Comparison of Solution Properties of Polymethylated DOTA-like Lanthanide Complexes with Opposite Chirality of the Pendant Arms

Polymethylated lanthanide 4 S 4 R -M4DOTMA complexes, bearing the ring methyl groups oriented in the SSSS position and the arm methyl groups in the RRRR configuration, exist exclusively as the SAP [Λ(δδδδ)] isomer in solution throughout the lanthanide series. This observation is in contrast to Ln-8 S -M4DOTMA, which was recently reported to adopt the SAP [Λ(δδδδ)] isomer in the early lanthanides, while the late lanthanides adopt the TSAP [Δ(δδδδ)] isomer. The methyl groups on the ring and the arm are both oriented in the SSSS configuration for Ln-8 S -M4DOTMA ( Dalton Trans. 2016 , 45 , 4673 - 4687 , DOI: 10.1039/C5DT03210E ). Quantum chemical calculations for Pr- and Yb-4 S 4 R -M4DOTMA indicate that the SAP isomer is significantly more stable. The luminescence profiles of Eu-8 S -M4DOTMA and Eu-4 S 4 R -M4DOTMA showed similar profiles signifying identical coordination environments. The hydration state, q , of the Eu(III) complexes was q = 0.91-0.95, while Tb-8 S -M4DOTMA had q = 0.86. A much lower q value was obtained for Tb-4 S 4 R -M4DOTMA ( q = 0.67), which indicates an elongation of the Ln-Ow bond. At 400 MHz, the relaxivity of Gd-8 S -M4DOTMA is 5.1 ± 0.1 mM -1 s -1 and 3.9 ± 0.1 mM -1 s -1 at 25 and 37 °C, respectively, whereas the relaxivity of Gd-4 S 4 R -M4DOTMA is 4.6 ± 0.1 mM -1 s -1 at 25 °C and 3.6 ± 0.1 mM -1 s -1 at 37 °C. At 45 MHz, the relaxivity of Gd-8 S -M4DOTMA is 5.4 ± 0.1 mM -1 s -1 , and the relaxivity of Gd-4 S 4 R -M4DOTMA is 4.5 ± 0.1 mM -1 s -1 at 25 °C. The temperature dependence of the 17 O NMR transverse relaxation rate of the Gd complexes revealed a 7-fold increase in the bound water residence lifetime of Gd-8 S -M4DOTMA (1/ k ex = τ M = 9.0 ± 0.5 ns and 1/ k ex = τ M = 60 ± 3 ns). The Pr(III) complex of 8 S -M4DOTMA crystallized as TSAP isomer with an apical water. The presence of the apical water for the TSAP of Pr-8 S -M4DOTMA was further confirmed with the observation that the fluoride ion replaces the bound water from the TSAP isomer of Pr-8 S -M4DOTMA. This was shown by the disappearance of the TSAP peaks and appearance of a new set of less shifted resonances, which exchange with the SAP isomer as confirmed by NMR exchange spectroscopy (EXSY).