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 comp lex 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 ( L-1: Zn2+ log beta(ML) 5.04, Ca2+ 3.91, Mg2+ 2.1, L-3: 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 [LnL(2)], exce pt for zinc binding for which a slightly enhanced affinity was observed. Ch anges in the form of ligand absorption and emission spectra and in the inte nsity of delayed lanthanide luminescence characterised metal ion binding. W ith [LnL(2)], 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 mu M (295 K).