Protein concentration with PEG

Dr Engelbert Buxbaum via (by engelbert_buxbaum from
Sat May 19 11:23:07 EST 2012

In article <jnphnv$npj$2 from>, dk from 
> >You are probably aware of the protein concentration method by
> >"dialyzing" against polyethyleneglycol.
> >
> >We just got some concerns now that some of the PEG could make it
> >across the membrane. 
> I'd advise you against using PEG. All PEGs are "dirty" and you will inevitably 
> get something into your protein. The same idea but much cleaner is to use 
> dried polyacrylamide gel that swells a lot. That's what we do with proteins 
> that cannot be concentrated by standard ultrafiltration. Buy this and be 
> happy: 

This of course has the disadvantage to cost some $20 per 5 ml sample. 

There are two things to consider here: low molecular PEG that can cross 
the membrane and other contaminants. The former are probably relatively 
benign, especially when further purification steps follow. PEG actually 
stabilizes protein conformation, that is why it is used in protein 

Other contaminants are a different story, in particular peroxides. It 
might be worthwhile to test each batch for those and to keep the PEG 
dark, cool and dry. 

That said, I have used the procedure many times, and never had any 
problems. However, that was with 20 kDa PEG and a 6 kDa membrane rather 
than 14 kDa as described by the OP.

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