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 β_ML 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).