Impedance analysis of supported lipid bilayer membranes: a scrutiny of different preparation techniques

Abstract

One topic of this study is the comparison of different preparation techniques to build up solid supported lipid bilayers onto gold substrates. The deposited lipid bilayers were investigated by a.c. impedance spectroscopy. Three different strategies were applied: (1) The gold surface was initially covered with a chemisorbed monolayer of octadecanethiol or 1,2-dimyristoyl- sn-glycero-3-phosphothioethanol (DMPTE). The second monolayer consisting of phospholipids was then deposited onto this hydrophobic surface by (i) the Langmuir-Schaefer-technique, (ii) from lipid solution in n-decane/isobutanol, (iii) by the lipid/detergent dilution technique or (iv) by fusion of vesicles. (2) Charged molecules carrying thiol-anchors for attachment to the gold surface by chemisorption were used. Negatively charged surfaces of 3-mercaptopropionic acid were found to be excellent substrates that allow the attachment of planar lipid bilayers by applying positively charged dimethyldioctadecylammoniumbromide (DODAB) vesicles or negatively charged 1,2-dipalmitoyl- sn-glycero-3-phosphoglycerol vesicles in the presence of chelating Ca 2+-ions. If positively charged first monolayers of mercaptoethylammoniumhydrochloride were used we were able to attach mixed 1,2-dimyristoyl- sn-glycero-3-phosphoglycerol/1,2-dimyristoyl- sn-glycero-3-phosphoethanolamine vesicles to form planar lipid bilayers via electrostatic interaction. (3) Direct deposition of lipid bilayers is possible from vesicles containing 1,2-dimyristoyl- sn-glycero-3-phosphothioethanol (DMPTE). A critical amount of more than 50 mol% of DMPTE was found to be necessary to form a solid supported lipid bilayer. Bilayers obtained with these different preparation techniques were scrutinized with respect to their capacitances, kinetics of formation and their long-term stabilities by impedance spectroscopy. The second feature of this paper is the application of the supported bilayers to study ion transport through channel-forming peptides. We used a DODAB-bilayer for the reconstitution of gramicidin D channels. By circular dichroism measurements we verified that the peptide is in its channel conformation. The ion transport of Cs +-ions through the channels was recorded by impedance analysis.