Electron Paramagnetic Resonance, Electronic Ground State, and Electron Spin Relaxation of Seven Lanthanide Ions Bound to Lanmodulin and the Bioinspired Chelator, 3,4,3‐LI(1,2‐HOPO)

Abstract

The electron paramagnetic resonance (EPR) spectra of lanthanide(III) ions besides Gd(III), bound to small‐molecule and protein chelators, are uncharacterized. Here, we systematically investigated the EPR properties of 7 lanthanide(III) ions bound to the natural lanthanide‐binding protein, lanmodulin (LanM), and the synthetic small‐molecule chelator, 3,4,3‐LI(1,2‐HOPO) (“HOPO”). Echo‐detected pulsed EPR spectra reveal intense signals from ions for which the normal continuous‐wave first‐derivative spectra are negligibly different from zero. Spectra of Kramers lanthanide ions Ce3+, Nd3+, Sm3+, Er3+, and Yb3+, and non‐Kramers Tb3+ and Tm3+, bound to LanM are more similar to the ions in dilute aqueous:ethanol solution than to those coordinated with HOPO. Lanmodulins from two bacteria, with distinct metal‐binding sites, had similar spectra for Tb3+ but different spectra for Nd3+. Spin‐echo dephasing rates (1/Tm) are faster for lanthanides than for most transition metals and limited detection of echoes to temperatures below ~6 to 12 K. Dephasing rates were environment dependent and decreased in the order water:ethanol > LanM > HOPO, which is attributed to decreasing librational motion. Our results demonstrate that the EPR spectra and relaxation times of lanthanide(III) ions are sensitive to coordination environment, motivating wider application of these methods for characterization of both small‐molecule and biomolecule interactions with lanthanides.