Chemistry Forums for Students > Organic Chemistry Forum for Graduate Students and Professionals
Phenylfluorenyl PG removal + isolation of small aminoacid
rolnor:
You could use some MeOH in the eluent to push out the product from the resin if you have lipophilic groups in the product. 4M sounds really strong, I would try 0,5M for sensitive compounds. But if it works good, there is no reason to change anything offcourse.
kriggy:
@Babcock_Hall:
Thank you very much for your reply and the references, it was very helpful.
I have few question towards the work with dowex. At first, I need to wash with HCl to make sure its in the H+ cycle, then with water untill the elution is neutral. Then add my aminoacid in water (free base not salt) and elute with water and then with HCl. ?
If my reaction mixture contains various salts (NaIO4) I suppose they are converted into HIO4 and still elute while the Na+ is retained on the ion exchange?
Thank you.
Babcock_Hall:
When converting from a more tightly bound ion to a less tightly bound ion, one has to use a greater volume of the new ion. "As a general rule, use 1 bed volume of 1 M counterion solution for each unit difference in relative selectivity. For example, converting AG 50W-X8 resin from the K+ form (relative selectivity 2.5) to the H+ form (relative selectivity 1.0) would require 2-3 bed volumes of 1 M HCl. The conversion is complete when all the K+ ions are displaced by the H+ ions." This information is useful when one reuses Dowex-50 that is in the ammonium form (see below).
Regarding the rinse after the conversion (for example) from H+ to Na+, BioRad recommends 4 bed volumes of water and checking the pH. The protons on Dowex-50 should protonate your amino acid. The sodium ions will also stick. Therefore when you calculate how much resin you need, you will need to sum the moles of amino acid and the moles of NaIO4. I typically use 20-fold capacity of resin, relative to this sum, but a lower ratio may also work. You can elute with acid or with ammonia. In the presence of a sensitive group, I have seen a few authors elute with a lower concentration of ammonia than is typical (0.5 M vs. 2.0 M), but if HCl is compatible with your functional group, then it should work fine. We have used HCl in the presence of either an aldehyde or a Michael acceptor.
Babcock_Hall:
http://www.jbc.org/content/213/1/39.citation
Black and Wright, J. Biological Chemistry (1955) 213:39-50 converted allylglycine into aspartate beta-semialdehyde and then purified over Dowex, using 4 M HCl. There are probably other examples of acid elution in the literature, but my paper copies of articles on this subject are in arrears (I would start by looking up papers by Stein and Moore, who did a great deal of analytics biochemistry on amino acids and proteins many years ago). I think that 4 M is overkill and 1 M would probably work fine, but we used 4 M twice, both in the presence of a Michael acceptor. The alternative to HCl elution is elution with ammonia, which deprotonates the bound amino acid.
rolnor:
Yes, if it works its fine. I guess you have to rotavape a lot of acid, othervise it does nor matter.
Navigation
[0] Message Index
[*] Previous page
Go to full version