Isoquinoline-derivatized tris(2-pyridylmethyl)amines as fluorescent zinc sensors with strict Zn²⁺/Cd²⁺ selectivity

Tris(2-pyridylmethyl)amine-based fluorescent ligands, N,N-bis(1-isoquinolylmethyl)-2-pyridylmethylamine (1-isoBQPA) and N,N-bis(7-methoxy-1-isoquinolylmethyl)-2-pyridylmethylamine (7-MeO-1-isoBQPA), have been prepared and the Zn(2+)-induced fluorescence enhancement has been investigated. Upon excitation at 324 nm, 1-isoBQPA exhibits a very weak emission (ϕ = ~0.010) in DMF-H2O (1 : 1). Upon Zn(2+) addition, the 1-isoBQPA fluorescence increases (ϕ(Zn) = 0.055) at 357 nm and 464 nm. The fluorescence enhancement at longer wavelengths is Zn(2+)-specific, whereas Cd(2+) induces a small emission increase at 464 nm (I(Cd)/I0 = 1.1, I(Cd)/I(Zn) = 14%). The Zn(2+)/Cd(2+) selectivity of the fluorescent response correlates with the Cd-N(isoquinoline) and Zn-N(isoquinoline) bond distances measured in the crystal structures. Introduction of methoxy groups into the 1-isoBQPA chromophore enhances the fluorescence significantly (ϕ(Zn) = 0.213), which affords 7-MeO-1-isoBQPA properties amenable for fluorescence microscopy in living cells.