Phase separation

Harry Powell hrp1000 at
Thu Sep 5 04:51:50 EST 1996

On Wed, 4 Sep 1996, H.C. Whitaker wrote:

> I wonder if anyone can help me? Can anyone explain to me (in laymans
> terms please!) what exactly is happening when phase separation occurs 
> in the drop when attempting to crystallise a protein?
In layman's terms;

If you have a solution that is unsaturated in the protein, it stays in 
solution. If it is concentrated so that it is at least saturated (and in 
the absence of any nuclei, e.g. shards of glass, bits of beard... often 
supersaturated) the protein will start to precipitate. If the 
precipitation process is done carefully, the precipitation will be in the 
form of crystals.

The drop contains the protein in solution. The reservoir contains pretty 
much the same solution with two major differences; 

 (1) it doesn't contain protein
 (2) it's more concentrated in at least one of the other components.

The solvent in the drop will tend to migrate from the drop to the 
reservoir (tending towards an equilibrium, i.e. the concentrations of 
the drop and reservoir will tend to equalize), so the concentration of 
everything in the drop increases, (hopefully) past the saturation point, 
causing precipitation. 

How's that? 

Dr Harry Powell
University Chemical Laboratory
Lensfield Road

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