Introduction

As noted in earlier chapters, intensive studies were carried out in the early 1990s to determine the effects on the phase diagrams of adding salt, decreasing temperature, or changing the pH towards the isoelectric value. These studies were typically performed on globular proteins with low molecular weight (such as lysozyme and the γ-crystallins). The reader should not think, however, that lysozyme or the γ-crystallins are typical of globular proteins. There is in fact a wide variability in the behavior of globular proteins. To illustrate this, we summarize the properties of a few additional proteins. We begin by presenting some results of an experimental study of the role of liquid–liquid phase separation in the crystallization of glucose isomerase. This study shows the possible complexity of crystallization processes, including its sensitivity to the choice of initial conditions in the phase boundary, as well as to the physicochemical properties of the system. It also illustrates the role of protein-rich liquid droplets in crystallization.

We also discuss results for α-crystallins, urate oxidase, bovine pancreas trypsin inhibitor (BPTI), and apoferritin, to illustrate the diversity of possible behavior. Most oligomeric proteins are not compact, which increases the repulsive hard core contribution to the interactions and correspondingly decreases the relative contribution of the short range van der Waals attractive interaction.