Synthesis and complexation of Gd3+, Ca2+, Cu2+ and Zn2+ by 3,6,10-tri(carboxymethyl)-3,6,10-triazadodecanedioic acid

Abstract

3,6,10-Tri(carboxymethyl)-3,6,10-triazadodecanedioic acid (H₅L), was synthesized and its protonation constants were determined by potentiometric titration in 0.10 mol dm^–3 Me₄NNO₃ and by NMR pH titration at 25.0 ± 0.1 °C. Stability and selectivity constants have been measured to evaluate the possibility of using the corresponding gadolinium(III) complex as a magnetic resonance imaging contrast agent. The formations of gadolinium(III), copper(II), zinc(II) and calcium(II) complexes were investigated quantitatively by potentiometry. The stability constant for the gadolinium(III) complex is larger than those of Ca^II, Zn^II and Cu^II for this octadentate ligand. The selectivity constants and modified selectivity constants of the ligand for Gd³⁺ over endogenously available metal ions were calculated. The spin–lattice relaxivity R₁ for the gadolinium(III) complex was also determined. It was found to decrease with increasing pH below 4 and became invariant with respect to pH over the range 4–10. Oxygen-17 NMR shifts showed that the [DyL]^2– complex had one inner-sphere water molecule. The water proton spin–lattice relaxation rate for the [GdL]^2– complex was also consistent with one inner-sphere co-ordination position.