On the deposition of phospholipids onto planar supports with the Langmuir—Blodgett technique using factorial experimental design 1. Screening of various factors and supports

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.

From liposomes to supported, planar bilayer structures on hydrophilic and hydrophobic surfaces: an atomic force microscopy study

The sequence of events involved in the transition from attached liposomes to bilayer patches on hydrophilic and hydrophobic solid supports were visualized in situ by Tapping Mode atomic force microscopy in liquid. In a smooth manner, the attached liposomes spread and flattened from the outer edges toward the center until the two membrane bilayers were stacked on top of each other. The top bilayer then either rolls or slides over the bottom bilayer, and the adjacent edges join to form a larger membrane patch. This is clearly visible from the apparent height of 6.0-7.5 nm of the single bilayer, measured in situ. The addition of calcium appeared to increase the rate of the processes preventing the visualization of the intermediate stages. The same intermediate steps appeared to be present on hydrophobic surfaces, although the attached liposomes seemed to be distorted and the resultant membrane edges were uneven. This work has provided visual and detailed information on liposome coalescence (fusion) onto solid supports and demonstrated how the atomic force microscope can be used to study the process.

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.