Fourier transform infrared-attenuated total reflection spectra of dipalmitoylphosphatidylcholine monomolecular films

Abstract

Single monomolecular films of 1,2- dipalmitoyl-3-sn- phosphatidylcholine (DPPC) were transferred onto a germanium plate by the Langmuir-Blodgett technique at various surface pressures encompassing a plateau region in a surface pressure(π)-area( A) isotherm. Molecular orientation and structure of the monomolecular films were investigated by the Fourier transform infrared-attenuated total reflection spectroscopy. Appreciable changes in frequency and halfbandwidth of the antisymmetric and symmetric CH 2 stretching bands of the palmitoyl chains were found at the onset of the plateau region in the π-A isotherm . By analyzing polarized infrared-attenuated total reflection spectra, the palmitoyl chains of DPPC films were proved to be oriented vertically to the germanium surface with all-trans conformation, irrespective of the surface pressure on the film transfer. This finding suggests the existence of islands or surface micelles on the surface throughout the surface pressure examined. The changes in the spectral parameters mentioned above were explained by changes in the state of packing of these islands. The plateau is ascribable to a transition process from a close-packed island phase to a molecularly homogeneous phase.