Characterization of Dipalmitoylphosphatidylcholine/Cholesterol Langmuir-Blodgett Monolayers Investigated by Atomic Force Microscopy and Fourier Transform Infrared Spectroscopy

Abstract

The addition effects of cholesterol on the dipalmitoylphosphatidylcholine (DPPC) Langmuir-Blodgett (LB) monolayer have been investigated by atomic force microscopy (AFM) and infrared reflection absorption spectroscopy (IRRAS). The phase transformation from pure DPPC to the DPPC/cholesterol phase proceeds through two stages: initial drastic changes in the surface morphology and the conformation of the DPPC acyl chains below 10% cholesterol, and the gradual homogenization of the morphology towards the liquid-order phase up to 35% cholesterol. The IRRAS peak position indicates that the conformational disorder of the acyl chain becomes almost that of the liquid level at 10% cholesterol addition. In the homogeneous liquid-order phase at 35% cholesterol, the terminal methyl groups of the DPPC are aligned in good order similarly to the solidlike gel phase, whereas the acyl chains have a liquid-level disordered conformation.