Complexation and sensing behavior of dansyl-appended cyclodextrins and cyclodextrin dimers with bile salts

The dansyl-modified cyclodextrin derivatives 2 and 3 form complexes with the steroidal bile salts. The selectivity of the monomeric derivative 3 is similar to that of native g-cyclodextrin. All binding constants with 3 are lowered compared to g-cyclodextrin because of the competition for the cavity between the steroids and the dansyl moiety. Cholate (4a) and deoxycholate (4b) form weak complexes, the other bile salts (4c-e) are complexed far more strongly. The difference is attributed to the absence of a 12-hydroxy group in the latter steroids. Data for dimer 2 reveal strongly enhanced binding of 4a and 4b and only slightly stronger complexes with the other steroids. Due to the low binding affinity of 3 for 4a and 4b, this receptor could not be used for their detection by fluorescence spectroscopy. Steroids 4c-e showed a decrease in fluorescence intensity. The detection of all steroids 4a-e was possible using 2. The fluorescence intensity of the dimer increased or decreased, depending on which steroid was added.