Gd(III) based MRI contrast agents: improved physical meaning in a combined analysis of EPR and NMR data?

A new global analysis of EPR, 17O NMR relaxation and chemical shift and 1H NMRD profiles with physically meaningful parameters for [Gd(DOTA)(H2O)]− and for [Gd(DTPA)(H2O)]2− in aqueous solution is presented (DOTA=1,4,7,10-tetraaza-1,4,7,10-tetrakis(carboxymethyl)-cyclododecane; DTPA=diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid). The recent developments of an improved EPR relaxation theory, the inclusion of the internal motion of the bound water molecule are the principal modifications. Furthermore the better knowledge of the quadrupolar coupling constant of the bound water molecule, the neglect of the outer-sphere contribution to the chemical shift and the consideration of different isomers for the DOTA complex allowed for an improved analysis. The water exchange and parameters of rotational motion are only slightly changed. Comparison of the contributions of static zero-field-splitting shows that a more symmetric environment of the Gd(III) ion should lead to slower electron spin relaxation, a feature which can become important if all other parameters (rotational correlation time and water exchange rate) are optimised. In the actual stage the improved combined analysis of Gd(III) poly(amino carboxylate) is limited by the approximations of Redfield’s relaxation theory, i.e., very low frequency NMRD-data and slowly tumbling complexes cannot be analysed with the method presented.