Synchrotron X-ray study of lung surfactant-specific protein SP-B in lipid monolayers

This work reports the first x-ray scattering measurements to determine the effects of SP-B1-25, the N-terminus peptide of lung surfactant-specific pro tein SP-B, on the structure of palmitic acid (PA) monolayers. In-plane diff raction shows that the peptide fluidizes a portion of the monolayer but doe s not affect the packing of the residual ordered phase. This implies that t he peptide resides in the disordered phase, and that the ordered phase is e ssentially pure lipid, in agreement with fluorescence microscopy studies. X -ray reflectivity shows that the peptide is oriented in the lipid monolayer at an angle of similar to 56 degrees relative to the interface normal, wit h one end protruding past the hydrophilic region into the fluid subphase an d the other end embedded in the hydrophobic region of the monolayer. The qu antitative insights afforded by this study lead to a better understanding o f the lipid/protein interactions found in lung surfactant systems.