Bis(β-cyclodextrin)s Linked with an Aromatic Diamine as Fluorescent Sensor for the Molecular Recognition of Bile Salts

The binding behavior of three aromatic diamino-bridged bis(beta-cyclodextrin)s (2-4) with four bile salts {cholate (CA), deoxycholate (DCA), glycocholate (GCA) and taurocholate (TCA)} has been investigated at 25 A degrees C in a phosphate buffer (pH=7.20) by fluorescence and 2D NMR spectroscopy. The results indicate that these bis(beta-cyclodextrin)s act as fluorescent sensors. From the ROESY spectra, it is deduced that the phenyl moieties of bis(beta-cyclodextrin)s 2-4 are partially self included in the cyclodextrin cavity, and are not expelled from the cavity upon complexation with bile guests. Owing to the cooperative host-tether-guest binding mode in which the linker and guest are co-included in the two cyclodextrin cavities, these bis(beta-cyclodextrin)s significantly enhance the binding ability and selectivity as compared with the native beta-cyclodextrin 1. Possessing suitable tether length, bis(beta-cyclodextrin) 3 gives the highest K (S) values, ranging up to 39,900 mola <...L(-1), for complexation with CA. The complex stability constants are discussed from the viewpoint of multiple recognitions between host and guest.