Crystallization of Proteins: Principles and Methods - Molecules: Nucleation, Aggregation and Crystallization - page 114

Introduction
Protein crystallization has gained a new strategic and commercial rele-
vance in the post-genomics era because of its pivotal role in structural
genomics (Chayen, 2005). Crystallization is a phase transition phenome-
non and occurs under non-equilibrium conditions.
The principle of inducing protein crystallization depends on the basic
strategy of bringing a system into a state of limited supersaturation, which
is the ratio of the protein concentration over its solubility value (Ducruix,
Giege, 1999). Knowledge of protein solubility in the presence of a crys-
tallizing reagent enables one to effectively use the crystallizing reagent in
a way that will promote crystallization (Anderson et al ., 2006).
Principles of Crystallization
Biocrystallization follows the same rules of crystallization as inorganic or
organic small molecules, but is multiparametric, making it a more com-
plex process. The fact that proteins are extremely sensitive to external
conditions accentuates this complexity. The three classical steps for crys-
tal formation include nucleation, growth and cessation of growth.
Nucleation is a prerequisite and the first step to crystal growth.
The crystallization phase diagram
The multiparametric nature of crystallization and the limited knowledge
of the mechanisms of crystal growth of proteins (McPherson, 1995) have
restricted most investigations to empirical work. Studies on model macro-
molecules show that phase diagrams can be useful for this purpose but
they have only infrequently been applied to find high-quality crystals of
proteins for structure determination (Ataka, Tanaka, 1986; Feher, Kam,
1985; Saridakis et al ., 1994).
During crystallization, the parameters characterizing the solution
may change with time, and the system will follow a particular path which
can be illustrated by a phase diagram. The phase diagram is a map which