Bis(2-quinolylmethyl)ethylenediaminediacetic acids (BQENDAs), TQEN–EDTA hybrid molecules as fluorescent zinc sensors

Abstract

Molecular hybrids of TQEN (N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine) and EDTA (ethylenediamine-N,N,N',N'-tetraacetic acid) were examined as fluorescent Zn(2+) sensors. Upon the addition of Zn(2+), N,N-BQENDA (N,N-bis(2-quinolylmethyl)ethylenediamine-N',N'-diacetic acid, 1a) exhibits a 30-fold emission enhancement at 456 nm (λex = 315 nm, ϕZn = 0.018) in buffer (HEPES, pH = 7.5, 100 mM KCl). The fluorescence enhancement is Zn(2+)-specific as Cd(2+) induces much smaller increases (ICd/I0 = 5 and ICd/IZn = 16%). These spectroscopic properties, as well as the excellent water-solubility, represent a significant improvement compared to the parent TQEN sensor (ϕZn = 0.007, ICd/IZn = 64%). The isoquinoline analog N,N-1-isoBQENDA (N,N-bis(1-isoquinolylmethyl)ethylenediamine-N',N'-diacetic acid, 1b) possesses a similar Zn(2+) fluorescence response to the parent 1-isoTQEN (N,N,N',N'-tetrakis(1-isoquinolylmethyl)ethylenediamine) sensor, but exhibits diminished fluorescence intensity. Apo 1a and 1b extract more than 50% of the Zn(2+) from an equimolar amount of [Zn(TPEN)](2+) (TPEN = N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine) or [Zn(EDTA)](2-), whereas TPEN and EDTA cannot effectively remove Zn(2+) from [Zn(1a)] and [Zn(1b)]. The reduction of steric crowding in [Zn(TQEN)](2+) resulting from the substitution of two quinolines with carboxylates enhances the interaction between the metal ion and the remaining quinoline nitrogen atoms. The stronger bonding interaction results in enhanced emission intensity, Zn(2+) selectivity and metal ion affinity. This is in contrast to [Zn(1-isoTQEN)](2+) where the isoquinoline-carboxylate replacement does not relieve any coordination distortion, therefore no significant changes in fluorescence or metal binding properties are observed.