Towards an improved understanding of the variables controlling crystal modification and phase transition in lysozyme crystals

The solubility of different lysozyme polymorphs is investigated to get more insight into the role of solution conditions in lysozyme crystallization and polymorphism. A simple method was developed to measure the solubility in a very short period of time. The solubility of each polymorph is measured over a wide range of temperature, pH and NaCl concentration for regions in the phase space where these polymorphs are stable and metastable. The boundaries between the different polymorphs were experimentally determined. Lysozyme crystals showed two different phase transitions, a temperature induced phase transition from the tetragonal or LTO to the High Temperature Orthorhombic form (HTO) and a pH induced phase transition from the tetragonal to the Low Temperature Orthorhombic form (LTO). The transition temperature between the tetragonal and the HTO forms was found to vary with pH and NaCl concentration whereas the transition pH was between the tetragonal and the LTO form is insensitive to temperature and NaCl concentration.