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

The complex between 17O-enriched DOTA (tetraazacyclododecanetetraacetic acid) and Pr(iii) (Pr3+) was studied in aq. soln. by variable-temp. 17O NMR at 14.1 T. PH effects as well as the influence of metal ions free in soln. were studied. At low temp., the so-called TSAP and SAP conformations give rise to distinct signals for the O atoms coordinated to the metal ion (O2); coalescence occurs 20-30°. In contrast, a single signal was detected for the noncoordinated O atoms (O1) in the entire studied temp. range, i.e. between -3 and 135°. At high temp., the spectra exhibit signal broadening that reveals the interchange of the O1 and O2 O atoms of the carboxylate groups. The linewidths measured for O1 were deconvolved into contributions from quadrupole relaxation and chem. exchange, allowing the corresponding activation barriers to be detd. The present 17O dynamic NMR study provides the 1st quant. exptl. data characterizing the interchange of the O 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 soln. These data support the results of a previous computational study according to which the exchange mechanism involves the internal rotation of the carboxylate groups. [on SciFinder(R)]