I don't have a foolproof way to prevent decomposition of these compounds,
but they need to be kept in the dark (which they usually are in a freezer),
as well as cold, to prevent light-catalyzed formation of free radicals,
leading to iodine. You are right, the yellow color is iodine which can
result in oxidation. To get rid of iodine, particularly when using
radiolabeled iodoacetic acid, I have always recrystallized iodoacetic acid
immediately before use. It's easy to recrystallize from cyclohexane. Add
iodoacetic acid to cyclohexane (about 70 mg per mL) in a glass centrifuge
or test tube and heat it in a hot water bath (less than 80 oC, the boiling
point of cyclohexane) until the crystals dissolve. Mix the solution, then
let it cool slowly, and beautiful white crystals will appear. Then spin
down the crystals, wash them with a little fresh cyclohexane, and dry them
under vacuum. It only takes a few minutes. I think iodoacetamide can be
crystallized the same way.
On the other hand, if I were carboxymethylating cysteine thiol groups after
treatment of the protein with a large excess of DTT or mercaptoethanol, I
wouldn't worry about a trace of free iodine (pale yellow crystals), because
the excess DTT will scavenge the iodine.
Richard Laursen
----------------------------------------
>Dear ABRF Colleagues,
>
>I am curious to hear how various labs approach the storage of Iodoacetamide
>or Iodoacetic acid?
>
>In the past, we have been careful to purge bottles of these compounds with
>inert gasses each time we use them and store them in the dark at
>refigerator or freezer temperatures. We still however only get a pretty
>short lifetime out of each container before it starts turning yellow. I am
>loath to use either of these compounds after they show this yellow
>decomposition color because I have been told that this represents free
>Iodine which can result in undesired oxidations or iodinations.
>
>Am I being too cautious in this regard or can any of you recommend a better
>way of handling these compounds?
>
>Ken
>
>********************************
>
>Ken I. Mitchelhill
>The John Holt Protein Structure Laboratory
>St. Vincent's Institute of Medical Research
>41 Victoria Parade
>Fitzroy 3065 Victoria
>AUSTRALIA
>
>Telephone: 61-3-9288 2480
>Facsimile: 61-3-9416 2676
>
>Email: k.mitchelhill@medicine.unimelb.edu.au
>
>Laboratory: http://www.medstv.unimelb.edu.au/WWWDOCS/SVIMRdocs/JHPSL.html
>ABRF: http://www.medstv.unimelb.edu.au/abrf.html
>
>***********************************
Richard A. Laursen
Department of Chemistry
Boston University
590 Commonwealth Ave.
Boston, MA 02215
Tel (617) 353-2491; FAX (617) 353-6466
email: <laursen@bu.edu>