To determine how the hydrophobic surfactant proteins promote insertion of the surfactant lipids into an air/water interface, we measured the effect of lysophosphatidylcholine (LPC) on adsorption. Existing models contend that the proteins function either by disordering the lipids or by stabilizing a negatively curved structure located between the adsorbing vesicle and the interface. Because LPC produces greater disorder but positive curvature, the models predict opposite effects. With vesicles containing either dioleoyl phosphatidylcholine (DOPC) or the neutral and phospholipids isolated from calf surfactant, LPC increased the initial rate at which surface tension fell. The final surface tension, however, remained well above the value of ∼25 mN/m expected for a saturated surface. With two preparations, dioleoyl phosphatidylethanolamine and gramicidin A-DOPC, which form the negatively curved hexagonal-II (HII) phase and adsorb rapidly, LPC instead had little effect on initial adsorption but delayed the fall of surface tension below ∼30 mN/m. LPC produced a similar inhibition of the late adsorption for extracted calf surfactant. Unlike dioleoyl phosphatidylethanolamine and gramicidin A-DOPC, small-angle x-ray scattering and 31P-nuclear magnetic resonance for extracted calf surfactant detected no evidence for the HII phase. Our results indicate that although LPC can promote the initial adsorption of vesicles containing only lamellar lipids, it inhibits the facilitation by the hydrophobic proteins of late adsorption. Our findings support a model in which the surfactant proteins accelerate adsorption by producing a focal tendency to stabilize a negatively curved kinetic intermediate without a general shift to the HII phase.
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