Surface rheology of monolayers of phospholipids and cholesterol measured with axisymmetric drop shape analysis

A biophysical study of tear film lipid layer model membranes

The tear film lipid layer (TFLL), the final layer of the human tear film is responsible for surface tension reduction while blinking, water evaporation retardation and maintaining the stability of the tear film. The study of the composition-structure-function relationship of TFLL is paramount, as a compromised structure of TFLL leads to the emergence of dry eye disease (DED) which is one the most prevalent ophthalmic surface diseases of the modern world, associated with chronic pain and reduced visual capability. In this model membrane study, a systematic approach is used to study the biophysical properties of TFLL model membranes as a function of composition. Three mixed-lipid model membranes are studied along with their individual components comprising cholesteryl oleate (CO), glyceryl trioleate (GT), L-α-phosphatidylcholine (egg PC) and a free fatty acid mixture. The models become progressively more complex from binary to quaternary mixtures, allowing the role of each individual lipid to be derived. Langmuir balance, Brewster Angle Microscopy (BAM) and Profile Analysis Tensiometer (PAT) are used to study the surface activity and compression-expansion cycles, morphology, and rheological behaviour of the model membranes, respectively. Evidence of multilayering is observed with inclusion of CO and a reversible collapse is associated with the GT phase transition. An initially more coherent film is observed due to the addition of polar PC. Notably, these individual behaviours are retained in the mixed films and suggest a possible role for each physiological component of TFLL.

Impact of Pollutant Ozone on the Biophysical Properties of Tear Film Lipid Layer Model Membranes

Ozone exposure from environmental smog has been implicated as a risk factor for developing dry eye disease (DED). The tear film lipid layer (TFLL), which is the outermost layer of the tear film and responsible for surface tension reduction while blinking, is in direct contact with the environment and serves as the first line of defense against external aggressors such as environmental pollution. The impact of exposure to ozone on the biophysical properties of three TFLL model membranes was investigated. These model membranes include a binary mixture of cholesteryl oleate (CO) and L-α-phosphatidylcholine (egg PC), a ternary mixture of CO, glyceryl trioleate (GT) and PC, as well as a quaternary mixture of CO, GT, a mixture of free fatty acids palmitic acid and stearic acid (FFAs) and PC. Biophysical impacts were evaluated as changes to the surface activity, respreadability, morphology and viscoelastic properties of the films. Expansion to higher molecular areas was observed in all the TFLL model membrane films which is attributable to the accommodation of the cleaved chains in the film. Significant morphological changes were observed, namely fluidization and the disruption of the phase transition behaviour of GT, and multilayer formation of CO. This fluidization reduces the hysteresis loops for the model membranes. On the other hand, the viscoelastic properties of the films exhibited differential impacts from ozone exposure as a function of composition. These findings are correlated to chemical changes to the lipids determined using ESI-MS.

Influence of Semifluorinated Alkane Surface Domains on Phase Behavior and Linear and Nonlinear Viscoelasticity of Phospholipid Monolayers

Semifluorinated alkanes self-assemble into 30-40 nm-large surface domains (hemimicelles) at the air/water interface. They have been drawing increasing attention to stabilize microbubbles coated with lipids, which are used for enhancing the contrast in sonographic imaging. Although previous studies suggested that semifluorinated alkanes increase the stability of phospholipid membranes, little is known about how semifluorinated alkanes influence phase behaviors and mechanical properties of lipid-coated microbubbles. As a well-defined model of microbubble surfaces, we prepared monolayers consisting of a mixture of phospholipids and semifluorinated alkanes at the air/water interface and investigated the influence of hemimicelles of semifluorinated alkanes on the phase behavior and interfacial viscoelastic properties of phospholipid monolayers. Hemimicelles are phase-separated from phospholipids and accumulate at the phase boundary, which strongly modulates the correlation between solid phospholipid domains. Intringuingly, we found that the mixed monolayer of semifluorinated alkanes and phospholipids possesses linear and nonlinear viscoelastic properties comparable to those of phospholipid monolayers. Since the mixing of semifluorinated alkanes and phospholipids enables one to overcome the intrinsically low stability of pure semifluorinated alkanes against the change in the surface area of microbubbles through the partial dissolution of gas into the aqueous phase, this is a promising strategy for the stable coating of microbubbles in ultrasound diagnosis.

Effect of saponins from quinoa on a skin-mimetic lipid monolayer containing cholesterol

The study discusses the effect of a quinoa seed coat extract on a cholesterol-based Langmuir monolayer mimicking the intercellular lipid mixture in the skin's outermost layer - stratum corneum. Besides cholesterol (CHOL), the monolayer contains also stearic acid (SA) and ceramide VI (CER), in a molar ratio of 10:14:14. Three quinoa extracts were tested for their surface activity: a) from the whole seed, b) from the dehulled seed, and c) from the seed coat. The latter shows significantly higher ability to reduce surface tension (increase surface pressure) than the others. Its adsorbed layers display also reasonable surface dilational elasticity (storage) modulus, E'. These observations are in line with the literature reports on the high concentrations of triterpenoid glycosidic biosurfactants - saponins, in quinoa seed, especially in its coat. The saponin-rich extract of quinoa seed coat was thus introduced underneath the pre-formed lipid monolayer compressed to surface pressure, Π = 30 mN/m in a Langmuir trough, in order to register the surface pressure response. The increase of both the surface pressure and surface dilational elasticity modulus suggests that saponins, and possibly other surface-active components of the extract, incorporate into the model lipid monolayer, without solubilizing it. This opens new perspectives for the saponin-rich quinoa seed extract as skin penetration-enhancing active components for cosmetics or pharmaceutical purposes.

Adsorption of phospholipids at oil/water interfaces during emulsification is controlled by stress relaxation and diffusion

Adsorption of phosphatidylcholines at oil/water interfaces strongly deviates from spread monolayers at air/water surfaces. Understanding its nature and consequences could vastly improve applications in medical nanoemulsions and biotechnologies. Adsorption kinetics at interfaces of water with different oil phases were measured by profile analysis tensiometry. Adsorption kinetics for 2 different phospholipids, DPPC and POPC, as well as 2 organic phases, squalene and squalane, show that formation of interfacial monolayers is initially dominated by stress-relaxation in the first minutes. Diffusion only gradually contributes to a decrease in interfacial tension at later stages of time and higher film pressures. The results can be applied for the optimization of emulsification protocols using mechanical treatments. Emulsions using phospholipids with unsaturated fatty acids are dominated much more strongly by stress-relaxation and cover interfaces very fast compared to those with saturated fatty acids. In contrast, phospholipid layers consisting of saturated fatty acids converge faster towards the equilibrium than those with unsaturated fatty acids.

Emergence of Strong Nonlinear Viscoelastic Response of Semifluorinated Alkane Monolayers

Viscoelasticity of monolayers of fluorocarbon/hydrocarbon tetrablock amphiphiles di(FnHm) ((C_nF_2n+1CH₂)(C_m-2H_2m-3)CH-CH(C_nF_2n+1CH₂)(C_m-2H_2m-3)) was characterized by interfacial dilational rheology under periodic oscillation of the moving barriers at the air/water interface. Because the frequency dispersion of the response function indicated that di(FnHm) form two-dimensional gels at the interface, the viscosity and elasticity of di(FnHm) were first analyzed with the classical Kelvin-Voigt model. However, the global shape of stress response functions clearly indicated the emergence of a nonlinearity even at very low surface pressures (π ≈ 5 mN/m) and small strain amplitudes (u₀ = 1%). The Fourier-transformed response function of higher harmonics exhibited a clear increase in the intensity only from odd modes, corresponding to the nonlinear elastic component under reflection because of mirror symmetry. The emergence of strong nonlinear viscoelasticity of di(FnHm) at low surface pressures and strain amplitudes is highly unique compared to the nonlinear viscoelasticity of other surfactant systems reported previously, suggesting a large potential of such fluorocarbon/hydrocarbon molecules to modulate the mechanics of interfaces using the self-assembled domains of small molecules.

Effects of Concentration and Surface Pressure on MBP Interaction with Cholesterol in Langmuir Films

Predicting the mechanism of MBP binding to cholesterol is meaningful in understanding how MBP participate in lateral membrane organization. The interaction of MBP with cholesterol monolayer was investigated at three surface pressures on 10 mM Tris-HCl buffer with the different concentrations of MBP. The results show that π-A isotherms shift to larger molecular area at all pressures. By means of analyzing π-T curves, a surface pressure increase was obtained. Results indicated that the greater the protein concentration in the subphase, the larger the increase of surface pressure. In addition, changes in monolayer surface morphology and domain formation were performed by AFM. These results provide more direct and convincing evidence for the MBP interaction with cholesterol. The MBP-cholesterol interaction suggests a significant concentrations and surface pressure dependence and is probably governed by hydrogen bonds. The date presented could help to understand at least one of the molecular mechanisms through which MBP affects lateral organization of the cholesterol membrane.

Model Lung Surfactant Films: Why Composition Matters

Lung surfactant replacement therapies, Survanta and Infasurf, and two lipid-only systems both containing saturated and unsaturated phospholipids and one containing additional palmitic acid were used to study the impact of buffered saline on the surface activity, morphology, rheology, and structure of Langmuir monolayer model membranes. Isotherms and Brewster angle microscopy show that buffered saline subphases induce a film expansion, except when the cationic protein, SP-B, is present in sufficient quantities to already screen electrostatic repulsion, thus limiting the effect of changing pH and adding counterions. Grazing incidence X-ray diffraction results indicate an expansion not only of the liquid expanded phase but also an expansion of the lattice of the condensed phase. The film expansion corresponded in all cases with a significant reduction in the viscosity and elasticity of the films. The viscoelastic parameters are dominated by liquid expanded phase properties and do not appear to be dependent on the structure of the condensed phase domains in a phase separated film. The results highlight that the choice of subphase and film composition is important for meaningful interpretations of measurements using model systems.

Effects of ambient hydrostatic pressure on the material properties of the encapsulation of an ultrasound contrast microbubble

Ultrasound contrast microbubbles experience widely varying ambient blood pressure in different organs, which can also change due to diseases. Pressure change can alter the material properties of the encapsulation of these microbubbles. Here the characteristic rheological parameters of contrast agent Definity are determined by varying the ambient pressure (in a physiologically relevant range 0-200 mm Hg). Four different interfacial rheological models are used to characterize the microbubbles. Effects of gas diffusion under excess ambient pressure are investigated in detail accounting for size decrease of contrast microbubbles. Definity contrast agent show a change in their interfacial dilatational viscosity (3.6 × 10(-8) Ns/m at 0 mm Hg to 4.45 × 10(-8) Ns/m at 200 mm Hg) and interfacial dilatational elasticity (0.86 N/m at 0 mm Hg to 1.06 N/m at 200 mm Hg) with ambient pressure increase. The increase results from material consolidation, similar to such enhancement in bulk properties under pressure. The model that accounts for enhancement in material properties with increasing ambient pressure matches with experimentally measured subharmonic response as a function of ambient pressure, while assuming constant material parameters does not.

Surface dilational rheological properties in the nonlinear domain

The interfacial tension response to dilational deformation of interfacial area exhibits a (more or less) nonlinear behavior, depending on the amplitude of the deformation. Studies of such observable interfacial properties in the nonlinear domain suggest valuable information about the two-dimensional microstructure of the interfacial layer, as well as about the structure time-evolution. In this article, the emphasis is centered on the available mathematical methods for quantitatively analyzing and describing the magnitude and the characteristics of the nonlinear interfacial viscoelastic properties. Specifically, in periodic oscillation experiments the nonlinear behavior can be represented by the combination of a linear part (the surface dilational modulus), with an additional complementary Fourier analysis parameterizing the nonlinearity. Also asymmetric Lissajous plots, of interfacial tension versus deformation, are useful tools for expanding the response nonlinearity into four distinct components relevant to significant points of the cyclic loop. In connection with the mathematical methods, nonequilibrium thermodynamic formulations provide a powerful theoretical framework for investigating the interfacial dynamic properties of multiphase systems. Experimental results for adsorption layers of complex components, available in the literature, show notable nonlinear interfacial viscoelastic behavior. In particular in this review, data are illustrated for solutions of polymers and of polyelectrolyte/surfactant complexes. The observed nonlinear findings reveal formation of complexes, patches, and other different interfacial structures.

Nonlinear stress deformation behavior of interfaces stabilized by food-based ingredients

Interfaces stabilized by food-based ingredients, such as proteins or glycolipids, often display nonlinear behavior when subjected to oscillatory dilatational deformations, even at the lowest deformation amplitudes which can currently be applied experimentally. Here we show that classical approaches to extract dilatational properties, based on the Young-Laplace equation, may not always be suitable to analyze data. We discuss a number of examples of food-ingredient stabilized interfaces (interfaces stabilized by protein fibrils, protein-polysaccharide complexes and oligosaccharide-fatty aid conjugates) and show how an analysis of the dynamic surface tension signal using Lissajous plots and a protocol which includes deformation amplitude and droplet size variations, can be used to obtain a more detailed and accurate description of their nonlinear dilatational behavior.

Adsorption of egg phosphatidylcholine to an air/water and triolein/water bubble interface: use of the 2-dimensional phase rule to estimate the surface composition of a phospholipid/triolein/water surface as a function of surface pressure

Phospholipid monolayers play a critical role in the structure and stabilization of biological interfaces, including all membranes, the alveoli of the lungs, fat droplets in adipose tissue, and lipoproteins. The behavior of phospholipids in bilayers and at an air-water interface is well understood. However, the study of phospholipids at oil-water interfaces is limited due to technical challenges. In this study, egg phosphatidylcholine (EPC) was deposited from small unilamellar vesicles onto a bubble of either air or triolein (TO) formed in a low-salt buffer. The surface tension (gamma) was measured using a drop tensiometer. We observed that EPC binds irreversibly to both interfaces and at equilibrium exerts approximately 12 and 15 mN/m of pressure (Pi) at an air and TO interface, respectively. After EPC was bound to the interface, the unbound EPC was washed out of the cuvette, and the surface was compressed to study the Pi/area relationship. To determine the surface concentration (Gamma), which cannot be measured directly, compression isotherms from a Langmuir trough and drop tensiometer were compared. The air-water interfaces had identical characteristics using both techniques; thus, Gamma on the bubble can be determined by overlaying the two isotherms. Both TO and EPC are surface-active, so in a mixed TO/EPC monolayer, both molecules will be exposed to water. Since TO is less surface-active than EPC, as Pi increases, the TO is progressively ejected. To understand the Pi/area isotherm of EPC on a TO bubble, a variety of TO-EPC mixtures were spread at the air-water interface. The isotherms show an abrupt break in the curve caused by the ejection of TO from the monolayer into a new bulk phase. By overlaying the compression isotherm above the ejection point with a TO bubble compression isotherm, Gamma can be estimated. This allows determination of Gamma of EPC on a TO bubble as a function of Pi.

Molecular view of the interaction between iota-carrageenan and a phospholipid film and its role in enzyme immobilization

Proteins incorporated into phospholipid Langmuir-Blodgett (LB) films are a good model system for biomembranes and enzyme immobilization studies. The specific fluidity of biomembranes, an important requisite for enzymatic activity, is naturally controlled by varying phospholipid compositions. In a model system, instead, LB film fluidity may be varied by covering the top layer with different substances able to interact simultaneously with the phospholipid and the protein to be immobilized. In this study, we immobilized a carbohydrate rich Neurospora crassa alkaline phosphatase (NCAP) in monolayers of the sodium salt of dihexadecylphosphoric acid (DHP), a synthetic phospholipid that provides very condensed Langmuir films. The binding of NCAP to DHP Langmuir-Blodgett (LB) films was mediated by the anionic polysaccharide iota-carrageenan (iota-car). Combining results from surface isotherms and the quartz crystal microbalance technique, we concluded that the polysaccharide was essential to promote the interaction between DHP and NCAP and also to increase the fluidity of the film. An estimate of DHP:iota-car ratio within the film also revealed that the polysaccharide binds to DHP LB film in an extended conformation. Furthermore, the investigation of the polysaccharide conformation at molecular level, using sum-frequency vibrational spectroscopy (SFG), indicated a preferential conformation of the carrageenan molecules with the sulfate groups oriented toward the phospholipid monolayer, and both the hydroxyl and ether groups interacting preferentially with the protein. These results demonstrate how interfacial electric fields can reorient and induce conformational changes in macromolecules, which may significantly affect intermolecular interactions at interfaces. This detailed knowledge of the interaction mechanism between the enzyme and the LB film is relevant to design strategies for enzyme immobilization when orientation and fluidity properties of the film provided by the matrix are important to improve enzymatic activity.

The importance of aspect ratio in profile analysis tensiometry

The latest developments in profile analysis tensiometry (PAT) for determining surface tension and interfacial viscoelastic parameters involve the determination of a digital interface profile and its best fit with the Young-Laplace equation. In this short communication, we show that the results for surface tension and other interfacial parameters determined by PAT are extremely sensitive to the (aspect) ratio of length to width of a pixel. Fine calibration (to five decimal digits) for the aspect ratio required to obtain physically consistent results is not always achieved with conventional numerical procedures due to nanometer resolution limit of optical imaging devices but can be manually adjusted using the known surface tension of pure water and/or surfactant solutions at reference temperature.

Surface rheology and phase transitions of monolayers of phospholipid/cholesterol mixtures

The dynamic surface elasticity and the surface dilational viscosity of three binary phospholipid/cholesterol mixtures were determined with axisymmetric drop shape analysis on a harmonically oscillating pendent drop. Dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and dioleoylphosphatidylcholine were used to explore the rheological properties and phase transitions of mixtures of saturated and unsaturated phospholipids with cholesterol. The growth rates for surface dilational viscosity and dynamic elasticity are parallel for all film pressures studied. Characteristic breaks and plateaus could be found for these growth rates, indicating phase transitions. For dipalmitoylphosphatidylcholine/cholesterol and dimyristoylphosphatidylcholine/cholesterol mixtures, phase diagrams with six regions separated by phase boundaries were found, which are in good agreement with phase transitions reported in the literature for static measurements of isotherms and isobars on a Langmuir film balance and from fluorescence microscopy. Some phase boundaries were only found by dynamic, but not by static, elasticity measurements. Imaging methods revealed phase separations produced by the formation of condensed stoichiometric complexes leading to micron-sized and mostly circular domains. The effects of these complexes on monolayer rheology in liquid/liquid phases is described. Furthermore, liquid/solid and solid phase transitions are discussed.