Department of Inorganic Chemistry, Faculty of Science, Universita Karlova (Charles University), Hlavova, 2030, 128 43 Prague 2, Czech Republic.
Dalton Trans. 2011 Mar 7;40(9):1945-51. doi: 10.1039/c0dt01328e. Epub 2011 Jan 27.
The kinetics of transmetallation of Mn(nota) and Mn(dota) was investigated in the presence of Zn(2+) (5-50-fold excess) at variable pH (3.5-5.6) by (1)H relaxometry. The dissociation is much faster for Mn(nota) than for Mn(dota) under both experimental and physiologically relevant conditions (t(½) = 74 h and 1037 h for Mn(nota) and Mn(dota), respectively, at pH 7.4, c(Zn(2+)) = 10(-5) M, 25 °C). The dissociation of the complexes proceeds mainly via spontaneous (Mn(nota)k(0) = (2.6 ± 0.5) × 10(-6) s(-1); Mn(dota)k(0) = (1.8 ± 0.6) × 10(-7) s(-1)) and proton-assisted pathways (Mn(nota)k(1) = (7.8 ± 0.1) × 10(-1) M(-1) s(-1); Mn(dota)k(1) = (4.0 ± 0.6) × 10(-2) M(-1) s(-1), k(2) = (1.6 ± 0.1) × 10(3) M(-2) s(-1)). The observed suppression of the reaction rates with increasing Zn(2+) concentration is explained by the formation of a dinuclear Mn(2+)-L-Zn(2+) complex which is about 20-times more stable for Mn(dota) than for Mn(nota) (K(MnLZn) = 68 and 3.6, respectively), and which dissociates very slowly (k(3)∼10(-5) M(-1) s(-1)). These data provide the first experimental proof that not all Mn(2+) complexes are kinetically labile. The absence of coordinated water makes both Mn(nota) and Mn(dota) complexes inefficient for MRI applications. Nevertheless, the higher kinetic inertness of Mn(dota) indicates a promising direction in designing ligands for Mn(2+) complexation.
通过（1）H 弛豫测量法，在 pH 值为 3.5-5.6 的可变条件下，研究了 [Mn(nota)]-和 [Mn(dota)]2-在 Zn2+（5-50 倍过量）存在下的转金属反应动力学。在实验和生理相关条件下，[Mn(nota)]-的离解速度比 [Mn(dota)]2-快得多（在 pH 值为 7.4、c(Zn2+) = 10-5 M、25°C 时，t1/2 = 74 h 和 1037 h）。配合物的离解主要通过自发途径（[Mn(nota)]-k0 = (2.6 ± 0.5) × 10-6 s-1；[Mn(dota)]2--k0 = (1.8 ± 0.6) × 10-7 s-1）和质子辅助途径（[Mn(nota)]-k1 = (7.8 ± 0.1) × 10-1 M-1 s-1；[Mn(dota)]2--k1 = (4.0 ± 0.6) × 10-2 M-1 s-1，k2 = (1.6 ± 0.1) × 103 M-2 s-1）进行。随着 Zn2+浓度的增加，反应速率的观察抑制可以通过形成双核 Mn2+-L-Zn2+配合物来解释，对于 [Mn(dota)]2-，这种配合物比 [Mn(nota)]-稳定约 20 倍（K(MnLZn) = 68 和 3.6），并且离解非常缓慢（k3∼10-5 M-1 s-1）。这些数据首次提供了实验证据，证明并非所有 Mn2+配合物都是动力学不稳定的。由于没有配位水，[Mn(nota)]-和 [Mn(dota)]2-配合物都不适合用于 MRI 应用。然而，[Mn(dota)]2-的更高动力学惰性表明，设计用于 Mn2+络合的配体具有有希望的方向。
