To functionalize a target

We typically use a biotinylated NHS ester that will bind to amines (R-N-H2 groups). Pierce makes the EZ-Link NHS-SS-Biotin. For this to work, you must have a target with an N-terminal amine (i.e every polypeptide) or a protein with an amino acid that contains amines (i.e. cysteine). The problem with functionalizing proteins this way is that you have to have the ratio of reagent to target just right to ensure you are attaching one or two biotins to the protein without wasting to much non-functionalized protein or biotinlyating the crap out of it. In addition, it can be difficult to assay the concentration properly.