High-resolution NMR and relaxometric studies of Ln(III) complexes of relevance to MRI

The solution structure and dynamics and the relaxation properties of the lanthanide(III) complexes with ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA) are studied by NMR techniques. The longitudinal water proton relaxation rates as a function of the magnetic field in a solution of Gd(EGTA)^- are investigated and analysed. The experimental results are compared with existing data for other linear and macrocyclic complexes and interpreted as indicative of the presence of a single coordinated water molecule and of an unusual symmetry and stereochemical rigidity of the coordination cage. Variable temperature high resolution ¹H and ¹³C NMR spectra for the Eu(III), Ho(III) and Yb(III) complexes confirm the above hypothesis and suggest a dodecahedral geometry for the complexes as well as a marked increase of rigidity along the lanthanide series.