Crystallization of Proteins: Principles and Methods - Molecules: Nucleation, Aggregation and Crystallization - page 116

Fig. 1. A classical phase diagram; the extent of supersaturation is mapped with regards
to the system parameters. Knowledge of phase diagrams is a prerequisite to the design
of crystallization experiments. The figure represents the phase diagram showing the dif-
ferent routes of reaching supersaturation by different methods. The four major crystal-
lization methods are represented, showing their different routes to the nucleation and
metastable zones. The starting conditions are represented by: (a) batch, (b) vapor diffu-
sion, (c) dialysis, and (d) free interface diffusion (FID). Two alternative starting points
are shown for FID and dialysis because the undersaturated protein solution can contain
either protein alone or protein mixed with a low concentration of the precipitating
agents. The solubility is defined as the concentration of protein in the solute that is in
equilibrium with crystals. The supersolubility curve is defined as the line separating
conditions where spontaneous nucleation (or phase separation, precipitation) occurs
from conditions where the crystallization solution remains clear if left undisturbed
(Chayen, 2005).
Nucleation zone
This is the zone of the phase diagram where crystals appear. Nucleation is
the process by which molecules or non-crystalline aggregates which are free
in solution come together in such a way as to produce thermodynamically