Tetrakis(2-quinolinylmethyl)ethylenediamine (TQEN) as a new fluorescent sensor for zinc

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Abstract
A new fluorescent sensor for zinc that binds 1 equivalent of zinc ion, N,N,N′,N′-tetrakis(2-quinolylmethyl)ethylenediamine (TQEN), has been prepared and characterized. Zinc-bound TQEN exhibits fluorescence around 383 nm upon excitation at 317 nm, while free TQEN emits very weak fluorescence. UV-Vis and 1H NMR spectral changes also detected the binding of TQEN with zinc ion. The crystal structure of zinc complex with TQEN was determined by X-ray crystallography and compared with that of the TPEN–Zn complex (TPEN = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine). The binding affinity of TQEN with zinc ion is very high (Kd < 1 µM in aqueous dmf solution). Competition experiments reveal that the zinc-binding affinity of TQEN is lower than the parent, strong metal ion chelator TPEN, and comparable to EGTA (EGTA = ethylene glycol-bis(2-aminomethyl)-N,N,N′,N′-tetraacetic acid).