Structures of Palmitoyl-l and dl-Lysine Monolayers at the Air-Water Interface - Polarization Modulation Infrared Reflection Absorption Spectroscopic Study

Polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) was applied to measure the IR spectra of palmitoyl-dl-lysine (l-PL) and palmitoyl-dl-lysine (dl-PL) at the air−water interface. The spectra in the amide I and II regions were simulated by using the extinction coefficients of the amide I and II bands of l-PL and dl-PL determined by the analyses of the IR external reflection spectra of the Langmuir−Blodget (LB) films prepared on a Ge plate (Yasukawa et al. J. Mol. Struct. 2005, 735−736, 53), indicating the angle between the plane of the secondary amide group (the amide plane) and the surface normal in the l-PL monolayer to be about 20° and the angle in the dl-PL monolayer to be about 37°. Comparison of the tilt angles with the corresponding angles in the LB films (about 20° for the LB film of l-PL; about 49° for the LB film of dl-PL) indicated that, upon being transferred to the solid substrate from the air−water interface, the l-PL monolayer keeps the orientation of the amide plane virtually unchanged, while the dl-PL monolayer changes the orientation appreciably to a horizontal direction. The orientation change of the amide plane was interpreted as due to the accommodation of irregularly oriented palmitoyl groups into the LB films of dl-PL on the solid substrate.