HOOK™ Biotin-PEG₂-Amine, and its long chain form, HOOK™ Biotin-PEG₃-Amine, are carboxyl reactive biotinylation reagents. These agents contain terminal amines and react with carboxyl groups found at the carboxyl termini, aspartate, and glutamate side chains. The reaction is mediated by a water-soluble carbodiimide (EDC). The carbodiimide activates the carboxyl group and reacts with the amines (-NH₂) on the biotinylation agent to form an amide bond. This reaction is rapid and takes just a few minutes to complete. Under these conditions, hydrazide-derivatives of biotin reagents may also react with the carboxyls.