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

• Published in: Langmuir Vol. 22; no. 22; pp. 9208 - 9213
• Main Authors: Itoh, Koichi, Oguri, Hukuhiro
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 24.10.2006
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Gas liquid interface; Plane surface; Monolayer; Polarization; Infrared reflection; Extinction; Film; Infrared absorption; Air water interface; Orientation; Solid; Plate; Infrared spectrometry; Substrate; Tilt angle; Lysine; Accommodation; Structure
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la060536j

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.