Re: protein solution

[Chris Colbert <colbc@wladziu.bio.purdue.edu>]

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Hi Lan,

I am not an expert, but I do have a few comments concerning protein solubility.

1)  Some proteins do exhibit cold solubility.  This is one reason why cold water
is used to immediately remove blood from clothing.  So, what you are seeing, 
the protein being soluble at 1mg/ml at RT and 4C, but only soluble at 4C when 
concentrated at 20mg/ml, could be this type of behavior, cold solubility. 
This is conterintuitive if you try to think of proteins as being salts, which they are not.

2)  In crystallization we use salt concentration to influence protein solubility.
The most common way is to use high salt concentrations to force our proteins out
of solution at high ionic strength.  However, very low concentrations of salt also
help protein solubility.  This is commonly referred to as "salting in."  You may want
to add some NaCl to your solution and test your solubilities.

3)  pH can also change the solubility of the protein.  What is the pH of your buffer
if any?  What is the pI of your protein?  If your pH is very close to your pI, then
your protein is at its least soluble in aqueous solution.  Change the pH.  How much?
I'm not sure.

It sounds like your protein is fairly manageable.  Crystallization at 4C is done all
the time.  As was mentioned earlier, you may be able to exploit this behavior to 
crystallize your protein.  You may use this property in the event you eventually wish
to use a seeding technique.  There are lots of things that can effect protein solubility.
I'm sure someone has a reference they could post for general interest to the ccp4bb???

Hope this helps,

Chris


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Christopher L. Colbert
Purdue University Macromolecular Crystallography         
Lilly Hall of Life Sciences                         
West Lafayette, IN 47907-1392			    
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