Effect of Mg and Ca on the Stability of the MRI Contrast Agent Gd–DTPA in Seawater
Effect of Mg and Ca on the Stability of the MRI Contrast Agent Gd–DTPA in Seawater
复制标题
Mg和Ca对MRI造影剂Gd-DTPA在海水中稳定性的影响
作者：
J. Schijf;I
二亚乙基三胺五乙酸钆（Gd-DTPA）是一种广泛应用于医学MRI的对比增强剂。由于Gd-DTPA在废水处理厂中的捕获量很小，或在紫外线和其他氧化过程中降解，因此自1987年引入以来，全球河流中的浓度增加了几个数量级。该复合物似乎也不受河口清除的影响，并开始出现在沿海沃茨，但它是未知的，它的稳定性是如何改变的DTPA配体从主要的海水阳离子的竞争。我们进行电位滴定在海水离子强度（0.7 M NaClO 4），以确定五个DTPA羧酸基团的解离常数，以及镁，钙，钆配合物的稳定常数与完全去质子化和单质子化配体。这些是在一般协议与文献值在低离子强度，并确认与Ca的络合物比与Mg更稳定。一个新的发现，镁和钙的DTPA络合物似乎在升高的pH值下水解，这意味着它们在这些螯合物中的配位小于六齿，使额外的竞争与Gd从双核镁和钙物种。从我们的研究结果计算出的微量金属的海水，副反应系数表明，较高丰度的镁和钙可能显着不稳定的钆-DTPA在沿海沃茨，造成解离和释放高达15%的有机络合Gd从配体。这种影响可能会提高敏感的河口栖息地的颗粒反应性和生物利用度的人为Gd，表明迫切需要进一步研究这种污染物在海洋环境中的命运。
Gadolinium diethylenetriaminepentaacetic acid (Gd–DTPA) is widely applied as a contrast enhancer in medical MRI. As Gd–DTPA is only minimally captured in wastewater treatment plants (WTPs) or degraded by UV light and other oxidative processes, concentrations in rivers have increased globally by orders of magnitude following its introduction in 1987. The complex also seems impervious to estuarine scavenging and is beginning to emerge in coastal waters, yet it is unknown how its stability is changed by competition for the DTPA ligand from major seawater cations. We performed potentiometric titrations at seawater ionic strength (0.7 M NaClO4) to determine dissociation constants of the five DTPA carboxylic acid groups, as well as stability constants of Mg, Ca, and Gd complexes with the fully deprotonated and single-protonated ligand. These are in general agreement with literature values at low ionic strength and confirm that complexes with Ca are more stable than with Mg. A new finding, that the DTPA complexes of Mg and Ca appear to be hydrolyzed at elevated pH, implies that their coordination in these chelates is less than hexadentate, enabling additional competition with Gd from dinuclear Mg and Ca species. Side-reaction coefficients for trace-metal-free seawater, calculated from our results, suggest that the higher abundance of Mg and Ca may significantly destabilize Gd–DTPA in coastal waters, causing dissociation and release of as much as 15% of the organically complexed Gd from the ligand. This effect could enhance the particle-reactivity and bioavailability of anthropogenic Gd in sensitive estuarine habitats, indicating an urgent need to further study the fate of this contaminant in marine environments.