Laboratoire de RMN haute résolution, Université Libre de Bruxelles, CP 160/08, Av. F.-D. Roosevelt 50, 1050 Brussels, Belgium.
Dalton Trans. 2014 Jan 21;43(3):967-72. doi: 10.1039/c3dt52533c. Epub 2013 Oct 25.
The complex between (17)O-enriched DOTA (tetraazacyclododecanetetraacetic acid) and praseodymium(III) (Pr(3+)) was studied in aqueous solution by variable-temperature (17)O 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 (O2); coalescence occurs between 20 and 30 °C. In contrast, a single signal was detected for the noncoordinated oxygen atoms (O1) in the entire investigated temperature range, i.e. between -3 and 135 °C. At high temperature, the spectra exhibit signal broadening that reveals the interchange of the O1 and O2 oxygen atoms of the carboxylate groups. The linewidths measured for O1 were deconvolved into contributions from quadrupole relaxation and chemical exchange, allowing the corresponding activation barriers to be determined. The present (17)O 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 Pr(3+) 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.
在 14.1 T 的磁场下，通过变温（17）O NMR 研究了（17）O 富集的 DOTA（四氮杂环十二烷四乙酸）与镨（III）（Pr（3+））在水溶液中的络合物。研究了 pH 值的影响以及溶液中游离金属离子的影响。在低温下，所谓的 TSAP 和 SAP 构象会导致与金属离子配位的氧原子（O2）产生明显的信号；在 20 至 30°C 之间发生合并。相比之下，在整个研究温度范围内（即-3 至 135°C），均检测到非配位氧原子（O1）的单个信号。在高温下，光谱表现出信号展宽，这揭示了羧酸盐基团的 O1 和 O2 氧原子的交换。O1 的线宽经过分解，得出了来自四极矩弛豫和化学交换的贡献，从而可以确定相应的活化势垒。目前的（17）O 动态 NMR 研究提供了表征镧系金属离子 DOTA 配合物中氧原子交换的第一个定量实验数据。该过程的活化熵可以忽略不计，并且发现活化焓在 66 至 77 kJ mol（-1）之间变化，具体取决于 pH 值以及溶液中游离 Pr（3+）离子的存在。这些数据支持先前计算研究的结果，根据该结果，交换机制涉及羧酸盐基团的内部旋转。
