Enantiopure Cryptophane- 129 Xe Nuclear Magnetic Resonance Biosensors Targeting Carbonic Anhydrase.

The (+) and (-) enantiomers for a cryptophane-7-bond-linker-benzenesulfonamide biosensor (C7B) were synthesised and their chirality was confirmed by electronic circular dichroism spectroscopy. Biosensor binding to carbonic anhydrase II (CAII) was characterised for both enantiomers by hyperpolarised (HP) Xe-129 NMR spectroscopy. Our previous study of the racemic (+/-) C7B biosensor-CAII complex [Chambers, J.M.; Hill, P.A.; Aaron, J.A.; Han, Z.H.; Christianson, D.W.; Kuzma, N.N.; Dmochowski, I.J. J. Am. Chem. Soc.2009, 131, 563-569] identified two 'bound' Xe-129@C7B peaks by HP Xe-129 NMR (at 71 and 67ppm, relative to 'free' biosensor at 64ppm), which led to the initial hypothesis that (+) and (-) enantiomers produce diastereomeric peaks when coordinated to Zn2+ at the chiral CAII active site. Unexpectedly, the single enantiomers complexed with CAII also identified two 'bound' Xe-129@C7B peaks: (+) 72, 68ppm and (-) 68, 67ppm. These results are consistent with X-ray crystallographic evidence for benzenesulfonamide inhibitors occupying a second site near the CAII surface. As illustrated by our studies of this model protein-ligand interaction, HP Xe-129 NMR spectroscopy can be useful for identifying supramolecular assemblies in solution.