Abstract
Lanthanides (Ln), also known as rare earth metals, have been utilized for industrial and biological purposes and are commonly encountered in the 3+ oxidation state as Ln3+. The presence of Ln in spent nuclear fuel introduces problems during the transmutation and partitioning of actinides (An) due to the Ln high neutron cross-sections. Therefore, designing ligands for Ln3+ complexation, can be helpful for their separation from An3+ in nuclear technology. Also, the biomimetic roles of Ln3+ could be suitable for studying the biological macromolecules, such as the Ca2+-binding EF-hand proteins that are otherwise inaccessible for detailed structural analysis. In this study, we report a bis(quinoline)-dipicolinamide ligand (DQPDH2), as well as ortho- nitrobenzyl cage ligands (DM-nitrophen and H2-cage) that bind and/or separate Ln3+. We studied DREAM, a Ca2+ EF-hand protein, to analyze its interaction with Ln3+. UV- Vis absorption and fluorescence spectroscopy, circular dichroism, extraction, isothermal titration calorimetry (ITC) and photophysical spectroscopy were used for studying the interaction of Ln3+ and ligands/proteins. Our results indicate that DQPDH2 showed high binding affinity to Ln3+ with 1-1 complexation ratio, as confirmed by spectroscopic and solvent extraction studies. The X-ray crystal structure of the Nd3+-DQPDH2 complex indicated a 1-1 binding pattern, which is consistent with our spectroscopic studies. DM-nitrophen, showed high binding affinity to Ln3+ by absorption spectroscopy and ITC. Photoacoustic calorimetry has been performed on DM-nitrophen and Tb3+DM-nitrophen photodissociation and the resulting kinetic and thermodynamic data indicated successful release of Tb3+ upon photocleavage of DM-nitrophen. H2-cage showed modest binding affinity with Ln3+ by UV-Vis absorption spectroscopy, yet it was also shown to be an effective Ln3+ extractant. Fluorescence spectroscopy studies of Ca2+ binding proteins with Ln3+ showed efficient energy transfer from the protein to central Ln3+ and possible conformational changes upon Ln3+ binding to protein by observing a decrease in tryptophan emission and an increase in emission of hydrophobic probe and DREAM complex.
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