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 Ca2+-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.

The structures formed by a pulmonary surfactant model system of dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and recombinant surfactant-associated protein C (SP-C) were studied using scanning force microscopy (SFM) on Langmuir-Blodgett films. The films appeared to be phase separated, in agreement with earlier investigations by fluorescence light microscopy. There were smooth polygonal patches of mostly lipid, surrounded by a corrugated rim rich in SP-C. When the films were compressed beyond the equilibrium surface pressure, the protein-rich phase mediated the formation of layered protrusions. The height of these multilamellar structures embodied equidistant steps slightly higher than a DPPC double layer in the gel phase. At the air-water interface too, a high compressibility at low surface tension was indicative of the exclusion of matter. The exclusion process proved to be fully reversible. The present study demonstrates that some of the matter of the model pulmonary surfactant can move in and out of the active monolayer. The SFM images revealed a lipid-protein complex that was responsible for the reversible exclusion of double-layer structures. This mechanism may be important in the natural system too, to keep the surface tension of the alveolar air/water interface constantly low over the range of area encountered upon breathing.

This study focuses on the structural organization of surfactant protein B (SP-B) containing lipid monolayers. The artificial system is composed of the saturated phospholipids dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) in a molar ratio of 4:1 with 0.2 mol% SP-B. The different "squeeze-out" structures of SP-B were visualized by scanning probe microscopy and compared with structures formed by SP-C. Particularly, the morphology and material properties of mixed monolayers containing 0.2 mol% SP-B in a wide pressure range of 10 to 54 mN/m were investigated revealing that filamentous domain boundaries occur at intermediate surface pressure (15-30 mN/m), while disc-like protrusions prevail at elevated pressure (50-54 mN/m). In contrast, SP-C containing lipid monolayers exhibit large flat protrusions composed of stacked bilayers in the plateau region (app. 52 mN/m) of the pressure-area isotherm. By using different scanning probe techniques (lateral force microscopy, force modulation, phase imaging) it was shown that SP-B is dissolved in the liquid expanded rather than in the liquid condensed phase of the monolayer. Although artificial, the investigation of this system contributes to further understanding of the function of lung surfactant in the alveolus.

The structure of an artificial pulmonary surfactant was studied by scanning force- and fluorescence light microscopy (SFM, and FLM, respectively). The surfactant--a mixture of dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG) and recombinant surfactant-associated protein C (SP-C)--was prepared at the air-water interface of a Langmuir film balance and imaged by FLM under various states of compression. In order to visualize their topography by SFM, the films were transferred onto a solid mica support by the Langmuir-Blodgett (LB) technique. We found that a region of high film compressibility of the spread monolayer close to its equilibrium surface pressure (pi = 50 mN/m) was due to the exclusion of layered protrusions with each layer 5.5 to 6.5 nm thick. They remained associated with the monolayer and readily reinserted upon expansion of the film. Comparison with the FLM showed that the protrusions contained the protein in high concentration. The more the film was compressed, the larger was the number of layers on top of each other. The protrusions arose from regions of the monolayer with a distinct microstructure that may have been responsible for their formation. The molecular architecture of the microstructure remains to be elucidated, although some of it can be inferred from spectroscopic data in combination with the SFM topographical images. We illustrate our current understanding of the film structure with a molecular model.

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