P212121

The phase diagram in protein crystallization is a schematic representation of how protein and precipitate concentration are related. Protein crystals are formed in supersaturated solutions. As shown below, low protein and/or precipitate concentrations will cause undersaturation that will not produce protein crystals.

The red line that separates undersaturated conditions from supersaturated is known as the solubility curve. A benefit of determining the solubility curve is that it can help guide you when analyzing your crystal growth conditions. A crystallization setup that is undersaturated or in the metastable zone will appear clear, however, the latter has the possibility of crystal growth if seeded.

The phase diagram is often broken down into 4 distinct zones one of which undersaturated we have already covered. Precipitation is when the protein comes out of solution as an aggregate and therefore is not useful for crystallographic studies. The labile zone (or nucleation zone) is important since this is where crystal nucleation and initial growth occur. As the crystal forms the protein concentration will be depleted causing one to move from the labile to metastable zone.