THE PHASE-BEHAVIOR OF LIPID MONOLAYERS CONTAINING PULMONARY SURFACTANT PROTEIN-C STUDIED BY FLUORESCENCE LIGHT-MICROSCOPY

Three compounds of the pulmonary surfactant - dipalmitoylphosphatidylc holine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and the surfact ant associated protein C (SP-C) - were spread at the air-water interfa ce of a Langmuir trough as a model system to mimic the properties of n atural surfactant. Fluorescence microscopical images of the film forme d at the interface were obtained during compression using a fluorescen ce dye bound covalently either to phosphatidylcholine or to SP-C. The images were quantified using statistical methods in respect to relativ e areas and relative fluorescence intensities of the domains found. In the early stage of compression, film pressure rose slightly and was a ccompanied by a phase separation which could be recognized in the imag es by the formation of bright and dark domains. On further compression , after a steep increase of film pressure, a plateau region of constan t film pressure started abruptly. During compression in the plateau re gion, fluorescence intensity of the bright domain formed in the early stage of compression increased. The increasing fluorescence intensity, the non-Gaussian intensity distribution of the bright domain, and the small mean molecular area of the film in the plateau region gave rise to the assumption that multilayer structures were formed in the late stage of compression. The formation of the multilayer structures was f ully reversible in repeated compression-expansion cycles including the plateau region of the phase diagram. The ability of lipid/SP-C mixtur es to form reversible multilayer structures during compression may be relevant to stability in lungs during expiration and inhalation.