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) A, b = 8.45(7) A, c = 11.20(8) A, beta = 90.11(6) degrees, V = 1733.6(2) A(3). 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 (lambda(ex) = 352 nm, lambda(em) = 453 nm, Phi(F) = 0.17). The complex was tested with several anions, such as F(-), Br(-), I(-), AcO(-), HCO(3)(-), H(2)PO(4)(-), HPO(4)(2-), PO(4)(3-), and a variety of biologically relevant phosphate anions, such as adenosine monophosphate (AMP), cyclic adenosine monophosphate (cAMP), inorganic pyrophosphate (PPi, P(2)O(7)(4-)), 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 > or = ATP > ADP.