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 yield (0.32), a single long lifetime (1.25 ms) which was not influenced by coupling to protein, very high stability in the presence of chelators like EDTA or EGTA, and no interaction with cofactors like ATP or GTP. A special feature is the minimal linker length between the Eu(III)-ion and the maleimide or hydrazide functions which allow 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.