Elucidating Polyphosphate Anion Binding to Lanthanide\nComplexes Using EXAFS and Pulsed EPR Spectroscopy

Reversible anion\nbinding to lanthanide complexes in aqueous solution\nhas emerged as an effective method for anion sensing. Through careful\ndesign of the organic ligand, luminescent lanthanide complexes capable\nof binding biologically relevant anions in a bidentate or monodentate\nmanner can be realized. While single-crystal X-ray diffraction analyses\nand NMR spectroscopy have revealed the structural geometry of several\nhost–guest complexes, the challenge remains in designing preorganized\nlanthanide receptors with enhanced anion selectivity for broader applications\nin diagnostics and bioimaging. To address this challenge, innovative\nand complementary methods to investigate host-anion binding geometry\nare becoming increasingly important. Herein, we demonstrate the combined\nuse of Eu L<sub>3</sub>-edge extended X-ray absorption fine structure\n(EXAFS) and electron paramagnetic resonance (EPR) spectroscopy to\nelucidate the binding of nucleoside phosphates (ATP, ADP, and AMP)\nto a cationic lanthanide complex. We establish that ATP unequivocally\nbinds the lanthanide center in a bidentate manner in water, while\nADP adopts both bidentate and monodentate modes, and AMP binds in\na monodentate manner. This interdisciplinary approach provides deeper\ninsight into lanthanide host–guest chemistry in solution, laying\nthe groundwork for designing emissive probes that undergo specific\nanion-induced structural changes and elicit desired optical responses\nupon binding.