| Abstract
| - Binding behavior of l- and d-tryptophan-modified β-cyclodextrins (l/d-Trp-β-CD) (1 and 2) withfour bile acids, i.e., cholate (CA), deoxycholate (DCA), glycocholate (GCA), and taurocholate (TCA),has been investigated by fluorescence, circular dichroism, and 2D-NMR spectroscopies andfluorescence lifetime measurement, as well as isothermal titration microcalorimetry. From theinduced circular dichroism (ICD) and 2D NMR spectra, it is deduced that the d-Trp moiety of 2attached to β-CD is more deeply self-included in the cavity than that of the antipodal l-Trp moietyof 1, indicating appreciably enantioselective binding of the chiral sidearm by β-CD. Interestingly,the original difference in conformation between 1 and 2 led to quite a large difference in affinitytoward DCA, giving 3.3 times higher binding ability for 2 than for 1. Thermodynamically, theinclusion complexation of 1 and 2 with bile acids was entirely driven by favorable enthalpy change(ΔH°) with accompanying negative entropy change (ΔS°). The stronger binding of bile acids byl/d-Trp-β-CD is attributable to higher enthalpic gains. The combined use of the calorimetric andNMR ROESY spectral examinations revealed the correlation between the thermodynamicparameters and the role of sidearm conformation in modified β-cyclodextrins.
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