Visualization of the Orientation of Amphipathic Peptides at the Air-Water Interface by X-Ray Reflectometry

X-ray reflectivity measurements were performed for the leucine and lysine-based LKα14 peptide designed to adopt an α-helical conformation at the air-water interface. The electron density profiles along the surface normal were calculated from the atomic coordinates predicted by an electronic structure program to fit the X-ray reflectivity curve. At the concentration of the monolayer formation, the long axis of the α-helix adsorbed parallel to the water surface, and the central part was revealed to be submerged in water. On the other hand, at 100 times higher than the surface area density of the monolayer formation, the double layer is formed in which the long axis of the α-helix is parallel to the water surface and only the peptide in the second layer is submerged in water.