Optical birefringence and x-ray measurements are reported for single crystals (0.5 mm X 2 mm X 5 micrometers) of dipalmitoylphosphatidylcholine as a function of temperature and relative humidity. Large single crystals grown in CHCl3/acetone at a low water concentration undergo an irreversible phase transformation at 20 degrees C and 70% relative humidity or at 60 degrees C and 65% relative humidity. After this transformation occurs, the crystals can be made to undergo a reversible transformation into and out of the L alpha phase. The x-ray-determined mosaic spread in the hydrated crystal is less than approximately 0.04 degrees both above and below the main transition.
