I have ever autoclaved an extraction buffer with 8 M urea for solubilize my protein that expressed in IB form, it so smelly (urea breaks down when heated, releasing NH3) and I worry about protein refolding process.
As Vladimir points out, the problem with urea is that it will isomerize to cyanate. The reaction is boosted by heat, thus urea solutions should be kept in the cold and never autoclaved nor heated in a microwave oven. However, since urea is more easily solubilized at high temperature, and since solubilization will absorb heat, it is possible to speed up the process by adding lukewarm water (~30-40 °C max) to the urea powder. It will cool upon dissolving urea. I don't bother to make urea solutions fresh each time, but I keep my stock solution over a spoonful of mixed-bed ion exchange resin, e.g. AG501-X8 from Bio-Rad in order to to mop up traces of cyanate.
Do not autoclave the solution. The solution has to be prepared freshly each time and you may use autoclaved but overall milli-Q grade water. Check also the quality of your urea powder: if it is too old get a new batch.
You don't want to autoclave Urea as with temperature and time it has a tendency to isomerise, so you end up with an effective concentration that is not what expected. Use freshly prepared solution at all time. If you were considering autoclaving to get the Urea in solution, you have just to be patient and avoid heating it up too.
No! Urea decomposes to form an ion cyanate (NCO-), which reacts with NH2, SH-groups of proteins. This process is faster at elevated temperatures. In addition, at an autoclaving temperature, you lower the concentration of urea.
As Vladimir points out, the problem with urea is that it will isomerize to cyanate. The reaction is boosted by heat, thus urea solutions should be kept in the cold and never autoclaved nor heated in a microwave oven. However, since urea is more easily solubilized at high temperature, and since solubilization will absorb heat, it is possible to speed up the process by adding lukewarm water (~30-40 °C max) to the urea powder. It will cool upon dissolving urea. I don't bother to make urea solutions fresh each time, but I keep my stock solution over a spoonful of mixed-bed ion exchange resin, e.g. AG501-X8 from Bio-Rad in order to to mop up traces of cyanate.
I do not both the sterility of Urea solution. No need to autoclave.
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