HOOK™-Biotin-PEG3-Amine reacts with protein carboxyl groups in the presence of the heterobifunctional cross-linker EDC. HOOK™-Biotin-Pentylamine, HOOK™-Biotin-PEG2-Amine and HOOK™-Biotin-PEG3-Amine are water soluble, due to its polyethylene glycol (PEG) spacer arm. These agents contain terminal amines and react with carboxyl groups found at the carboxyl termini, aspartate, and glutamate side chains.