Methoxy-substituted isoTQEN family for enhanced fluorescence response toward zinc ion

Abstract

Previously, we have reported that 1- and 3-isoTQENs (N,N,N′,N′-tetrakis(1- or 3-isoquinolylmethyl)ethylenediamines) exhibit a specific fluorescence enhancement toward zinc ion. In this study, three methoxy-substituted derivatives of 1-isoTQEN were synthesized and their fluorescent response toward zinc ion was studied. The substitution pattern of the methoxy group significantly changes the solubility of compounds in aqueous DMF, λ_max in the absorption spectra, excitation/emission wavelengths and fluorescence intensity of zinc complexes. In the presence of zinc ion, 7-MeO-1-isoTQEN exhibits higher fluorescence intensity and longer excitation/emission wavelengths (λ_ex = 342 nm, λ_em = 526 nm) than 6-MeO-1-isoTQEN (λ_ex = 303 nm, λ_em = 469 nm) and 5,6,7-triMeO-1-isoTQEN (λ_ex = 340 nm, λ_em = 504 nm). The fluorescence intensity of a zinc complex of 7-MeO-1-isoTQEN (ϕ = 0.122) is four times higher than the parent 1-isoTQEN (ϕ = 0.034) under the same conditions. The crystal structure of 7-MeO-1-isoTQEN-Zn complex reveals that all six nitrogen atoms participate to the metal coordination with ideal octahedral geometry, affording significantly high metal binding affinity comparable with TPEN (N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine). 7-MeO-1-isoTQEN detects zinc ion concentration change in cells by fluorescence microscopic analysis.