Department of Chemistry, University of Texas at Dallas, 800 W. Campbell Road, Richardson, TX, 75080, USA.
J Biol Inorg Chem. 2014 Feb;19(2):161-71. doi: 10.1007/s00775-013-1031-3. Epub 2013 Aug 27.
Many Eu(III) complexes formed with DOTA-tetraamide ligands (where DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) have sufficiently slow water exchange kinetics to meet the slow-to-intermediate condition required to serve as chemical exchange saturation transfer (CEST) contrast agents for MRI. This class of MRI contrast agents offers an attractive platform for creating biological sensors because water exchange is exquisitely sensitive to subtle ligand stereochemistry and electronic effects. Introduction of carboxyl groups or carboxyl ethyl ester groups on the amide substituents has been shown to slow water exchange in these complexes, but less is known about the orientation or position of these side-chain groups relative to the inner-sphere Eu(III)-bound water molecule. In this study, a series of Eu(III) complexes having one or more carboxyl groups or carboxyl esters at the δ-position of the pendant amide side chains were prepared. Initial attempts to prepare optically pure EuDOTA-[(S)-Asp]4 resulted in a chemically pure ligand consisting of a mixture of stereochemical isomers. This was traced to racemization of (S)-aspartate diethyl ester during the synthetic procedure. Nevertheless, NMR studies of the Eu(III) complexes of this mixture revealed that each isomer had a different water exchange rate, differing by a factor of 2 or more. A second controlled synthesis and CEST study of EuDOTA-[(S)-Asp]4 and cis-EuDOTA-[(S)-Asp]2[(R)-Asp]2 confirmed that the water exchange rates in these diastereomeric complexes are controlled by the axial versus equatorial orientation of the carboxyl groups on the amide side chains. These observations provide new insights toward the development of even more slowly water exchanging systems which will be necessary for practical in vivo applications.
许多 Eu(III) 配合物与 DOTA-四酰胺配体形成（其中 DOTA 是 1,4,7,10-四氮杂环十二烷-1,4,7,10-四乙酸），其水交换动力学足够缓慢，可满足作为 MRI 的化学交换饱和传递 (CEST) 对比剂的慢到中等条件。该类 MRI 对比剂为创建生物传感器提供了一个有吸引力的平台，因为水交换对配体立体化学和电子效应的细微变化非常敏感。在酰胺取代基上引入羧基或羧基乙酯基团已被证明会降低这些配合物中的水交换速度，但对于这些侧链基团相对于内球结合水分子的取向或位置知之甚少。在这项研究中，制备了一系列在侧链酰胺的 δ-位具有一个或多个羧基或羧基酯的 Eu(III) 配合物。最初尝试制备光学纯 EuDOTA-[(S)-Asp]4 导致化学纯配体，由立体异构体混合物组成。这可追溯到合成过程中 (S)-天冬氨酸二乙酯的外消旋化。尽管如此，对该混合物的 Eu(III) 配合物的 NMR 研究表明，每个异构体的水交换速率不同，相差 2 倍或更多。对 EuDOTA-[(S)-Asp]4 和顺式-EuDOTA-[(S)-Asp]2[(R)-Asp]2 的第二次受控合成和 CEST 研究证实，这些非对映异构体配合物的水交换速率受酰胺侧链上羧基的轴向与赤道取向的控制。这些观察结果为开发甚至更慢水交换系统提供了新的见解，这对于实际的体内应用是必要的。
