ZP4, an Improved Neuronal Zn2+ Sensor of the Zinpyr Family

Abstract

A second-generation fluorescent sensor for Zn²⁺ from the Zinpyr family, ZP4, has been synthesized and characterized. ZP4 (Zinpyr-4, 9-(o-carboxyphenyl)-2-chloro-5-[2-{bis(2-pyridylmethyl)aminomethyl}-N-methylaniline]-6-hydroxy-3-xanthanone) is prepared via a convergent synthetic strategy developed from previous studies with these compounds. ZP4, like its predecessors, has excitation and emission wavelengths in the visible range (∼500 nm), a dissociation constant (K_d) for Zn²⁺ of less than 1 nM and a high quantum yields (Φ = ∼0.4), making it well suited for biological applications. A 5-fold fluorescent enhancement is observed under simulated physiological conditions corresponding to the binding of the Zn²⁺ cation to the sensor, which inhibits a photoinduced electron transfer (PET) quenching pathway. The metal-binding stereochemistry of ZP4 was evaluated through the synthesis and X-ray structural characterization of [M(BPAMP)(H₂O)_n]⁺ complexes, where BPAMP is [2-{bis(2-pyridylmethyl)aminomethyl}-N-methylaniline]-phenol and M = Mn²⁺, Zn²⁺ (n = 1) or Cu²⁺ (n = 0).