Outside the box: quantifying interactions of anions with the exterior surface of a cationic coordination cage

Abstract

We describe a study of the binding of anions to the surface of an octanuclear coordination cageHW, which carries a 16+ charge, in aqueous solution. Anionic aromatic fluorophores such as fluorescein (and derivatives) and hydroxypyrene tris-sulfonate (HPTS) bind strongly to an extent depending on their charge and hydrophobicity. Job plots indicated binding of up to six such fluorescent anions toHW, implying that one anion can bind to each face of the cubic cage, as previously demonstrated crystallographically with small anions such as halides. The quenching of these fluorophores on association with the cage provides the basis of a fluorescence displacement assay to investigate binding of other anions: addition of analyte (organic or inorganic) anions in titration experiments to anHW/fluorescein combination results in displacement and restoration of the fluorescence from the bound fluorescein, allowing calculation of 1:1 binding constants for theHW/anion combinations. Relative binding affinities of simple anions for the cage surface can be approximately rationalised on the basis of ease of desolvation (e.g.F−< Cl− < dianions), and extent of hydrophobic surface. The interaction of a di-anionic pH indicator (bromocresol purple) withHWresults in a pKashift, with the surface-bound di-anionic form stabilised by approximately 1 pKaunit compared to the non-bound neutral form due to the charge on the cage.