Relative Ligand Exchange Rates in Gd‐based MRI Contrast Agent Formation as Probed by Gd–XO Complex

A novel methodology involving the use of Gd³⁺–Xylenol Orange (Gd–XO), a solution colorimetric probe, was used to determine the conditional relative rates of complex formation between the Gd³⁺ ion and polyamino carboxylate ligands used in MRI contrast agent development. Among the ligands tested, the order of rate of Gd‐complex formation was found to be DTPA ≈ DOTP > DOTA > DO3A > DOTAM. The observed rates are an effect of ligand denticity, backbone structure, and nature of the ligand donor groups.