desalting/concentrating low MW proteins

[Dima Klenchin]

In article <79abc8$ulb$1 at nnrp1.dejanews.com>, ardibat at wanadoo.fr wrote:
>In article <798aqf$lqu$1 at enyo.uwa.edu.au>,
>  Eugene Hone <eugeneh at cyllene.uwa.edu.au> wrote:
>> Hi,
>>
>> Does anyone know of/have a method for concentrating and desalting a solution
>> containing a 4kDa protein?
>>
>> Cheers
>>
>
>One solution for your problem (the problem is that a 4kD protein is too small
>for dialyse) is to use a G25 resin.
>Just put the resin in a tube with a very small hole on the bottom, pour your
>protein solution and give a very short centrifuge tour to collect the
>concentrated protein.
>For desalting... I don't know...
>

Keep in mind that G-25 has nominal cut-off of 5K for globular proteins. 
Another thing is that old dry G-25 leaches something (dextranes? 
crosslinkers?) which inhibits various enzymatic reactions. For 
chromatography it's OK since the column gets washed anyway but
for for the purpose of concentrating, this can be a reservation. 

        - Dima