The solution thermodynamic stability of desferrioxamine B (DFO) with Zr(IV)

Desferrioxamine B (DFO, [H4L](+) ligand) is currently the preferred chelator for Zr-89(IV), however the biological studies suggest that it releases the metal ion in vivo. Herein, we present the solution thermodynamics of complexes formed between Zr(IV) and this hexadentate chelating agent, the data surprisingly not yet available in the literature. Several techniques including electrospray ionization mass spectrometry (ESI-MS), potentiometry, UV Vis spectroscopy and isothermal titration calorimetry (ITC) were used to determine the stoichiometry and thermodynamic stability of complexes formed in solution over pH range 1-11, overcoming all the difficulties with the characterisation of the aqueous solution chemistry of Zr(IV) complexes, like strong hydrolysis and lack of spectral information. A model containing only mononuclear complexes, i.e. [ZrHL](2+) [ZrL](+), [ZrLH-1] throughout the entire measured pH range is proposed. The stability constants and pM (Zr(IV)) value determined for Zr(IV)-DFO system, place DFO among good Zr(IV) chelators, however the formation of 6-coordinate unsaturated complexes (i.e. with coordination sphere of 8-coordinate Zr(IV) completed by water molecules), together with the susceptibility of coordinated water molecule to deprotonation, are suggested to be the reason of in vivo instability of Zr-89(IV)-DFO complexes.