Pressure versus pH phase diagrams of two lysozymes crystallized in agarose gel

The effect of hydrostatic pressure and pH on the crystallization of tetragonal hen lysozyme crystals (HeL, M r 14,300) and hexagonal turkey lysozyme crystals (TeL, M r 14,200) in agarose gel was studied. Samples (adjusted to a pH in the range 4–6) were pressurized at 0.1–100 MPa for nine days at 293 K. The morphology and number of crystals as well as protein solubility were analyzed after depressurization. For both proteins and whatever the pH, a higher pressure resulted in greater numbers of crystals and greater solubility values. At any pressure, the solubility and number of crystals were lower at the highest pH. Thus the physical chemical parameters and the outcome of the crystallization process are more affected by pressure at pH 4.0 than at pH 6.0. In the case of HeL, either high pressure or high pH induces a transition from the initial tetragonal form to an orthorhombic one. The observed effects are related to minor differences in the macromolecular structures.