NMR relaxometric studies of Gd(III) complexes with heptadentate macrocyclic ligands

The water ¹H and ¹⁷O NMR relaxation properties of solutions containing Gd(III) chelates of the heptadentate DO3A, PCTA[12] and PCTP[12] ligands were thoroughly investigated and the results obtained are compared with those previously reported for other Gd(III) complexes with octadentate ligands {H₃DO3A = 1,4,7,10-tetraazacyclododecane 1,4,7-triacetic acid; H₃PCTA[12] = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid; H₆PCTP[12] = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9- tris(methanephosphonic) acid}. The observed behaviour is consistent with a hydration number q = 2 in the case of GdDO3A and GdPCTA[12] and q = 1 in the case of PCTP[12]. The high relaxivity of the latter complex is accounted for in terms of the occurrence of an additional contribution arising from water molecules tightly bound to the phosphonate moieties on the surface of the paramagnetic chelate. Furthermore, it was found that the decreased relaxation rates observed at basic pH in the case of GdDO3A and GdPCTA[12] can probably be ascribed to a partial decrease in their hydration. The measurement of ¹⁷O NMR transverse relaxation rates, in the temperature range 273-342 K, allowed the assessment of the water exchange rate between the coordination site and the bulk solvent. A particularly short exchange lifetime was measured for the octacoordinate GdPCTP[12], which suggests the occurrence of an associative exchange mechanism. Further insights into the understanding of the structural properties of the three complexes were gained by measuring the magnetic field dependence (NMRD profiles) of the proton relaxivity on a Koenig-Brown field cycling relaxometer.