Thermodynamic and real-space structural evidence of a 2D critical point in phospholipid monolayers

Hierarchical structure growth across different length scales in the two-phase coexistence region of myristic acid Langmuir monolayers: correlation of static and dynamic heterogeneities

We investigated the hierarchical structure growth of myristic acid monolayers at the air-water interface across different length scales in the two-phase coexistence region of the first order liquid expanded (LE)-liquid condensed (LC) phase transition. A combined study of surface pressure-area (π-A) isotherm measurements with Brewster angle microscopy (BAM) observations was done at different temperatures. At the nanometer scale, the analysis of the π-A isotherm by application of a thermodynamic cluster equation allowed us to obtain the π dependence of cluster size (cluster distribution) in the LE-LC coexistence region. The cluster distributions showed a peak at the midpoint pressure of the transition. At higher temperature the larger nanocluster size was obtained at the transition midpoint. At the micrometer scale, BAM showed that LC domains have characteristic textures depending on the temperature. At low temperature domain density was lower and the average size of circular domains was larger. A large number of circular domains revealed a virtual boojum texture from the initial to the late stage of the transition. At the final stage some circular domains coalesced to form larger circular stripe domains and others coalesced to each other without the formation of stripe domains, finally resulting in a uniform texture over the entire water surface. At high temperature the domain texture was predominantly uniform, and a small number of domains only included straight line defects from the intermediate to the late stage of the transition. All domains coalesced to each other without the development of any texture including the stripe, different from the case at low temperature. The phase boundary line tension is highly likely to play a key role for understanding the hierarchical growth and coarsening (coalescence) process in the LE-LC transition between the different length scales from the nanometer to the micrometer scale consistently together.

Understanding interactions of plasticisers with a phospholipid monolayer

The use of DEHP (diethylhexyl phthalate) is now banned for most applications in Europe; the exception is for blood bags, where its toxicity is overshadowed by its ability to extend the storage life of red blood cells. Another plasticiser, BTHC (butanoyl trihexyl citrate), is used in paediatric blood bags but does not stabilise blood cells as effectively. Interactions between plasticisers and lipids are investigated with a phospholipid, DMPC, to understand the increased stability of blood cells in the presence of DEHP as well as bioaccumulation and identify differences with BTHC. Mixed monolayers of DMPC and DEHP or BTHC were studied on Langmuir troughs where surface pressure/area isotherms can be measured. Neutron reflection measurements were made to determine the composition and structure of these mixed layers. A large amount of plasticiser can be incorporated into a DMPC monolayer but once an upper limit is reached, plasticiser is selectively removed from the interface at high surface pressures. The upper limit is found to occur between 40-60 mol% for DEHP and 20-40 mol% for BTHC. The areas per molecule are also different with DEHP being in the range of 50-100 Å2 and BTHC being 65-120 Å2. Results indicate that BTHC does not fit as well as DEHP in DMPC monolayers which could help explain the differences observed with regards to the stability of blood cells.

Aβ Amyloid Fibers Drastically Alter the Topography and Mechanical Properties of Lipid Membranes

Alzheimer's disease (AD) is related to the fibrillation of the Aβ peptides at neuronal membranes, a process that depends on the lipid composition and may impart different physical states to the membrane. In the present work, we study the properties of the Aβ peptide when mixed with a zwitterionic lipid (DMPC), using the Langmuir monolayer technique as an approach to control membrane physical conditions. First, we build on previous characterizations of pure Aβ monolayers and observe that, in addition to high shear, these films present a pronounced compressional hysteresis. When Aβ is assembled with DMPC in a binary film, the resulting membranes become heterogeneous, with a peptide-enriched phase distributed in a network-like pattern, and they exhibit a lateral transition that depends on the Aβ content. At lower peptide proportions, the films segregate into two well-defined phases: one consisting of lipids and another enriched with peptides. The reflectivity of these phases differs from that obtained for pure Aβ films. Thus, the formed fibers effectively cover most of the interface area and remain stable at higher pressures (from 20 to 30 mN m-1 depending on Aβ content) compared to pure peptide films (17 mN m-1). Furthermore, such structures induce a compressional hysteresis in the film, similar to that of pure peptide films (which is nonexistent in the pure lipid monolayer), even at low peptide proportions. We claim that the mechanical properties at the interface are governed by the size of the fibril-like structures. Based on the low molar fractions and surface packing at which these phenomena were observed, we postulate that as a consequence of peptide intermolecular interactions, Aβ may have drastic effects on the molecular arrangement and mechanical properties of a lipid membrane.

Dexamethasone and Dexamethasone Phosphate: Effect on DMPC Membrane Models

Dexamethasone (Dex) and Dexamethasone phosphate (Dex-P) are synthetic glucocorticoids with high anti-inflammatory and immunosuppressive actions that gained visibility because they reduce the mortality in critical patients with COVID-19 connected to assisted breathing. They have been widely used for the treatment of several diseases and in patients under chronic treatments, thus, it is important to understand their interaction with membranes, the first barrier when these drugs get into the body. Here, the effect of Dex and Dex-P on dimyiristoylphophatidylcholine (DMPC) membranes were studied using Langmuir films and vesicles. Our results indicate that the presence of Dex in DMPC monolayers makes them more compressible and less reflective, induces the appearance of aggregates, and suppresses the Liquid Expanded/Liquid Condensed (LE/LC) phase transition. The phosphorylated drug, Dex-P, also induces the formation of aggregates in DMPC/Dex-P films, but without disturbing the LE/LC phase transition and reflectivity. Insertion experiments demonstrate that Dex induces larger changes in surface pressure than Dex-P, due to its higher hydrophobic character. Both drugs can penetrate membranes at high lipid packings. Vesicle shape fluctuation analysis shows that Dex-P adsorption on GUVs of DMPC decreases membrane deformability. In conclusion, both drugs can penetrate and alter the mechanical properties of DMPC membranes.

Statistical mechanical determination of nanocluster size distributions in the phase coexistence region of a first order phase transition from the isotherms of DMPC monolayers at the air-water interface

A statistical mechanical deconvolution procedure for the experimentally measured surface pressure-area isotherms has been presented to obtain the surface pressure dependence of the liquid expanded (LE) and liquid condensed (LC) nanocluster size distributions in the LE-LC phase coexistence region of the first order phase transition of Dimyristoyl phosphatidylcholine (DMPC) monolayers at the air-water interface. This study presents the application of the deconvolution formulation introduced originally by Freire and Biltonen for the experimentally measured specific heat to calculate the submicroscopic lipid cluster distribution function in the phase coexistence region [E. Freire, R. L. Biltonen, Biopolymers, 1978, 17, 481-496] and extends their formulation to surface pressure isotherms. The present procedure involves the extraction of the pressure partition function calculated from the isotherm and utilizes the general relation between molecular density fluctuations and macroscopic lateral compressibility. In this procedure the high-density LC phase boundary has been determined uniquely. The average nanoscopic cluster sizes obtained in this study have been compared with the results from previous experimental studies. The cause of the finite difference between the values of the LC phase boundary area obtained from the present deconvolution procedure and the conventional extrapolation method on the same isotherm has been discussed from the viewpoint of slow hierarchical growth from nanoscopic clusters to macroscopic domains in the coexistence region.

Nanoscale Chemical Imaging of Supported Lipid Monolayers using Tip-Enhanced Raman Spectroscopy

Visualizing the molecular organization of lipid membranes is essential to comprehend their biological functions. However, current analytical techniques fail to provide a non‐destructive and label‐free characterization of lipid films under ambient conditions at nanometer length scales. In this work, we demonstrate the capability of tip‐enhanced Raman spectroscopy (TERS) to probe the molecular organization of supported DPPC monolayers on Au (111), prepared using the Langmuir–Blodgett (LB) technique. High‐quality TERS spectra were obtained, that permitted a direct correlation of the topography of the lipid monolayer with its TERS image for the first time. Furthermore, hyperspectral TERS imaging revealed the presence of nanometer‐sized holes within a continuous DPPC monolayer structure. This shows that a homogeneously transferred LB monolayer is heterogeneous at the nanoscale. Finally, the high sensitivity and spatial resolution down to 20 nm of TERS imaging enabled reproducible, hyperspectral visualization of molecular disorder in the DPPC monolayers, demonstrating that TERS is a promising nanoanalytical tool to investigate the molecular organization of lipid membranes.

Anisotropic line tension of domains in lipid monolayers

We formulate a simple effective model to describe molecular interactions in a lipid monolayer and calculate the line tension between coexisting domains. The model represents lipid molecules in terms of two-dimensional anisotropic particles on the plane of the monolayer. These particles interact through forces that are believed to be relevant for the understanding of fundamental properties of the monolayer: van der Waals interactions originating from lipid chains and dipolar forces between dipole groups in the molecular heads. The model stresses the liquid-crystalline nature of the ordered phase in lipid monolayers and explains coexistence properties between ordered and disordered phases in terms of molecular parameters. Thermodynamic and interfacial properties of the model are analyzed using density-functional theory. In particular, the line tension at the interface between ordered and disordered phases turns out to be highly anisotropic with respect to the angle between the nematic director and the interface separating the coexisting phases. This important feature mainly results from the tilt angle of lipid chains and, to a lesser extent, from dipolar interactions perpendicular to the monolayer. The role of the two dipolar components, parallel and perpendicular to the monolayer, is assessed by comparing with computer simulation results for lipid monolayers.

Emergence of a linear slope region of the isotherm in the first-order liquid-expanded-liquid-condensed phase transition in Langmuir monolayers

A nonhorizontal slope in the isotherm has been observed in the two-phase coexisting region of the first-order liquid-expanded (LE)-liquid-condensed (LC) phase transition in Langmuir monolayers for many decades. We show that the simple analysis of a phenomenological Landau free energy involving the coupling-energy contributions of molecular lateral density (ρ) with spontaneous collective chain tilt (θ) and two-dimensional strain (ɛ_{s}) inside the LC domain enables one to understand the origin of a nonhorizontal straight-line slope in the LE-LC phase coexistence region of the isotherm. The presence of ρ-ɛ_{s} coupling must be essential for the appearance of the straight-line shape of a nonhorizontal plateau in the isotherm. Moreover, it is found from the comparison of the two-dimensional contour plots of the free energy that an LE phase may persist significantly even at the later stage of the straight-line regime beyond a transition midpoint surface pressure in the presence of this coupling. The persistence of the LE phase may lead to the delay of transition progress as manifested more clearly by the appearance of a compressibility plateau in the coexistence region that indicates the existence of persistent equilibrium density fluctuations in the monolayer.

Phase Transitions in Phospholipid Monolayers: Theory Versus Experiments

The Doniach lattice gas (DLG) represents a ternary-mixture statistical model, whose components, water molecules (w), ordered-chain lipids (o), and disordered-chain lipids (d)-the latter carrying a high degenerescence ω ≫ 1-are located at each site of a two-dimensional lattice. The DLG model was introduced to describe phospholipid Langmuir films at the air-water interface and can be mapped into a spin-1 model, with the single-site states s _i = 0, +1, and -1 representing the three types of molecules in the system (w, o, and d), respectively. The model allows lipid-density fluctuations and has been analyzed at the mean-field approximation (Guidi, H. S.; Henriques, V. B. Phys. Rev. E 2014, 90, 052705) as well as at the pair approximation (de Oliveira, F. O.; Tamashiro, M. N. Phys. Rev. E 2019, 99, 012147). In this work, we focus on performing an explicit comparison of the theoretical predictions obtained for the DLG model at the pair approximation with isothermal monolayer compression experiments (Nielsen, L. K.; Bjørnholm, T.; Mouritsen, O. G. Langmuir 2007, 23, 11684) for the two most commonly studied saturated zwitterionic phospholipids, DMPC (1,2-dimyristoyl- sn-glycero-3-phosphocholine) and DPPC (1,2-dipalmitoyl- sn-glycero-3-phosphocholine). The model parameters obtained by fitting to the experimental data yield phase diagrams that are qualitatively consistent with the observed phase transitions on DMPC and DPPC monolayers, with the absence of a low-density gas phase. Quantitative agreement, however, was less significant partially because of the challenging reproducibility of Langmuir monolayer compression experiments, claimed in the literature to be influenced by kinetic effects.

Phase transitions in phospholipid monolayers: Statistical model at the pair approximation

A Langmuir film, consisting of a phospholipid monolayer at the air-water interface, was modeled as a two-dimensional lattice gas corresponding to a ternary mixture of water molecules (w), ordered-chain lipids (o), and disordered-chain lipids (d). The statistical problem is formulated in terms of a spin-1 model, in which the disordered-chain lipid states possess a high degenerescence ω≫1, and was termed Doniach lattice gas (DLG). Motivated by some open questions in the analysis of the DLG model at the mean-field approximation (MFA) [Phys. Rev. E 90, 052705 (2014)PLEEE81539-375510.1103/PhysRevE.90.052705], we have reconsidered it at the pair-approximation level by solving the model on a Cayley tree of coordination z. The attractors of the corresponding discrete-map problem are associated with the thermodynamic solutions on the Bethe lattice (the central region of an asymptotically infinite Cayley tree). To check the thermodynamic stability of the possible phases, the grand-potential density was obtained by the method proposed by Gujrati [Phys. Rev. Lett. 74, 809 (1995)PRLTAO0031-900710.1103/PhysRevLett.74.809]. In general, the previous MFA results are confirmed at the pair-approximation level, but a novel staggered phase, overlooked in the MFA analysis, was found when the condition ε_{wd}>1/2(ε_{ww}+ε_{dd}) is satisfied, where ε_{xy} represents the nearest-neighbor intermolecular interactions between single-site states x and y. Model parameters obtained by fitting to experimental data for the two most commonly studied zwitterionic phospholipids, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), yield phase diagrams consistent with the phase transitions observed on Langmuir films of the same lipids under isothermal compression, which present a liquid-condensed to a liquid-expanded first-order transition line ending at a critical point.

Effect of polyols on the DMPC lipid monolayers and bilayers

In this study, the effect of polyols, erythritol, xylitol, mannitol, on a model membrane systems composed of DMPC was investigated using differential scanning calorimetry and Fourier transform infrared spectroscopy. Generally, it is considered that polyols possess strong hydrophilic properties, and either does not interact with the hydrophobic environment at all, or these interactions are very weak. To better understand the mutual interactions between polyols and the lipid system, the Langmuir technique was used to examine the molecular organization of monolayers and to calculate their thickness in the presence of polyols at the subphase. The detailed description of the interactions between polyols and DMPC molecules was complemented by the analysis of the morphology of monolayers with the application of Brewster angle microscopy. From ATR FTIR, the significant spectral shift is observed only for the PO₂^- stretching band, which correlates strongly with the polyol chain-length. The longer the polyol chain, the weaker the observed interactions with lipid molecules. The most important findings, obtained from thickness measurements, reveal that short-chain polyols may prevent the formation of bilayers by the DMPC molecules under high surface pressure. The changes in the organization of DMPC monolayers on the surface, as visualized by Brewster angle microscopy, showed that the domains observed for phospholipid film spread on pure water differ substantially from those containing polyols in the subphase.

Melting of the Dipalmitoylphosphatidylcholine Monolayer

Langmuir monolayer self-assembled at the air-water interface represents an excellent model for studying phase transition and lipid polymorphism in two dimensions. Compared with numerous studies of phospholipid phase transitions induced by isothermal compression, there are very scarce reports on two-dimensional phase transitions induced by isobaric heating. This is mainly due to technical difficulties of continuously regulating temperature variations while maintaining a constant surface pressure in a classical Langmuir-type film balance. Here, with technological advances in constrained drop surfactometry and closed-loop axisymmetric drop shape analysis, we studied the isobaric heating process of the dipalmitoylphosphatidylcholine (DPPC) monolayer. It is found that temperature and surface pressure are two equally important intensive properties that jointly determine the phase behavior of the phospholipid monolayer. We have determined a critical point of the DPPC monolayer at a temperature of 44 °C and a surface pressure of 57 mN/m. Beyond this critical point, no phase transition can exist in the DPPC monolayer, either by isothermal compression or by isobaric heating. The melting process of the DPPC monolayer studied here provides novel insights into the understanding of a wide range of physicochemical and biophysical phenomena, such as surface thermodynamics, critical phenomena, and biophysical study of pulmonary surfactants.

Critical Temperature of 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine Monolayers and Its Possible Biological Relevance

Because transmembrane proteins (TMPs) can be obtained with sufficient purity for X-ray diffraction studies more frequently than decades ago, their mechanisms of action may now be elucidated. One of the pending issues is the actual interplay between transmembrane proteins and membrane lipids. There is strong evidence of the involvement of specific lipids with some membrane proteins, such as the potassium crystallographically sited activation channel (KcsA) of Streptomyces lividans and the secondary transporter of lactose LacY of Escherichia coli, the activities of which are associated with the presence of anionic phospholipids such as the phosphatidylglycerol (PG) and phosphatidyethanolamine (PE), respectively. Other proteins such as the large conductance mechanosensitive channel (MscL) of E. coli seem to depend on the adaptation of specific phospholipids to the irregular surface of the integral membrane protein. In this work we investigated the lateral compressibility of two homoacid phosphatidylethanolamines (one with both acyl chains unsaturated (DOPE), the other with the acyl chains saturated (DPPE)) and the heteroacid phosphatidyletanolamine (POPE) and their mixtures with POPG. The liquid expanded (LE) to liquid condensed (LC) transition was observed in POPE at a temperature below its critical temperature (T_c = 36 °C). Because T_c lies below the physiological temperature, the occurrence of this phase transition may have something to do with the functioning of LacY. This magnitude is discussed within the context of the experiments carried out at temperatures below the T_c of POPE at which the activity of Lac Y and other TMPs are frequently studied.

Temperature dependent heterogeneous rotational correlation in lipids

Lipid structures exhibit complex and highly dynamic lateral structure; and changes in lipid density and fluidity are believed to play an essential role in membrane targeting and function. The dynamic structure of liquids on the molecular scale can exhibit complex transient density fluctuations. Here the lateral heterogeneity of lipid dynamics is explored in free standing lipid monolayers. As the temperature is lowered the probes exhibit increasingly broad and heterogeneous rotational correlation. This increase in heterogeneity appears to exhibit a critical onset, similar to those observed for glass forming fluids. We explore heterogeneous relaxation in in a single constituent lipid monolayer of 1, 2-dimyristoyl-sn-glycero-3-phosphocholine  by measuring the rotational diffusion of a fluorescent probe (1-palmitoyl-2-[1]-sn-glycero-3-phosphocholine), which is embedded in the lipid monolayer at low labeling density. Dynamic distributions are measured using wide-field time-resolved fluorescence anisotropy. The observed relaxation exhibits a narrow, liquid-like distribution at high temperatures (τ ∼ 2.4 ns), consistent with previous experimental measures (Dadashvand et al 2014 Struct. Dyn. 1 054701, Loura and Ramalho 2007 Biochim. Biophys. Acta 1768 467-478). However, as the temperature is quenched, the distribution broadens, and we observe the appearance of a long relaxation population (τ ∼ 16.5 ns). This supports the heterogeneity observed for lipids at high packing densities, and demonstrates that the nanoscale diffusion and reorganization in lipid structures can be significantly complex, even in the simplest amorphous architectures. Dynamical heterogeneity of this form can have a significant impact on the organization, permeability and energetics of lipid membrane structures.

Phase Transitions in Dipalmitoylphosphatidylcholine Monolayers

A self-assembled phospholipid monolayer at an air-water interface is a well-defined model system for studying surface thermodynamics, membrane biophysics, thin-film materials, and colloidal soft matter. Here we report a study of two-dimensional phase transitions in the dipalmitoylphosphatidylcholine (DPPC) monolayer at the air-water interface using a newly developed methodology called constrained drop surfactometry (CDS). CDS is superior to the classical Langmuir balance in its capacity for rigorous temperature control and leak-proof environments, thus making it an ideal alternative to the Langmuir balance for studying lipid polymorphism. In addition, we have developed a novel Langmuir-Blodgett (LB) transfer technique that allows the direct transfer of lipid monolayers from the droplet surface under well-controlled conditions. This LB transfer technique permits the direct visualization of phase coexistence in the DPPC monolayer. With these technological advances, we found that the two-dimensional phase behavior of the DPPC monolayer is analogous to the three-dimensional phase transition of a pure substance. This study has implications in the fundamental understanding of surface thermodynamics as well as applications such as self-assembled monolayers and pulmonary surfactant biophysics.

Critical behavior of a two-dimensional complex fluid: Macroscopic and mesoscopic views

Liquid disordered (L_{d}) to liquid ordered (L_{o}) phase transition in myristic acid [MyA, CH_{3}(CH_{2})_{12}COOH] Langmuir monolayers was studied macroscopically as well as mesoscopically to locate the critical point. Macroscopically, isotherms of the monolayer were obtained across the 20^{∘}C-38^{∘}C temperature (T) range and the critical point was estimated, primarily from the vanishing of the order parameter, at ≈38^{∘}C. Mesoscopically, domain morphology in the L_{d}-L_{o} coexistence regime was imaged using the technique of Brewster angle microscopy (BAM) as a function of T and the corresponding power spectral density function (PSDF) obtained. Monolayer morphology passed from stable circular domains and a sharp peak in PSDF to stable dendritic domains and a divergence of the correlation length as the critical point was approached from below. The critical point was found to be consistent at ≈38^{∘}C from both isotherm and BAM results. In the critical regime the scaling behavior of the transition followed the two-dimensional Ising model. Additionally, we obtained a precritical regime, over a temperature range of ≈8^{∘}C below T_{c}, characterized by fluctuations in the order parameter at the macroscopic scale and at the mesoscopic scale characterized by unstable domains of fingering or dendritic morphology as well as proliferation of a large number of small sized domains, multiple peaks in the power spectra, and a corresponding fluctuation in the peak q values with T. Further, while comparing temperature studies on an ensemble of MyA monolayers with those on a single monolayer, the system was found to be not strictly ergodic in that the ensemble development did not strictly match with the time development in the system. In particular, the critical temperature was found to be lowered in the latter. These results clearly show that the critical behavior in fatty acid monolayer phase transitions have features of both complex and nonequilibrium systems.

Water-binding phospholipid nanodomains and phase-separated diacylglycerol nanodomains regulate enzyme reactions in lipid monolayers

Phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) nanodomains covered with bound water as well as diacylglycerol 1-palmitoyl-2-oleoyl-sn-glycerol (POG) nanodomains separated from a lipid membrane were studied, using monolayer surfaces of POPC hydrolyzed by phospholipase C (PLC). The investigation was based on the analysis of compression isotherms and on atomic force microscope (AFM) observations of Langmuir-Blodgett (LB) films and Langmuir-Schaefer (LS) films. The results included reaction rate constants obtained by kinetic analysis of phosphocholine at surface pressures from 0.1 to 31 mN/m and determined by a luminol-enhanced chemiluminescence method. Monolayer elastic modulus values and fluorescence microscopic images confirmed that hydrolysis by PLC progressed in the intermediate monolayer between a liquid-expanded (L1) film and a liquid-condensed (L2) film at 2-17 mN/m. Furthermore, the intermediate film was confirmed to consist of L1 film and the POPC nanodomains in the L2 state are covered with bound water, conclusions based on the following AFM results: (1) nanodomains in POPC LS films were catalyzed by PLC, (2) POG nanodomains extended out from LB films of mixed POPC/POG 9/1 (mol/mol) monolayers, and (3) POPC LS films were covered with bound water, as indicated by cross-sectional analysis. At the optimal surface pressure of 10 mN/m, when POPC nanodomains (L2), with internal diameters of ∼75 nm, were hydrolyzed by PLC, they shrank down into pockets of the same size as those that appeared with POG. The resulting pocket sizes on LS films were in agreement with POG nanodomain sizes on LB films. This study demonstrated that PLC reacted with POPC nanodomains (L2) dispersed in L1/L2 mixed phase monolayers selectively and that POG nanodomains were phase-separated from the monolayer as hydrolysis proceeded.

Lattice solution model for order-disorder transitions in membranes and Langmuir monolayers

Lipid monolayers and bilayers have been used as experimental models for the investigation of membrane thermal transitions. The main transition takes place near ambient temperatures for several lipids and reflects the order-disorder transition of lipid hydrocarbonic chains, which is accompanied by a surface density gap. Equivalence between the transitions in the two systems has been argued by several authors. The two-state statistical model adopted by numerous authors for different properties of the membrane, such as permeability, diffusion, and mixture or insertion of cholesterol or protein, is inadequate for the description of charged membranes, since it lacks a proper description of surface density. We propose a lattice solution model which adds interactions with water molecules to lipid-lipid interactions and obtain its thermal properties under a mean-field approach. Density variations, although concomitant with chain order variations, are independent of the latter. The model presents both chain order and gas-liquid transitions, and extends the range of applicability of previous models, yielding Langmuir isotherms in the full range of pressures and areas.

Microphase formation at a 2D solid-gas phase transition

Density modulated micro-separated phases (microphases) occur at 2D liquid interfaces in the form of alternating regions of high and low density domains. Brewster angle microscopy (BAM) images demonstrate the existence of microphases in cluster, stripe, and mosaic morphologies at the buried interface between hexane and water with fluoro-alkanol surfactant dissolved in the bulk hexane. At high temperature, the surfactant assembles at the interface in a 2D gaseous state. As the system is cooled additional surfactants condense onto the interface, which undergoes a 2D gas-solid phase transition. Microphase structure is observed within a few degrees of this transition in the form of clusters and labyrinthine stripes. Microphases have been observed previously in a number of other systems; nevertheless, we demonstrate that adsorption transitions at the liquid-liquid interface provide a convenient way to observe a full sequence of temperature-dependent 2D phases, from gas to cluster to stripe to mosaic to inverted stripe phases, as well as coexistence between some of these microphases. Cracking and fracture of the clusters reveal that they are a solid microphase. Theories of microphases often predict a single length scale for cluster and stripe phases as a result of the competition between an attractive and a repulsive interaction. Our observation that two characteristic length scales are required to describe clusters whose diameter is much larger than the stripe period, combined with the solid nature of the clusters, suggests that a long-range elastic interaction is relevant. These results complement earlier X-ray measurements on the same system.

Crystallization, structural diversity and anisotropy effects in 2D arrays of icosahedral viruses

We investigate two-dimensional (2D) assembly of the icosahedral turnip yellow mosaic virus (TYMV) under cationic lipid monolayers at the aqueous solution–vapor interface. The 2D crystallization of TYMV has been achieved by enhancing electrostatically induced interfacial adsorption, an approach recently demonstrated for another virus. In situ X-ray scattering reveals two close-packed 2D crystalline phases of TYMV that are distinct from the previously reported hexagonal and centered square (√2 × √2) arrays of TYMV. One of the newly observed phases arises from either a dimeric double-square (2 × 1) or tetrameric square (2 × 2) unit cell. The other is a rhombic crystal with a lattice angle of 80°. The two observed crystal phases are substantially less dense (by over 10%) than a 2D lattice of TYMV could be according to its known size and shape, indicating that local anisotropic interparticle interactions play a key role in stabilizing these crystals. TYMV's anisotropy attributes and numerical analysis of 2D arrays of virus-shaped particles are used to derive a model for the rhombic crystal in which the particle orientation is consistent with the electrostatic lipid–TYMV attraction and the interparticle contacts exhibit steric complementarity. The interplay between particle anisotropy and packing is contrasted between the rhombic crystal model and the square (√2 × √2) crystal. This study highlights how the high symmetry and subtle asphericity of icosahedral particles enrich the variety and complexity of ordered 2D structures that can be generated through self-assembly.

Critical dynamics of an isothermal compressible nonideal fluid

A pure fluid at its critical point shows a dramatic slow-down in its dynamics, due to a divergence of the order-parameter susceptibility and the coefficient of heat transport. Under isothermal conditions, however, sound waves provide the only possible relaxation mechanism for order-parameter fluctuations. Here we study the critical dynamics of an isothermal, compressible nonideal fluid via scaling arguments and computer simulations of the corresponding fluctuating hydrodynamics equations. We show that, below a critical dimension of 4, the order-parameter dynamics of an isothermal fluid effectively reduces to "model A," characterized by overdamped sound waves and a divergent bulk viscosity. In contrast, the shear viscosity remains finite above two dimensions. Possible applications of the model are discussed.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: theory versus experiment

Aqueous dispersions of phosphatidylglycerol (PG) lipids may present an anomalous chain-melting transition at low ionic strengths, as seen by different experimental techniques such as calorimetry or light scattering. The anomaly disappears at high ionic strengths or for longer acyl-chain lengths. In this article, we use a statistical model for the bilayer that distinguishes both lipid chain and headgroup states in order to compare model and experimental thermotropic and electrical properties. The effective van der Waals interactions among hydrophobic chains compete with the electrostatic repulsions between polar headgroups, which may be ionized (counterion dissociated) or electrically neutral (associated with counterions). Electric degrees of freedom introduce new thermotropic charge-ordered phases in which headgroup charges may be spatially ordered, depending on the electrolyte ionic strength, introducing a new rationale for experimental data on PGs. The thermal phases presented by the model for different chain lengths, at fixed ionic strength, compare well with an experimental phase diagram constructed on the basis of differential scanning calorimetry profiles. In the case of dispersions of DMPG (dimyristoyl phosphatidylglycerol) with added monovalent salt, the model properties reproduce the main features displayed by data from differential scanning calorimetry as well as the characteristic profile for the degree of ionization of the bilayer surface across the anomalous transition region, obtained from the theoretical interpretation of electrokinetic (conductivity and electrophoretic mobility) measurements.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: a statistical model

We propose a statistical model to account for the gel-fluid anomalous phase transitions in charged bilayer- or lamellae-forming ionic lipids. The model Hamiltonian comprises effective attractive interactions to describe neutral-lipid membranes as well as the effect of electrostatic repulsions of the discrete ionic charges on the lipid headgroups. The latter can be counterion dissociated (charged) or counterion associated (neutral), while the lipid acyl chains may be in gel (low-temperature or high-lateral-pressure) or fluid (high-temperature or low-lateral-pressure) states. The system is modeled as a lattice gas with two distinct particle types--each one associated, respectively, with the polar-headgroup and the acyl-chain states--which can be mapped onto an Ashkin-Teller model with the inclusion of cubic terms. The model displays a rich thermodynamic behavior in terms of the chemical potential of counterions (related to added salt concentration) and lateral pressure. In particular, we show the existence of semidissociated thermodynamic phases related to the onset of charge order in the system. This type of order stems from spatially ordered counterion association to the lipid headgroups, in which charged and neutral lipids alternate in a checkerboard-like order. Within the mean-field approximation, we predict that the acyl-chain order-disorder transition is discontinuous, with the first-order line ending at a critical point, as in the neutral case. Moreover, the charge order gives rise to continuous transitions, with the associated second-order lines joining the aforementioned first-order line at critical end points. We explore the thermodynamic behavior of some physical quantities, like the specific heat at constant lateral pressure and the degree of ionization, associated with the fraction of charged lipid headgroups.

Correlated volume-energy fluctuations of phospholipid membranes: a simulation study

This paper reports all-atom computer simulations of five phospholipid membranes (DMPC, DPPC, DMPG, DMPS, and DMPSH) with focus on the thermal equilibrium fluctuations of volume, energy, area, thickness, and chain order. At constant temperature and pressure, volume and energy exhibit strong correlations of their slow fluctuations (defined by averaging over 0.5 ns). These quantities, on the other hand, do not correlate significantly with area, thickness, or chain order. The correlations are mainly reported for the fluid phase, but we also give some results for the ordered (gel) phase of two membranes, showing a similar picture. The cause of the observed strong correlations is identified by splitting volume and energy into contributions from tails, heads, and water, and showing that the slow volume-energy fluctuations derive from van der Waals interactions of the tail region; they are thus analogous to the similar strong correlations recently observed in computer simulations of the Lennard-Jones and other simple van der Waals type liquids (U. R. Pedersen et al., Phys. Rev. Lett. 2008, 100, 015701). The strong correlations reported here confirm one crucial assumption of a recent theory for nerve signal propagation proposed by Heimburg and Jackson (T. Heimburg and A. D. Jackson, Proc. Natl. Acad. Sci. 2005, 102, 9790-9795).

Reversible formation of nanodomains in monolayers of DPPC studied by kinetic modeling

Dipalmitoylphosphatidylcholine (DPPC) is the most abundant component in pulmonary surfactants and is believed to be responsible for maintaining low surface tension in alveoli during breathing. In this work, a kinetic model is introduced that describes the phase separation in DPPC films that produces the liquid-condensed (LC) and liquid-expanded (LE) fractions, which differ according to the area density of DPPC. The phase separation in an initially homogeneous film has been investigated numerically. Furthermore, explicit simulations of periodic compression-expansion cycles are reported. In this process, a moderate change of the surface area resulted in a dramatic change in the total amount of LC fraction, as well as in the surface morphology. Depending on the extent of the film's compression, the simulated surface morphologies comprised individual nanosized LC domains embedded in the LE fraction, interconnected networks of such domains, or continuous LC films with nanopores. Equilibration of the total area of the LC nanodomains occurred over a few milliseconds, indicating that the rate of the LE-LC phase transformation is sufficient for maintaining low surface tension during breathing, and that nanoscale LC domains are likely to play a major role in this process. Unlike the total content of the LC fraction, which stabilized quickly, the average size of LC nanodomains showed a tendency to increase slowly, at a rate determined by the diffusivity of DPPC. The computed average domain size seems to be compatible with published experiments for DPPC films. The numeric results also elucidate the distinction between thermodynamically determined and kinetically limited structural properties during phase separation in the major structure-forming component of pulmonary surfactants.

Diffusion and partitioning of fluorescent lipid probes in phospholipid monolayers

The pressure-dependent diffusion and partitioning of single lipid fluorophores in DMPC and DPPC monolayers were investigated with the use of a custom-made monolayer trough mounted on a combined fluorescence correlation spectroscopy (FCS) and wide-field microscopy setup. It is shown that lipid diffusion, which is essential for the function of biological membranes, is heavily influenced by the lateral pressure and phase of the lipid structure. Both of these may change dynamically during, e.g., protein adsorption and desorption processes. Using FCS, we measured lipid diffusion coefficients over a wide range of lateral pressures in DMPC monolayers and fitted them to a free-area model as well as the direct experimental observable mean molecular area. FCS measurements on DPPC monolayers were also performed below the onset of the phase transition (Pi < 5 mN/m). At higher pressures, FCS was not applicable for measuring diffusion coefficients in DPPC monolayers. Single-molecule fluorescence microscopy and differential scanning calorimetry clearly showed that this was due to heterogeneous partitioning of the lipid fluorophores in condensed phases. The results were compared with dye partitioning in giant lipid vesicles. These findings are significant in relation to the application of lipid fluorophores to study diffusion in both model systems and biological systems.

Lipid domain morphologies in phosphatidylcholine-ceramide monolayers

In cells, one of the main roles of ceramide-enriched membrane domains is to recruit or exclude intracellular signaling molecules and receptors, thereby facilitating signal transduction cascades. Accordingly, in model membranes, even low contents of ceramide segregate into lateral domains. The impact of the N-acyl chain on this segregation and on the morphology of the domains remains to be explored. Using Langmuir monolayers, we have systematically studied binary mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and ceramide (2:1, molar ratio) and varied the N-acyl chain length of ceramide from 2 to 24 carbon atoms (Cer2 to Cer24). Fluid Cer2, Cer6, and Cer8/DMPC mixtures were miscible at all surface pressures. Longer ceramides, however, formed surface pressure-dependent immiscible mixtures with DMPC. The domain morphology under fluorescence microscopy after including a trace amount of fluorescent NBD-phosphatidylcholine into DMPC/Cer mixtures was found to be very sensitive to the N-acyl chain length. Shorter ceramides (Cer10-Cer14) formed flower-like (seaweed) domains, whereas longer ceramides (N-acyl chain length>14 carbon atoms) formed round and regular domains. We attribute the formation of the flower patterns to diffusive morphological instabilities during domain growth.

Infrared reflection absorption spectroscopy coupled with Brewster angle microscopy for studying interactions of amphiphilic triblock copolymers with phospholipid monolayers

Novel water-soluble amphiphilic triblock copolymers poly(glycerol monomethacrylate)-b-poly(propylene oxide)-b-poly(glycerol monomethacrylate) (PGMA-b-PPO-b-PGMA) were synthesized because of their expected enhanced ability to interact with biological membranes compared to the well-known poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) block copolymers. Their bulkier hydrophilic PGMA blocks might induce a disturbance in the packing of liquid-crystalline lipid bilayers in addition to the effect caused by the hydrophobic PPO block alone. To gain a better insight into the polymer-membrane interactions at the molecular level, the adsorption kinetics and concomitant interactions of (PGMA14)(2-)PPO(34) with model membranes of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were monitored using infrared reflection absorption spectroscopy (IRRAS) coupled with Brewster angle microscopy (BAM) and surface pressure (pi) measurements. The maximum penetration surface pressure of ca. 39 mN/m suggests that (PGMA14)(2-)PPO(34) is able to insert into lipid monolayers even above the so-called monolayer-bilayer equivalent pressure of 30-35 mN/m. Copolymer adsorption to a liquid-expanded DPPC-d62 monolayer proceeds in a two-step mechanism: (i) initially only the more hydrophobic PPO middle block penetrates the lipid monolayer; (ii) following the liquid-expanded-liquid-condensed (LE-LC) phase transition, the bulky PGMA hydrophilic blocks are dragged into the headgroup region as the PPO block inserts further into the fatty acid region. The adsorption kinetics is considerably faster for DMPC-d54 monolayers due to their higher fluidity. Copolymer adsorption to an LC-DPPC-d62 monolayer leads to a change in the monolayer packing by forcing the lipid alkyl chains into a more vertical orientation, their tilt angle with respect to the surface normal being reduced from initially 30 degrees +/- 3 degrees to 18 degrees +/- 3 degrees. BAM images rule out macroscopic phase separation and show that coalescence of DPPC-d62 LC domains takes place at relatively low surface pressures of pi > or = 23 mN/m, suggesting that (PGMA14)(2-)PPO (34) partitions into both LE as well as LC domains.

Line tensions, correlation lengths, and critical exponents in lipid membranes near critical points

Membranes containing a wide variety of ternary mixtures of high chain-melting temperature lipids, low chain-melting temperature lipids, and cholesterol undergo lateral phase separation into coexisting liquid phases at a miscibility transition. When membranes are prepared from a ternary lipid mixture at a critical composition, they pass through a miscibility critical point at the transition temperature. Since the critical temperature is typically on the order of room temperature, membranes provide an unusual opportunity in which to perform a quantitative study of biophysical systems that exhibit critical phenomena in the two-dimensional Ising universality class. As a critical point is approached from either high or low temperature, the scale of fluctuations in lipid composition, set by the correlation length, diverges. In addition, as a critical point is approached from low temperature, the line tension between coexisting phases decreases to zero. Here we quantitatively evaluate the temperature dependence of line tension between liquid domains and of fluctuation correlation lengths in lipid membranes to extract a critical exponent, nu. We obtain nu = 1.2 +/- 0.2, consistent with the Ising model prediction nu = 1. We also evaluate the probability distributions of pixel intensities in fluorescence images of membranes. From the temperature dependence of these distributions above the critical temperature, we extract an independent critical exponent of beta = 0.124 +/- 0.03, which is consistent with the Ising prediction of beta = 1/8.

Current perspectives in pulmonary surfactant--inhibition, enhancement and evaluation

Pulmonary surfactant (PS) is a complicated mixture of approximately 90% lipids and 10% proteins. It plays an important role in maintaining normal respiratory mechanics by reducing alveolar surface tension to near-zero values. Supplementing exogenous surfactant to newborns suffering from respiratory distress syndrome (RDS), a leading cause of perinatal mortality, has completely altered neonatal care in industrialized countries. Surfactant therapy has also been applied to the acute respiratory distress syndrome (ARDS) but with only limited success. Biophysical studies suggest that surfactant inhibition is partially responsible for this unsatisfactory performance. This paper reviews the biophysical properties of functional and dysfunctional PS. The biophysical properties of PS are further limited to surface activity, i.e., properties related to highly dynamic and very low surface tensions. Three main perspectives are reviewed. (1) How does PS permit both rapid adsorption and the ability to reach very low surface tensions? (2) How is PS inactivated by different inhibitory substances and how can this inhibition be counteracted? A recent research focus of using water-soluble polymers as additives to enhance the surface activity of clinical PS and to overcome inhibition is extensively discussed. (3) Which in vivo, in situ, and in vitro methods are available for evaluating the surface activity of PS and what are their relative merits? A better understanding of the biophysical properties of functional and dysfunctional PS is important for the further development of surfactant therapy, especially for its potential application in ARDS.