protein precipitation

Emir Khatipov ekhatipo at NOSPAMmidway.uchicago.edu
Mon Oct 2 17:17:25 EST 2000

>      Do you think the membrane is not really mwc 12000 or
>      my PEG is not really as high molecular weight as I thought

Both PEG is never exactly 20,000, and the MW cut off value of the membrane
indicated just the size of the biggest molecule that penetrates the membrane
freely. Depending on the properties of the membrane, more or less higher MW
molecules would still be able to squeeze through it. For the purposes of
concentrating with PEG20k, I would take the membrane with MWCO ~3,000 Da.
Sorry I missed that point in my earlier post.
>      OR should I not really worry about getting PEG mixed up with my
>      protein.

A little bit of PEG (<10%) in your solution would not hurt. Higher
concentrations may precipitate some proteins. To get rid of PEG I would pass
my protein soln. through NAP 25 or similar mol. sieve column. However, that
will result in diluting the soln. ~1.5 times.

So, 20k PEG and <3k MWCO membrane is the best in terms of avoiding troubles.


> Mike.
> On Fri, 29 Sep 2000, Emir Khatipov wrote:
> > Just an old method for concentrating protein solutions: pour your
> > into dialysis tube and dig the tube into the beaker with dry PEG 20,000.
> > will absorb the liquid really fast (2-3-hrs).

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