Epifluorescence optical microscopy has been used to show the formation of solid phase domains from fluid phase domains on compression of dipalmitoylphosphatidylcholine (DPPC) monolayers at the air-water interface. These monolayers can be transferred to solid substrates, and in certain conditions exhibit X-ray diffraction characteristic of highly ordered two-dimensional crystals. In previous work, the crystal domains, visualized by the exclusion of fluorescent lipid probes, were often round and arrayed within a continuous fluid phase domain in a hexagonal pattern. We report here that when DPPC monolayers are more rapidly compressed, at a rate of the order of a 2% decrease in area per second, chiral solid domains of lipid are formed. The handedness of the solid domains is directly related to the enantiomorphic configuration of the lipids composing the monolayer. The shape of these domains provides direct visual evidence for long range orientational order in two-dimensional crystals.
