EXTERNAL REFLECTION-ABSORPTION INFRARED-SPECTROSCOPY STUDY OF LUNG SURFACTANT PROTEINS SP-B AND SP-C IN PHOSPHOLIPID MONOLAYERS AT THE AIR WATER INTERFACE/

The interactions of the hydrophobic pulmonary surfactant proteins SP-B and SP-C with 1,2-dipalmitoylphosphatidylcholine in mixed, spread mon olayer films have been studied in situ al the air/water interface with the technique of external reflection absorption infrared spectroscopy (IRRAS). SP-C has a mostly alpha-helical secondary structure both in the pure state and in the presence of lipids, whereas SP-B secondary s tructure is a mixture of alpha-helical and disordered forms. When film s of SP-B/1,2-dipalmitoylphosphatidylcholine are compressed to surface pressures (pi) greater than similar to 40-43 mN/m, the protein is par tially (15-35%) excluded from the surface, as measured by intensity ra tios of the peptide bond amide I/lipid C=O stretching vibrations. The extent of exclusion increases as the protein/lipid ratio in the film i ncreases. In contrast, SP-C either remains at the surface at high pres sures or leaves accompanied by lipids. The amide I peak of SP-C become s asymmetric as a result of the formation of intermolecular sheet stru ctures (1615-1630 cm(-1)) suggestive of peptide aggregation. The power of the IRRAS experiment for determination of film composition and mol ecular structure, i.e., as a direct test of the squeeze-out hypothesis of pulmonary surfactant function, is evident from this work.