The molecular structure of indium-DTPA

The x-ray structural characterization of Na2In(DTPA).7 H2O is described. The structure is notable for two reasons. It is the first In3+-complex with relevance to antibody attachment to be structurally characterized. Surprisingly, the complex has coordination number eight and is the first monomeric In3+-complex with such a high coordination number. The potentially octadentate ligand binds through three amino groups and five deprotonated carboxylate groups to the metal ion. The ligand atoms surround the central indium atom in a slightly distorted Archimedian antiprismatic configuration. High resolution nuclear magnetic resonance studies in solution show that this high coordination number also pertains in aqueous medium. Consequently, one may conclude that octadentate ligands are optimal for the thermodynamic and kinetic stabilization of In3+ in living systems. It is also indicated that bifunctional diethylenetriaminepentaacetic acid (DTPA) is superior to amide bound (= sevendentate) DTPA in antibody labeling.