Analysis of lung surfactant model systems with time-of-flight secondary ion mass spectrometry

An often-used model lung surfactant containing dipalmitoylphosphatidylcholi ne (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and the surfactant prote in C (SP-C) was analyzed as Langmuir-Blodgett film by spatially resolved ti me-of-flight secondary ion mass spectrometry (TOF-SIMS) to directly visuali ze the formation and composition of domains. Binary lipid and lipid/SP-C sy stems were probed for comparison. TOF-SIMS spectra revealed positive second ary ions (S1) characteristic for DPPC and SP-C, but not for DPPG. SI mappin g results in images with domain structures in DPPC/DPPG and DPPG/SP-C, but not in DPPC/SP-C films. We are able to distinguish between the fluid and co ndensed areas probably due to a matrix effect. These findings correspond wi th other imaging techniques, fluorescence light microscopy (FLM), scanning force microscopy (SFM), and silver decoration. The ternary mixture DPPC/DPP G/SP-C transferred from the collapse region exhibited SP-C-rich domains sur rounding pure lipid areas. The results obtained are in full accordance with our earlier SFM picture of layered protrusions that serve as a compressed reservoir for surfactant material during expansion. Our study demonstrates once more that SP-C plays a unique role in the respiration process.