Pentetic acid is an aminopolycarboxylic acid consisting of a diethylenetriamine backbone with five carboxymethyl groups. The molecule can be viewed as an expanded version of EDTA and is used similarly. It is a white, water-soluble solid. The conjugate base of DTPA has a high affinity for metal cations. Thus, the penta-anion DTPA5- is potentially an octadentate ligand assuming that each nitrogen centre and each COO–-group counts as a centre for coordination. The formation constants for its complexes are about 100 greater than those for EDTA. As a chelating agent, DTPA wraps around a metal ion by forming up to eight bonds. Transition metals, however, usually form less than eight coordination bonds. So, after forming a complex with a metal, DTPA still has the ability to bind to other reagents, as is shown by its derivative pendetide.
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