Microstructure of phospholipid monolayers studied by dark field electron and fluorescence microscopy

The preparation method of horizontal transfer onto hydrophobic carbon substrates was used to investigate pure dipalmitoyl phosphatidylcholine (DPPC) monolayers at various surface pressures by high contrast dark field electron microscopy. A microporous structure of a continuous monolayer containing lipid free regions of about 0.1 ∼ 2 μm in diameter was found. The lipid-free regions on the substrate, which decrease in size with growing surface pressure, are related to parts of the phospholipid monolayer in the liquid phase at the air/water interface. The pressure dependence of the density of these liquid phase regions is described by a simple two-phase model for the pressure range from the liquid-expanded/liquid-condensed (LE/LC) phase transition up to high pressures. For evaluating the horizontal transfer properties, DPPC monolayers containing ∼ 1 mol% fluorescent dye molecules were investigated in the same way and by fluorescence microscopy. The structure of these mixed monolayers consisted of small solid phase domains with diameters of about 0.02 ∼ 5 μm arranged in periodic patterns at a submicroscopic scale in a way similar to that known from fluorescence observations (1, 2, 4). Evidence is provided that different contents of impurities may be responsible for the formation of the observed structures of the pure and mixed system. In addition, from high magnification dark field electron micrographs the submicrometer sizes of solid phase domains in DPPC monolayers were estimated for the first time.