Fluorescent Chemosensors by Luling Wu;Adam C Sedgwick;Xiao-Peng He;Tony D James;

Author:Luling Wu;Adam C Sedgwick;Xiao-Peng He;Tony D James;
Language: eng
Format: epub
ISBN: 9781839167331
Publisher: Lightning Source Inc. (Tier 2)
Published: 2023-07-15T00:00:00+00:00

10.2 Phosphate-binding Interactions

10.2.1 Charge–Charge Interactions

Early work in the development of chemosensors for phosphate species was influenced by the discovery that polyalkylammonium species, including natural polyamines such as spermine and spermidine that are protonated at neutral pH, bind to adenosine nucleotides (AMP, ADP, and ATP).44 Thus followed the development of a wide range of cyclic and acyclic polyammonium receptors for phosphate species. Binding of phosphate species was typically determined by careful pH or NMR titrations, and predominantly evaluated in aqueous solution. For the most part, only binding affinities of these receptors towards nucleotides and inorganic phosphate species were examined, and comparisons with other anions that could potentially interact competitively—such as carboxylates—were not reported.

The first receptor to provide a fluorescence response to the binding of phosphate species (or indeed to any anion) was developed by Czarnik and co-workers in 1989.45 Chemosensor 1 utilized the affinity of polyammonium groups for phosphates, coupled with an anthracene fluorophore to provide a response to binding. A “receptor–spacer–fluorophore” design strategy was used to provide a fluorescent response to binding. PeT, which quenches the anthracene fluorescence, was inhibited in the presence of phosphate species, leading to an increase in fluorescence intensity. The selectivity of this chemosensor and of most related polyammonium-based chemosensors was found to correlate most strongly with the overall charge of the phosphate species: the higher the charge on the phosphate species the greater the affinity. For example, chemosensor 1 was found to bind HPO42− with a log Ka of 0.82, whereas affinity for the trivalent anion ATP was determined to be more than three orders of magnitude greater (log Ka 4.2). Discrimination based on the size and shape of phosphate species was also possible, and these properties were exploited as chemosensor designs became more advanced and as understanding of the factors influencing selectivity increased.

Given that polyammonium receptors rely on charge–charge interactions (in addition to hydrogen bonds) between an anionic phosphate species and a protonated amine group, they are also highly sensitive to changes in pH. Chemosensor 1 was found to be most effective at pH 6, at which point the chemosensor is triprotonated and H2PO4− is the predominant form of inorganic phosphate. A successor compound, double-armed polyamine chemosensor 2 exhibited higher pKa values and hence was able to be used at pH 7 with each arm triprotonated.46 This chemosensor exhibited 2200 times greater selectivity for PPi over Pi (log Ka PPi: 5.54; log Ka Pi: 2.20). Although both species are present as dianions at this pH, the structure of PPi is presumed to be better suited to fit between the two polyammonium arms than is Pi.

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