NMR Evidence of a Long Exchange Lifetime for the Coordinated Water in Ln(III)-Bis(methyl amide)-DTPA Complexes (Ln = Gd, Dy)

The magnetic field dependence of the water proton longitudinal relaxation rate for the paramagnetic complex Gd–BMA-DTPA has been measured and analyzed at different temperatures. Interestingly we have found that the solvent relaxivity is largely affected by the residence lifetime (tm) of the water molecule in the coordination site on the metal chelate. The calculated values for τ_M at 298 K is 0.81 gs, i.e. 4-fold longer than the values recently reported for Gd–DTPA and much longer than the values commonly assumed in the analysis of NMRD profiles of poly (amino carboxylate) complexes. This result has been also confirmed by an alternative approach for the structurally analogous Dy(III) complex. In this latter case, from the measurement of the paramagnetic contribution to ¹H NMR chemical shift and transverse relaxation rate of the solvent water protons, we evaluated the residence lifetime τ_M by solving iteratively the Swift–Connick equations.