Two types of conformational dynamics and thermo-sensor properties of praseodymium-DOTA by 1H/13C NMR

Abstract

In aqueous medium, [Pr(H2O)(DOTA)]− (I) undergo a intramolecular interconversion between two conformers (here DOTA = 1, 4, 7, 10-tetraaza-1, 4, 7, 10-tetrakis (carboxymethyl)cyclododecan; n = 0, 1, 2). Two types of molecular dynamic processes were found {enantiomerization and interconversion of the square-anti-prismatic (SAP) to twisted-square-anti-prismatic conformers (TSAP)}. The free energies of activation of the SAP to TSAP interconversion (for I complex) are found to be 50 ± 3 and 54 ± 4 kJ mol−1 (according to 1H and 13C NMR respectively). And, the activation free energy of enantiomerization is ΔG≠298 = 65 ± 2 kJ mol−1. It was evaluated that the activation energy of enantiomerization in [Gd(H2O) (DOTA)]− (II) is 64 ± 2 kJ mol−1 (by analysis of the dependence of the free activation energy on the ionic radius of other lanthanide ions). The experimental paramagnetic LIS values of I are well fitted by linear dependence on 1/T. It was found that I may be used as NMR thermometric sensors for controlling the local temperature in aqueous medium.