Current methods for metal chelation are generally based on multidentate organic ligands, which are generated through cumbersome multistep synthetic processes that lack flexibility for systematically varying metal-binding motifs. Octadentate ligands incorporating hydroxypyridinone or catecholamide binding moieties onto a spermine scaffold are known to display some of the highest affinities towards f-elements. Enhancing binding affinity for specific lanthanide or actinide ions however, necessitates ligand architectures that allow for modular and high throughput synthesis. Here we introduce a high-throughput combinatorial library of 16 tetrameric N-substituted glycine oligomers (peptoids) containing hydroxypyridinone or catecholamide chelating units linked via an ethylenediamine bridge and, for comparison, we also synthesized the corresponding mixed ligands derived from the spermine scaffold: 3,4,3-LI(1,2-HOPO)2(CAM)2 and 3,4,3-LI(CAM)2(1,2-HOPO)2. Coordination-based luminescence studies were carried out with Eu3+ and Tb3+ to begin probing the properties of the new ligand architecture and revealed higher sensitization efficiency with the spermine scaffold as well as different spectroscopic features among the structural peptoid isomers. Solution thermodynamic properties of selected ligands revealed different coordination properties between the spermine and peptoid analogues with Eu3+ stability constants log β110 ranging from 28.88 ± 3.45 to 43.97 ± 0.49. The general synthetic strategy presented here paves the way for precision design of new specific and versatile ligands, with a variety of applications tailored towards the use of f-elements, including separations, optical device optimization, and pharmaceutical development.
Bibliographical noteFunding Information:
Early peptoid library development was supported by the Fuel Cycle Research and Development Campaign (FCRD)/Fuel Resources Program, Office of Nuclear Energy, the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231 at the Lawrence Berkeley National Laboratory (LBNL). Spermine-based ligand development and all spectroscopic characterization were supported by the DOE, Office of Science Early Career Research Program and Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division at LBNL under Contract DE-AC02-05CH11231. Peptoid ligand purication and analysis were performed at the Molecular Foundry, a User Facility supported by the Office of Science, Office of Basic Energy Sciences, of the DOE under Contract No. DE-AC02-05CH11231. The authors thank Drs Ronald Zuckermann and Michael Connolly from the LBNL Molecular Foundry for invaluable help with peptoid preparation.
© 2019 The Royal Society of Chemistry.