Synthesis of a water-soluble 2,2′-biphen[4]arene and its efficient complexation and sensitive fluorescence enhancement towards palmatine and berberine

A water-soluble 2,2'-biphen[4]arene (2,2'-CBP4) containing eight carboxylato moieties was synthesized and characterized. Its complexation behavior towards two alkaloids, palmatine (P) and berberine (B), was investigated by means of fluorescence and H-1 NMR spectroscopy in aqueous phosphate buffer solution (pH 7.4). In the presence of 2,2'-CBP4, H-1 NMR signals of P and B displayed very large upfield shifts, indicating the formation of inclusion complexes with strong binding affinities. Fluorescence titration experiments showed that P and B exhibited dramatic fluorescence enhancement of more than 600 times upon complexation with 2,2'-CBP4. Particularly, the fluorescence intensity is strong enough to be readily distinguished by the naked eye. Although the two guests have similar structures, the association constant of B with 2,2'-CBP4 (K-a = (2.29 +/- 0.27) x 10(6) M-1) is 3.9 times larger than that of P (K-a = (5.87 +/- 0.24) x 10(5) M-1).