Direct excitation luminescence spectroscopy of Eu(iii) complexes of 1,4,7-tris(carbamoylmethyl)-1,4,7,10- tetraazacyclododecane derivatives and kinetic studies of their catalytic cleavage of an RNA analog
作者:Kido Nwe、John P. Richard、Janet R. Morrow
DOI:10.1039/b710072h
日期:——
The macrocycles 1,4,7-tris(carbamoylmethyl)-1,4,7,10-tetrazacyclododecane (1), 1,4,7-tris[(N-ethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane (2), 1,4,7-tris[(N,N-diethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane (3) and their Eu(III) complexes are prepared. Studies using direct Eu(III) excitation luminescence spectroscopy show that all three Eu(III) complexes exhibit only one predominant isomer with two bound waters under neutral to mildly basic conditions (Eu(X)(H2O)2 for X = 1–3). There are no detectable ligand ionizations over the pH range 5.0–8.0 for Eu(3), 5.0–8.5 for Eu(2) or 5.0–9.5 for Eu(1). The three Eu(III) complexes show a linear dependence of second-order rate constants for the cleavage of 4-nitrophenyl-2-hydroxyethylphosphate (HpPNP) on pH in the range 6.5–8.0 for Eu(3), 7.0–8.5 for Eu(2) and 7.0–9.0 for Eu(1). This pH–rate profile is consistent with the Eu(III) complex–substrate complex being converted to the active form by loss of a proton and with Eu(III) water pKa values that are higher than 8.0 for Eu(3), 8.5 for Eu(2) and 9.0 for Eu(1). Inhibition studies show that Eu(1) binds strongly to the dianionic ligand methylphosphate (Kd = 0.28 mM), and more weakly to diethylphosphate (Kd = 7.5 mM), consistent with a catalytic role of the Eu(III) complexes in stabilizing the developing negative charge on the phosphorane transition state.
大环 1,4,7-三(氨基甲酰基甲基)-1,4,7,10-四氮杂环十二烷 (1)、1,4,7-三[(N-乙基)氨基甲酰基甲基]-1,4,7、10-四氮杂环十二烷 (2)、1,4,7-三[(N,N-二乙基)氨基甲酰基甲基]-1,4,7,10-四氮杂环十二烷 (3) 及其 Eu(III) 复合物。利用直接 Eu(III)激发发光光谱进行的研究表明,在中性至弱碱性条件下,所有三种 Eu(III)配合物都只显示出一种主要异构体,其中有两个结合水(Eu(X)(H2O)2,X = 1-3)。在 pH 值为 5.0-8.0(Eu(3))、5.0-8.5(Eu(2))或 5.0-9.5(Eu(1))的范围内,均检测不到配体电离。三种 Eu(III)配合物裂解 4-硝基苯基-2-羟乙基磷酸(HpPNP)的二阶速率常数与 pH 值呈线性关系,Eu(3) 为 6.5-8.0,Eu(2) 为 7.0-8.5,Eu(1) 为 7.0-9.0。这种 pH 值-速率曲线与 Eu(III)络合物-底物络合物因失去一个质子而转化为活性形式,以及 Eu(III) 水 pKa 值高于 8.0(Eu(3))、8.5(Eu(2))和 9.0(Eu(1))相一致。抑制研究表明,Eu(1) 与二阴离子配体甲基磷酸的结合力很强(Kd = 0.28 mM),而与二乙基磷酸的结合力较弱(Kd = 7.5 mM),这与 Eu(III) 复合物在稳定磷烷过渡态上正在形成的负电荷方面的催化作用是一致的。