Combined High Resolution NMR and1H and17O Relaxometric Study Sheds Light on the Solution Structure and Dynamics of the Lanthanide(III) Complexes of HPDO3A

GdHPDO3A is one of the most used MRI contrast agents (CAs) for clinical use. However, unlike most of the other commercially available Gd-based CAs, only limited information is available on its solution structure and dynamics. 600 MHz high resolution 1H NMR spectra of nine LnHPDO3A complexes (Ln = Pr, Nd, Eu, Tb, Dy, Ho, Er, Tm, and Yb) have been recorded at 298 K and neutral pH. Because of the low symmetry of the Ln-chelates, each proton gives rise to a different peak. Despite the very crowded spectra, it is possible to detect the presence of two sets of resonances associated with different isomers in solution in slow exchange in the NMR time scale. In principle, the LnHPDO3A complexes may be present in solution as eight isomeric forms (four enantiomeric pairs) differing in the layout of the acetate arms (Δ or Λ), in the conformation of the macrocyclic ring (δδδδ or λλλλ) and in the configuration of the chiral center (R or S). 1D- and 2D proton NMR spectra were measured as a function of temperature across the Lanthanide series. The data allow identifying the nature of the most abundant isomeric species in solution (e.g., Λ(λ λλλ)-R/Λ(δδδδ)-R and their enantiomeric forms Δ(δδδδ)-S/ Δ(λλλλ)-S) and their interconversion process. Analysis of the data led us to identify the presence in solution of a third isomeric species, lacking the coordinated water molecule (q = 0), whose population becomes more relevant for the heavier lanthanides (Ln = Er-Lu). Moreover, we have introduced an innovative way of modeling the thermodynamic equilibrium between the various isomeric forms of LnHPDO3A that can be extended to a number of other systems. This analysis enabled us to calculate the molar fractions of the two isomeric forms for GdHPDO3A (χ = 0.7 and 0.30, for SAP and TSAP, respectively). This information has allowed interpreting the slightly anomalous relaxometric properties of GdHPDO3A. In particular, we observed that the temperature dependence of the 17O NMR transverse relaxation rate of GdHPDO3A, R2, reveals an unusual trend at low temperatures and at high magnetic field strength (>9.4 T). This behavior has been attributed to the occurrence of a very large difference in the rate of water exchange, k ex, for the two isomeric species (1/kex = τM = 640 ± 35 ns and 8.9 ± 0.5 ns, for the major and minor isomer respectively).