Amphiphilic Ditopic Bis-Aqua Gd-AAZTA-like Complexes Enhance Relaxivity of Lipidic MRI Nanoprobes

Two amphiphilic mono- and dimeric GdAAZTA-like chelates composed of stable bis-aquo Gd^III complexes (q=2) linked to one (for the monomer) or two dodecyl aliphatic chains (for the dimer) were synthesized. Both chelates showed high relaxivity when incorporated into the lipid bilayer of liposomes or after interaction with human serum albumin (HSA). The ditopic complex shows a significantly decreased internal motion relative to the monomeric complex, associated with an enhanced relaxivity (r₁≈60 mm⁻¹ s⁻¹, at 30 MHz and 310 K). The presence of two metal-bound water molecules in fast exchange and the restricted rotational freedom make the relaxivity of this system the highest measured for paramagnetic liposomes.