Gd–XO: a colourimetric probe for the complexation of Gd³⁺ with DO3A-type ligands

Small molecule Gd-based MRI contrast agents possess both thermodynamic stability and kinetic inertness, and have been used safely in the clinic. Since the kinetics of Gd³⁺-complex formation with macrocyclic ligands is slow, convenient methods to investigate their rates of formation are desirable. To this end, heptadentate macrocyclic ligands 1 and 2 were synthesized, and their binding with Gd³⁺ was investigated using the Gd³⁺–Xylenol Orange (Gd–XO) complex as a colourimetric probe. A significantly faster rate of complex formation was observed with 1 in comparison with 2, demonstrating that the hydrophobic functionality of 2 plays a role in reducing the rate of complex formation of the ligand with Gd³⁺. Other macrocyclic ligands can be conveniently assessed for their binding and kinetic properties using the methods described.