Editing Urea (section)

====Aqueous stability====
In aqueous solution, urea slowly equilibrates with ammonium cyanate. This [[elimination reaction]]<ref name="AlexandrovaJorgensen2007">{{cite journal |last1=Alexandrova |first1=Anastassia N. |author-link1=Anastassia Alexandrova |last2=Jorgensen |first2=William L. |author-link2=William L. Jorgensen |title=Why Urea Eliminates Ammonia Rather than Hydrolyzes in Aqueous Solution |journal=The Journal of Physical Chemistry B |date=1 February 2007 |volume=111 |issue=4 |pages=720–730 |doi=10.1021/jp066478s|pmid=17249815 |pmc=2995377 }}</ref> cogenerates [[isocyanic acid]], which can [[Isocyanic acid#Reactions|carbamylate]] proteins, in particular the N-terminal amino group, the side chain amino of [[lysine]], and to a lesser extent the side chains of [[arginine]] and [[cysteine]].<ref name="SA_PDF">{{cite web |last1=Aldrich |first1=Sigma |title=Urea Solution Product Information |url=https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/392/609/u4883dat.pdf |access-date=7 February 2023}}</ref><ref name="Burgess Deutscher 2009">{{cite book | last1=Burgess | first1=Richard R. | last2=Deutscher | first2=Murray P. | title=Guide to protein purification | publisher=Academic Press/Elsevier | publication-place=San Diego, Calif | date=2009 | isbn=978-0-12-374536-1 | oclc=463300660 | page=819}}</ref> Each carbamylation event adds 43 [[Dalton (unit)|daltons]] to the mass of the protein, which can be observed in [[protein mass spectrometry]].<ref name="Burgess Deutscher 2009"/> For this reason, pure urea solutions should be freshly prepared and used, as aged solutions may develop a significant concentration of cyanate (20&nbsp;mM in 8&nbsp;M urea).<ref name="Burgess Deutscher 2009"/> Dissolving urea in ultrapure water followed by removing ions (i.e. cyanate) with a mixed-bed [[ion-exchange resin]] and storing that solution at 4&nbsp;°C is a recommended preparation procedure.<ref name="Deutscher 1990">{{cite book | last=Deutscher | first=M.P. | title=Guide to Protein Purification | publisher=Academic Press | series=Methods in enzymology | year=1990 | isbn=978-0-12-182083-1 | url=https://books.google.com/books?id=zTiRJHpKIQoC&pg=PR11 | access-date=2023-02-24 | page=267}}</ref> However, cyanate will build back up to significant levels within a few days.<ref name="Burgess Deutscher 2009"/> Alternatively, adding 25–50 mM [[ammonium chloride]] to a concentrated urea solution decreases formation of cyanate because of the [[common ion effect]].<ref name="Burgess Deutscher 2009"/><ref>{{cite journal | vauthors = Sun S, Zhou JY, Yang W, Zhang H | title = Inhibition of protein carbamylation in urea solution using ammonium-containing buffers | journal = Analytical Biochemistry | volume = 446 | pages = 76–81 | date = February 2014 | pmid = 24161613 | pmc = 4072244 | doi = 10.1016/j.ab.2013.10.024 }}</ref>