Fluorescent and Electrochemical Sensing of Polyphosphate Nucleotides by Ferrocene Functionalised with Two Zn-II(TACN)(pyrene) Complexes

The [Fc-bis{Zn-II(TACN)(Py)}] complex, comprising two Zn-II(TACN) ligands (Fc=ferrocene; Py=pyrene; TACN=1,4,7-triazacyclononane) bearing fluorescent pyrene chromophores linked by an electrochemically active ferrocene molecule has been synthesised in high yield through a multistep procedure. In the absence of the polyphosphate guest molecules, very weak excimer emission was observed. indicating that the two pyrene-bearing Zn-II(TACN) units are arranged in a trans-like configuration with respect to the ferrocene bridging unit. Binding of a variety of polyphosphate anionic guests (PPi and nucleotides di- and triphosphate) promotes the interaction between pyrene units and results in an enhancement in excimer emission. Investigations of phosphate binding by P-31 NMR spectroscopy, fluorescence and electrochemical techniques confirmed a 1:1 stoichiometry for the binding of PPi and nucleotide polyphosphate anions to the bis(Zn-II(TACN)) moiety of [Fc-bis{Zn-II(TACN)(Py)}] and indicated that binding induces a trans to cis configuration rearrangement of the bis(Zn-II(TACN)) complexes that is responsible for the enhancement of the pyrene excimer emission. Pyrophosphate was concluded to have the strongest affinity to [Fc-bis{Zn-II(TACN)(Py)}] among the anions tested based on a six-fold fluorescence enhancement and 0.1 V negative shift in the potential of the ferrocene/ferrocenium couple. The binding constant for a variety of polyphosphate anions was determined from the change in the intensity of pyrene excimer emission with polyphosphate concentration, measured at 475 nm in CH3CN/Tris-HCl (1:9) buffer solution (10.0 mm, pH 7.4). These measurements confirmed that pyrophosphate binds more strongly (K-b=(4.45 +/- 0.41) x 10(6) M-1) than the other nucleotide di- and triphosphates (K-b=1-50 x 10(5) M-1) tested.