Structure of SP-B/DPPC Mixed Films Studied by Neutron Reflectometry.

The structures of films of SP-B and mixtures of SP-B and DPPC at the air/water interface have been studied by neutron reflectometry (NR) and Langmuir film balance methods. From the film balance studies we observe that at very high pressures the isotherms of pure DPPC and SP-B/DPPC mixtures very nearly overlay one another, suggesting that the SP-B is being excluded from the film. The use of multiple contrasts with NR at a range of surface pressures has enabled the mixing and 'squeeze out' of the DPPC and SP-B mixtures to be studied. We are able to identify the SP-B component of the interfacial structure and its position as a function of surface pressure. The mixtures are initially a homogeneous layer at low surface pressures. At higher surface pressures the SP-B is squeezed out of the lipid layer into the subphase, with the first signs detected at 30 mN m(-1). At 50 mN m(-1) the subphase is almost completely excluded from the DPPC layer with the SP-B content significantly reduced. Only a small amount of DPPC appears to be associated with the squeezed out SP-B.