>> Hi. I am having some problems purifying bromo and iodo DNA
> oligomers (10 and 12 mers). During purification by anion exchange
> in 7M urea with KCl gradient,
> I usually see 2-3 significant peaks which overlap. I don't see these
> peaks when purifying non-modified DNA. What are the components of
> these peaks? And what is a good way to identify the correct peak?
As there is a difference in pKa between halogenated bases and
unmodified bases, what you are seeing is the resolution of oligos that
have lost the halogen (photo-labile) and the appropriately modified
DNA. On an anion exchange resin, the halogenated DNA should be
retained the best (ie. it corresponds to the last peak in your
Try a shallower gradient to resolve the peaks further.