The Phase Diagram in Protein Crystallization
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.
BR
November 15th, 2009 at 11:13 PM #
Hi All -
the situation is not quite as described here, and actually more complicated. I suggest to consult Chapter 3 in my book and to revise this posting accordingly. The difference between thermodynamic phase diagrams and the tentative (to conjectural) kinetic information pasted into the ‘crystallization diagrams’ is explained there. For those who are curious, the underlying thermodynamic theory is on http://www.ruppweb.org/Xray/tutorial/PC_of_crystallization.htm
Best, BR
Sean
November 15th, 2009 at 11:54 PM #
I agree the situation is quite complicated and this post does not cover all the details.
As I am sure you already know this type of phase diagram has been used quite often such as in this paper on Protein crystallization & phase diagrams, a paper by Chayen, an explanation by Jena Bioscience or this figure from Cambridge.
I do not know of any incorrect statements in this post, but will gladly update it if they are specifically brought to my attention. Thanks for sharing the link on the physical chemistry of protein crystallization. Unfortunately, I do not have a copy of your book to consult.
BR
November 16th, 2009 at 1:52 AM #
Most of these diagrams are drawn incorrectly. Think about where the solubility line originates from – can the graph as drawn be parallel to an axis in principle? In practice, examine the solubility functions for salts and PEGS. There is also no difference thermodynamically between the metastable and ‘labile’ zone – everything between the solubility line and the spinodal decomposition line is metastable and supersaturated. ‘Labile’ means something different in thermodynamics than in colloquial terms. The correct description of this part of the metastable region is one of spontaneous homogeneous nucleation. Consult the schematic G/x diagrams that illustrate how the solubility line, metastable region, and instability line are derived.
BR