A model system using modulation of lanthanide luminescence to signal Zn2+ in competitive aqueous media†
Two pentadentate tribasic ligand systems containing aniline or benzylamine nitrogens covalently linked to a proximate kinetically stable Eu or Tb complex are described. The affinity of these complexes and their non-conjugated analogues for Zn2+, Ca2+ and Mg2+ ions has been measured at ambient pH in a high salt background. Apparent binding constants for the parent ligands (L1: Zn2+ log βML 5.04, Ca2+ 3.91, Mg2+ 2.1, L3: Zn2+ 5.93, Ca2+ 5.00, Mg2+ 3.60) were slightly lowered in the aniline-based terbium conjugate [TbL4], and were the same for the benzylamine-based conjugate [LnL2], except for zinc binding for which a slightly enhanced affinity was observed. Changes in the form of ligand absorption and emission spectra and in the intensity of delayed lanthanide luminescence characterised metal ion binding. With [LnL2], a 42 and 26% increase in emission at 700 nm (Eu) and 545 nm (Tb) accompanied zinc binding in a simulated extracellular background, with an apparent dissociation constant of 0.6 μM (295 K).