Supramolecular Assembly of an Amphiphilic GdIII Chelate: Tuning the Reorientational Correlation Time and the Water Exchange Rate

Abstract

We report the synthesis and characterization of the novel ligand H5EPTPA-C16 ((hydroxymethylhexadecanoyl ester)ethylenepropylenetriaminepentaacetic acid). This ligand was designed to chelate the GdIII ion in a kinetically and thermodynamically stable way while ensuring an increased water exchange rate (kex) on the GdIII complex owing to steric compression around the water-binding site. The attachment of a palmitic ester unit to the pendant hydroxymethyl group on the ethylenediamine bridge yields an amphiphilic conjugate that forms micelles with a long tumbling time (tauR) in aqueous solution. The critical micelle concentration (cmc = 0.34 mM) of the amphiphilic [Gd(eptpa-C16)(H2O)]2- chelate was determined by variable-concentration proton relaxivity measurements. A global analysis of the data obtained in variable-temperature and multiple-field 17O NMR and 1H NMRD measurements allowed for the determination of parameters governing relaxivity for [Gd(eptpa-C16)(H2O)]2-; this is the first time that paramagnetic micelles with optimized water exchange have been investigated. The water exchange rate was found to be = 1.7×108 s-1, very similar to that previously reported for the nitrobenzyl derivative [Gd(eptpa-bz-NO2)(H2O)]2- ( = 1.5×108 s-1). The rotational dynamics of the micelles were analysed by using the Lipari-Szabo approach. The micelles formed in aqueous solution show considerable flexibility, with a local rotational correlation time of = 330 ps for the GdIII segments, which is much shorter than the global rotational correlation time of the supramolecular aggregates, = 2100 ps. This internal flexibility of the micelles is responsible for the limited increase of the proton relaxivity observed on micelle formation (r1 = 22.59 mM-1 s-1 for the micelles versus 9.11 mM-1 s-1 for the monomer chelate (20 MHz; 25 °C)).