Effect of neutral lipids on coexisting phases in monolayers of pulmonary surfactant

We previously established that compression of monolayers containing the lipids in pulmonary surfactant, with or without the surfactant proteins, initially leads to phase separation. On further compression, however, phase coexistence terminates at a critical point that requires the presence of cholesterol. The studies reported here address the changes in the phospholipid phase diagram produced by cholesterol. We used the two systems of the lipids from calf surfactant with and without the surfactant proteins. For both systems, we began with the postulate that cholesterol had no effect on the composition of other constituents in the two phases, and then used the known behavior of interfacial tension at a critical point to test the two extreme cases in which the cholesterol partitions exclusively into condensed domains or into the surrounding film. Measurements of surface potential along with the fraction of the nonfluorescent area and the radius of the domains, both obtained by fluorescence microscopy, for films with and without cholesterol allowed calculation of the interfacial tension between the two phases. Only the model that assumes the presence of cholesterol within the domains accurately predicts a decreasing line tension during film compression toward the critical point. That model, however, also predicts an unlikely decrease during compression of the dipole moment density for the condensed phase. Our results are best explained in terms of cholesterol partitioning predominantly into the condensed domains, with a resulting partial redistribution of the phospholipids between the two phases.