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

ABSTRACT: Polymethylated lanthanide 4S4R-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-8S-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-8S-M4DOTMA ( Dalton Trans. 2016 , 45 , 4673 - 4687 , DOI: 10.1039/C5DT03210E ). Quantum chemical calculations for Pr- and Yb-4S4R-M4DOTMA indicate that the SAP isomer is significantly more stable. The luminescence profiles of Eu-8S-M4DOTMA and Eu-4S4R-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-8S-M4DOTMA had q = 0.86. A much lower q value was obtained for Tb-4S4R-M4DOTMA (q = 0.67), which indicates an elongation of the Ln-Ow bond. At 400 MHz, the relaxivity of Gd-8S-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-4S4R-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-8S-M4DOTMA is 5.4 ± 0.1 mM^-1 s^-1, and the relaxivity of Gd-4S4R-M4DOTMA is 4.5 ± 0.1 mM^-1 s^-1 at 25 °C. The temperature dependence of the ¹⁷O NMR transverse relaxation rate of the Gd complexes revealed a 7-fold increase in the bound water residence lifetime of Gd-8S-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 8S-M4DOTMA crystallized as TSAP isomer with an apical water. The presence of the apical water for the TSAP of Pr-8S-M4DOTMA was further confirmed with the observation that the fluoride ion replaces the bound water from the TSAP isomer of Pr-8S-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).

SUBMITTER: Opina ACL

PROVIDER: S-EPMC8371676 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The coordination geometry adopted by the lanthanide complexes of DOTA-tetraamides is a critical factor in determining their water exchange kinetics. Controlling the water exchange kinetics of DOTA-tetraamide complexes, and by extension their coordination geometry, is of particular interest because of the potential application of this class of complex as PARACEST MRI contrast agents. To facilitate the maximum CEST effect at the lowest pre-saturation powers much slower exchange kinetics are required than are commonly observed with these types of chelates. Complexes that adopt the more slowly exchanging square antiprismatic coordination geometry are therefore preferred; however, the factors that govern which coordination geometry is preferred remain unclear. A series of DOTA-tetraamide complexes with butyl amide substituents in different regioisomeric configurations provides some insight into these factors. The population of each coordination geometry was found to vary substantially depending upon the regiochemistry of the butyl amide substituent. It was observed that the twisted square antiprism coordination geometry, usually favored in complexes with the larger lanthanide ions only, is also increasingly favored for certain DOTA-tetraamide complexes with the smaller lanthanides. This is in marked contrast to simple DOTA-tetraamide complexes such as DOTAM. The effect was more prevalent in complexes formed with more bulky and more electron donating amide butyl substituents. It is also associated with loss of an inner-sphere water molecule from the complexes of later lanthanides that adopt the twisted square antiprismatic geometry. The complexes with sec-butyl substituents are inherently more complicated because of the introduction of a stereochemical center into each pendant arm. Unlike chiral complexes with larger amide substituents there is no "locking" effect of the orientation of the pendant arms in these complexes and up to four diastereoisomeric coordination isomers can be observed.

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

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