Structure and orientation of lung surfactant SP-C and L-alpha-dipalmitoylphosphatidylcholine in aqueous monolayers

SP-C, a pulmonary surfactant-specific protein, aids the spreading of the main surfactant phospholipid L-alpha-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 similar to 24 degrees to the interface normal in lipid bilayers to similar to 70 degrees in the mixed monolayer films, whereas the acyl chain tilt angle of DPPC decreased from similar to 26 degrees in pure lipid monolayers (comparable to bilayers) to similar to 10 degrees 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 ol SP-C orientation in Langmuir films, a paradigm widely used to simulate processes at the air/alveolar interface.