Stable Europium(III) Complexes with Short Linkers for Site‐Specific Labeling of Biomolecules

In this study, two new terpyridine-based EuIII complexes were synthesized, the structures of which were optimized for luminescence resonance energy-transfer (LRET) experiments. The complexes showed high quantum yields (32 %); a single long lifetime (1.25 ms), which was not influenced by coupling to protein; very high stability in the presence of chelators such as ethylenediamine-N,N,N′,N′-tetraacetate and ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid; and no interaction with cofactors such as adenosine triphosphate and guanosine triphosphate. A special feature is the short length of the linker between the EuIII ion and the maleimide or hydrazide function, which allows for site-specific coupling of cysteine mutants or unnatural keto amino acids. As a consequence, the new complexes appear particularly suited for accurate distance measurements in biomolecules by LRET.