Ternary complexes between cationic GdIII chelates and anionic metabolites in aqueous solution:: An NMR relaxometric study
Ternary complexes between cationic GdIII chelates and anionic metabolites in aqueous solution:: An NMR relaxometric study
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DOI：
10.1002/chem.200204475
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发表时间：
2003-05-09
影响因子：
4.3
通讯作者：
Terreno, E
中科院分区：
文献类型：
--
作者：
Botta, M;Aime, S;Terreno, E
已在水介质中研究了 Gd-III 与大环七齿三酰胺配体 L-1 及其 N-甲基化类似物 L-2 形成的两种阳离子配合物的 H-1 和 O-17 NMR 弛豫特性与 pH、温度和磁场强度的函数关系。该配合物在其内部配位层中具有两个水分子，N-甲基化衍生物的交换速率明显更快，并可以通过电子效应（金属中心电荷密度降低）和外部水合层中氢键水分子网络的扰动来解释。质子弛豫率显示出对 pH 值的显着依赖性，并且在 pH 值 6.5 至 9.0 范围内降低约 6 个单位。这是通过溶解的碳酸盐（充当螯合阴离子）置换两个水分子来解释的。在 20 MHz 和 pH 6.3 下，通过 H-1 NMR 弛豫滴定监测了与乳酸盐、丙二酸盐、柠檬酸盐、乙酸盐、氟化物和磷酸氢盐形成的三元络合物，并且可以获得亲和常数 K 的值和加合物的弛豫率。乳酸、丙二酸和柠檬酸与复合物发生强烈相互作用（log K 大于或等于 3.7），并通过置换两个水分子以双链模式进行配位。已测量到 GdL2 具有较大的亲和常数。乙酸盐、氟化物和磷酸氢盐形成单水三元络合物，对其弛豫性能进行了详细研究。核磁共振分散（NMRD）曲线表明属于配合物第二水合壳层并与阴离子氢键结合的水分子对加合物的弛豫性有很大贡献。 VT O-17 NMR 研究表明，结合后水交换速率显着增加，这是由于阴离子在赤道位点的配位所致，从而使水分子处于顶端位置，更容易与第二水合壳的溶剂分子相互作用，从而促进交换过程。
The H-1 and O-17 NMR relaxometric properties of two cationic complexes formed by Gd-III with a macrocyclic heptadentate triamide ligand, L-1, and its N-methylated analogue, L-2, have been investigated in aqueous media as a function of pH, temperature and magnetic field strength. The complexes possess two water molecules in their inner coordination sphere for which the rate of exchange has been found to be sensibly faster for the N-methylated derivative and explained in terms of electronic effects (decrease of the charge density at the metal center) and perturbation of the network of hydrogen-bonded water molecules in the outer hydration sphere. The proton relaxivity shows a marked dependence from pH and decreases of about six units in the pH range 6.5 to 9.0. This has been accounted for by the displacement of the two water molecules by dissolved carbonate which acts as a chelating anion. The formation of ternary complexes with lactate, malonate, citrate, acetate, fluoride and hydrogenphosphate has been monitored by H-1 NMR relaxometric titrations at 20 MHz and pH 6.3 and the value of the affinity constant, K, and of the relaxivity of the adducts could be obtained. Lactate, malonate and citrate interact strongly with the complexes (log K greater than or equal to 3.7) and coordinate in a biden-date mode by displacing both water molecules. Larger affinity constants have been measured for GdL2. Acetate, fluoride and hydrogenphosphate form monoaqua ternary complexes which were investigated in detail with regard to their relaxometric properties. The NMR dispersion (NMRD) profiles indicate a large contribution to the relaxivity of the adducts from water molecules belonging to the second hydration shell of the complexes and hydrogen-bonded to the anion. A VT O-17 NMR study has shown a marked increase of the rate of water exchange upon binding which is explained by coordination of the anion in an equatorial site, thus leaving the water molecule in an apical position, more accessible for interactions with the solvent molecules of the second hydration shell which facilitate the exchange process.