Water soluble cryptophanes showing unprecedented affinity for xenon: Candidates as NMR-based biosensors

Cryptophanes bearing OCH2COOH groups in place of the methoxy groups represent a new class of xenon-carrier molecules soluble in water at biological pH. By using H-1 and Xe-129 NMR (thermally-and laser-polarized dissolved gas), the structural and dynamical behaviors of these host molecules as well as their interaction with xenon are studied. They are shown to exist in aqueous solution under different conformations in very slow exchange. A saddle form present for one of these conformations could explain the H-1 NMR spectra. Whereas the cryptophanes in such a conformation are unable to complex xenon, unprecedented high binding constants are found for cryptophanes in the other canonical crown-crown conformation. These host molecules could therefore be valuable candidates for biosensing using Xe-129 MRI.