Induced fit activity-based sensing: a mechanistic study of pyrophosphate detection with a "flexible" Fe-salen complex

Activity-based sensing of biological targets is attracting increasing attention. In this work, we report detailed UV-Vis and fluorescence mechanistic studies on an Fe-salen based probe, [Fe^III{salenMeCl₂(SO₃)₂}OH₂]^- for pyrophosphate (PPi) detection. In the presence of PPi as an analyte, the probe disassembles into its molecular subunits and releases a fluorescent signal. Our studies illustrate that the aqua form of the complex (1-OH2) is the active species and that upon substitution of Fe-coordinated H₂O and an initial end-on coordination of HP₂O₇ ^3-, the "trapped" pyrophosphate species switches from a monodentate to a bidentate coordination mode (i.e. linkage isomerism) via a probable equilibrium process. The elusive intermediate is further stabilized by a hydrogen bonding interaction that activates the probe for the subsequent final irreversible rate-limiting step, and allows selective discrimination between the other pyrophosphate (H₂P₂O₇ ^2- and P₂O₇ ^4-) species in favour of the HP₂O₇ ^3-. The flexible mode of molecular recognition and binding of HP₂O₇ ^3- by the tetradentate probe 1-OH2 is unexpected and most effective at physiological pH, and has precedence in enzymatic catalysis (i.e. induced fit principle). These binding properties explain the previously observed outstanding selectivity of 1-OH2 for pyrophosphate over other (poly)oxophosphates and potentially competing analytes.