On the deposition of phospholipids onto planar supports with the Langmuir—Blodgett technique using factorial experimental design 3. Optimizing lipid composition for maximal adhesion to chromium substrates

Phase separation in polymer-based biomimetic structures containing planar membranes

Phase separation in biological membranes is crucial for proper cellular functions, such as signaling and trafficking, as it mediates the interactions of condensates on membrane-bound organelles and transmembrane transport to targeted destination compartments. The separation of a lipid bilayer into phases and the formation of lipid rafts involve the restructuring of molecular localization, their immobilization, and local accumulation. By understanding the processes underlying the formation of lipid rafts in a cellular membrane, it is possible to reconstitute this phenomenon in synthetic biomimetic membranes, such as hybrids of lipids and polymers or membranes composed solely of polymers, which offer an increased physicochemical stability and unlimited possibilities of chemical modification and functionalization. In this article, we relate the main lipid bilayer phase transition phenomenon with respect to hybrid biomimetic membranes, composed of lipids mixed with polymers, and fully synthetic membranes. Following, we review the occurrence of phase separation in biomimetic hybrid membranes based on lipids and/or direct lipid analogs, amphiphilic block copolymers. We further exemplify the phase separation and the resulting properties and applications in planar membranes, free-standing and solid-supported. We briefly list methods leading to the formation of such biomimetic membranes and reflect on their improved overall stability and influence on the separation into different phases within the membranes. Due to the importance of phase separation and compartmentalization in cellular membranes, we are convinced that this compiled overview of this phenomenon will be helpful for any researcher in the biomimicry area.

A quantitative determination of lipid bilayer deposition efficiency using AFM

The efficacy of a number of different methods for depositing a dimyristoylphosphatidylcholine (DMPC) lipid bilayer or DMPC-cholesterol (3 : 1) mixed bilayer onto a silicon substrate has been investigated in a quantitative manner using atomic force microscopy (AFM) image analysis to extract surface coverage. Complementary AFM-IR measurements were used to confirm the presence of the lipids. For the Langmuir-Blodgett/Schaefer deposition method at temperatures below the chain-melting transition temperature (T m), a large number of bilayer defects resulted when DMPC was deposited from a water subphase. Addition of calcium ions to the trough led to smaller, more frequent defects, whereas addition of cholesterol to the lipid mixture led to a vast improvement in bilayer coverage. Poor coverage was achieved for deposition at temperatures above T m. Formation of the deposited bilayer from vesicle fusion proved a more reliable method for all systems, with formation of near-complete bilayers within 60 seconds at temperatures above T m, although this method led to a higher probability of multilayer formation and rougher bilayer surfaces.

Thermodynamic characteristics and Langmuir–Blodgett deposition behavior of mixed DPPA/DPPC monolayers at air/liquid interfaces

The role of dipalmitoylphosphatic acid (DPPA) as a transfer promoter to enhance the Langmuir-Blodgett (LB) deposition of a dipalmitoylphosphatidylcholine (DPPC) monolayer at air/liquid interfaces was investigated, and the effects of Ca2+ ions in the subphase were discussed. The miscibility of the two components at air/liquid interfaces was evaluated by surface pressure-area per molecule isotherms, thermodynamic analysis, and by the direct observation of Brewster angle microscopy (BAM). Multilayer LB deposition behavior of the mixed DPPA/DPPC monolayers was then studied by transferring the monolayers onto hydrophilic glass plates at a surface pressure of 30 mN/m. The results showed that the two components, DPPA and DPPC, were miscible in a monolayer on both subphases of pure water and 0.2 mM CaCl2 solution. However, an exception occurs between X(DPPA)=0.2 and 0.5 at air/CaCl2-solution interface, where a partially miscible monolayer with phase separation may occur. Negative deviations in the excess area analysis were found for the mixed monolayer system, indicating the existence of attractive interactions between DPPA and DPPC molecules in the monolayers. The monolayers were stable at the surface pressure of 30 mN/m for the following LB deposition as evaluated from the area relaxation behavior. It was found that the presence of Ca2+ ions had a stabilization effect for DPPA-rich monolayers, probably due to the association of negatively charged DPPA molecules with Ca2+ ions. Moreover, the Ca2+ ions may enhance the adhesion of DPPA polar groups to a glass surface and the interactions between DPPA polar groups in the multilayer LB film structure. As a result, Y-type multilayer LB films containing DPPC could be fabricated from the mixed DPPA/DPPC monolayers with the presence of Ca2+ ions.

Planar lipid bilayers on solid supports from liposomes--factors of importance for kinetics and stability

One method to create planar lipid bilayers on solid substrates involves the transfer of lipids from liposomes to the support. We have varied the composition of liposomes systematically using factorial experimental designs and analyzed the adsorption behaviour of lipids from these liposomes onto solid supports. The hydrophilic supports were either used plain or modified with a monolayer of a lipid mixture, exposing hydrophobic groups. The monolayer-covered supports were used to identify factors important for adhesion and stability. Lipid adsorption kinetics was primarily studied on plain silicon supports in an ellipsometric cell or on a silicon nitride surface in a resonant mirror system (IAsys), using the systematic approach. Saturated phospholipids were essential for the required stability. Mixtures of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, dipalmitoylphosphatidylethanolamine and cholesterol in combination with proteins were investigated in further detail as regards kinetics. The propensity to form a supported planar bilayer could be manipulated by the presence of calcium ions.

Retained activities of some membrane proteins in stable lipid bilayers on a solid support

Highly stable lipid bilayers, composed of biologically relevant lipids such as phosphatidylcholine, phosphatidylethanolamine and cholesterol, were formed on platinum surfaces. Bacteriorhodopsin isolated from purple membrane (PM) from Halobacterium halobium, cytochrome oxidase from bovine heart, acetylcholinesterase from bovine brain and the nicotinic acetylcholine receptor from Torpedo electric organ were also incorporated into these reconstituted membranes. The proteins retained their biological activities. Some of them were active several weeks after the reconstitution and after several cycles of assay, washing and storage.