Preparation and structural analysis of Langmuir-Blodgett films of acidic and zwitterionic phospholipids

Abstract

Langmuir-Blodgett (LB) films were prepared from acidic and zwitterionic phospholipids on hydrophobic solid substrates. Dipalmitoyl phosphatidic acid, which is negatively charged at physiological conditions, was more easily transferred onto the solid substrate in the presence of divalent cations as compared to the zwitterionic phospholipids dipalmitoyl-phosphatidylcholine and its ether analog dihexadecyl-phosphatidylcholine The structure across the film was characterized by X-ray reflectivity and X-ray diffraction, whereas the in-plane structure was investigated by electron diffraction. X-ray experiments revealed a repeat unit of 5.35 nm for the LB film of phosphatidic acid. A model for the Y-type structure of this film is presented based on a modeling technique of the X-ray reflectivity data. The substructure of the film along the normal to the substrate plane is described in detail using this new approach. Electron diffraction indicated a hexagonal packing of the hydrocarbon chains normal to the substrate surface with a spacing of 0.425 nm. These parameters correlate well with data from bilayer structures. It can be supposed that LB films of acidic phospholipids are useful as membrane mimetic systems. Deposition of the zwitterionic phospholipids was of poor reproducibility, which could be improved in the presence of polymers. A spacing of about 7 nm was deduced for dipalmitoyl-phosphatidylcholine films indicating that the polymer was transferred with the lipid onto the solid support.