It depends on what you want to do with them afterwards. Usually it may be necessary to use some chaotropic agent (urea, GuHCl, etc) and/or detergent, and even some heating.

For SDS-PAGE, the safest bet would be Laemmli sample buffer containing 7-8 M urea.

Thanks for the answer Alejandro. The problem is that later I want to do a chromatography and I need the buffer which I have used. I can try to heat the sample.

Is it essential to use acetone precipitation for your samples?

Dear Juan

I need to precize you to be careful in your process:

-Before use acetone, so cold it at  -20°C 

-It is necessary allow the acetone to evaporate  at room temperature for 30 minutes. 

- To optimize precipation rate  please add buffer appropriate for the downstream process and vortex thoroughly to dissolve protein pellet. 

-After precipitation you can use rehydration buffer  as 8 M Urea, 2% (w/v) CHAPS, 0.5% (v/v) IPG buffer) to redisolve the protein pellet.

Hoping to be helpful

Marius