I meant application of GF as an ion exchange column. You can use special ion exchange columns, but our lab often uses preparative GF columns for this task.  We just load the column, keeping sample volume <  the void volume. Thus, we do not  concentrate a protein before an ion exchange, only after it. But that is inevitable. When I am afraid to loose a protein during its concentrating, I concentrate shoulders of the eluted peak first, then add a central part. 

My point was that it might be okay to exchange buffers by concentrating a protein, but other molecules like Peg3K would not penetrate the membrane as well as water or salts do, as a result their reduction in concentration will be unreliable. Like, you do a 10 fold concentrating/delusion of a solution, but the final concentration of PEG3K will drop only by 3 fold... 

Alex

Hi Alex, 

I disagree with you even though GF is always the last step in my purifications. 

Because it involves concentration before and after the GF so during the concentration you can already be doing the buffer exchange.

You use GF when you want to purify other protein impurities if they are different sizes. Of course it has other uses too. But not quite practical for just changing buffer also considering the amount of protein you could be loosing along the process. If one is careful, centripreps are best for concentrating and changing the buffer. I tell you this from experience with large hard to express proteins.

Best of luck,
Remie

On Aug 19, 2014, at 10:45 AM, Alexander Aleshin <[log in to unmask]> wrote:

Remie,

Actually, concentrating of a protein solution is not the best approach to removing low MW impurities, gel filtration chromatography is  more reliable and ... faster.

Regards,

Alex

On Aug 19, 2014, at 7:03 AM, Remie Fawaz-Touma wrote:

Hi Reza, I had to do this before. 

This protocol works for any PEG and any chemical to be removed from a solution: buffer exchange into the new buffer you want your protein to be in. There are ways to do that by 15 mL Amicon concentrators from millipore for large volumes, or if your protein is already concentrated, there are some small 0.5 mL concentrators from millipore as well.

The key is to keep your spinning at low speeds (concentrators manuals will tell you) so you don’t precipitate or loose your protein. Check your protein concentration every 2 hours just to make sure you are not loosing it on concentrator surfaces and so on. 

Good Luck,

Remie

On Aug 19, 2014, at 9:55 AM, Reza Khayat <[log in to unmask]> wrote:

Hi,

Does anyone have a protocol for getting rid of PEG3350 from a protein sample?

Best wishes,
Reza

Reza Khayat, PhD
Assistant Professor
The City College of New York
Department of Chemistry, MR-1135
160 Convent Avenue
New York, NY  10031
Tel. (212) 650-6070
www.khayatlab.org