Aqueous Fluorometric and Colorimetric Sensing of Phosphate Ions by a Fluorescent Dinuclear Zinc Complex

A fluorescent dinuclear zinc complex (1) has been prepared by an easy “one pot” method in which the lysine-based Schiff base ligand is generated in situ. The compound is characterized in detail by various spectroscopic techniques and single-crystal X-ray diffraction. Crystal data: monoclinic, C2, a = 18.31(13) Å, b = 8.45(7) Å, c = 11.20(8) Å, β = 90.11(6)°, V = 1733.6(2) ų. Z = 2, R1 = 0.0442, wR2 = 0.1068, Flack(x) parameter = 0.03(2). Compound 1 is highly fluorescent, in both the solution phase and as a solid. Under neutral aqueous conditions (0.01 M HEPES buffer, pH 7.4), compound 1 exhibits intense blue fluorescence (λ_ex = 352 nm, λ_em = 453 nm, Φ_F = 0.17). The complex was tested with several anions, such as F⁻, Br⁻, I⁻, AcO⁻, HCO₃⁻, H₂PO₄⁻, HPO₄²⁻, PO₄³⁻, and a variety of biologically relevant phosphate anions, such as adenosine monophosphate (AMP), cyclic adenosine monophosphate (cAMP), inorganic pyrophosphate (PPi, P₂O₇⁴⁻), adenosine diphosphate (ADP), and adenosine triphosphate (ATP), in water at physiological pH. The dizinc species was found to act as a highly selective sensor for PPi in particular by fluorescence ON-OFF signaling. Both the fluorometric and colorimetric (indicator displacement assays) titration results suggest that compound 1 is a highly selective sensor for phosphate ions with the order PPi ≥ ATP > ADP.