Lipari-Szabo approach as a tool for the analysis of macromolecular gadolinium(III)-based MRI contrast agents illustrated by the [Gd(EGTA-BA-(CH2)(12))](n)(n+) polymer

The parameters governing the water proton relaxivity of the [Gd(EGTA-BA-(CH 2)(12))](n)(n+) polymeric complex were determined through global analysis o f O-17 NMR, EPR and nuclear magnetic relaxation dispersion (NMRD) data [EGT A-BA(2-)=3,12-bis(carbamoylmethyl)-6,9-dioxa-3,12-diazatetradecanedioate(2- )]. The Lipari-Szabo approach that distinguishes the global motion of the p olymer (tau (g)) from the local motion of the Gd(III)-water vector (tau (1) ) was necessary to describe the H-1 and O-17 longitudinal relaxation rates: therefore for the first time it was included in the global simultaneous an alysis of the EPR,O-17 NMR and NMRD data. The polymer consists on average o f only five monomeric units, which limits the intramolecular hydrophobic in teractions operating between the (CH,),, groups. Hence the global rotationa l correlation time is not very high (tau (298)(g)=3880+/-750ps) compared to the corresponding DTPA-BA-based polymer (about 15 monomeric units), where tau (298)(g)=6500 ps. AS a consequence, the relaxivity is limited by the ro tation, which precludes the advantage obtained from the fast exchanging che lating unit (k(ex)(298)=2.2+/-0.1x10(6) s(-1)).