Anions sensing ensemble

Many chemosensor ensembles were reported by Ans-lyn and coworkers. Compound 14, with three gua-nidinium groups displayed in the same direction (Fig. 5). binds to the citrate tricarboxylate anion. Anionic fluorophore 5-carboxy-fluorescein 15 binds with modest affinity to this receptor and shows increased fluorescence intensity due to the decreased pKa of its phenol group. As citrate is added, the fluorophore is displaced from the receptor, and the emission intensity diminishes. The same fluorophore was used in another sensing ensemble for inositol-1,4,5-triphosphate (IP3), an important second messenger involved in many cell regulatory functions. The fluorophore bound to positively charged hexa-gua-nidinium receptor 16 is displaced by added IP3, and the fluorescence intensity decreases. 

A competitive method for carbonate sensing is based on dicopper complex 17 and coumarin 343 as anion receptor and external fluorophore, respectively. The fluorophore 18 coordinates to the copper atoms through its carboxylic group, and energy transfer between coumarin ring and copper atoms quenches the fluorescence emission. Carbonate showed strong binding to the metal complex and coupled the displacement of the bound fluorophore with concomitant recovery of the coumarin emission. This ensemble showed selective response to carbonate over phosphate or carboxylate. 

The hexaamide receptor (24) and the coumarin phosphate (25) formed an ensemble for effective sensing of geranyl pyrophosphate (26) (GPP). The fluorescence resonance energy transfer in the (24 + 25) ensemble diminished when (25) was replaced by GPP. The receptor (24) bound selectively with GPP over other anions, including fatty acids. 

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