Biology Reference
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represents the state of a material (e.g. solid and liquid) as a function of the
ambient conditions (e.g. temperature and concentration) (Asherie, 2004).
Phase diagrams form the basis for the design of crystal growth. Knowledge
about them is central to understanding the principles of protein crystal-
lization (Haas, Drenth, 1999).
Phase diagrams can be constructed by setting up a number of crystal-
lization trials where a minimum of two parameters are varied, and plotting
the outcome after a set period of time (Sauter et al ., 1999a). Figure 1 rep-
resents a 2D solubility phase diagram using two of the most commonly
varied parameters, the concentrations of protein and the crystallizing
agent.
The two curves of the phase diagram are:
The solubility curve
This divides the phase diagram into two major concentration zones.
(a) The undersaturated zone, where crystallization does not occur as the
solution is thermodynamically stable.
(b) The supersaturated zone, which corresponds to the regions above
the solubility curve. Crystals can only form in supersaturated
solutions.
The supersolubility curve
This is less well defined than the solubility curve but can be found exper-
imentally to a reasonable approximation much more easily. It can be fur-
ther subdivided into three zones:
Precipitation zone
Precipitation occurs at very high supersaturation. Excess protein
molecules immediately separate from solution to form amorphous
aggregates.
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