Pentetic acid

Coordination properties

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. For example, in its complex with copper(II), DTPA binds in a hexadentate manner utilizing the three amine centres and three of the five carboxylates.

Applications

Like the more common EDTA, DTPA is mainly used for sequestering metal ions that otherwise decompose hydrogen peroxide, which is used to bleach pulp in paper making. Several million kilograms are produced for this purpose annually.

Its chelating properties are useful in deactivating calcium and magnesium ions in hair products. DTPA is used in over 150 cosmetic products. Additionally, DTPA is used in MRI contrasting agents. DTPA improves MRI images by forming a complex with a gadolinium ion, which alters the properties of nearby water molecules.

DTPA has been considered for treatment of radioactive materials such as plutonium, americium, and other actinides. In theory, these complexes are more apt to be eliminated in urine. It is normally administered as the calcium or zinc salt, since these ions are readily displaced by more highly charged cations. DTPA forms complexes with thorium(IV), uranium(IV), neptunium(IV), and cerium(III/IV).

In August, 2004 the US Food and Drug Administration (USFDA) determined zinc-DTPA and calcium-DTPA to be safe and effective for treatment of those who have breathed in or otherwise been contaminated internally by plutonium, americium, or curium. The recommended treatment is for an initial dose of calcium-DTPA, as this salt of DTPA has been shown to be more effective in the first 24 hours after internal contamination by plutonium, americium, or curium. After that time has elapsed both calcium-DTPA and zinc-DTPA are similarly effective in reducing internal contamination with plutonium, americium or curium, and zinc-DTPA is less likely to deplete the body's normal levels of zinc and other metals essential to health. Each drug can be administered by nebulizer for those who have breathed in contamination, and by intravenous injection for those contaminated by other routes.

DTPA is also used as a chelate for aquarium plant fertilizer, specifically iron, an essential micronutrient typically needed in substantial quantities by all plants. Chelates are dissolved organic substances that bind to metals and prevent them from forming larger molecules through oxidation. FeDTPA is often sold under the name iron chelate 10% or 11% when used for the purpose of aquarium plant fertilization. Iron typically found in the aquarium water column has been converted into the ferric state (Fe³⁺) since it is in the presence of dissolved oxygen. However plants require iron in the ferrous state (Fe²⁺), therefore additional energy must be expended in order to extract the ferric iron from the water column and convert it to the ferrous form. When used to chelate iron fertilizer DTPA ensures that the iron is kept in the ferrous state (Fe²⁺) over time so it can be utilized by aquatic plants without expending valuable energy.

Related compounds

Compounds that are structurally related to DTPA are used in medicine, taking advantage of the high affinity of the triaminopentacarboxylate scaffold for metal ions.