The structure of a model pulmonary surfactant as revealed by scanning force microscopy

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.