Highly Emissive Eu(III) Probes for Biological Assays

Abstract

Luminescent lanthanide complexes are important tools for molecular sensing and cellular staining due to their unique photophysical properties, including their long luminescence lifetimes that permit the use of time-gated measurements. However, a common drawback of such complexes is the non-specific binding associated with their low solubility in biological media.
A new class of bright, highly water soluble, and negatively charged sulfonate or carboxylate derivatives of substituted aryl–alkynyl triazacyclononane complexes has been synthesised and their photophysical properties analysed. In addition, new synthetic methodologies have been explored for the introduction of solubilising moieties into the ligand system and for the incorporation of a linkage point for conjugation with biomolecules.
Each complex exhibits extremely high quantum yields (up to 38 % in H2O), large extinction coefficients (60,000 M-1 cm-1) and long luminescence lifetimes (1.1 ms). Introduction of the charged solubilising moieties suppresses cellular uptake or adsorption to living cells, making them applicable for labelling and performing assays on membrane receptors.
These Eu(III) complexes have been applied to monitor fluorescent ligand binding on cell-surface proteins (G-protein coupled receptors) with time-resolved fluorescence resonance energy transfer (TR-FRET) assays and TR-FRET microscopy.
In addition, the introduction of a linkage point for conjugation on the macrocyclic ring provided complete control of the stereochemistry of the final complex. Direct and selective formation of chiral complexes was observed with > 95 % optical purity resulting in an intense circularly polarised luminescence signal.