The crystallization, including both the phase diagram and the phase transition of hen egg white lysozyme (HEWL) was investigated. A tetragonal modification and a needle modification were obtained during crystallization. The phase diagram and stability of two modifications in both acid and basic pH solutions (pH 4.5, 8.0 and 9.0) were determined. Besides in acid solutions, the well-known tetragonal crystals can also be obtained in basic solutions at low temperature (7 °C) while the needle like modification can only crystallize in a basic solution. Based on the phase diagram, phase transfer behavior was found to exist between the two modifications. In basic solutions, tetragonal modification can transfer to needle shaped crystals. This process can be affected by a changing of pH and temperature. While in acid buffer, the needle shaped crystals dissolve and tetragonal crystals crystallize and remain in solution.
