New complexing agents for labeling of proteins with metals

The applicability of bifunctional chelating agents in the labeling of proteins was analysed by using Eu-labeled antibodies as a model system. The requirements set for chelate stability are defined mainly by the system and metals used. The chelates used for in vitro applications cannot necessarily be utilized in in vivo experiments in which the competing chelating agents and metals set great demands on the inertness of the chelates against ligand changes. An isothiocyanatobenzyl derivative of diethyelenetriaminetetraacetic acid showed excellent stability in the conditions under investigation. The coupling reaction is mild and allows simple addition of metals within or after the coupling of the chelator to the protein, which makes it a good choice for the labeling of antibodies with metals like Gd3+.