In my experience, either urea or guanidinium crashes out in gels. I
can't remember--which one is it? I am thinking guanidinium. (If the
answer to this email saves one grad student from the aggravation of
such a phenomenon, it will have been worth it...)
It's GuHCl and what crashes is dodecyl sulfate salts. Urea is fine (recall
that many gels are run with urea in them and soem loading buffers contain
urea). High salts is bad for gels but they are easy to remove from the
samples by methanol/chloroform precipitation:
http://www.abrf.org/ResearchGroups/EdmanSequencing/EPosters/ABRFhand1.doc
(With low protein amounts, I'd suggest modifying this protocol by
increasing centrifugation times: 10 min in the interphase forming step and
5-10 min in the last step of pelleting precipitated protein).
Dima
