Bright, highly water-soluble triazacyclononane europium complexes to detect ligand binding with time-resolved FRET microscopy

Luminescent europium complexes are used in a broad range of applications as a result of their particular emissive properties. The synthesis and application of bright, highly water-soluble, and negatively charged sulfonic- or carboxylic acid derivatives of para-substituted aryl-alkynyl triazacyclononane complexes are described. Introduction of the charged solubilizing moieties suppresses cellular uptake or adsorption to living cells making them applicable for labeling and performing assays on membrane receptors. These europium complexes are applied to monitor fluorescent ligand binding on cell-surface proteins with time-resolved Förster resonance energy transfer (TR-FRET) assays in plate-based format and using TR-FRET microscopy. Bright and selective: Hydrophilic Eu^III complexes have been synthesized that suppress nonspecific binding and permit their use in bioconjugates using time-resolved assays, as exemplified for G protein-coupled receptor antagonists.