Prototropic vs whole water exchange contributions to the solvent relaxation enhancement in the aqueous solution of a cationic Gd3+ macrocyclic complex
Prototropic vs whole water exchange contributions to the solvent relaxation enhancement in the aqueous solution of a cationic Gd3+ macrocyclic complex
复制标题
DOI：
10.1021/ja963743m
复制
发表时间：
1997-05-21
影响因子：
15
通讯作者：
DeSousa, AS
中科院分区：
文献类型：
--
作者：
Aime, S;Barge, A;DeSousa, AS
Gd3+配合物增强溶剂水质子核弛豫速率的能力是金属离子的未配对电子与核自旋之间有效的偶极磁耦合的结果，这种耦合被认为是通过三个过程发生的：1 (i)顺磁配合物附近水分子的扩散（外球项）；（ii）水分子从配位点到体的交换（内球项）；（iii）涉及配合物和/或配位水本身的可移动质子的原生交换。在这些体系中，后一种贡献很难评估，因为配位水分子的交换速率通常比原性交换速率快得多。最近研究表明，Gd3+配合物与单配位水分子（水合数q） 1的交换寿命τM值较大。此外，当τM从带负电荷的[Gd （DTPA)] 2-(DTPA)二乙烯三胺五乙酸酯）过渡到中性的[Gd (DTPA- bma)]（DTPA- bma) 1,7 -二[（n -甲基氨基甲酰）甲基]- 1,4,7 -三氮杂庚烷- 1,4,7 -三乙酸酯）络合物时，τM大约增加了1个数量级。我们当时认为，在以更慢的水交换速率为特征的配合物中，可能有一种可能的途径来评估原向性交换对总体水交换的贡献。因此，观察到τM随着负剩余电荷的减少而增加，促使我们考虑三正电荷[Gd (DTMA)] 3+络合物[DTMA) DOTA tetrakis (methyllamide)]的三酸盐。6,7 dtma(1)作为中性的八齿配体对镧系（III）离子起作用，从而形成能够进一步配位一个水分子的阳离子配合物。[Eu (DTMA)] 3+在水溶液中的1H和13C NMR谱与一个具有高度立体化学刚性的主要优势异构体的存在相一致。
The ability shown by Gd3+ complexes to enhance the nuclear relaxation rate of solvent water protons is the result of the efficient dipolar magnetic coupling between the unpaired electrons of the metal ion and the nuclear spins which is believed to occur through three processes: 1 (i) diffusion of water molecules in the proximity of the paramagnetic complex (outersphere term);(ii) exchange of the water molecules from the coordination sites to the bulk (inner-sphere term);(iii) prototropic exchange involving mobile protons of the complex and/or of the coordinated water itself. In these systems, the latter contribution is extremely difficult to evaluate because the exchange rate of the coordinated water molecules is usually much faster than the rate of prototropic exchange. 2, 3 Recently, it has been shown that Gd3+ complexes with a single coordinated water molecule (hydration number q) 1) may display rather large values of the exchange lifetime τM. 4 Moreover τM increases approximately by 1 order of magnitude on passing from the negatively charged [Gd (DTPA)] 2-(DTPA) diethylenetriaminepentaacetate) to the neutral [Gd (DTPA-BMA)](DTPA-BMA) 1, 7-bis [(N-methylcarbamoyl) methyl]-1, 4, 7-triazaheptane-1, 4, 7-triacetate) complex. 5 We thought then that a possible route to assess the contribution of the prototropic exchange to the overall water exchange may be possible in complexes characterized by an even slower water exchange rate. Thus, the observation that τM increases as the negative residual charge decreases prompted us to consider the triflate salt of the tripositively charged [Gd (DTMA)] 3+ complex [DTMA) DOTA tetrakis (methylamide)]. 6, 7DTMA (1) acts as a neutral octadentate ligand toward lanthanide (III) ions, thus forming cationic complexes capable of further coordinating one water molecule. The solution 1H and 13C NMR spectra of [Eu (DTMA)] 3+ in aqueous solution are consistent with the presence of one main, largely dominant isomer possessing a high degree of stereochemical rigidity.