Gadolinium-based linear polymer with temperature-independent proton relaxivities: a unique interplay between the water exchange and rotational contributions
Résumé
Macromolecular complexes of Gd(III) chelates are widely investigated as MRI contrast agents. In addition to the potential increase in relaxivity, they have a further advantage over the Gd(III) chelates of an extended lifetime in the blood pool, which is necessary for magnetic resonance angiography applications. When designing macromolecular complexes of Gd(III) chelates, it is important to know how the parameters that determine relaxivity are affected in comparison with those of the chelate. This paper reports variable‐temperature EPR, variable‐temperature and ‐pressure, multiple field 17O NMR and variable‐temperature NMRD studies on a linear Gd(DTPA–bisamide)–poly(ethylene glycol) copolymer. The rate [kex298=(4.8±0.1)×105 s‐1] and mechanism (dissociatively activated) of the water exchange are identical with those on the corresponding chelate. The rotational correlation time (τR=232 ps) is not much longer than that of the monomer unit restricted to rotate around a single axis, indicating large flexibility of the ethylene glycol chain. The proton relaxivities of the linear polymer complex are virtually independent of temperature, a result of an offset between the opposite dependences of the outer‐ and inner‐sphere contributions with temperature. ©1998 John Wiley & Sons, Ltd.