Fine-tuning water exchange on Gd-III poly(amino carboxylates) by modulation of steric crowding

In the objective of optimizing water exchange rate on stable, nine-coordinate, monohydrated Gd-III poly(amino carboxylate) complexes, we have prepared monopropionate derivatives of DOTA(4-) (DO3A-Nprop(4-)) and DTPA(5-)(DTTA-Nprop(5-)). A novel ligand, EPTPA-BAA(3-), the bisamylamide derivative of ethylenepropylenetriamine-pentaacetate (EPTPA(5-)) was also synthesized. A variable temperature O-17 NMR study has been performed on their Gd-III complexes, which, for [Gd(DTTA-Nprop)(H2O)](2-) and [Gd(EPTPA-BAA)(H2O)] has been combined with multiple field EPR and NMRD measurements. The water exchange rates, k(ex)(298), are 8.0x10(7) s(-1), 6.1x10(7) s(-1) and 5.7x10(7) s(-1) for [Gd(DTTA-Nprop)(H2O)](2-), [Gd( DO3A-Nprop)(H2O)](-) and [Gd(EPTPA-BAA)(H2O)], respectively, all in the narrow optimal range to attain maximum proton relaxivities, provided the other parameters (electronic relaxation and rotation) are also optimized. The substitution of an acetate with a propionate arm in DTPA(5-) or DOTA(4-) induces increased steric compression around the water binding site and thus leads to an accelerated water exchange on the Gd-III complex. The k(ex) values on the propionate complexes are, however, lower than those obtained for [Gd(EPTPA)(H2O)](2-) and [Gd(TRITA)(H2O)](-) which contain one additional CH2 unit in the amine backbone as compared to the parent [Gd(DTPA)(H2O)](2-) and [Gd(DOTA)(H2O)](-). In addition to their optimal water exchange rate, [Gd(DTTA-Nprop)(H2O)](2-) has, and [Gd(DO3A-Nprop)(H2O)](-) is expected to have sufficient thermodynamic stability. These properties together make them prime candidates for the development of high relaxivity, macromolecular MRI contrast