The kinetics of exchange between a lanthanide ion and the gadolinium complex of N,N''-bis(2-methoxyethylamide-carbamoylmethyl)diethylenetriamine-N,N',N''-triacetate

The exchange kinetics of Gd(DTPA-BMEA) with the radioactive tracer Eu(III) have been studied in aqueous solution of 0.15 M ionic strength and 25 degrees C. The exchange is a first-order reaction with the rate expression containing both acid-dependent and acid-independent terms. At pH > 6, the exchange is governed by direct reaction of Eu(III) ions with Gd(DTPA-BMEA), while below pH 6, the rate determining step is proton-catalyzed dissociation of the complex with a first-order dependence on [H+]. The relatively slow association and dissociation of the Ln(DTPA-BMEA) complexes can be attributed to steric effects related to the binding of the two methoxyethylamide groups.