Abstract:

β-cyclodextrin (β-CD) has a central cavity into which organic molecules can enter to form inclusion complexes. We report here ¹H NMR features of a variety of β-CD complexes formed with tryptophan (Try) in a molar ration of  1: 1 which illustrate the enantioselective binding of Try by β-CD . The complex ation between the β-CD  and the racemic Try induced remarkable change in the ¹H chemical shifts of both host and guest. β-CD  inner protons (H3、H5 、Hab) and Try protons shift upfield due to anisotropic shielding. It can be consid ered to be a proof that the complexes are of inclusion-type, involving the inse rt ion of an aromatic group of the guest molecule inside the macrocycle cavity, primarily from the secondary rim of the β-CD. The observed Δδ of proton signals , when enantiopure D-Try or L-Try were used as guest molecules, were entirely  comparable to those induced by the corresponding recemic DL-Try and Δδ of D- Try+β-CD  complex were slightly larger than those of L-Try+β-CD complex. This suggests that the binding modes for both enantiomers of each compound are ess entially the same and the interaction between D-Try and β-CD  is stronger tha n that between L-Try and β-CD  which has been verified by our previous study.  The enantiodiscrimination must be found in subtle geometrical differences.

Key words: ¹H NMR, β-cyclodextrin, Tryptophan, Host-gue st complexes, Enantiodiscrimination