Recognition mechanism of d- and l-tryptophan enantiomers using 2-hydroxypropyl-α- or β-cyclodextrins as chiral selectors

This work investigates the chiral recognition of d- and l-tryptophan enantiomers using 2-hydroxypropyl-α- or β-cyclodextrins (HPαCD and HPβCD) as selectors. Capillary electrophoresis experiments showed that only the α-form was effective for enantiomeric separation. NMR measurements (T1 and ROESY) and circular dichroism experiments were carried out to provide information about the topologies of interaction between the tryptophan isomers and cyclodextrins. For HPβCD, the inclusion of both isomers was verified, while for HPαCD only d-tryptophan was inserted in the cavity. These results agree with the stability constant values obtained from NMR diffusion experiments. We were able to determine that, together with charged groups of tryptophan (NH3+ and COO−), the indole moiety is the third guest interaction point with the cyclodextrin.