The unique magnetic and optical properties of the lanthanide ions (Ln3+) render it useful as magnetic resonance imaging and paramagnetic chemical exchange saturation transfer contrast agents as well as NMR shift probes. In a previous work, we have demonstrated that the polymethylated 4S-4S-M4DOTMA complexed with early and late lanthanides predominantly exists as a square antiprismatic (SAP) and a twisted square antiprismatic (TSAP) isomer, respectively, in solution. Unexpectedly, when the stereochemistry of the pendent arm of 4S-4S-M4DOTMA was changed to 4S-4R-M4DOTMA, only a single isomer was observed across the lanthanide series. Accordingly, we have carried out both NMR and quantum chemical studies to elucidate the structures of the single isomer. The NOE data and the QM calculations at the level of M06L/6-31+G(d)-SDD have unambiguously identified the conformation of the single isomer as SAP. The geometry and energetics of Lu-4S-4S-M4DOTMA were also compared to the newly synthesized Lu-4S-4R-M4DOTMA and the structural requirements that modulate the thermochemical stability of one isomer over the other are discussed.

Structures and energetics of polymethylated DOTA complexed with lanthanides

2019 Spring National Meeting

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