SP-C, a pulmonary surfactant-specific protein, aids the spreading of the main surfactant phospholipid L-α-dipalmitoylphosphatidylcholine (DPPC) across air/water interfaces, a process that has possible implications for in vivo function. To understand the molecular mechanism of this process, we have used external infrared reflection-absorption spectroscopy (IRRAS) to determine DPPC acyl chain conformation and orientation as well as SP-C secondary structure and helix tilt angle in mixed DPPC/SP-C monolayers in situ at the air/water interface. The SP-C helix tilt angle changed from ~24°to the interface normal in lipid bilayers to ~70°in the mixed monolayer films, whereas the acyl chain tilt angle of DPPC decreased from ~26°in pure lipid monolayers (comparable to bilayers) to ~10°in the mixed monolayer films. The protein acts as a 'hydrophobic lever' by maximizing its interactions with the lipid acyl chains while simultaneously permitting the lipids to remain conformationally ordered. In addition to providing a reasonable molecular mechanism for protein-aided spreading of ordered lipids, these measurements constitute the first quantitative determination of SP-C orientation in Langmuir films, a paradigm widely used to simulate processes at the air/alveolar interface.
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