What happens, when urea containing solutions are stored for
a longer period, is that carbamine acid is formed by
deamination (so you have NH2-CO-COOH instead of NH2-CO-NH2).
The same happens quite rapidly if you heat urea-containing
solutions. If amino acids or proteins are present, they are
carbamylated, which causes a big alteration in the
isoelectric point of the protein affected. For example, you
will get a lot of spots arranged like pearls on a string
due to increasing modification of the respective protein.
This modification can also affect protein functionality. So
it is best, to make urea solutions fresh or at least stirr
it with a mixed-bed ion-exchange resin (e.g. Dowex AG1X8)for
1 hour at RT to remove most ions.
Good luck
Thomas
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