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 (1)H NMR spectroscopy in aqueous phosphate buffer solution (pH 7.4). In the presence of 2,2’-CBP4, (1)H 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) × 10(6) M(−1)) is 3.9 times larger than that of P (K(a) = (5.87 ± 0.24) × 10(5) M(−1)).