Relaxometric and modelling studies of the binding of a lipophilic Gd-AAZTA complex to fatted and defatted human serum albumin

A new lipophilic gadolinium chelate consisting of a long aliphatic chain bound to the AAZTA coordination cage (Gd-AAZTAC17) has been synthesised. It possesses two coordinated water molecules (q = 2) in fast exchange with the solvent (τ_M²⁹⁸ = 67 ns), which yields a relaxivity of 10.2 mM^-1 s^-1. At concentrations greater than 0.1 mM, it forms micelles (average diameter 5.5 nm) characterised by a relaxivity of approximately 30 mM^-1 s^-1 at 20 MHz and 298 K. The latter value appears to be "quenched" by magnetic interactions among the Gd^III ions on the surface of the micelle that cause a decrease in the electronic relaxation time. A relaxivity of 41 mM^-1 s^-1 was recorded for this micellar system when 98% of the Gd^III ions were replaced by diamagnetic Y^III. Gd-AAZTAC17 exhibits a better affinity for fatted human serum albumin (HSA) than for defatted HSA, whereas the relaxivities of the supramolecular adducts are reversed. The relaxivity shown by Gd-AAZTAC17/defatted HSA (r₁^b(20 MHz, 298 K) = 84 mM ^-1 s^-1) is by far the highest relaxivity reported so far for non-covalent paramagnetic adducts with slow-moving substrates. As shown by molecular docking calculations, the gadolinium complex enters a hydrophobic pocket present in fatted HSA more extensively than the corresponding adduct with defatted HSA. Interestingly, no marked difference was observed in either the relaxation enhancement or the binding affinity between fatted and defatted HSA when the binding titrations were carried out at a Gd-AAZTAC17 concentration higher than its critical micellar concentration (cmc). This behaviour has been attributed to the formation of an association between the negatively charged micelle of the lipophilic metal complexes and the positive residues on the surface of the protein.