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Keywords:

  • Gallium;
  • Indium;
  • Copper;
  • Complexation;
  • Thermodynamics;
  • Kinetics­;
  • Radiopharmaceuticals;
  • Imaging agents

Abstract

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 (log KML) 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(H2AAZTA)]·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 βGaLH–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 t1/2 = 23 h at pH = 7.5 and 25 °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.