Optimized Lanthanide(III) Tags for Rapid and Efficient Protein Labeling in Electron Paramagnetic Resonance and Luminescence Applications

Lanthanide(III) complexes are invaluable tools for probing protein structure and dynamics, enabling precise distance measurements and site‐specific labeling across electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and luminescence‐based techniques. Designing an optimal lanthanide tag requires high thermodynamic and kinetic stability, minimal linker flexibility, and rapid, quantitative reactivity under mild conditions. Herein, a short series of Ln(III) complexes developed as efficient protein tags is reported, each showing high chemoselectivity for cysteine and enabling labeling in aqueous solution under mild conditions (pH 8, 37 °C). Subtle modifications to the reactive arm of the parent Ln.L1 complex has a pronounced effect on tagging kinetics, with Tb.QuNO achieving >95% labeling of the free amino acid cysteine within 10 min. The more compact and readily synthesized Gd.PyNO also labels efficiently and exhibits a narrow EPR linewidth (3.1 mT average at Q‐band across three protein‐labeled samples), making it an optimal choice for many EPR applications. These findings highlight the critical role of linker design in lanthanide‐based probes for EPR spectroscopy and biological imaging.