In this paper we study the interaction of cholesterols with L/sub /spl alpha// phase of DPPC monolayer prepared by the langmuir-Blodgett method and characterization of dipalmitoylphosphatidylcholine (DPPC)/cholesterol Langmuir-Blodgett monolayers by AFM and FT-IR spectra.
Using atomic force microscopy, we have investigated the formation of the dipalmitoylphosphatidylcholine (DPPC) membrane by the vesicle fusion method on SiO2 surfaces modified with self-assembled monolayer (SAM) islands of octadecyltrichlorosilane (OTS) with sizes comparable to those of the vesicles. OTS-SAM islands with various sizes and coverages can be constructed on the SiO2 surfaces prepared by thermal oxidation followed by partial hydroxylation in a H2O2/H2SO4 solution. When vesicles are sufficiently smaller than the SiO2 domains, DPPC bilayers and DPPC/OTS layers form on the SiO2 and OTS domains, respectively. However, the adhesion of larger vesicles onto SiO2 is prevented by the OTS islands; therefore only DPPC/OTS layers form without formation of DPPC bilayers on the SiO2 domains. On surfaces with domains on the scale of tens to hundreds of nanometers, the relative size between the hydrophilic domains and the vesicles becomes an important factor in the membrane formation by the fusion of vesicles.
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.
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