A high-affinity fluorescent Zn2+ sensor improved by the suppression of pyridine-pyridone tautomerism and its application in living cells

A high-affinity and low-molecular-weight fluorescent Zn2+ sensor 1 based on a 2,2'-bipyridine scaffold, which functions as both the chelating moiety for Zn2+ and the fluorophore, was developed and evaluated in biological applications. Controlling the occurrence of tautomerism of the chelating moiety for Zn2+ by introducing an amino group at the 6-position of the pyridine ring dramatically increased the binding affinity toward Zn2+. Fluorescent sensor 1 exhibited a nanomolar-range dissociation constant (K-d = 2.2 nM), a large Stokes shift (140 nm), and an 8.6-fold turn-on response to Zn2+ under physiological conditions. Fluorescence images revealed that fluorescent sensor 1 exhibits good properties with respect to aqueous solubility and cell permeability and can quantitatively detect the Zn2+ levels in living cells. Furthermore, a 65Zn(2+) radioactive zinc isotope uptake study revealed the real concentration of accumulated Zn2+ at the detection limit. The novel fluorescent sensor 1 is a promising sensor for use in biological applications. (C) 2015 Elsevier B.V. All rights reserved.