Relaxivity Studies on a Gadolinium(III) Complex of a Macrocyclic DTPA Derivative

Abstract

The dinuclear Gd3+ complex of the 30-membered cyclic DTPA derivative, cy(DTPA-en-DTPA-en), was studied by various NMR spectroscopic techniques in order to evaluate the parameters governing its relaxivity. The values obtained were compared with those for Gd(DTPA-BMA), which is an acyclic mononuclear analogue. Because of the increase in molecular weight the relaxivity r1 =5.25 s−1mM−1 (20 MHz, 37 °C) is higher than that of the monomeric analogue (4.2 s−1mM−1). The rotational correlation time at 298 K, τR298 was determined to be 175−188 ps, which is significantly higher than that expected, based on the increase in the molecular volume of the acyclic compared to the cyclic compound. Possibly, the cyclic nature of the ligand induces steric constraints in Gd[cy(DTPA-en-DTPA-en)], which reduce the local mobility. The water exchange rate for this complex (kex298 = 2.9 × 105 s−1) is somewhat smaller than that of Gd(DTPA-BMA) (kex298 = 4.5 × 105 s−1), which suggests that the water exchange rates are different in the various isomers of acyclic Gd-DTPA-bisamides.