Synthesis of some N,N''-bis(amide) derivatives of diethylenetriaminepentaacetic acid and the stabilities of their complexes with Gd3+, Ca2+, Cu2+ and Zn2+

Three N,N ''-bis(amide) derivatives of H(5)dtpa (diethylenetriamine-N,N,N',N '',N ''-pentaacetic acid), H(3)L(1) = bis(isopropylamide), H(3)L(2) = bis(tert-butylamide) and H(3)L(3) = bis(benzylamide), were synthesized. Their protonation constants were determined by potentiometric titration in 0.10 mol dm(-3) KCl and by NMR pH titration at 25 +/- 0.1 degrees C. Stability and selectivity constants were measured to evaluate the possibility of using the corresponding gadolinium(III) complexes as magnetic resonance imaging contrast agents. The formation of the gadolinium(III), copper(II), zinc(II) and calcium(Ii) complexes were investigated quantitatively by potentiometry. The stability constant determined for Gd-III is larger than those for Ca-II, Zn-II and Cu-II for these octadentate ligands. The selectivity constants and modified selectivity constants of the amides for Gd3+ over endogenously available metal ions were calculated.