Relaxometric Study of a Series of Monoaqua GdIII Complexes of Rigidified EGTA-Like Chelators and Their Noncovalent Interaction with Human Serum Albumin

The lanthanide(III) complexes of five EGTA [ethyleneglycol-bis(2-aminoethyl) ether-N,N,N′,N′-tetraacetic acid] derivatives fused with aromatic (benzene: L1; naphthalene: L2) and functionalized aromatic (L3–L5) moieties on the oxyethylene bridge were investigated in aqueous solution. 1H and 13C NMR spectra of the LaIII and LuIII complexes of L1 and L2 indicate that the structural differences across the Ln series found for the [Ln(EGTA)]– complexes is maintained in the new derivatives. The presence of the aromatic moiety favours an increased stereochemical rigidity, as assessed by VT 13C NMR spectra. Temperature-dependent 17O NMR spectroscopic data and 1/T11H NMRD profiles of the GdIII complexes of L1–L3 were measured and globally analyzed to obtain the parameters influencing the water exchange rate, kex, and rotational dynamics. The kex values are ca. twice as high as the corresponding value of [Gd(EGTA)]– and very close to optimal. The lipophilic complexes [GdL4]– and [GdL5]– form micellar aggregates whose NMRD profiles were measured and analyzed in terms of a model that allows separation of global motion and local rotation. The noncovalent binding interaction of the complexes to human serum albumin and the relaxivity of the macromolecular conjugates were investigated with relaxometric techniques at 20 MHz and 298 K, and the importance of a restricted local motion of the complex at the binding site for attaining large relaxivity enhancement outlined. Theoretical docking simulations and relaxometric competition experiments provided insights into the binding modes of the complexes.