Stereoselective interaction of ketoprofen enantiomers with beta-cyclodextrin: ground state binding and photochemistry

The chiral recognition ability of beta-cyclodextrin (beta-CyD) vs. S- and R-ketoprofen (KP) enantiomers has been studied by circular dichroism (CD), isothermal titration calorimetry (ITC) and NMR. The association constants of the 1 : 1 complexes obtained from CD and ITC titration experiments resulted to be the same for both enantiomers within the experimental uncertainty. Well differentiated CD spectra were determined for the diastereomeric complexes. Their structure was assessed by molecular mechanics and molecular dynamics calculations combined with quantum mechanical calculation of the induced rotational strengths in the low energy KP:beta-CyD associates, upon comparison of the calculated quantities with the corresponding experimental CD. The inclusion geometry is similar for both enantiomers with the aromatic carbonyl inserted in the CyD cavity, the monosubstituted ring close to the primary CyD rim and the carboxylate group exposed to the solvent close to the secondary rim. NMR spectra fully confirmed the geometry of the diastereomeric complexes. Tiny structural differences were sensibly probed by CD and confirmed by 2D ROESY spectra. Photoproduct studies with UV absorption and MS detection as well as nanosecond laser flash photolysis evidenced lack of chiral discrimination in the photodecarboxylation of KP within the cavity and formation of a photoaddition product to beta-CyD by secondary photochemistry of 3-ethylbenzophenon