How Does Internal Motion Influence the Relaxation of the Water Protons in LnIIIDOTA-like Complexes?
Taking advantage of the Curie contribution to the relaxation of the protons in the Tb(III) complex, and the quadrupolar relaxation of the 17O and 2H nuclei on the Eu(III) complex, the effect of the internal motion of the water molecule bound to [Ln(DOTAM)(H2O)]3+ complexes was quantified. The determination of the quadrupolar coupling constant of the bound water oxygen X_O(1 + eta_O^2/3)1/2 = 5.2 ± 0.5 MHz allows a new analysis of the 17O and 1H NMR data of the [Gd(DOTA)(H2O)]- complex with different rotational correlation times for the Gd(III)-Owater and Gd(III)-Hwater vectors. The ratio of the rotational correlation times for the Ln(III)-Hwater vector and the overall rotational correlation time is calculated tauRH/tauRO = 0.65 ± 0.2. This could have negative consequences on the water proton relaxivity, which we discuss in particular for macromolecular systems. It appears that the final effect is actually attenuated and should be around 10% for such large systems undergoing local motion of the chelating groups.