1H AND 17O NMR RELAXATION STUDIES OF THE AQUA IONS OF GdIII, MnII AND FeIII

A detailed re-examination was carried out on the relaxometric properties of the aqua ions of GdIII, MnII and FeIII, by measuring 1H longitudinal relaxation data from 0.01 up to 500 MHz and 17O transverse relaxation rates (R2) and shift () at 11.7 T, as a function of temperature. The availability of (i) a wider range of magnetic fields, (ii) relaxation data as a function of temperature and (iii) 17O shift and relaxation data measured at high magnetic field strength enable to obtain very accurate value of the structural and dynamic parameters that adequately describe the behaviour of the paramagnetic complexes in solution. A thorough and accurate knowledge is preliminary and necessary for research on contrast enhancing agents for clinical and pre-clinical MRI. The efficiency of a contrast agent is evaluated in vitro in terms of its relaxivity, which represents the relaxation rate enhancement of water proton nuclei per mM concentration of the paramagnetic ion. Relaxivity depends upon a large number of parameters that can hardly be determined by analyzing relaxivity data alone. Indeed, variable temperature 17O NMR measurements of R2 and Dw constitute a valuable tool to investigate the parameters influencing relaxivity. 17O NMR data provide information on the water exchange kinetics of the complex, and depend on the hyperfine coupling constant AO/ħ between the electron spin of the metal ion and the 17O nuclear spin. Additionally, the Nuclear Magnetic Relaxation Dispersion (NMRD) profiles recorded over a wide range of frequencies allows obtaining an accurate description of the rotational dynamics. The occurrence in solution of hydration equilibria has also been explicitly considered.