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This study proposes a novel route to improved contrast agents for magnetic resonance imaging (MRI) applications based on the formation of a non-covalent adduct between a paramagnetic complex and an exogeneous macromolecule. For this purpose a 12-membered pyridine-containing triacetate macrocyclic ligand with a p-bromobenzyloxy substituent on the pyridine moiety was synthesized.

: 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 MH 3 DO3A \ 1,4,7-triacetic acid ; 1,4,7,10-tetraazacyclododecane H 3 PCTA[12] \ 3,6,9,15-tetraazabicyclo[9.3.1]pentadecaacid ; 1(15),11,13-triene-3,6,9-triacetic H 6 PCTP[12] \ 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-trieneacidN. The observed behaviour is consistent with a hydration number q \ 2 in the 3,6,9-tris(methanephosphonic) 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 273È342 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 Ðeld dependence (NMRD proÐles) of the proton relaxivity on a KoenigÈBrown Ðeld cycling relaxometer.

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