An oxygen-17 dynamic NMR study of the Pr-DOTA complex

作者：Fusaro Luca^*; Luhmer Michel

来源：Dalton Transactions, 2014, 43(3): 967-972.

DOI：10.1039/c3dt52533c

The complex between O-17-enriched DOTA (tetraazacyclododecanetetraacetic acid) and praseodymium(III) (Pr3+) was studied in aqueous solution by variable-temperature O-17 NMR at 14.1 T. pH effects as well as the influence of metal ions free in solution were investigated. At low temperature, the so-called TSAP and SAP conformations give rise to distinct signals for the oxygen atoms coordinated to the metal ion (O-2); coalescence occurs between 20 and 30 degrees C. In contrast, a single signal was detected for the non-coordinated oxygen atoms (O-1) in the entire investigated temperature range, i.e. between -3 and 135 degrees C. At high temperature, the spectra exhibit signal broadening that reveals the interchange of the O-1 and O-2 oxygen atoms of the carboxylate groups. The linewidths measured for O-1 were deconvolved into contributions from quadrupole relaxation and chemical exchange, allowing the corresponding activation barriers to be determined. The present O-17 dynamic NMR study provides the first quantitative experimental data characterizing the interchange of the oxygen atoms in a DOTA chelate of a lanthanide metal ion. The activation entropy of this process is negligible and the activation enthalpy is found to range between 66 and 77 kJ mol(-1), depending on the pH and the presence of free Pr3+ ions in solution. These data support the results of a previous computational study according to which the exchange mechanism involves the internal rotation of the carboxylate groups.

• 出版日期2014