Equilibrium and NMR Relaxometric Studies on the s-Triazine-Based Heptadentate Ligand PTDITA Showing High Selectivity for Gd3+ Ions

A complete potentiometric and NMR relaxometric solution study on the heptadentate 2,2',2 '',2'''-[(6-piperidinyl-1,3,5-triazine-2,4-diyl)dihydrazin-2-yl-1-ylidene]tetraacetic acid (PTDITA) ligand has been carried out. This ligand is based on the 1,3,5-triazine ring with two hydrazine-N,N-diacetate groups in positions 2 and 4 and a piperidine moiety in position 6. The introduction of the triazine ring into the ligand backbone is expected to modify its flexibility and then to affect the stability of the corresponding complexes with transition-metal and lanthanide ions. Thermodynamic stabilities have been determined by pH potentiometry, UV spectrophotometry, and H-1 NMR spectroscopy for formation of the complexes with Mg2+, Ca2+, Cu2+, Zn2+, La3+, Gd3+, and Lu3+ ions. PTDITA shows a good binding affinity for Gd3+ (logK = 18.49, pGd = 18.6) and an optimal selectivity for Gd3+ over the endogenous Ca2+, Zn2+, and Cu2+ (K-sel = 6.78 x 10(7)), which is 3 orders of magnitude higher that that reported for Gd(DTPA) (K-sel = 2.85 x 10(4)). This is mainly due to the lower stability of the Cu-II- and Zn-II(PTDITA) complexes compared to the corresponding DTPA complexes, which suggests an important role of the triazine ring on the selectivity for the Gd3+ ion. The relaxometric properties of Gd(PTDITA) have been investigated in aqueous solution by measuring the H-1 relaxivity as a function of the pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion profile). Variable-temperature O-17 NMR data have provided direct information on the kinetic parameters for exchange of the coordinated water molecules. A simultaneous fit of the data suggests that the high relaxivity value (r(1) = 10.2 mM(-1) s(-1)) is a result of the presence of two inner-sphere water molecules along with the occurrence of relatively slow rotation and electronic relaxation. The water residence lifetime, (298)tau(M) = 299 ns, is quite comparable to that of clinically approved magnetic resonance imaging contrast agents. The displacement of the inner-sphere water molecules by bidentate endogeneous anions (citrate, phosphate, and carbonate) has also been evaluated by H-1 relaxometry. In general, the binding interaction is markedly weak, and only in the case of citrate, a ca. 35% decrease in relaxivity was observed in the presence of 60 equiv of the anion. Phosphate and carbonate also interact with the paramagnetic ion, likely as monodentate ligands, but formation of the ternary complex is accompanied by a modest increase of r(1) due to the contribution of second-sphere water molecules.