Phase Diagrams

The solubility of proteins can be represented in phase diagrams.

The phase diagram plots the solubility curve of a protein.

Phase DiagramPhase Diagram for a typical protein
There is an energy barrier to crystallization

How can the protein become supersaturated?
Why doesn't the protein immediately precipitate or crystallise as soon as saturation is achieved?
Because in order for a protein to crystallize it must overcome an energy barrier analogous to that for conventional chemical reactions.

Phase DiagramEnergy Diagram for Crystallization

The probability of nucleation increases with increasing supersaturation

The more supersaturated the protein solution,

This can be represented on a phase diagram by dividing the supersaturated zone into regions of increasing probability of nucleation and precipitation.

Phase Diagram showing zones for crystal nucleation, growth and precipitation.

© 1999-2005 Airlie J McCoy, University of Cambridge. All rights reserved.

Last updated: 7 June, 2005