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.
We have investigated the crystal nucleation behavior of potassium hexacyanoferrate(III) (KFC) in an isolated aqueous microdroplet surrounded by a decanol medium which is isothermally driven to supersaturation by water transport from the droplet, to examine the influence upon KFC nucleation by self-assembled surfactant monolayers at the liquid-liquid interface. The nucleation behavior was studied by measurement of the level of supersaturation required for the onset of crystallization (S onset ) based on microscopic observation. For KFC crystallization in the absence of surfactant, it was found that S onset was 1.74 ( 0.13. This value was independent of droplet initial size (80-160 µm) and initial KFC concentration (10-40%). In the presence of a minimum threshold concentration of octadecylamine (ODA) in decanol, the S onset value was diminished significantly (1.41 ( 0.06), indicative of templated nucleation acceleration. Additionally, the crystal habit for KFC grown in the presence of ODA monolayer was markedly more regular than in the absence of monolayer. Neither octadecylammonium chloride (ODA • HCl) nor stearic acid showed any templating effect. Mixed ODA-ODA • HCl monolayers exhibited intermediate effects. The results are explained in terms of ODA monolayer as a template for nucleation due to a combination of both electrostatic interactions and structural matching.
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.
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 antiinflammatory 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.
Conspectus 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.
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