Posted 04 January 2011 - 06:08 AM
It is generally possible to add a label to any protein even after purification. There are a number of vendors that sell labeling kits (Invitrogen and Pierce come to mind, but no doubt there are others as well) for flourescently labeling your protein. The only thing that could seriously hinder labeling is if your protein were in a Tris buffer, because the majority of labeling reactions go via the amino groups. Because Tris is an amine it acts as a decoy for the labeling and hence lowers the yield of successfully labeled protein dramatically. If this is the case you can dialyze your protein and you should be OK.