STUDY ON CHIRAL DISCRIMINATION ABILITY OF CYCLODEXTRINS TOWARD DL-ALANINE BETA-NAPHTHYLAMIDE IN MIXTURES OF D2O AND VARIOUS ORGANIC-SOLVENTS BY H-1-NMR SPECTROSCOPY

H-1-NMR spectroscopy was used to investigate the chiral discrimination mechanism of cyclodextrins (mainly 2,6-dimethylated beta-cyclodextrin, DM-beta-CyD) toward DL-alanine beta-naphthylamide (DL-AN) in a mixture of D2O and various perdeuterated organic solvents such as acetone, acetonitrile (MeCN), dimethylformamide (DMF), dimethylsulfoxide (DMSO), dioxane, tetrahydrofuran (THF), methanol, ethanol, and 2-propanol. We have found that the chiral discrimination ability of the cyclodextrins toward the guest enantiomers can be evaluated from the splitting of their proton signals, especially that of the naphthyl a-proton, along with a large and lower magnetic field shift. In a D2O solution of DM-beta-CyD and DL-AN, a signal splitting of each proton of DL-AN was not observed, although DM-beta-CyD can interact strongly with DL-AN. However, the addition of each organic solvent to a D2O solution of DM-beta-CyD and DL-AN has been found to enhance the chiral discrimination for the enantiomers, in spite of a lowered stability of their inclusion complex. The highest chiral discrimination was obtained in the mixed solvents containing 20 approximately 30(v/v)% of organic solvents such as acetone, MeCN, DMF or DMSO. With dioxane and THF, the chiral discrimination was lower than that of the above organic solvents, possibly due to the strong interaction of DM-beta-CyD with these solvents. On the other hand, with trimethylated-beta-cyclodextrin (TM-beta-CyD) which has no hydroxyl group, chiral discrimination could not be observed. These results indicate that the hydroxyl groups of CyDs may play an important part in chiral discrimination.