Molecular organization revealed by time-of-flight secondary ion mass spectrometry of a clinically used extracted pulmonary surfactant

Pulmonary surfactant lowers the surface tension of the lung air-liquid interface to prevent alveolar collapse. It is composed mainly of saturated and unsaturated phosphatidyleholines and phosphatidylglycerols. Solvent-spread films of a pulmonary surfactant, bovine lipid extract surfactant (BLES), form liquid condensed and liquid expanded phases with changing surface pressures, as seen by fluorescence and atomic force microscopy. Time-of-flight secondary ion mass spectrometry conducted in an imaging mode below the static limit provides a novel means of mapping the identities of the molecular species present in each phase. Images can be obtained for cations, like calcium and sodium, as well as for phospholipid parent and fragment ions. The phospholipids identified include dipalmitoyl- and palmitoyloleoylphosphatidylcholine as positive ions and dipalmitoyl- and palmitoyloleoylphosphatidylglycerol as negative ions. We observe good miscibility of phospholipids with similar chain lengths but different headgroups. At higher surface pressures, dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol co-crystallize in the liquid condensed (LC) phase. As well, there is evidence of an intermediate phase, within the LC region. The results indicate that the details of phospholipid mixing and reorganization in surfactant films can be determined by static secondary ion mass spectrometry imaging at high resolution, thus providing insight into the relationship between composition and structure in surfactant films at the air-aqueous interface.