Given the practical advantages of the 68Ga isotope in positron emission tomography applications, gallium complexes are gaining increasing importance in biomedical imaging. However, the strong tendency of Ga 3+ to hydrolyze and the slow formation and very high stability of macrocyclic complexes altogether render Ga3+ coordination chemistry difficult and explain why stability and kinetic data on Ga3+ complexes are rather scarce. Here we report solution and solid-state studies of Ga3+ complexes formed with the macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, (DOTA)4-, and its mono(n-butylamide) derivative, (DO3AMBu)3-. Thermodynamic stability constants, log K(GaDOTA) = 26.05 and log K(GaDO3AM Bu) = 24.64, were determined by out-of-cell pH-potentiometric titrations. Due to the very slow formation and dissociation of the complexes, equilibration times of up to ∼4 weeks were necessary. The kinetics of complex dissociation were followed by 71Ga NMR under both acidic and alkaline conditions. The GaDOTA complex is significantly more inert (τ1/2 ∼12.2 d at pH = 0 and τ1/2 ∼6.2 h at pH = 10) than the GaDO3AMBu analogue (τ1/2 ∼2.7 d at pH = 0 and τ1/2 ∼0.7 h at pH = 10). Nevertheless, the kinetic inertness of both chelates is extremely high and approves the application of Ga3+ complexes of such DOTA-like ligands in molecular imaging. The solid-state structure of the GaDOTA complex, crystallized from a strongly acidic solution (pH < 1), evidenced a diprotonated form with protons localized on the free carboxylate pendants.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry