Neutral lipids induce critical behavior in interfacial monolayers of pulmonary surfactant

We have shown previously that lateral compression of pulmonary surfactant m onolayers initially induces separation of two phases but that these remix w hen the films become more dense (1). In the studies reported here, we used fluorescence microscopy to examine the role of the different surfactant con stituents in the remixing of the separated phases. Subfractions containing only the purified phospholipids (PPL), the surfactant proteins and phosphol ipids (SP&PL), and the neutral and phospholipids (N&PL) were obtained by ch romatographic separation of the components in extracted calf surfactant (ca lf lung surfactant extract, CLSE). Compression of the different monolayers produced nonfluorescent domains that emerged for temperatures between 20 an d 41 degrees C at similar surface pressures 6-8 mN/m higher than values obs erved for dipalmitoyl phosphatidylcholine (DPPC), the most prevalent compon ent of pulmonary surfactant. Comparison of the different preparations showe d that the neutral lipid increased the total nonfluorescent area at surface pressures up to 25 mN/m but dispersed that total area among a larger numbe r of smaller domains. The surfactant proteins also produced smaller domains , but they had the opposite effect of decreasing the total nonfluorescent a rea. Only the neutral lipids caused remixing. In images from static monolay ers, the domains for N&PL dropped from a maximum of 26 +/- 3% of the interf ace at 25 mN/m to 4 +/- 2% at 30 mN/m, similar to the previously reported b ehavior for CLSE. During continuous compression through a narrow range of p ressure and molecular area, in N&PL, CLSE, and mixtures of PPL with 10% cho lesterol, domains became highly distorted immediately prior to remixing. Th e characteristic transition in shape and abrupt termination of phase coexis tence indicate that the remixing caused by the neutral lipids occurs at or close to a critical point.