GADOLINIUM-BASED LINEAR POLYMER WITH TEMPERATURE-INDEPENDENT PROTON RELAXIVITIES - A UNIQUE INTERPLAY BETWEEN THE WATER EXCHANGE AND ROTATIONAL CONTRIBUTIONS

Macromolecular complexes of Gd(III) chelates are widely investigated a s MRI contrast agents. In addition to the potential increase in relaxi vity, they have a further advantage over the Gd(III) chelates of an ex tended lifetime in the blood pool, which is necessary for magnetic res onance angiography applications. When designing macromolecular complex es of Gd(III) chelates, it is important to know how the parameters tha t determine relaxivity are affected in comparison with those of the ch elate. This paper reports variable-temperature EPR, variable-temperatu re and -pressure, multiple field O-17 NMR and variable-temperature NMR D studies on a linear Gd(DTPA-bisamide)-poly(ethylene glycol) copolyme r. The rate [k(ex)(298) = (4.8 +/- 0.1) x 10(5) s(-1)] and mechanism ( dissociatively activated) of the water exchange are identical with tho se on the corresponding chelate. The rotational correlation time (tau( R) = 232 ps) is not much longer than that of the monomer unit restrict ed 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 offse t between the opposite dependences of the outer- and inner-sphere cont ributions with temperature.