Equilibrium, Kinetic and Structural Studies of AAZTA Complexes with Ga3+, In3+ and Cu2+

A detailed study of the structures, thermodynamic stabilities and kinetics of the dissociation of Ga3+, In3+ and Cu2+ complexes formed with the heptadentate ligand AAZTA is reported. The stability constants (logKML) of the AAZTA complexes of Ga3+, In3+ and Cu2+ are 22.18, 29.58 and 22.27, respectively, which suggests that the seven-membered-ring skeleton is suited to the accommodation of these metal ions. The solid-state structure of [Cu(H(2)AAZTA)]center dot H2O shows a distorted octahedral coordination. The equatorial coordination sites of Cu2+ are occupied by one of the ring N atoms, a water O atom, one of the carboxylate O atoms and the N atom of the iminodiacetate moiety. The other ring N atom and the carboxylate O atom of the iminodiacetate moiety coordinate to the Cu2+ in the axial positions. In the pH range 4.5-8.5, Ga3+ is present in the form of the highly stable [Ga(AAZTA)-OH](2-) (log beta(GaLH-1) = 17.69) The exchange reactions of [Ga(AAZTA) OH](2-) with Cu2+ and transferrin are very slow and mainly occur through the spontaneous dissociation of the complex close to physiological conditions. The half-life for the dissociation of [Ga(AAZTA) OH](2-) is t(1/2) = 23 h at pH = 7.5 and 25 degrees C in 0.025 M NaHCO3 and 0.15 M NaCl. The high conditional stability, fast formation and sufficiently slow dissociation of [Ga(AAZTA) OH](2-) represent promising properties for the complexation and diagnostic applications of radioactive Ga isotopes.