Changes in size and shape of liposomes undergoing chain melting transitions as studied by optical microscopy.

Changes in the shape and size of dipalmitoylphosphatidylcholine liposomes at the phase transition at 41.5 degrees C have been monitored by light microscopy. All liposomes change size or shape at the transition and those with simple topologies such as spheres and cylinders can be readily measured. The surface area of these is some 24% greater above the transition than below. This surface area change is virtually identical to that predicted by crystallographic measurements on this system. Also, the rate of transition from one state to another is seen to proceed more rapidly in the smaller liposomes. Optical microscopic observation provides a rapid simple method for monitoring the dependence of the lipid bilayer area on temperature.