Effect of monoglyceride structure and cholesterol content on water permeability of the droplet bilayer

Concentration-Dependent Effects of Curcumin on Membrane Permeability and Structure

Growing evidence suggests that many bioactive molecules can nonspecifically modulate the physicochemical properties of membranes and influence the action of embedded membrane proteins. This study investigates the interactions of curcumin with protein-free model membranes consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and DOPC with cholesterol (4/1 mol ratio). The focus is on the capability of curcumin to modify membrane barrier properties such as water permeability assayed through the droplet interface bilayer (DIB) model membrane. For pure DOPC, our findings show a concentration-dependent biphasic effect: a reduction in water permeability is observed at low concentrations (up to 2 mol %), whereas at high concentrations of curcumin, water permeability increases. In the presence of cholesterol, we observed an overall reduction in water permeability. A combination of complementary experimental methods, including phase transition parameters studied by differential scanning calorimetry (DSC) and structural properties measured by attenuated total reflectance (ATR)-FTIR, provides a deeper understanding of concentration-dependent interactions of curcumin with DOPC bilayers in the absence and presence of cholesterol. Our experimental findings align with a molecular mechanism of curcumin's interaction with model membranes, wherein its effect is contingent on its concentration. At low concentrations, curcumin binds to the lipid-water interface through hydrogen bonding with the phosphate headgroup, thereby obstructing the transport of water molecules. Conversely, at high concentrations, curcumin permeates the acyl chain region, inducing packing disorders and demonstrating evidence of phase separation. Enhanced knowledge of the impact of curcumin on membranes, which, in turn, can affect protein function, is likely to be beneficial for the successful translation of curcumin into effective medicine.

Differential Effects of Soy Isoflavones on the Biophysical Properties of Model Membranes

The effects that the main soy isoflavones, genistein and daidzein, have upon the biophysical properties of a model lipid bilayer composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or DOPC with cholesterol (4 to 1 mol ratio) have been investigated by transbilayer water permeability, differential scanning calorimetry, and confocal Raman microspectroscopy. Genistein is found to increase water permeability, decrease phase transition temperature, reduce enthalpy of transition, and induce packing disorder in the DOPC membrane with an increasing concentration. On the contrary, daidzein decreases water permeability and shows negligible impact on thermodynamic parameters and packing disorder at comparable concentrations. For a cholesterol-containing DOPC bilayer, both genistein and daidzein exhibit an overall less pronounced effect on transbilayer water permeability. Their respective differential abilities to modify the physical and structural properties of biomembranes with varying lipid compositions signify a complex and sensitive nature to isoflavone interactions, which depends on the initial state of bilayer packing and the differences in the molecular structures of these soy isoflavones, and provide insights in understanding the interactions of these molecules with cellular membranes.

Interfacial stresses on droplet interface bilayers using two photon fluorescence lifetime imaging microscopy

HYPOTHESIS

Response of lipid bilayers to external mechanical stimuli is an active area of research with implications for fundamental and synthetic cell biology. Developing novel tools for systematically imposing mechanical strains and non-invasively mapping out interfacial (membrane) stress distributions on lipid bilayers can accelerate research in this field.

EXPERIMENTS

We report a miniature platform to manipulate model cell membranes in the form of droplet interface bilayers (DIBs), and non-invasively measure spatio-temporally resolved interfacial stresses using two photon fluorescence lifetime imaging of an interfacially active molecular flipper (Flipper-TR). We established the effectiveness of the developed framework by investigating interfacial stresses accompanying three key processes associated with DIBs: thin film drainage between lipid monolayer coated droplets, bilayer formation, and bilayer separation.

FINDINGS

The measurements revealed fundamental aspects of DIBs including the existence of a radially decaying interfacial stress distribution post bilayer formation, and the simultaneous build up and decay of stress respectively at the bilayer corner and center during bilayer separation. Finally, utilizing interfacial rheology measurements and MD simulations, we also reveal that the tested molecular flipper is sensitive to membrane fluidity that changes with interfacial stress - expanding the scientific understanding of how molecular flippers sense stress.

Lipid Bilayer Reformation Using the Wiping Blade for Improved Ion Channel Analysis

The measurement of ion permeation activity across planar lipid bilayers is a useful technique for the functional analysis and drug evaluation of ion channels at the single-molecule level. To enhance the data throughput, parallelization of lipid bilayers is desirable. However, existing parallelized approaches face challenges in simultaneously and efficiently measuring ion channel activities under various conditions on one chip. In this study, we propose an approach to overcome these limitations by developing a device capable of repeated measurements of ion channels incorporated into individually arrayed lipid bilayers. Our device forms an array of a lipid bilayer at a micropore on a separator by merging two lipid monolayers assembled on the surface of aqueous droplets. We introduce a vertically moving, blade-shaped module─referred to as a "wiping blade"─which enables controlled disruption and reformation of the bilayer at the micropore. By optimizing the surface properties and clearance of the wiping blade, we successfully achieved repeated bilayer formation. The arrayed lipid bilayer device with the integrated wiping blade module demonstrates a 5-fold improvement in data throughput during ion channel activity measurements. Finally, we validate the practical utility of our device by evaluating the effects of an ion channel inhibitor. The developed device opens new avenues for high-throughput analysis and screening of ion channels, leading to significant advancements in drug discovery and functional studies of membrane proteins. It offers a powerful tool for researchers in the field and holds promise for accelerating drug development by targeting ion channels.

Aspirin Interacts with Cholesterol-Containing Membranes in a pH-Dependent Manner

Aspirin has been used for broad therapeutic treatment, including secondary prevention of cardiovascular disease associated with increased cholesterol levels. Aspirin and other nonsteroidal anti-inflammatory drugs have been shown to interact with lipid membranes and change their biophysical properties. In this study, mixed lipid model bilayers made from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) or 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) comprising varying concentrations of cholesterol (10:1, 4:1, and 1:1 mole ratio of lipid:chol), prepared by the droplet interface bilayer method, were used to examine the effects of aspirin at various pH on transbilayer water permeability. The presence of aspirin increases the water permeability of POPC bilayers in a concentration-dependent manner, with a greater magnitude of increase at pH 3 compared to pH 7. In the presence of cholesterol, aspirin is similarly shown to increase water permeability; however, the extent of the increase depends on both the concentration of cholesterol and the pH, with the least pronounced enhancement in water permeability at high cholesterol levels at pH 7. A fusion of data from differential scanning calorimetry, confocal Raman microspectrophotometry, and interfacial tensiometric measurements demonstrates that aspirin can promote significant thermal, structural, and interfacial property perturbations in the mixed-lipid POPC or DOPC membranes containing cholesterol, indicating a disordering effect on the lipid membranes. Our findings suggest that aspirin fluidizes phosphocholine membranes in both cholesterol-free and cholesterol-enriched states and that the overall effect is greater when aspirin is in a neutral state. These results confer a deeper comprehension of the divergent effects of aspirin on biological membranes having heterogeneous compositions, under varying physiological pH and different cholesterol compositions, with implications for a better understanding of the gastrointestinal toxicity induced by the long term use of this important nonsteroidal anti-inflammatory molecule.

Hydrophobin Bilayer as Water Impermeable Protein Membrane

One of the most important properties of membranes is their permeability to water and other small molecules. A targeted change in permeability allows the passage of molecules to be controlled. Vesicles made of membranes with low water permeability are preferable for drug delivery, for example, because they are more stable and maintain the drug concentration inside. This study reports on the very low water permeability of pure protein membranes composed of a bilayer of the amphiphilic protein hydrophobin HFBI. Using a droplet interface bilayer setup, we demonstrate that HFBI bilayers are essentially impermeable to water. HFBI bilayers withstand far larger osmotic pressures than lipid membranes. Only by disturbing the packing of the proteins in the HFBI bilayer is a measurable water permeability induced. To investigate possible molecular mechanisms causing the near-zero permeability, we used all-atom molecular dynamics simulations of various HFBI bilayer models. The simulations suggest that the experimental HFBI bilayer permeability is compatible neither with a lateral honeycomb structure, as found for HFBI monolayers, nor with a residual oil layer within the bilayer or with a disordered lateral packing similar to the packing in lipid bilayers. These results suggest that the low permeabilities of HFBI and lipid bilayers rely on different mechanisms. With their extremely low but adaptable permeability and high stability, HFBI membranes could be used as an osmotic pressure-insensitive barrier in situations where lipid membranes fail such as desalination membranes.

Homogeneous hybrid droplet interface bilayers assembled from binary mixtures of DPhPC phospholipids and PB-b-PEO diblock copolymers

Hybrid membranes built from phospholipids and amphiphilic block copolymers seek to capitalize on the benefits of both constituents for constructing biomimetic interfaces with improved performance. However, hybrid membranes have not been formed or studied using the droplet interface bilayer (DIB) method, an approach that offers advantages for revealing nanoscale changes in membrane structure and mechanics and offers a path toward assembling higher-order tissues. We report on hybrid droplet interface bilayers (hDIBs) formed in hexadecane from binary mixtures of synthetic diphytanoyl phosphatidylcholine (DPhPC) lipids and low molecular weight 1,2 polybutadiene-b-polyethylene oxide (PBPEO) amphiphilic block copolymers and use electrophysiology measurements and imaging to assess the effects of PBPEO in the membrane. This work reveals that hDIBs containing up to 15 mol% PBPEO plus DPhPC are homogeneously mixtures of lipids and polymers, remain highly resistive to ion transport, and are stable-including under applied voltage. Moreover, they exhibit hydrophobic thicknesses similar to DPhPC-only bilayers, but also have significantly lower values of membrane tension. These characteristics coincide with reduced energy of adhesion between droplets and the formation of alamethicin ion channels at significantly lower threshold voltages, demonstrating that even moderate amounts of amphiphilic block copolymers in a lipid bilayer provide a route for tuning the physical properties of a biomimetic membrane.

Challenges and opportunities in achieving the full potential of droplet interface bilayers

Model membranes can be used to elucidate the intricacies of the chemical processes that occur in cell membranes, but the perfectly biomimetic, yet bespoke, model membrane has yet to be built. Droplet interface bilayers are a new type of model membrane able to mimic some features of real cell membranes better than traditional models, such as liposomes and black lipid membranes. In this Perspective, we discuss recent work in the field that is starting to showcase the potential of these model membranes to enable the quantification of membrane processes, such as the behaviour of protein transporters and the prediction of in vivo drug movement, and their use as scaffolds for electrophysiological measurements. We also highlight the challenges that remain to enable droplet interface bilayers to achieve their full potential as artificial cells, and as biological analytical platforms to quantify molecular transport.

Trans-Resveratrol Decreases Membrane Water Permeability: A Study of Cholesterol-Dependent Interactions

Resveratrol (RSV), a biologically active plant phenol, has been extensively investigated for cancer prevention and treatment due to its ability to regulate intracellular targets and signaling pathways which affect cell growth and metastasis. The non-specific interactions between RSV and cell membranes can modulate physical properties of membranes, which in turn can affect the conformation of proteins and perturb membrane-hosted biological functions. This study examines non-specific interactions of RSV with model membranes having varying concentrations of cholesterol (Chol), mimicking normal and cancerous cells. The perturbation of the model membrane by RSV is sensed by changes in water permeability parameters, using Droplet Interface Bilayer (DIB) models, thermotropic properties from Differential Scanning Calorimetry, and structural properties from confocal Raman spectroscopy, all of which are techniques not complicated by the use of probes which may themselves perturb the membrane. The nature and extent of interactions greatly depend on the presence and absence of Chol as well as the concentration of RSV. Our results indicate that the presence of RSV decreases water permeability of lipid membranes composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), indicating a capability for RSV in stiffening fluidic membranes. When Chol is present, however, (at 4:1 and 2:1 mol ratio DOPC to cholesterol), the addition of RSV has no significant effect upon the water permeability. DSC thermograms show that RSV interacts with DOPC and DOPC/Chol bilayers and influences their thermotropic phase behavior in a concentration-dependent manner, by decreasing the main phase transition temperature and enthalpy, with a phase separation shown at the higher concentrations of RSV. Raman spectroscopic studies indicate an ordering effect of RSV on DOPC supported bilayer, with a lesser extent of ordering in the presence of Chol. Combined results from these investigations highlight a differential effect of RSV on Chol-free and Chol-enriched membranes, respectively, which results constitute a bellwether for increased understanding and effective use of resveratrol in disease therapy including cancer.

Physicochemical characteristics of droplet interface bilayers

Droplet interface bilayer (DIB) is a lipid bilayer formed when two lipid monolayer-coated aqueous droplets are brought in contact within an oil phase. DIBs, especially post functionalization, are a facile model system to study the biophysics of the cell membrane. Continued advances in enhancing and functionalizing DIBs to be a faithful cell membrane mimetic requires a deep understanding of the physicochemical characteristics of droplet interface bilayers. In this review, we provide a comprehensive overview of the current scientific understanding of DIB characteristics starting with the key experimental frameworks for DIB generation, visualization and functionalization. Subsequently we report experimentally measured physical, electrical and transport characteristics of DIBs across physiologically relevant lipids. Advances in simulations and mathematical modelling of DIBs are also discussed, with an emphasis on revealing principles governing the key physicochemical characteristics. Finally, we conclude the review with important outstanding questions in the field.

Differential Interaction of Cannabidiol with Biomembranes Dependent on Cholesterol Concentration

Cannabidiol (CBD), the major nonpsychoactive component of plant-derived cannabinoids, has been reported to have a broad range of potential beneficial pharmacological effects on the central nervous system (CNS). In this study, the droplet interface bilayer, a model cell membrane, is used to examine the effects of CBD on passive water permeability, a fundamental membrane biophysical property. The presence of CBD decreases the water permeability of model lipid membranes composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and at low concentrations of cholesterol (Chol) (20 mol %) in DOPC, whereas when higher concentrations of Chol are present (33 mol %), CBD has an opposing effect, increasing water permeability. The diametric effect in water permeability change upon addition of CBD to Chol-low and Chol-high bilayers signifies a variant interaction of CBD, depending on the initial state of bilayer packing and fluidity. Additionally, differential scanning calorimetry studies provide evidence that there are selective changes in thermotropic behavior for CBD with DOPC and with DOPC/Chol membranes, respectively, supportive of these varying membrane interactions of CBD dependent upon cholesterol. The intriguing ability of CBD to sensitively respond to membrane Chol concentrations in modifying physical properties highlights the significant impact that CBD can have on heterogeneous biomembranes including those of the CNS, the neurons of which are enriched in Chol to a point where up to a quarter of the body's total Chol is in the brain, and defective brain Chol homeostasis is implicated in neurodegenerative diseases.

Characterizing the Structure and Interactions of Model Lipid Membranes Using Electrophysiology

The cell membrane is a protective barrier whose configuration determines the exchange both between intracellular and extracellular regions and within the cell itself. Consequently, characterizing membrane properties and interactions is essential for advancements in topics such as limiting nanoparticle cytotoxicity. Characterization is often accomplished by recreating model membranes that approximate the structure of cellular membranes in a controlled environment, formed using self-assembly principles. The selected method for membrane creation influences the properties of the membrane assembly, including their response to electric fields used for characterizing transmembrane exchanges. When these self-assembled model membranes are combined with electrophysiology, it is possible to exploit their non-physiological mechanics to enable additional measurements of membrane interactions and phenomena. This review describes several common model membranes including liposomes, pore-spanning membranes, solid supported membranes, and emulsion-based membranes, emphasizing their varying structure due to the selected mode of production. Next, electrophysiology techniques that exploit these structures are discussed, including conductance measurements, electrowetting and electrocompression analysis, and electroimpedance spectroscopy. The focus of this review is linking each membrane assembly technique to the properties of the resulting membrane, discussing how these properties enable alternative electrophysiological approaches to measuring membrane characteristics and interactions.

Ibuprofen and the Phosphatidylcholine Bilayer: Membrane Water Permeability in the Presence and Absence of Cholesterol

The interactions between drugs and cell membranes can modulate the structural and physical properties of membranes. The resultant perturbations of the membrane integrity may affect the conformation of the proteins inserted within the membrane, disturbing the membrane-hosted biological functions. In this study, the droplet interface bilayer (DIB), a model cell membrane, is used to examine the effects of ibuprofen, a nonsteroidal anti-inflammatory drug (NSAID), on transbilayer water permeability, which is a fundamental membrane biophysical property. Our results indicate that the presence of neutral ibuprofen (pH 3) increases the water permeability of the lipid membranes composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). When cholesterol is present with the DOPC, however, the water permeability is not influenced by addition of ibuprofen, regardless of the cholesterol content in DOPC. Given the fact that cholesterol is generally considered to impact packing in the hydrocarbon chain regions, our findings suggest that a potential competition between opposing effects of ibuprofen molecules and cholesterol on the hydrocarbon core environment of the phospholipid assembly may influence the overall water transport phenomena. Results from confocal Raman microspectroscopy and interfacial tensiometry show that ibuprofen molecules induce substantial structural and dynamic changes in the DOPC lipid bilayer. These results, demonstrating that the presence of ibuprofen increases the water permeability of pure DOPC but not that of DOPC-cholesterol mixtures, provide insight into the differential effect of a representative NSAID on heterogeneous biological membranes, depending upon the local composition and structure, results which will signal increased understanding of the gastrointestinal damage and toxicity induced by these molecules.

Membrane determinants for the passive translocation of analytes through droplet interface bilayers

Understanding how small molecules cross cell membranes is crucial to pharmaceutics. Several methods have been developed to evaluate such a process, but they need improvement since many false-positive candidates are often selected. Robust tools enabling rapid and reproducible screening can increase confidence on hits, and artificial membranes based on droplet interface bilayers (DIBs) offer this possibility. DIBs consist in the adhesion of two phospholipid-covered water-in-oil droplets which reproduce a bilayer. By having donor and acceptor droplets, the permeability of an analyte can be studied. However, the relevance of this system relies on the comprehension of how well the physical chemistry of the produced bilayer recapitulates the behavior of cell membranes. This information is missing, and we address it here. Taking small fluorophores as model analytes, we studied their permeation through DIBs made of a wide range of phospholipids. We found that both the phospholipid acyl chain and polar head affect permeability. Overall, these parameters impact the phospholipid shape and thereupon the membrane lateral pressure, which is a major factor modulating with permeability in our system. These results depend on the nature of the chosen oil. We thereupon identified relevant physical chemistry conditions that best mimic the compactness and subsequent permeability of biological membranes.

Enhanced water permeability across a physiological droplet interface bilayer doped with fullerenes

We measure the water permeability across a physiological lipid bilayer produced by the droplet interface bilayer (DiB) technique. This lipid bilayer can be considered as physiologically relevant because it presents a lipidic composition close to human cell membranes. The measured water permeability coefficients across this lipid bilayer are reported as a function of the cholesterol concentration. It is found that the water permeability coefficients decreased with increasing cholesterol concentration, in agreement with the existing literature. And, consistently, the extracted corresponding activation energies increase with increasing cholesterol concentration in the lipid bilayer. Hence having demonstrated the robustness of the experimental system, we extend this study by exploring the influence of fullerenes on the water permeability of a physiological lipid bilayer. Interestingly, we observe a significant increase of the measured water permeability coefficients across this lipid bilayer for large fullerenes concentration. This enhanced permeability might be related to the conductive properties of fullerenes.

We measure the water permeability across a physiological lipid bilayer produced by the droplet interface bilayer technique.

Good to the Last Drop: Interfacial Droplet Chemistry, from Crystals to Biological Membranes

The study of the liquid-liquid interface has a long and storied history yet still holds important implications for science and technology. Although deep examination of this buried interface poses challenges, recent progress in experimental and theoretical methodology has allowed for advanced understanding of the molecular bases of such interfaces. This Account will focus on the behavior of surfaces of aqueous microdroplets immersed in an immiscible phase, exhibiting physicochemical behavior dependent on the presence of interfacial self-assembled structures. Amphiphiles spontaneously form self-assembled nanostructures at the liquid interface, creating a soft liquid surface for the aqueous microdroplet that can modulate its behavior. A prominent characteristic of a micron-sized droplet is its elevated surface area/volume ratio, a feature that presents opportunities for investigating the role of the interface in aspects of droplet chemistry. In two notable examples, a surfactant self-assembly can act as a template for crystal nucleation of droplet solutes at the monolayer level, while at the level of a bilayer, formed when two monolayer-covered droplets are made to adhere, the apposition of monolayers bears remarkable similarities to cell membranes. Each type of system provides arbitrary control of important factors, both for studying crystallization nucleation and for modeling semipermeable lipid membranes at an interdroplet contact zone, the droplet interface bilayer (DIB). The droplet bilayer allows for direct observation of species transport across an unsupported bilayer and versatile parameter control to expore the effects of membrane lipid structure on bilayer transport. It is demonstrated that molecular shape for monoglycerides and phospholipids influences the surface characteristics of monolayers and bilayers. Additionally, subtle interfacial interactions between aqueous contents (ions, solutes) and the monolayer/bilayer are shown to have a marked influence on lipid packing and permeability. It is anticipated that this successful demonstration of surface engineering at the micron scale will deliver cogent insights into many biologically relevant phenomena, such as membrane transport and biomineralization.

Effects of Acyl Chain Unsaturation on Activation Energy of Water Permeability across Droplet Bilayers of Homologous Monoglycerides: Role of Cholesterol

Cholesterol is an important component of total lipid in mammalian cellular membranes; hence, the knowledge of its association with lipid bilayer membranes will be essential to understanding membrane structure and function. A droplet interface bilayer (DIB) provides a convenient and reliable platform through which values for permeability coefficient and activation energy of water transport across the membrane can be extracted. In this study, we investigated the effect of acyl chain structure in amphiphilic monoglycerides on the permeability of water across DIB membranes composed of cholesterol and these monoglycerides, where the acyl chain length, number of double bonds, and the position of double bond are varied systematically along the acyl chains. To elucidate the role of cholesterol in these membranes, we investigated its influence on water permeability and associated activation energies at two different cholesterol concentrations. Our systematic studies show dramatic sensitivity and selectivity of specific interaction of cholesterol with the monoglyceride bilayer having structural variations in acyl chain compositions. Our findings allow us to delineate the exquisite interplay between membrane properties and structural components and understand the balanced contribution of each.

Quantitative Raman microspectroscopy for water permeability parameters at a droplet interface bilayer

Using confocal Raman microspectroscopy, we derive parameters for bilayer water transport across an isolated nanoliter aqueous droplet pair.

Phytosterols-induced viscoelasticity of oleogels prepared by using monoglycerides

Monoglycerides (MGs) and phytosterols (PS) are known to form firm oleogels with liquid oil. However, the oleogels are prone to undergo polymorphic transition over time that lead to crystals' aggregation thus, compromises physical properties. Thus, we combined MGs with PS to control the crystallization and modify the morphology of the combination oleogels, as both components are reported to interact together. The oleogels were prepared at different ratio combinations and characterized in their rheological, thermal, morphology, and diffraction properties. The results showed that the 8:2 MGP:PS exhibited higher storage modulus (G') than the MGP mono-component. The combination oleogels exhibited effects on the crystallization and polymorphic transition. Consequently, the effects led to change in the morphology of the combination oleogels which was visualized using optical and electron microscope. The resultant effect on the morphology is associated with crystal defect. Due to observable crystals of MGP and PS, it is speculated that the combination oleogels formed a mixed crystal system. This was confirmed with diffraction analysis in which the corresponding peaks from MGP and PS were observed in the combination oleogels. However, the 8:2 oleogel exhibited additional peak at 35.41Å. Ultimately, the 8:2 was the optimum combination observed in our study. Interestingly, this combination is inspired by nature as sterols (phytosterols) are natural component of lipid membrane whilst MGP has properties similar to phospholipids. Hence, the results of our study not only beneficial for oil structuring, but also for the fields of biophysical and pharmaceutical.

Enthalpic Effects of Chain Length and Unsaturation on Water Permeability across Droplet Bilayers of Homologous Monoglycerides

A deeper understanding of unassisted passive transport processes can better delineate basic lipid dynamics in biological membranes. A droplet interface bilayer (DIB) is made by contacting two aqueous droplets covered with a lipid monolayer, and has increasingly been employed as a model artificial biological membrane. In this study, we have investigated the effect of acyl chain structure of amphiphilic monoglycerides on the osmotic permeability of water across DIB membranes composed of these monoglycerides, where the acyl chain length (C14-C24), number of double bonds (1-4), and the position of double bond are varied systematically along the acyl chains. Both permeability values and activation energies have been extracted for water transport across a lipid bilayer formed of a homologous series of lipids, allowing us to make ready comparisons between the different lipids and potentially better elucidate the contributions that molecular motifs make to the permeation process.

Hydrodynamic trapping for rapid assembly and in situ electrical characterization of droplet interface bilayer arrays

The droplet interface bilayer (DIB) is a modular technique for assembling planar lipid membranes between water droplets in oil. The DIB method thus provides a unique capability for developing digital, droplet-based membrane platforms for rapid membrane characterization, drug screening and ion channel recordings. This paper demonstrates a new, low-volume microfluidic system that automates droplet generation, sorting, and sequential trapping in designated locations to enable the rapid assembly of arrays of DIBs. The channel layout of the device is guided by an equivalent circuit model, which predicts that a serial arrangement of hydrodynamic DIB traps enables sequential droplet placement and minimizes the hydrodynamic pressure developed across filled traps to prevent squeeze-through of trapped droplets. Furthermore, the incorporation of thin-film electrodes fabricated via evaporation metal deposition onto the glass substrate beneath the channels allows for the first time in situ, simultaneous electrical interrogation of multiple DIBs within a sealed device. Combining electrical measurements with imaging enables measurements of membrane capacitance and resistance and bilayer area, and our data show that DIBs formed in different trap locations within the device exhibit similar sizes and transport properties. Simultaneous, single channel recordings of ion channel gating in multiple membranes are obtained when alamethicin peptides are incorporated into the captured droplets, qualifying the thin-film electrodes as a means for measuring stimuli-responsive functions of membrane-bound biomolecules. This novel microfluidic-electrophysiology platform provides a reproducible, high throughput method for performing electrical measurements to study transmembrane proteins and biomembranes in low-volume, droplet-based membranes.

Water Permeability across Symmetric and Asymmetric Droplet Interface Bilayers: Interaction of Cholesterol Sulfate with DPhPC

Cellular membranes employ a variety of strategies for controlling the flow of small molecules into the cytoplasmic space, including incorporation of sterols for modulation of permeability and maintenance of lipid asymmetry to provide both sides of the membrane with differing biophysical properties. The specific case of cholesterol asymmetry, especially, is known to have profound effects in neurological cellular systems. Synthetic membrane models that can readily determine valuable physical parameters, such as water transport rates, for sterol-containing membranes of defined lipid composition remain in demand. We report the use of the droplet interface bilayer (DIB), composed of adherent aqueous droplets surrounded by a lipid monolayer and immersed in a hydrophobic medium, for measurement of water permeability across the membrane, with rapid visualization and ease of experimental setup. We studied droplet bilayer membranes composed of the prototypical synthetic membrane lipid (i.e., the archaeal lipid DPhPC) as well as of symmetric and asymmetric DIBs formed by DPhPC and sodium cholesterol sulfate (S-Chol). The presence of S-Chol in DPhPC in symmetric DIB reduced the passive water permeability rate (P(f)) at all concentrations and increased the activation energy (E(a)) to 17-18 kcal/mol. When only one side of the DIB contains S-Chol (asymmetric DIB), an E(a) of 14-15 kcal/mol was obtained, a value intermediate that of pure lipid and symmetrical DIB containing lipid and S-Chol. Our data are consistent with a capability for regulation of water transport by one leaflet independent of the other. The engineering of our various systems is believed to have implications for garnering detailed knowledge regarding the transport of small moieties across bilayers in a wide variety of lipid systems.

A refined model of water and CO₂ membrane diffusion: effects and contribution of sterols and proteins

Black lipid bilayers, a general model system for biomembranes were studied for diffusion rates of small molecules such as water or CO₂ using advanced analysis techniques and cell free synthesized proteins. We provide evidence that by simple insertion of proteins or sterols the diffusion rates of water or those of CO₂ decrease. Insertion of cell free synthesized water permeable aquaporins restored water diffusion rates as well as insertion of CO₂-facilitating aquaporins the CO₂ diffusion. Insertion of water or CO₂ impermeable proteins decreased the respective diffusion rates. Therefore, for normal high cellular CO₂ diffusion rates specific aquaporins are mandatory.