Grazing incidence X-ray diffraction studies of condensed double-chain phospholipid monolayers formed at the soft air/water interface

Ionizable lipids based on branched fatty acids - An explorative study on Langmuir monolayers

Ionizable lipids are a class of pharmaceutical excipients with a main application in lipid nanoparticles for nucleic acid delivery. New ionizable lipids are needed to tune characteristics of lipid-based nucleic acid delivery systems, e.g. stability, nucleic acid loading capacity and binding strength, as well as bio-distribution. Herein, we present the synthesis of three novel ionizable lipids as putative excipients for lipid-based nucleic acid delivery systems. Langmuir monolayer experiments with classical surface pressure/area isotherm evaluation were used to understand the self-assembly behavior of the lipids. Additional experiments with surface sensitive techniques, namely grazing incidence x-ray scattering and infrared reflection-absorption spectroscopy (IRRAS), were performed to understand structural characteristics of lipid associates. The latter technique was also used to investigate the nucleic acid binding process between DNA and the ionizable lipids. Finally, first transfection experiments with the novel lipids formulated as cationic liposomes were performed providing first efficacy data. Although the alkyl chain pattern was comparable for all three ionizable lipids, the results demonstrated that with increasing head-group size the DNA binding capacity changed and the alkyl chain fluidity was increased. The lipid with the lowest phase transition temperature and the smallest packing parameter showed the highest DNA transfer efficiency.

Statin Action Targets Lipid Rafts of Cell Membranes: GIXD/PM-IRRAS Investigation of Langmuir Monolayers

Lipid rafts are condensed regions of cell membranes rich in cholesterol and sphingomyelin, which constitute the target for anticholesterolemic drugs - statins. In this work, we use for the first time a combined grazing-incidence X-ray diffraction (GIXD)/polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS)/Brewster angle microscopy (BAM) approach to show the statin effect on model lipid rafts and its components assembled in Langmuir monolayers at the air-water interface. Two representatives of these drugs, fluvastatin (FLU) and cerivastatin (CER), of different hydrophobicity were chosen, while cholesterol (Chol) and sphingomyelin (SM), and their 1:1 mixture were selected to form condensed monolayers of lipid rafts. The effect of statins on the single components of lipid rafts indicated that both the hydrophobicity of the drugs and the organization of the layer determined the drug-lipid interaction. For cholesterol monolayers, only the most hydrophobic CER was effectively changing the film structure, while for the less organized sphingomyelin, the biggest effect was observed for FLU. This drug affected both the polar headgroup region as shown by PM-IRRAS results and the 2D crystalline structure of the SM monolayer as evidenced by GIXD. Measurements performed for Chol/SM 1:1 models proved also that the statin effect depends on the presence of Chol-SM complexes. In this case, the less hydrophobic FLU was not able to penetrate the binary layer at all, while exposure to the hydrophobic CER resulted in the phase separation and formation of ordered assemblies. The changes in the membrane properties were visualized by BAM images and GIXD patterns and confirmed by thermodynamic parameters of hysteresis in the Langmuir monolayer compression-decompression experiments.

Effect of Anionic Lipids on Mammalian Plasma Cell Membrane Properties

The effect of lipid composition on models of the inner leaflet of mammalian cell membranes has been investigated. Grazing incidence X-ray diffraction and X-ray and neutron reflectivity have been used to characterize lipid packing and solvation, while electrochemical and infrared spectroscopic methods have been employed to probe phase behavior in an applied electric field. Introducing a small quantity of the anionic lipid dimyristoylphosphatidylserine (DMPS) into bilayers of zwitterionic dimyristoylphosphatidylethanolamine (DMPE) results in a significant change in the bilayer response to an applied field: the tilt of the hydrocarbon chains increases before returning to the original tilt angle on detachment of the bilayer. Equimolar mixtures, with slightly closer chain packing, exhibit a similar but weaker response. The latter also tend to incorporate more solvent during this electrochemical phase transition, at levels similar to those of pure DMPS. Reflectivity measurements reveal greater solvation of lipid layers for DMPS > 30 mol %, matching the greater propensity for DMPS-rich bilayers to incorporate water. Taken together, the data indicate that the range of 10-35 mol % DMPS provides optimum bilayer properties (in flexibility and function as a barrier), which may explain why the DMPS content of cell membranes tends to be found within this range.

Characterization of lipid bilayers adsorbed to functionalized air/water interfaces

Lipid bilayers immobilized in planar geometries, such as solid-supported or "floating" bilayers, have enabled detailed studies of biological membranes with numerous experimental techniques, notably X-ray and neutron reflectometry. However, the presence of a solid support also has disadvantages as it complicates the use of spectroscopic techniques as well as surface rheological measurements that would require surface deformations. Here, in order to overcome these limitations, we investigate lipid bilayers adsorbed to inherently soft and experimentally well accessible air/water interfaces that are functionalized with Langmuir monolayers of amphiphiles. The bilayers are characterized with ellipsometry, X-ray scattering, and X-ray fluorescence. Grazing-incidence X-ray diffraction reveals that lipid bilayers in a chain-ordered state can have significantly different structural features than regular Langmuir monolayers of the same composition. Our results suggest that bilayers at air/water interfaces may be well suited for fundamental studies in the field of membrane biophysics.

The Effect of pH on the Structure and Lateral Organization of Cardiolipin in Langmuir Monolayers

Cardiolipin (CL) is a unique phospholipid featuring a dimeric structure. With its four alkyl chains, it has a large hydrophobic region and the charged hydrophilic head group is relatively small. Biological membranes exhibit CL exclusively in the inner bacterial and mitochondrial membranes. Alteration of CL packing can lead to structural changes and membrane instabilities. One environmental influence is the change in pH. Since the acidic properties of the phosphate head groups remain still controversial in literature, this work focusses on the influence of pH on the ionization degree of CL. For the analyses, surface pressure (π) - molecular area (A) isotherm experiments were combined with total reflection X-ray fluorescence (TRXF) and grazing incidence X-ray diffraction (GIXD). Continuous ionization with a high CL packing density was observed in the monolayer over a wide pH range. No individual pKa values can be assigned to the two phosphate groups, but mutual influence is observed.

pH-Dependent physicochemical properties of ornithine lipid in mono- and bilayers

In certain bacteria, phosphatidylethanolamine lipids (PEL) get largely replaced by phosphate-free ornithine lipids (OL) under conditions of phosphate starvation. It has so far been unknown how much these two lipid types deviate in their physicochemical properties, and how strongly bacteria thus have to adapt in order to compensate for the difference. Here, we use differential scanning calorimetry, X-ray scattering, and X-ray fluorescence to investigate the properties of OL with saturated C14 alkyl chains in mono- and bilayers. OL is found to have a greater tendency than chain-analogous PEL to form ordered structures and, in contrast to PEL, even a molecular superlattice based on a hydrogen bonding network between the headgroups. This superlattice is virtually electrically uncharged and persists over a wide pH range. Our results indicate that OL and PEL behave very differently in ordered single-component membranes but may behave more similarly in fluid multicomponent membranes.

Phase behavior and miscibility in lipid monolayers containing glycolipids

HYPOTHESIS

Glycolipids in biological membranes are ubiquitous and believed to be involved in the formation of ordered functional domains. However, our current knowledge about such glycolipid-enriched domains is limited because they are inherently difficult to characterize.

EXPERIMENTS

We use grazing-incidence X-ray diffraction, isotherm measurements, and Brewster angle microscopy to investigate the phase behavior and miscibility in Langmuir lipid monolayers containing glycolipids.

FINDINGS

Glycolipid-enriched domains give rise to distinct diffraction patterns that allow for a systematic structural investigation and reveal a rich phenomenology, ranging from near-complete demixing to the formation of mixed domains with unique features. The phase behavior is governed by the headgroup chemistry and by the length and saturation of the tails.

Self-Assembly of Graphene Oxide Nanoflakes in a Lipid Monolayer at the Air-Water Interface

The graphene family, especially graphene oxide (GO), has captured increasing prospects in the biomedical field due to its excellent physicochemical properties. Understanding the health and environmental impact of GO is of great importance for guiding future applications. Although their interactions with living organisms are omnipresent, the exact molecular mechanism is yet to be established. The cellular membrane is the first barrier for a foreign molecule to interact before entering into the cell. In the present study, a model system consisting of a lipid monolayer at the air-water interface represents one of the leaflets of this membrane. Surface pressure-area isotherms and advanced synchrotron X-ray scattering techniques have been employed to comprehend the interaction by varying the electrostatics of the membrane. The results depict a strong GO interaction with positively charged phospholipids, weak interaction with zwitterionic lipids, and interestingly negligible interaction with negatively charged lipids. GO flakes induce significant changes in the out-of-plane organization of a positively charged lipid monolayer with a minor influence on in-plane assembly of lipid chains. This interaction is packing-specific, and the influence of GO is much stronger at lower surface pressure. Even though for zwitterionic phospholipids, the GO flakes may partly insert into the lipid chains, the X-ray scattering results indicate that the flakes preferentially lie horizontally underneath the positively charged lipid monolayer. This in-depth structural description may pave new perspectives for the scientific community for the development of GO-based biosensors and biomedical materials.

Interface-Mediated Mechanism of Action-The Root of the Cytoprotective Effect of Immediate-Release Omeprazole

Omeprazole is usually administered under an enteric coating. However, there is a Food and Drug Administration-approved strategy that enables its release in the stomach. When locally absorbed, omeprazole shows a higher efficacy and a cytoprotective effect, whose mechanism was still unknown. Therefore, we aimed to assess the effect of the absorption route on the gastric mucosa. 2D and 3D models of dipalmitoylphosphatidylcholine (DPPC) at different pH values (5.0 and 7.4) were used to mimic different absorption conditions. Several experimental techniques, namely, fluorescence studies, X-ray scattering methodologies, and Langmuir monolayers coupled with microscopy, X-ray diffraction, and infrared spectroscopy techniques, were combined with molecular dynamics simulations. The results showed that electrostatic and hydrophobic interactions between omeprazole and DPPC rearranged the conformational state of DPPC. Omeprazole intercalates among DPPC molecules, promoting domain formation with untilted phospholipids. Hence, the local release of omeprazole enables its action as a phospholipid-like drug, which can reinforce and protect the gastric mucosa.

An Azidolipid Monolayer - Transitions, Miscibility, and UV Reactivity Studied by Infrared Reflection Absorption Spectroscopy

In this study, we characterized monolayers of an azide-modified lipid at the air-water interface, pure and in its mixtures with the model lipid DPPC, with the aim of proving its potential to be applied for photo-cross-linking with other molecules. We chose a phospholipid bearing a terminal azide group in one of its hydrophobic tails to study its monolayer characteristics with the Langmuir film balance technique. Furthermore, we performed infrared reflection absorption spectroscopy (IRRAS) to get detailed insights into the organization of those monolayers as well as high-resolution mass spectrometry (HRMS) to see the effects of UV-irradiation on the lipids' chemical structure and organization. Our results suggest that in expanded monolayers of pure azide-modified membrane lipids, the azido-terminated chain folds back toward the air-water interface. Above the LE/LC (liquid-expanded/liquid-condensed) phase transition, the chains stretched, and thus, the azide group detaches from the interface. From temperature-dependent monolayer compressions, we evaluated all relevant thermodynamic parameters of the monolayers, such as the phase transition pressure, the critical temperature, and the triple point, and compare them to those of model lipids. For future applications, we studied the miscibility of the azide-modified lipid with DPPC in monolayers and found at least a certain miscibility over all investigated mixing ratios ranging from 10 to 75% of the azidolipid. Finally, we irradiated the azidolipid monolayer with UV light at 305 nm and measured photodissociation of the azide, leading to chemical cross-linking with other lipids, which shows the potential to be used as a cross-linking agent within self-assembled lipid or lipid/protein layers.

Monolayer behavior of pure F-DPPC and mixed films with DPPC studied by epifluorescence microscopy and infrared reflection absorption spectroscopy

The monolayer behavior of a l-DPPC derivative with a single fluorination in one of its terminal methyl groups (F-DPPC) at air-water interface was investigated by epifluorescence microscopy and infrared reflection absorption spectroscopy (IRRAS). Epifluorescence microscopy was utilized to study the shape and morphology of liquid-condensed (LC) domains observed upon compression of the film. IRRAS was employed for the determination of chain order and orientation. The shapes of LC-domains in a monolayer of F-DPPC are more dependent on the rate of compression than those of DPPC. The LC domains of F-DPPC display pronounced fractal growth patterns depending on the compression speed. The evolution of LC domain occurs under dominating electrostatic dipolar forces in F-DPPC. IRRAS measurements with the analysis of the frequency of the methylene stretching vibrations as a function of film compression show that the acyl chains in an F-DPPC monolayer in the LE-phase are more disordered than those in a DPPC film. The reason for the higher chain disorder in LE phase F-DPPC monolayers is a back folding of the fluorinated sn-2 chain terminus towards the air-water interface leading to larger molecular area requirement. Angular dependent IRRA spectra of monolayers at a surface pressure of 30 mN m^-1 show that in the LC phase DPPC and F-DPPC exhibit a similar tilt of the acyl chains of ca. 28-30 ° relative to the surface normal. F-DPPC is ideally miscible with l-DPPC-d₆₂ having the same chirality, as indicated by epifluorescence images and by IRRAS. However, the LC domains in an equimolar mixture of d-DPPC and F-DPPC having opposite chirality show multi-lobed complex domain patterns indicating chiral phase separation within LC domains.

The Impact of Alkyl-Chain Purity on Lipid-Based Nucleic Acid Delivery Systems - Is the Utilization of Lipid Components with Technical Grade Justified?

The physicochemical properties and transfection efficacies of two samples of a cationic lipid have been investigated and compared in 2D (monolayers at the air/liquid interface) and 3D (aqueous bulk dispersions) model systems using different techniques. The samples differ only in their chain composition due to the purity of the oleylamine (chain precursor). Lipid 8 (using the oleylamine of technical grade for cost-efficient synthesis) shows lateral phase separation in the Langmuir layers. However, the amount of attached DNA, determined by IRRAS, is for both samples the same. In 3D systems, lipid 8 p forms cubic phases, which disappear after addition of DNA. At physiological temperatures, both lipids (alone and in mixture with cholesterol) assemble to lamellar aggregates and exhibit comparable DNA delivery efficiency. This study demonstrates that non-lamellar structures are not compulsory for high transfection rates. The results legitimate the utilization of oleyl chains of technical grade in the synthesis of cationic transfection lipids.

Licofelone-DPPC Interactions: Putting Membrane Lipids on the Radar of Drug Development

(1) Background: Membrane lipids have been disregarded in drug development throughout the years. Recently, they gained attention in drug design as targets, but they are still disregarded in the latter stages. Thus, this study aims to highlight the relevance of considering membrane lipids in the preclinical phase of drug development. (2) Methods: The interactions of a drug candidate for clinical use (licofelone) with a membrane model system made of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were evaluated by combining Langmuir isotherms, Brewster angle microscopy (BAM), polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and grazing-incidence X-ray diffraction (GIXD) measurements. (3) Results: Licofelone caused the expansion of the DPPC isotherm without changing the lipid phase transition profile. Moreover, licofelone induced the reduction of DPPC packing density, while increasing the local order of the DPPC acyl chains. (4) Conclusions: The licofelone-induced alterations in the structural organization of phosphatidylcholine monolayers may be related to its pharmacological actions. Thus, the combination of studying drug-membrane interactions with the pharmacological characterization that occurs in the preclinical stage may gather additional information about the mechanisms of action and toxicity of drug candidates. Ultimately, the addition of this innovative step shall improve the success rate of drug development.

Lysine-based amino-functionalized lipids for gene transfection: the influence of the chain composition on 2D properties

The influence of the chain composition on the physical-chemical properties will be discussed for five transfection lipids containing the same lysine-based head group. For this purpose, the chain composition will be gradually varied from saturated tetradecyl (C_14:0) and hexadecyl (C_16:0) chains to longer but unsaturated oleyl (C_18:1) chains with double bonds in the cis configuration. In this work, we investigated the lipids as Langmuir monolayers at the air-water-interface in the absence and presence of calf thymus DNA applying different techniques such as infrared reflection absorption spectroscopy (IRRAS) and grazing incidence X-ray diffraction (GIXD). The replacement of saturated tetradecyl (C_14:0) and hexadecyl (C_16:0) chains by unsaturated oleyl (C_18:1) chains increases the fluidity of the lipid monolayer: TH10 < TT10 < OH10 < OT10 < OO10 resulting in a smaller packing density. TH10 forms the stiffest and OO10 the most fluid monolayer in this structure-property study. OO10 has a higher protonation degree compared to the saturated lipids TT10 and TH10 as well as to the hybrids OT10 and OH10 because of a better accessibility of the amine groups. Depending on the bulk pH, different scenarios of DNA coupling to the lipid monolayers have been proposed.

Against the rules: pressure induced transition from high to reduced order

Envisioning the next generation of drug delivery nanocontainers requires more in-depth information on the fundamental physical forces at play in bilayer membranes. In order to achieve this, we combine chemical synthesis with physical-chemical analytical methods and probe the relationship between a molecular structure and its biophysical properties. With the aim of increasing the number of hydrogen bond donors compared to natural phospholipids, a phospholipid compound bearing urea moieties has been synthesized. The new molecules form interdigitated bilayers in aqueous dispersions and self-assemble at soft interfaces in thin layers with distinctive structural order. At lower temperatures, endothermic and exothermic transitions are observed during compression. The LC1 phase is dominated by an intermolecular hydrogen bond network of the urea moieties leading to a very high chain tilt of 52°. During compression and at higher temperatures, presumably this hydrogen bond network is broken allowing a much lower chain tilt of 35°. The extremely different monolayer thicknesses violate the two-dimensional Clausius-Clapeyron equation.

Synthesis and Biophysical Characterization of an Odd-Numbered 1,3-Diamidophospholipid

Nanomedicine suffers from low drug delivery efficiencies. Mechanoresponsive vesicles could provide an alternative way to release active compounds triggered by the basic physics of the human body. 1,3-Diamidophospholipids with C16 tails proved to be an effective building block for mechanoresponsive vesicles, but their low main phase transition temperature prevents an effective application in humans. As the main phase transition temperature of a membrane depends on the fatty acyl chain length, we synthesized a C17 homologue of a 1,3-diamidophospholipid: Rad-PC-Rad. The elevated main phase transition temperature of Rad-PC-Rad allows mechanoresponsive drug delivery at body temperature. Herein, we report the biophysical properties of Rad-PC-Rad monolayer and bilayer membranes. Rad-PC-Rad is an ideal candidate for advancing the concept of physically triggered drug release.

Malonic acid based cationic lipids - The way to highly efficient DNA-carriers

Cationic lipids play an important role as non-viral nucleic acid carriers in gene therapy since 3 decades. This review will introduce malonic acid derived cationic lipids as nucleic acid carriers which appeared in the literature dealing with lipofection 10years ago. The family of amino-functionalized branched fatty acid amides will be presented as well as different generations of malonic acid diamides. Both groups of cationic lipids yield lipid mixtures with highly efficient nucleic acid transfer activities in in-vitro cell culture models. The DNA transfer screening of lipid libraries with directed structural variations in the lipophilic as well as in the hydrophilic part of the amphiphiles yields structure/activity relationships. Furthermore, the detailed characterizations of selected lipid composites at the air/water interface and in bulk systems are summarized with regard to transfection determining physical-chemical properties. The findings are also discussed in comparison to results obtained with other families of cationic lipids.

The interaction of antimicrobial peptides with membranes

The interaction of antimicrobial peptides (AMPs) with biological membranes is in the focus of research since several years, and the most important features and modes of action of AMPs are described in this review. Different model systems can be used to understand such interactions on a molecular level. As a special example, we use 2D and 3D model membranes to investigate the interaction of the natural cyclic (Ar-1) and the synthetic linear molecule arenicin with selected amphiphiles and phospholipids. A panoply of sophisticated methods has been used to analyze these interactions on a molecular level. As a general trend, one observes that cationic antimicrobial peptides do not interact with cationic amphiphiles due to electrostatic repulsion, whereas with non-ionic amphiphiles, the peptide interacts only with aggregated systems and not with monomers. The interaction is weak (hydrophobic interaction) and requires an aggregated state with a large surface (cylindrical micelles). Anionic amphiphiles (as monomers or micelles) exhibit strong electrostatic interactions with the AMPs leading to changes in the peptide conformation. Both types of peptides interact strongly with anionic phospholipid monolayers with a preference for fluid layers. The interaction with a zwitterionic layer is almost absent for the linear derivative but measurable for the cyclic arenicin Ar-1. This is in accordance with biological experiments showing that Ar-1 forms well defined stable pores in phospholipid and lipopolysaccharide (LPS) membranes (cytotoxicity). The synthetic linear arenicin, which is less cytotoxic, does not affect the mammalian lipids to such an extent. The interaction of arenicin with bacterial membrane lipids is dominated by hydrogen bonding together with electrostatic and hydrophobic interactions.

Correlation of surface pressure and hue of planarizable push-pull chromophores at the air/water interface

It is currently not possible to directly measure the lateral pressure of a biomembrane. Mechanoresponsive fluorescent probes are an elegant solution to this problem but it requires first the establishment of a direct correlation between the membrane surface pressure and the induced color change of the probe. Here, we analyze planarizable dithienothiophene push-pull probes in a monolayer at the air/water interface using fluorescence microscopy, grazing-incidence angle X-ray diffraction, and infrared reflection-absorption spectroscopy. An increase of the lateral membrane pressure leads to a well-packed layer of the 'flipper' mechanophores and a clear change in hue above 18 mN/m. The fluorescent probes had no influence on the measured isotherm of the natural phospholipid DPPC suggesting that the flippers probe the lateral membrane pressure without physically changing it. This makes the flipper probes a truly useful addition to the membrane probe toolbox.

From Langmuir Monolayers to Multilayer Films

This feature article is intended to describe a route from Langmuir monolayers as the most suitable and well-defined models to polyelectrolyte multilayers. The latter are structurally controlled not with angstrom but with nanometer precision; however, they are very modular with regard to building blocks and function and are robust, therefore promising many diverse applications. There have been many methods developed to structurally characterize Langmuir monolayers; therefore, they serve as models in membrane biophysics and materials science as well as in general physics as two-dimensional model systems. Many of these methods as well as ideas to control interfaces could be taken over to study polyelectrolyte multilayers with their extended internal interfaces. Finally, as an outlook we try to sketch various aspects to transit toward systems with higher structural hierarchy, enabling the coupling of different functions and arriving at responsive three-dimensional systems.

Iron Ion and Iron Hydroxide Adsorption to Charge-Neutral Phosphatidylcholine Templates

Surface-sensitive X-ray scattering and spectroscopy techniques reveal significant adsorption of iron ions and iron-hydroxide (Fe(III)) complexes to a charge-neutral zwitterionic template of phosphatidylcholine (PC). The PC template is formed by a Langmuir monolayer of dipalmitoyl-PC (DPPC) that is spread on the surface of 2 to 40 μM FeCl3 solutions at physiological levels of KCl (100 mM). At 40 μM of Fe(III) as many as ∼3 iron atoms are associated with each PC group. Grazing incidence X-ray diffraction measurements indicate a significant disruption in the in-plane ordering of DPPC molecules upon iron adsorption. The binding of iron-hydroxide complexes to a neutral PC surface is yet another example of nonelectrostatic, presumably covalent bonding to a charge-neutral organic template. The strong binding and the disruption of in-plane lipid structure has biological implications on the integrity of PC-derived lipid membranes, including those based on sphingomyelin.

Impact of Structural Differences in Galactocerebrosides on the Behavior of 2D Monolayers

The molecular interactions of three biologically important galactocerebrosides have been studied in monolayers formed at the soft air/water interface as 2D model membranes. Highly surface-sensitive techniques as GIXD (grazing incidence X-ray diffraction), IRRAS (infrared reflection-absorption spectroscopy), and BAM (Brewster angle microscopy) have been used. The study reveals that small differences in the chemical structure have a relevant impact on the physical-chemical properties and intermolecular interactions. The presence of a 2-d-hydroxyl group in the fatty acid favored for GalCer C24:0 (2-OH) monolayers a higher hydration state of the headgroup at low lateral pressures (<25 mN/m) and a higher condensation effect above 30 mN/m. An opposite behavior was recorded for GalCer C24:0 and GalCer C24:1, for which the intermolecular interactions are defined by the weakly hydrated but strong H-bonded interconnected head groups. Additionally, the 15-cis-double bond in the fatty acid chain (nervonic acid) of GalCer C24:1 stabilized the LE phase but did not disturb the packing parameters of the LC phase as compared with the saturated compound GalCer C24:0.

Phase transitions in polymer monolayers: Application of the Clapeyron equation to PEO in PPO-PEO Langmuir films

In this paper we investigate the application of the two-dimensional Clapeyron law to polymer monolayers. This is a largely unexplored area of research. The main problems are (1) establishing if equilibrium is reached and (2) if so, identifying and defining phases as functions of the temperature. Once this is validated, the Clapeyron law allows us to obtain the entropy and enthalpy differences between two coexisting phases. In turn, this information can be used to obtain insight into the conformational properties of the films and changes therein. This approach has a wide potential for obtaining additional information on polymer adsorption at interfaces and the structure of their monolayer films. The 2D Clapeyron law was applied emphasizing polyethylene oxide (PEO) in polypropylene oxide (PPO)-PEO block copolymers, based on new well-defined data for their Langmuir films. Values for enthalpy per monomer of 0.12 and 0.23 kT were obtained for the phase transition of two different PEO chains (Neo of 2295 and 409, respectively). This enthalpy was estimated to correspond to 1.2±0.4 kT per EO monomer present in train conformation at the air/water interface.

Langmuir monolayers as models to study processes at membrane surfaces

The use of new sophisticated and highly surface sensitive techniques as synchrotron based X-ray scattering techniques and in-house infrared reflection absorption spectroscopy (IRRAS) has revolutionized the monolayer research. Not only the determination of monolayer structures but also interactions between amphiphilic monolayers at the soft air/liquid interface and molecules dissolved in the subphase are important for many areas in material and life sciences. Monolayers are convenient quasi-two-dimensional model systems. This review focuses on interactions between amphiphilic molecules in binary and ternary mixtures as well as on interfacial interactions with interesting biomolecules dissolved in the subphase. The phase state of monolayers can be easily triggered at constant temperature by increasing the packing density of the lipids by compression. Simultaneously the monolayer structure changes are followed in situ by grazing incidence X-ray diffraction or IRRAS. The interactions can be indirectly determined by the observed structure changes. Additionally, the yield of enzymatic reaction can be quantitatively determined, secondary structures of peptides and proteins can be measured and compared with those observed in bulk. In this way, the influence of a confinement on the structural properties of biomolecules can be determined. The adsorption of DNA can be quantified as well as the competing adsorption of ions at charged interfaces. The influence of modified nanoparticles on model membranes can be clearly determined. In this review, the relevance and utility of Langmuir monolayers as suitable models to study physical and chemical interactions at membrane surfaces are clearly demonstrated.