NMR study of optically active monosubstituted cryptophanes and their interaction with xenon

The study of the interaction between xenon and two cryptophane derivatives in organic solution has been carried out through H-1 and Xe-129 NMR. The cage molecules constituted by the grafting of (-)-camphanic acid are diastereomers through the orientation of the ethylenedioxy linkers joining the cyclotriveratrylene moieties. The availability of laser-polarized xenon has allowed us not only to perform Xe-129 spectra at a low concentration of dissolved gas but also to perform magnetization-transfer experiments from xenon to protons, confirming the inclusion of noble gas atoms into the cryptophane cavities. Binding constants of xenon in diastereomers were compared, as well as xenon exchange rates in the cryptophane cavities. A detailed structural and dynamic study performed with help of off-resonance ROESY experiments gave precious local information. The cryptophane derivative with the highest affinity for xenon seems to be the most able to distort its cavity to fit the xenon atom. In the complex, the cavity is elongated, and the noble gas atom can find different locations. This seems to indicate that the entropy contribution is also important in explaining the difference in affinity of xenon between these cage molecules. These results clearly show that in the design of new cage molecules for biosensing, the separation of cryptophane diastereomers is important because their affinity and their properties vis-a-vis xenon can be strongly different.