Liquid-expanded Phase Research Articles

Understanding the Retention of Vaping Additives in the Lungs: Model Lung Surfactant Membrane Perturbation by Vitamin E and Vitamin E Acetate.

Deviations from the normal physicochemical and functional properties of pulmonary surfactants are associated with the incidence of lung injury and other respiratory disorders. This study aims to evaluate the alteration of the 2D molecular organization and morphology of pulmonary surfactant model membranes by the electronic cigarette additives α-tocopherol (vitamin E) and α-tocopherol acetate (vitamin E acetate), which have been associated with lung injury, termed e-cigarette or vaping-use-associated lung injury (EVALI). The model membranes used contained a 7:3 molar ratio of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol) to which α-tocopherol and α-tocopherol acetate were added to form mixtures of up to 20 mol % additive. The properties of the neat tocopherol additives and DPPC/POPG (7:3) mixtures with increasing molar proportions of additive were evaluated by surface pressure-area isotherms, excess area calculations, Brewster angle microscopy, grazing incidence X-ray diffraction, X-ray reflectivity, and atomic force microscopy. The addition of either additive alters the essential phase balance of the model pulmonary surfactant membrane by generating a greater proportion of the fluid phase. Despite this net fluidization, both tocopherol additives have space-filling effects on the liquid-expanded and condensed phases, yielding negative excess areas in the liquid-expanded phase and reduced tilt angles in the condensed phase. Both tocopherol additives alter the stability of the fluid phase, pushing the eventual collapse of this phase to higher surface pressures than the model membrane in the absence of an additive.

Ruptures of mixed lipid monolayers under tension and supercooling: implications for nanobubbles in plants.

Mixed phospholipid and glycolipid monolayers likely coat the surfaces of pressurised gas nanobubbles within the hydraulic systems of plants. The lipid coatings bond to water under negative pressure and are thus stretched out of equilibrium. In this work, we have used molecular dynamics simulations to produce trajectories of a biologically relevant mixed monolayer, pulled at mild negative pressures (-1.5 to -4.5 MPa). Pore formation within the monolayer is observed at both 270 and 310 K, and proceeds as an activated process once the lipid tails fully transition from the two dimensional liquid condensed to liquid expanded phase. Pressure:area isotherms showed reduced surface pressure under slight supercooling (T = 270 K) at all observed areas per lipid. Finally, Rayleigh-Plesset simulations were used to predict evolving nanobubble size using the calculated pressure:area isotherms as dynamic surface tensions. We confirm the existence of a second critical radius with respect to runaway growth, above the homogeneous cavitation radius.

Evaluation of the Effect of Antibacterial Peptides on Model Monolayers.

The aim of the study was to assess the effect of the synthesized antibacterial peptides: P2 (WKWK)2-KWKWK-NH2, P4 (C12)2-KKKK-NH2, P5 (KWK)2-KWWW-NH2, and P6 (KK)2-KWWW-NH2 on the physicochemical properties of a model biological membrane made of azolectin or lecithin. The Langmuir Wilhelmy method was used for the experiments. Based on the compressibility factor, it was determined that the monolayers formed of azolectin and peptides in the aqueous subphase are in the condensed liquid phase. At the boundary between the condensed and expanded liquid phases, there was a monolayer made of lecithin and P4, P5 or P6 in the aqueous subphase. In turn, the film consisting of lecithin alone (37.7 mN/m) and lecithin and P2 (42.6 mN/m) in the water subphase was in the expanded liquid phase. All peptides change, to varying degrees, the organization and packing of molecules in the monolayer, both those made of azolectin and of lecithin. The test results can be used for further research to design a system with the expected properties for specific organisms.

Interfacial Aβ fibril formation is modulated by the disorder-order state of the lipids: The concept of the physical environment as amyloid inductor in biomembranes

The behavior of amphiphilic molecules such as lipids, peptides and their mixtures at the air/water interface allow us to evaluate and visualize the arrangement formed in a confined and controlled surface area. We have studied the surface properties of the zwitterionic DPPC lipid and Aβ(1–40) amyloid peptide in mixed films at different temperatures (from 15 to 40 °C). In this range of temperature the surface properties of pure Aβ(1–40) peptide remained unchanged, whereas DPPC undergoes its characteristic liquid-expanded → liquid-condensed bidimensional phase transition that depends on the temperature and lateral pressure. This particular property of DPPC makes it possible to dynamically study the influence of the lipid phase state on amyloid structure formation at the interface in a continuous, isothermal and abrupt change on the environmental condition. As the mixed film is compressed the fibril-like structure of Aβ(1–40) is triggered specifically in the liquid-expanded region, independently of temperature, and it is selectively excluded from the well-visible liquid condensed domains of DPPC. The Aβ amyloid fibers were visualized by using BAM and AFM and they were Thio T positive. In mixed DPPC/Aβ(1–40) films the condensed domains (in between 11 mN/m to 20 mN/m) become irregular probably due to the fibril-like structures is imposing additional lateral stress sequestering lipid molecules in the surrounding liquid-expanded phase to self-organize into amyloids.

Influence of Surface Flows on the Shape of Fractal Domains.

Tidal breathing is associated with a 30% change of the surfactant-covered alveolar surface occurring about 16 times per minute. To model this highly dynamic process, erucic acid monolayers at the air-water interface were compressed fast. Brewster angle microscopy imaged the fractal liquid-condensed (LC) domains and quantified the surface flow in size, direction, and duration. Radial branch distribution of the domains has a minimum in the flow direction, as was shown with directionality histograms. The fast Fourier transform of the domains shows a preferential growth perpendicular to the flow direction. Additionally, at the beginning of the flow, the downstream side of the domain grows faster than the upstream side. Surface flows act on the mm to cm scale, cause an anisotropic flow in the liquid expanded phase surrounding the LC domain, and affect the overall domain shape. On the μm-scale, the dendritic or seaweed domains' branches were only slightly disturbed. These results may help to understand pulmonary surfactant layers.

Effect of γ-Oryzanol on the LE-LC Phase Coexistence Region of DPPC Langmuir Monolayer.

We have studied the effect of relative composition of γ-Oryzanol (γ-Or) on the liquid expanded-liquid condensed phase coexistence region in the mixed Langmuir monolayer of γ-Or and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) molecules at air-water interface. The surface manometry studies at a fixed temperature show that the mixture of γ-Or and DPPC forms a stable monolayer at air-water interface. As the relative composition of γ-Or increases the range of area per molecule over which the coexistence of liquid expanded (LE)-liquid condensed (LC) phases exists reduces. Although the LE-LC phase coexistence corresponds to the first-order phase transition, the slope of the surface pressure-area per molecule isotherm is non-zero. Earlier studies have attributed the non-zero slope in LE-LC phase coexistence region to the influence of the strain between the ordered LC phase and disordered LE phase. The effect of strain on the coexistence of LE-LC phases can be studied in terms of molecular density-strain coupling. Our analysis of the liquid condensed-liquid expanded coexistence region in the isotherms of mixed monolayers of DPPC and γ-Or shows that with the increase in the mole fraction of sterol in the mixed monolayer the molecular lateral density-strain coupling increases. However, at 0.6 mole fraction of γ-Or in the mixed monolayer the coupling decreases. This is corroborated by the observation of minimum Gibb's free energy of the mixed monolayer at this relative composition of γ-Or indicating better packing of molecules.

Surfactant Proteins SP-B and SP-C in Pulmonary Surfactant Monolayers: Physical Properties Controlled by Specific Protein-Lipid Interactions.

The lining of the alveoli is covered by pulmonary surfactant, a complex mixture of surface-active lipids and proteins that enables efficient gas exchange between inhaled air and the circulation. Despite decades of advancements in the study of the pulmonary surfactant, the molecular scale behavior of the surfactant and the inherent role of the number of different lipids and proteins in surfactant behavior are not fully understood. The most important proteins in this complex system are the surfactant proteins SP-B and SP-C. Given this, in this work we performed nonequilibrium all-atom molecular dynamics simulations to study the interplay of SP-B and SP-C with multicomponent lipid monolayers mimicking the pulmonary surfactant in composition. The simulations were complemented by z-scan fluorescence correlation spectroscopy and atomic force microscopy measurements. Our state-of-the-art simulation model reproduces experimental pressure-area isotherms and lateral diffusion coefficients. In agreement with previous research, the inclusion of either SP-B and SP-C increases surface pressure, and our simulations provide a molecular scale explanation for this effect: The proteins display preferential lipid interactions with phosphatidylglycerol, they reside predominantly in the lipid acyl chain region, and they partition into the liquid expanded phase or even induce it in an otherwise packed monolayer. The latter effect is also visible in our atomic force microscopy images. The research done contributes to a better understanding of the roles of specific lipids and proteins in surfactant function, thus helping to develop better synthetic products for surfactant replacement therapy used in the treatment of many fatal lung-related injuries and diseases.

Towards predicting shear-banding instabilities in lipid monolayers

Langmuir monolayers are advantageous systems used to investigate how lipid membranes get involved in the physiology of many living structures, such as collapse phenomena in alveolar structures. Much work focuses on characterizing the pressure-bearing capacity of Langmuir films, expressed in the form of isotherm curves. These show that monolayers experience different phases during compression with an according evolution of their mechanical response, incurring into instability events when a critical stress threshold is overcome. Although well-known state equations, which establish an inverse relationship between surface pressure and area change, are able to properly describe monolayer behaviour during liquid expanded phase, the modelling of their nonlinear behaviour in the subsequent condensed region is still an open issue. In this regard, most efforts are addressed to explain out-of-plane collapse by modelling buckling and wrinkling mainly resorting to linearly elastic plate theory. However, some experiments on Langmuir monolayers also show in-plane instability phenomena leading to the formation of the so-called shear bands and, to date, no theoretical description of the onset of shear banding bifurcation in monolayers has been yet provided. For this reason, by adopting a macroscopic description, we here study material stability of the lipid monolayers and exploit an incremental approach to find the conditions that kindle shear bands. In particular, by starting from the widely assumed hypothesis that monolayers behave elastically in the solid-like region, in this work a hyperfoam hyperelastic potential is introduced as a new constitutive strategy to trace back the nonlinear response of monolayer response during densification. In this way, the obtained mechanical properties together with the adopted strain energy are successfully employed to reproduce the onset of shear banding exhibited by some lipid systems under different chemical and thermal conditions.

Effect of Newly Synthesized Structures of Peptides on the Stability of the Monolayers Formed.

The aim of the study was to evaluate the effect of the peptide structure (WKWK)2-KWKWK-NH2, P4 (C12)2-KKKK-NH2, P5 (KWK)2-KWWW-NH2, P6 (KK)2-KWWW-NH2 on their physicochemical properties. The thermogravimetric method (TG/DTG) was used, which made it possible to observe the course of chemical reactions and phase transformations occurring during the heating of solid samples. Based on the DSC curves, the enthalpy of the processes occurring in the peptides was determined. The influence of the chemical structure of this group of compounds on their film-forming properties was determined using the Langmuir-Wilhelmy trough method and was followed by molecular dynamics simulation. Evaluated peptides showed high thermal stability and the first significant mass loss occurred only at about 230 °C and 350 °C. The analysis of the compressibility coefficient of individual peptides indicates that all formed peptide monolayers were in the expanded liquid phase. Their maximum compressibility factor was less than 50.0 mN/m. Its highest value of 42.7 mN/m was achieved in a monolayer made of P4. The results obtained in molecular dynamic simulation indicate that non-polar side chains played an important role in the properties of the P4 monolayer, and the same applies to P5, except that a spherical effect was observed here. A slightly different behavior was observed for the P6 and P2 peptide systems, where the type of amino acids present had an influence. The obtained results indicate that the structure of the peptide affected its physicochemical and layer-forming properties.

The role of cholesterol in curvature and stripe width evolution in model lung surfactant monolayers.

Lung surfactant (LS) is a lipid-protein monomolecular layer coating the alveolar air-liquid interface. A primary role of LS is to reduce the interfacial tension to facilitate breathing. Simple two and three component systems are used to model LS in studies to test the relationship between composition, morphology, rheological, and phase behavior. Failure of LS is associated with acute respiratory distress syndrome (ARDS). The primary lipid in LS is dipalmitoylphosphatidylcholine (DPPC) which accounts for 40-80 percent of native and replacement LS therapies. The addition of fatty acids, fatty alcohols, and cholesterol to replacement LS therapies is still an area of research. We utilize mixtures of DPPC and hexadecanol (HD) or palmitic acid (PA) with small mole fractions of dihydrocholesterol (DChol). The morphology of 2-dimensional self-assembled monolayers with coexisting crystalline-like domains (liquid-condensed phase) within a liquid-like matrix (liquid-expanded phase) is investigated. With the addition of DChol to the DPPC/HD or DPPC/PA system, a finger instability is seen during monolayer compression followed by a time dependent circle-to-stripe transition at a fixed interfacial tension. In this poster we will discuss recently published results on the equilibrium nature of these films (REF: Valtierrez et al., Sci. Adv. v:8:14, 2022) and extend this work to explore the role of cholesterol in determining morphology and curvature.

Identification of an optimized ratio of amyloid and non-amyloid fractions in engineered fibril solutions from whey protein isolate for improved foaming

Engineered fibril solutions from whey protein beta-lactoglobulin are known for their excellent foaming capacity. These amyloid aggregates solutions (AAS) usually contain a polydisperse mixture of different protein structures. An optimized ratio of amyloid (AF, fibrils) and non-amyloid fractions (n-AF) in AAS may improve foaming, particularly by interactions at the air-water interface. Foamability, surface activity, and monolayer phase behavior at the air-water interface of isolated AF and n-AF as well as AAS with different AF/n-AF-ratios were investigated using drop tensiometry, Langmuir trough and foam analysis. N-AF exhibited faster migration, twice as fast adsorption and thus faster spreading at the air-water interface than the fibrils (AF). N-AF required less energy to assemble in a liquid-expanded phase in a monolayer, i.e., they were more compressible in the monolayer than AF. This resulted in rapid stabilization of lamellae in foam. High surface hydrophobicity of AF results in faster adsorption and formation of capillary forces between adsorbed fibrils, improving the attraction for additional fibrils. Orientation of semi-flexible and larger fibrils in AF consumes high energy. In combination with n-AF, the energy needed for orientation and assembly of fibrils is equivalent, however, the yield of AF in AAS was only 20 %, indicating the interplay of amyloid and non-amyloid proteins at the air-water interface. N-AF can be incorporated into a fibrillar film, which increase the network's density and stiffness and the interfacial film stability. Consequently, in AAS fibrils and non-amyloid material acted synergistically at the air-water interface, whereby only small amounts of amyloid aggregates are required to stabilize foams.

Interactions of polycyclic aromatic hydrocarbons and their nitro derivatives with bilayer and monolayer models of fungal membranes

Polycyclic aromatic hydrocarbons (PAH) are ubiquitous persistent organic pollutants forming as byproducts in combustion processes. These compounds can become nitrated in the environment forming even more toxic nitro derivatives. Both PAHs and nitro-PAHs accumulate in the soils which can lead to the pollution of vast areas of arable land. The best means of their elimination from the environment is bioremediation that is the degradation by soil microorganisms. Important key players with high prospects are non-ligninolytic fungi, which can incorporate PAHs and their derivatives into their plasma membranes, oxidize and metabolize to nontoxic products. The accumulation of PAHs in the membrane can, however, be toxic to the fungal cell, but the mechanisms of the PAH-membrane lipid interactions and how they influence the membrane phase state and properties are still poorly understood. To shed light on these questions we employed fungal plasma membrane models and studied their interactions with selected PAHs and nitro-PAHs. Two complementary model systems were investigated: giant unilamellar vesicles (GUV) and Langmuir monolayers. We find that the native PAHs induce phase separation and the formation of solid ordered phase in the GUVs, whereas the nitro-derivatives do not disturb the liquid crystalline state of the membranes. The Langmuir monolayer studies indicate that some PAHs preferably partition in the liquid-expanded phase while others in the liquid-condensed domains. The possibility of the incorporation of PAHs into the condensed domains implies that these compounds can influence the order in the lipid bilayer and affect the function of the lipid rafts in plasma membranes.

Chemical Imaging of Lipid Segregation: Determining Different Length Scales of Heterogeneity with Compressive-Sensing Sum Frequency Generation Microscopy and Brewster Angle Microscopy

Membranes of various phospholipids may separate into different domains at micrometer length scales at the air-water interface. A significant challenge is to visualize the molecular organization and obtain chemical information on this surface. Langmuir-Blodgett monolayers of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) on fused silica were investigated with compressive sensing sum frequency generation (SFG) microscopy. Before and after mixing, SFG spectra for DSPC and DLPC revealed significant differences, indicating a structural change. Brewster angle microscopy images show phase separations that directly correlate the morphology of phospholipid mixtures with SFG images. Exploiting vibrational contrast in SFG images, exchange between the two phases was discovered, and quantitative thermodynamic analysis of lipid compositions in liquid-condensed and liquid-expanded (LE) phases was provided. Local SFG spectra reveal significant differences from one another, indicating the heterogeneity resulting from domain areas with distinct molecular orientation and conformation. Further heterogeneity across the domain boundary was presented on a finer scale, revealing an effect on DLPC due to the condensed phase DSPC, and the terminal methyl of DLPC perturbs the ordering of DSPC in the LE phase. This work demonstrates the heterogeneity of a two-dimensional binary lipid system mainly due to the aliphatic chain length and transition temperature difference.

Ionic environment, thickness and line tension as determinants of phase separation in whole Purified Myelin Membranes monolayers

Purified myelin membranes (PMM) were spread as monomolecular films at the air/aqueous solution interface, and visualized by Brewster Angle Microscopy (BAM) at different lateral pressures (π) on three specific aqueous solutions: absence of salts, physiological conditions and presence of calcium. Coexistence of Liquid-Expanded (LE) and Liquid Ordered (LO) phases persisted up to collapse in the presence of salts, whereas monolayers became homogeneous at π ≥ 35−40 mN/m when salts are absent. This PMM phase-mixing behavior in monolayers is similar to the previously reported behavior of PMM multilamellar vesicles. Reflectivities (Rp) of p-polarized light from both phases were assessed throughout the whole π -range, and film thicknesses (t) were calculated from the Rp values and measured film refractive indices (n). The LO phase was found to be more reflective and thicker than the LE phase at π ≤ 15 mN/m, but less reflective and thinner at higher π.We also determined the line tension (λ) of PMM monolayers at the domain boundaries from the rate of domain shape relaxation, which turned out to be of the order of picoNewtons (pN) and decreased as π increased. A correlation between λ and thickness differences (Δt) was found, suggesting that Δt is a molecular determinant for λ in PMM monolayers.Both λ and Δt were found to increase markedly when calcium was present in the subphase. This result corroborates the concept of divalent cations as a stabilizing factor for phase separation, in line with earlier studies on this mixture forming multilamellar membrane arrangements.

Polyhydroxyalkanoate Nanoparticles for Pulmonary Drug Delivery: Interaction with Lung Surfactant.

Polyhydroxyalkanoates (PHA) are polyesters produced intracellularly by many bacterial species as energy storage materials, which are used in biomedical applications, including drug delivery systems, due to their biocompatibility and biodegradability. In this study, we evaluated the potential application of this nanomaterial as a basis of inhaled drug delivery systems. To that end, we assessed the possible interaction between PHA nanoparticles (NPs) and pulmonary surfactant using dynamic light scattering, Langmuir balances, and epifluorescence microscopy. Our results demonstrate that NPs deposited onto preformed monolayers of DPPC or DPPC/POPG bind these surfactant lipids. This interaction facilitated the translocation of the nanomaterial towards the aqueous subphase, with the subsequent loss of lipid from the interface. NPs that remained at the interface associated with liquid expanded (LE)/tilted condensed (TC) phase boundaries, decreasing the size of condensed domains and promoting the intermixing of TC and LE phases at submicroscopic scale. This provided the stability necessary for attaining high surface pressures upon compression, countering the destabilization induced by lipid loss. These effects were observed only for high NP loads, suggesting a limit for the use of these NPs in pulmonary drug delivery.

First quantitative assessment of the adsorption of a fluorocarbon gas on phospholipid monolayers at the air/water interface

HypothesisFluorocarbon gases introduced above monolayers of phospholipids at the air/water interface were recently found to promote the adsorption of diverse molecular compounds, with potential application in drug-loaded microbubble design. Quantitative determination of the fluorocarbon present in the monolayers is strongly needed for the development of such applications. We hypothesized that neutron reflectometry (NR) and ellipsometry experiments would allow quantification of the fluorocarbon trapped in the monolayers. ExperimentsWe report the first quantitative determination of the extents of adsorption of perfluorohexane (F-hexane) on different phospholipid monolayers with respect to both their phase and isotopic form. To this aim, we applied an approach based on co-modeling the data obtained from NR and ellipsometry. FindingsWe found that F-hexane adsorbs strongly in monolayers of dipalmitoylphosphatidylcholine (DPPC) when they are both in the liquid expanded (LE) and liquid condensed (LC) phases, but to different extents according to the isotopic form of the phospholipid. Kinetic resolution of the interfacial composition from data on both isotopic contrasts (assuming chemical identicality) was therefore not possible using NR alone, so an alternative NR/ellipsometry co-modeling treatment was applied to data from each isotopic contrast. F-hexane adsorbs more abundantly on monolayers of hydrogenous DPPC than chain-deuterated DPPC when they are in the LE phase, whilst the opposite was observed when they monolayers are in the LC phase. The extents of adsorption of F-hexane in monolayers of dimyristoylphosphatidylcholine (DMPC, LE phase) and distearoylphosphatidylcholine (DSPC, LC phase) concurs with the strong dependence of those with phospholipids of different isotopic contrasts according to the monolayer phase. This new methodology can lead to advances in the novel characterization of fluorocarbons interacting with phospholipid monolayers of relevance to applications such as in the shells of fluorocarbon-stabilized medically-oriented microbubbles.

Mass sensitivity of Langmuir-Blodgett monolayer film coated surface acoustic wave resonators to volatile organic solvents

Langmuir-Blodgett (LB) monolayer films deposited from a Dipalmytoyl Phosphatidyl Ethanolamine head labelled with Nitrbenzoxadiazole (DPPE-NBD), have been studied for their ability to be used as sensing layer for chemical gas sensors. DPPE-NBD molecules are known for their similarity to bio membrane molecules. Therefore, it is expected that they can serve as a matrix for immobilization of proteins, enzymes, aptamers, while preserving their function for selective reaction with organic analytes. In this study, LB monolayers of DPPE-NBD were deposited on Rayleigh Surface Acoustic Wave (RSAW) resonant devices working at 411 MHz and simultaneously on ultra-flat Si wafer substrates for Atomic Force Microscopy (AFM) inspection. Depositions were carried out both at a low surface pressure where the liquid-expanded phase dominates, and at a high surface pressure, where the liquid-condensed (solid) phase dominates. AFM topography reveals the liquid and solid phase coexistence as well as the formation of 3D pyramids of 3 to 30 nm height and 50 to 1000 nm in diameter. In a vapour sensing experiment, the RSAW devices were exposed subsequently to vapours of 6 volatile organic compounds (VOCs) and water. The most significant resonant frequency shift of 225 kHz which corresponds to 11.42 ng mass change was observed with chloroform vapours when the substrate with the solid phase LB monolayer was used as a sensing layer. Adsorption and desorption of the vapours was very fast (a few seconds) and completely reversible. The higher-pressure deposited LB film demonstrates higher sensitivity to all gases. Mechanisms of this behaviour are discussed.

On the smectic order of polymer monolayers at the air–water interface

The McMillan theory of the smectic-A phase is extended to two dimensions. Its predictions, which strongly differ from those in three dimensions, are found to be in disagreement with the experiment on monolayers of the polymer PLA [poly(l-lactic acid)] at the air–water interface. In contrast to the latter, the theory predicts that the smectic A-phase does not melt into the isotropic phase but first goes through a nematic phase. In addition, the effect of CdSe nanoparticles and Fe2O3 beads on the phase transition from the isotropic liquid-expanded phase to the smectic liquid-condensed phase of PLA monolayers at the air–water interface is investigated experimentally using the Langmuir–Blodgett method. It is found that depending on the size and concentration of the nanoparticles and beads, this transition can be shifted or suppressed altogether.

Physico-chemical behaviors of human and bovine milk membrane extracts and their influence on gastric lipase adsorption

Milk fat globule membrane conditions the reactivity and enzymatic susceptibility of milk lipids. The use of bovine membrane extracts to make infant formulas more biomimetic of human milk has been suggested recently. A comparison of the physico-chemical behavior of human and bovine milk membrane extracts and their interaction with gastric lipase is here undertaken using biophysical tools. Milk membrane extracts (70% of polar lipids) were obtained either pooling of mature human milk (n = 5) or bovine buttermilk. Human extract contained more anionic glycerophospholipids, less phosphatidylethanolamine and more unsaturated fatty acids (57% versus 46%) than bovine extract. Human extract presented a higher compressibility, with slower increase of surface pressure, than bovine extract. Micronic liquid condensed (LC) domains were evidenced in both extracts at 10 mN/m, but the evolution differs upon compression. Upon gastric lipase addition, an adsorption preference for liquid expanded phase (LE) was observed for both extracts. However, insertion was more homogeneous in terms of height level in human extract and impacted less its lipid lateral organization than in bovine extract. Both membrane extracts share close physico-chemical properties, however human membrane higher compressibility may favour gastric lipase insertion and higher interfacial reactivity in gastric conditions.

Strong Headgroup Interactions Drive Highly Directional Growth and Unusual Phase Co-Existence in Self-Assembled Phenolic Films.

Self-assembled materials as surface coatings are used to confer functional properties to substrates, but such properties are highly dependent on molecular organization that can be controlled through tailoring the noncovalent interactions. For monomolecular films, it is well-known that strong, dipolar interactions can oppose line tension generating noncircular domain growth. While many surfactant films exhibit liquid crystalline arrangement of the alkyl chains, there are relatively few reports of crystalline headgroups. Here, we report the self-assembly of phenolic surfactants where the combination of hydrogen bonding and π-stacking leads to a herringbone arrangement of the headgroups, generating a molecular super-lattice that can be observed using grazing incidence X-ray diffraction; such an arrangement has been previously proposed for related phenolic systems but never experimentally observed. We also investigated using pH to modulate the intermolecular interactions and the response of the system in terms of molecular organization. The first hydroxyl deprotonation does not appear to impact the structure but has significant impact on the domain size and morphology. Higher pH generates both strong directional domain growth and a loss of the molecular lattice structure, attributed to a second deprotonation. In contrast, a shorter chain surfactant, lauryl gallate, forms a liquid expanded phase that can contract upon deprotonation. In the condensed phase, the deprotonation kinetics are unusually slow for which an internal charge re-organization is proposed. The slow kinetics leads to the co-existence of three distinct phases for a single component system over relatively long timescales and provides evidence of a liquid-mediated polymorphic transformation process in two-dimensional, soft-matter films. This work has implications for understanding the long-range ordering in aromatic self-assembled structures and the mechanisms underlying Langmuir monolayer polymorphism.