Actions and Interactions of Mirror-Image Cyclodextrins

Cyclodextrins (CDs) were the first identified molecules and arguably the most consequential molecules in the areas of macrocyclic science and technology. As carbohydrates, their intrinsic architecture and chirality have played a fundamental role in their response to and effect on guest molecules. Indeed, the nature of inclusion complexation was formalized with CD-based hosts. A recent report on the first synthesis of unnatural mirror-image l-cyclodextrins foreshadows a new era of stereochemical research involving macrocyclic entities. Here, we show the inherently difficult, but rapid and sensitive, separation of enantiomeric CDs. The absolute configurations of CD antipodes have been investigated and distinguished by vibrational circular dichroism (VCD). Results indicated that the most intense VCD band at 1150 cm^-1 arises from the C-O stretching of the glucosidic bonds lining the CD torus. It follows that chiroptical radiation can excite either d- or l-CD selectively. The enzymatic susceptibilities of mirror-image CDs are vastly different, suggesting the possibility of different biochemical and medicinal uses. The electrophoretic migration of small chiral molecules, e.g., amino acids is easily and predictably reversed with d- and l-CDs.