositions. During subsequent expansion up to 35 mN m(-1), the protrusions retain their height and lateral diameter for the DPPG/Hel 13-5 system, whereas the protrusions become smaller for the DPPC/Hel 13-5 and DPPC/DPPG/Hel 13-5 systems due to a reentrance of the ejected Hel 13-5 into the surface. In this work we detected for the first time, to our knowledge, a remarkably large hysteresis loop for cyclic DeltaV-A isotherms of the binary DPPG/Hel 13-5 preparation. This exciting phenomenon suggests that the specific interaction triggers two completely independent processes for Hel 13-5 during repeated compression and expansion: 1), squeezing-out into the subsolution; and 2), and close packing as a monolayer with DPPG at the interface. These characteristic processes are also strongly supported by atomic force microscopy observations. The data presented here provide complementary information on the mechanism and importance of the specific interaction between the phosphatidylglycerol headgroup and the polarized moiety of native surfactant protein B for biophysical functions of pulmonary surfactants.

Comparing experimental and simulated pressure-area isotherms for DPPC

Although pressure-area isotherms are commonly measured for lipid monolayers, it is not always appreciated how much they can vary depending on experimental factors. Here, we compare experimental and simulated pressure-area isotherms for dipalmitoylphosphatidylcholine (DPPC) at temperatures ranging between 293.15 K and 323.15 K, and explore possible factors influencing the shape and position of the isotherms. Molecular dynamics simulations of DPPC monolayers using both coarse-grained (CG) and atomistic models yield results that are in rough agreement with some of the experimental isotherms, but with a steeper slope in the liquid-condensed region than seen experimentally and shifted to larger areas. The CG lipid model gives predictions that are very close to those of atomistic simulations, while greatly improving computational efficiency. There is much more variation among experimental isotherms than between isotherms obtained from CG simulations and from the most refined simulation available. Both atomistic and CG simulations yield liquid-condensed and liquid-expanded phase area compressibility moduli that are significantly larger than those typically measured experimentally, but compare well with some experimental values obtained under rapid compression.

Langmuir monolayer of artificial pulmonary surfactant mixtures with an amphiphilic peptide at the air/water interface: comparison of new preparations with surfacten (Surfactant TA)

Interfacial behavior was studied on the pulmonary lipid mixture containing a newly designed amphiphilic alpha-helical peptide (Hel 13-5) that consists of 13 hydrophobic and 5 hydrophilic amino acid residues. Moreover, the data obtained were compared with those of commercially available Surfacten (Surfactant TA) which has been clinically used for neonatal respiratory distress syndrome (NRDS) in Japan. Surface pressure (pi)-A and surface potential (DeltaV)-area (A) isotherms were measured for our synthetic preparations and Surfacten. Herein, a mixture of dipalmitoylphosphatidylcholine (DPPC)/egg-phosphatidylglycerol (PG)/palmitic acid (PA) (68:22:9 by weight) was used as the constituent of basic preparations. Monolayers were spread on 0.02 M Tris buffer (pH 7.4) with 0.13 M NaCl at the air/liquid interface, and the surface behavior was investigated by employing the Wilhelmy method, an ionizing electrode method, and fluorescence microscopy (FM). Cyclic compression and expansion isotherms of the prepared materials (or products) (DPPC/PG/PA/Hel 13-5) were examined to confirm the spreading and respreading ability. For the prepared products, a plateau region exists on pi-A and DeltaV-A isotherms at approximately 42 mN m(-1), indicating that Hel 13-5 is squeezed out of surface monolayers together with fluid components (PG) upon lateral compression. That is, the squeeze-out phenomenon induces a 2D-3D phase transformation. In particular, the inclination of the pi-A isotherms at X(Hel 13-5) = 0.1 in the plateau region was almost zero irrespective of the molecular area. As proposed in the earlier report (Nakahara, H.; Lee, S.; Sugihara, G.; Shibata, O. Langmuir 2006, 22, 5792-5803), an observed refluorescence phenomenon was discussed for FM measurements. This phenomenon provides evidence of the squeeze-out motion with fluid molecules. Furthermore, the cyclic pi-A and DeltaV-A isotherms show larger hysteresis areas and better respreading abilities in comparison with the previous ternary systems (DPPC/PG/Hel 13-5 and DPPC/PA/Hel 13-5) that are very important properties in pulmonary functions. FM photographs and the temperature dependence of pi-A and DeltaV-A isotherms suggest that the phase behavior of the present preparation product is very similar to that of Surfacten in terms of the domain size and in parameters such as collapse pressures, maximum DeltaV values, and so on. These results demonstrate that PG and PA even in the present preparations work well for compression-expansion cycling as is the case in the previous ternary systems, and the present preparations show comparable properties to Surfacten in vitro.

Purification and thermal analysis of perfluoro-n-alkanoic acids

Purification of perfluoro-n-alkanoic acids (C(n)F(2n+1)COOH, n=7, 9, 11, 13, 15 and 17) was made by repeated recrystallizations from n-hexane/acetone mixed solvent, and their purity was found to be more than 99.5% by GC-MS, NMR, and elemental analysis. The thermal behaviors such as melting point and enthalpy change of fusion were investigated using differential scanning calorimetry (DSC). The melting point monotonously increased with increasing carbon number (n) of the acids, while the enthalpy change showed irregularity at n=14. The crystal structure of these acids was found to be dependent upon solvent used for recrystallization; that is, the acids recrystallized from the above solvent becomes more stable energetically, indicating their higher enthalpy change of fusion than that of the solidified acids from fused ones. The solid state was also found to vary depending upon the thermal history, indicating that a few crystal structures of the solid state are quite similar energetically. The melting points (T(m)) of perfluoro-n-alkanoic acids are higher than those of corresponding n-alkanoic acids, and the difference in T(m) increases with increasing carbon number in the acids.

Two-component Langmuir monolayers of single-chain partially fluorinated amphiphiles with dipalmitoylphosphatidylcholine (DPPC)

The surface pressure (pi)-area (A) and surface potential (DeltaV)-A isotherms were measured for two-component monolayers made of dipalmitoylphosphatidylcholine (DPPC)/single-chain (perfluorooctyl)pentanol (F8C5OH) and DPPC/single-chain (perfluorooctyl)pentylphosphocholine (F8C5PC) on a substrate solution of 0.15 M NaCl at 293.2 K as a function of the composition of the two components. The Langmuir method and the ionizing electrode method were used. The data for these systems were analyzed using an additivity rule. Assuming a regular surface mixture, the Joos equation, which allows description of the collapse pressure of a monolayer made of two miscible components, was used to establish the miscibility within the monolayer. An interaction parameter and an interaction energy were calculated. The two-component DPPC/F8C5OH and DPPC/F8C5PC monolayers were miscible. Furthermore, the mean molecular area, surface potential, and phase diagrams enabled us to determine the molecular orientation of DPPC/F8C5OH and DPPC/F8C5PC in the monolayer. Two types of phase diagrams were obtained and classified into the positive azeotropic and negative azeotropic types. Fluorescence microscopy (FM) and Brewster angle microscopy (BAM) for the DPPC/F8C5OH and DPPC/F8C5PC systems show that both systems can dissolve the ordered micrometer-size solid DPPC domains. However, morphological analyses using atomic force microscopy (AFM) suggest partial miscibility or phase separation for DPPC and the partially fluorinated compounds on the nanometer scale. In particular, triskelion-shaped domains were evidenced for F8C5OH. These results indicate that the partially fluorinated amphiphiles investigated here fluidize the DPPC monolayer upon lateral compression and that these chemicals may be useful to develop their innovative applications in the biomedical field.

Water evaporation rates across hydrophobic acid monolayers at equilibrium spreading pressure

The effect of alkanoic acid [CH(3)(CH(2))(n-2)COOH; HCn] and perfluoroalkanoic acid [CF(3)(CF(2))(n-2)COOH; FCn] monolayers on the water evaporation rate was investigated by thermogravimetry tracing the decrease in amount of water with time. The evaporation rate from the surface covered by a monolayer was measured as a function of temperature and hydrophobic chain length of the acids, where the monolayer was under an equilibrium spreading pressure. From thermal behavior of the crystallized acids, their solid states are C-type in crystalline state over the temperature range from 298.2 to 323.2 K. The dry air was flowed through a furnace tube of a thermogravimetry apparatus at the flow rate of 80 mL min(-1), where the evaporation rate becomes almost constant irrespective of the flow rate. The temperature dependence of the evaporation rate was analyzed kinetically to evaluate the activation energy and thermodynamics values for the activated complex, which demonstrated that these values were almost the same for both alkanoic acids and perfluoroalkanoic acids, although the effect of perfluoroalkanoic acids on the evaporation rate was smaller than that of corresponding hydrogenated fatty acids. The difference in the evaporation rate between FCn and HCn was examined by atomic force microscopy (AFM), Brewster angle microscopy (BAM), surface potential (DeltaV) at equilibrium spreading pressure, and Langmuir curve (pi-A isotherm), and their results were consistent and supported the difference.

Nanofibers and nanospirals fabricated through the interfacial organization of a partially fluorinated compound

Nanostructures composed of fluorinated compounds are of great interest both for fundamental investigations and practical applications. In this paper, we have investigated the supramolecular assembly of two carbamate derivatives, 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-henicosa fluorododecyl 1-naphthylcarbamate (F10C2Np) and 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-henicosa fluorododecyl phenylcarbamate (F10C2Ph), which bear partially fluorinated alkyl tails on their molecular skeletons, through the air/water interfacial organization. It has been found that F10C2Np could form nanofiber or nanospiral structure with a dimension of several micrometers, while F10C2Ph formed straight ribbon-like structures. More interestingly, when the multilayer films of both of the compounds were subjected to the circular dichroism (CD) measurements, distinct CD signals could be detected, although the compounds themselves are optically inactive. Based on a series of characterizations on the films and the structural features of the molecules, a size mismatching effect was suggested to explain the interesting phenomenon.

Langmuir monolayer properties of perfluorinated double long-chain salts with divalent counterions of separate electric charge at the air-water interface

The novel perfluorinated double long-chain salts with divalent counterions of separate electric charge, 1,1-(1,omega-alkanediyl)-bispyridinium perfluorotetradecane- carboxylate [CnBP(FC14)2 : n = 2, 6, 10, 14], were newly synthesized and their interfacial behavior was investigated by Langmuir monolayer methods. Surface properties [surface pressure (pi)-, surface potential (DeltaV)-, dipole moment (micro perpendicular)-area (A) isotherms] and morphological images of CnBP(FC14)2 monolayers on a subphase of water and on various NaCl concentrations were measured by employing the Wilhelmy method, the ionizing electrode method, fluorescence microscopy (FM), and Brewster angle microscopy (BAM). CnBP(FC14)2 formed a stable monolayer on water at 298.2 K, where these pi-A isotherms shifted to a larger molecular area with increasing charge separation and had no transition point from a disordered phase to an ordered one. On the contrary, the pi-A isotherms on NaCl solutions moved to the smaller areas, showed the transition and higher collapse pressures compared to the pi-A isotherms on water. These results suggested that a sodium chloride subphase induced the condensation of CnBP(FC14)2 molecules upon compression. In addition, it is quite noticeable that a dissociation of CnBP counterion from CnBP(FC14)2 occurs on NaCl solutions, depending on the extent of charge separation. This phenomenon was supported by the changes of the limiting area, transition pressure, collapse pressure, repeated compression-expansion cycle curve, and DeltaV behavior of perfluorotetradecanoic acid (FC14). Furthermore, temperature dependence of these monolayers was investigated, and an apparent molar quantity change on the phase transition was evaluated on 0.15 M NaCl. The morphological behavior of CnBP(FC14)2 and FC14 monolayers was also confirmed by FM and BAM images.

Influence of Perfluorinated Compounds on the Properties of Model Lipid Membranes

The influence of selected perfluorinated compounds (PFCs), perfluorooctanoic acid (PFOA) or perfluorooctanesulfonic acid (PFOS), on the structure and organization of lipid membranes was investigated using model membranes-lipid monolayers and bilayers. The simplest model--a lipid monolayer--was studied at the air-water interface using the Langmuir-Blodgett technique with surface pressure and surface potential measurements. Lipid bilayers were characterized by NMR techniques and molecular dynamics simulations. Two phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), characterized by different surface properties have been chosen as components of the model membranes. For a DPPC monolayer, a phase transition from the liquid-expanded state to the liquid-condensed state can be observed upon compression at room temperature, while a DMPC monolayer under the same conditions remains in the liquid-expanded state. For each of the two lipids, the presence of both PFOA and PFOS leads to the formation of a more fluidic layer at the air-water interface. Pulsed field gradient NMR measurements of the lateral diffusion coefficient (DL) of DMPC and PFOA in oriented bilayers reveal that, upon addition of PFOA to DMPC bilayers, DL of DMPC decreases for small amounts of PFOA, while larger additions produce an increased DL. The DL values of PFOA were found to be slightly larger than those for DMPC, probably as a consequence of the water solubility of PFOA. Furthermore, 31P and 2H NMR showed that the gel-liquid crystalline phase transition temperature decreased by the addition of PFOA for concentrations of 5 mol % and above, indicating a destabilizing effect of PFOA on the membranes. Deuterium order parameters of deuterated DMPC were found to increase slightly upon increasing the PFOA concentration. The monolayer experiments reveal that PFOS also penetrates slowly into already preformed lipid layers, leading to a change of their properties with time. These experimental observations are in qualitative agreement with the computational results obtained from the molecular dynamics simulations showing a slow migration of PFCs from the surrounding water phase into DPPC and DMPC bilayers.

Gramicidin A Channel in a Matrix from a Semifluorinated Surfactant Monolayer

Gramicidin A, a polypeptide antibiotic forming transmembrane ion channels, has been incorporated into a Langmuir monolayer formed by a semifluorinated alkane (SFA). In this work, partially fluorinated tetracosane, perfluorohexyloctadecane (F6H18), has been applied, aiming at finding a suitable matrix for gramicidin A to be transferred onto solid support for a biosensor design. For this purpose, the physiological conditions were of special interest (mixed monolayers containing low gramicidin proportion and the surface pressure of 30 mN/m). Mixed monolayers of gramicidin and SFA were found to be miscible within the whole range of mole fractions. A very significant increase of the stability of SFA monolayer has been found in the presence of gramicidin, even at such a low proportion as X(gramicidin) = 0.1, which is reflected in a 3.5-fold increase of the collapse pressure value of mixed monolayer as compared to the film from pure SFA. This interesting phenomenon has been interpreted as being due to the existence of a strong dipole-dipole interaction between both film-forming molecules. Opposite sign of the measured electric surface potential for gramicidin and SFA, resulting from different directions of the dipole moment vectors in both film molecules, implies that the ordered, antiparallel orientation of the dipole moments in the mixed gramicidin/SFA system can be responsible for its extremely high stability.

Mode of interaction of ganglioside Langmuir monolayer originated from echinoderms: Three binary systems of ganglioside/DPPC, ganglioside/DMPE, and ganglioside/cholesterol

The surface pressure (pi)-area (A), the surface potential (DeltaV)-A, and the dipole moment (mu( perpendicular))-A isotherms were obtained for monolayers made from a ganglioside originated from echinoderms [Diadema setosum ganglioside (DSG-1)], dipalmitoylphosphatidylcholine (DPPC), dimyristoylphosphatidylethanolamine (DMPE), cholesterol (Ch), and their combinations. Monolayers spread on several different substrates were investigated at the air/water interface by the Wilhelmy method, ionizing electrode method, fluorescence microscopy (FM) and atomic force microscopy (AFM). Surface potentials (DeltaV) of pure components were analyzed using the three-layer model proposed by Demchak and Fort [R.J. Demchak, T. Fort, J. Colloid Interface Sci. 46 (1974) 191-202]. The new finding was that DSG-1 was stable and showed a liquid-expanded film and that its monolayer behavior of DeltaV was sensitive for the change of the NaCl concentration in the subphase. Moreover, the miscibility of DSG-1 and three major lipids in the two-component monolayers was examined by plotting the variation of the molecular area and the surface potential as a function of the DSG-1 molar fraction (X(DSG-1)), using the additivity rule. From the A-X(DSG-1) and DeltaV(m)-X(DSG-1) plots, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined at the discrete surface pressure. The PMA and APSP with the mole fraction were extensively discussed for the miscible system. The miscibility was also investigated from the two-dimensional phase diagrams. Furthermore, a regular surface mixture, for which the Joos equation was used for the analysis of the collapse pressure of two-component monolayers, allowed calculation of the interaction parameter (xi) and the interaction energy (-Deltavarepsilon) between them. The observations using fluorescence microscopy and AFM image also provide us the miscibility in the monolayer state.