Surface pressure induced structural transitions of an amphiphilic peptide in pulmonary surfactant systems by an in situ PM-IRRAS study.

Pulmonary surfactant model peptide, Hel 13-5, in binary and ternary lipid mixtures has been characterized employing the polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) in situ at the air-water interface for a monolayer state and the polarized ATR-FTIR for a bilayer film. In the bilayer form, Hel 13-5 predominantly adopts an α-helical secondary structure in the lipid mixtures. It had been made clear from CD measurements that the Hel 13-5 structure is mainly in the α-helical form in aqueous solutions. In the monolayer state, however, the secondary structure of Hel 13-5 exhibits an interconversion of the α-helix into β-sheet with increasing surface pressures. The difference in the secondary structure is attributed to formation of a surface-associated reservoir just below the surface monolayer. The reservoir formation is a key function of pulmonary surfactants and is induced by a squeeze-out of the fluid components in their monolayers. Compression and expansion cycles of the monolayers generate a hysteresis in molecular orientation of the lipid monolayer as well as in peptide structure. The formation and deformation of reservoirs are, in common, deeply related to the hysteresis behavior. Thus, the transition of peptide structures across the interface is a quite important matter to clarify the role and its mechanism of the reservoirs in pulmonary functions. The present study primarily reveals roles of the anionic lipids in control of the peptide secondary structure. Accordingly, it is demonstrated that they prevent the protein structure transition from α-helix into β-sheet by incorporating the peptide during the squeeze-out event.