Changes in membrane elasticity caused by the hydrophobic surfactant proteins correlate poorly with adsorption of lipid vesicles.

To establish how the hydrophobic surfactant proteins, SP-B and SP-C, promote adsorption of lipids to an air/water interface, we used X-ray diffuse scattering (XDS) to determine an order parameter of the lipid chains (S) and the bending modulus of the lipid bilayers (K). Samples contained different amounts of the proteins with two sets of lipids. Dioleoylphosphatidylcholine (DOPC) provided a simple, well characterized model system.

The Equilibrium Spreading Tension of Pulmonary Surfactant.

Monomolecular films at an air/water interface coexist at the equilibrium spreading tension (γ(e)) with the bulk phase from which they form. For individual phospholipids, γ(e) is single-valued, and separates conditions at which hydrated vesicles adsorb from tensions at which overcompressed monolayers collapse. With pulmonary surfactant, isotherms show that monolayers compressed on the surface of bubbles coexist with the three-dimensional collapsed phase over a range of surface tensions.