Multinuclear and multifrequency NMR study of gadolinium(III) complexes with bis-amide derivatives of ethylenedioxydiethylenedinitrilotetraacetic acid

New cationic gadolinium complexes have been synthesized and their H-1 and O -17 relaxometric properties investigated in view of their possible use as M RI contrast agents. The reaction of the bicyclic anhydride of ethylenedioxy diethylenedinitrilotetraacetic acid (H(4)egta) with NH3, isobutylamine and phenylpropylamine gave the octadentate bis-amide ligands 1-3 in high yields . The gadolinium(III) complexes were prepared from GdCl3 and the acid form of the ligands. The stability constants of the complexes were obtained from potentiometric data and are about five orders of magnitude lower than for the parent [Gd(egta)](-) complex. The magnetic field dependence of the prot on relaxivity at 6, 25 and 39 degrees C was quantitatively analysed in orde r to obtain the relaxation parameters and indicated that one water molecule occupies a site of the co-ordination sphere of the complexes. The temperat ure dependence of the relaxivity, measured at 20 MHz, suggested that the wa ter exchange rate, k(ex) = 1/tau(M), is sensibly reduced compared to that o f the parent compound. O-17 NMR data were acquired at 2.1 T and allowed an accurate estimation of tau(M) which, at 298 K, resulted to be 230, 133 and 211 ns for Gd.1, Gd.2 and Gd.3 respectively. The two complexes bearing hydr ophobic residues on the amide nitrogens form non-covalent adducts with seru m albumin whose relaxivity is limited by the water exchange rate.