Reversible formation of plastic two-dimensional lipid crystals

Individual dipalmitoylphosphatidylcholine (DPPC) crystals formed in a monomolecular film of DPPC at the air-water interface have been studied by epifluorescence microscopy. For this purpose a new trough design was developed to enable the observation and tracking of a chosen crystal over periods of hours. We found that various observed crystal shapes are multiples of a basic domain structure. Each domain has a distinct in-plane director but no macroscopic symmetry. The complex crystal shapes were found to be reversible with respect to growth and decay. Trace amounts of cholesterol drastically alter the shapes of the crystals but change none of the above-mentioned properties. Upon monolayer compression, the crystals grow uniformly at the fluid-crystal interface while simultaneously elongating and thinning. This remarkable effect requires a high lateral plasticity of these two-dimensional crystals.