Infrared external reflection absorption spectroscopic study on the structures of the Langmuir–Blodgett films of palmitoyl-l and dl-lysine on a Ge plate

Abstract

The p-polarized infrared reflection absorption spectra were observed for the Langmuir–Blodgett (LB) film of palmitoyl-l-lysine (l-PL) and the film of a 1:1 mixture of palmitoyl-l- and d-lysine (dl-PL) prepared on a germanium plate as a function of incident angles. The spectra were simulated by assuming a three-layered model consisting of an air/optically anisotropic LB film/Ge substrate, to determine the orientation angles relative to the surface normal for the transition moments of the CH2 asymmetric stretching, CH2 symmetric stretching, amide I, and amide II modes. The results gave detailed information about the orientation of the palmitoyl as well as peptide groups in the LB-films of l-PL and dl-PL, elucidating the structural difference between the two LB films. In the l-PL film the palmitoyl groups exist in a uniaxial orientation state with the tilt angle of about 29° relative to the surface normal and the peptide groups form an ordered hydrogen-bonded network with the peptide plane tilting from the surface normal of about 19°. On the other hand, the peptide groups form an ordered hydrogen-bonded network with the tilt angle of about 49°, while the palmitoyl groups exist in an irregularly orientated state. A structural model consistent with the above-mentioned results was proposed to the dl-PL film.