Cryptophane

Abstract: The interaction of xenon with cryptophane-A in 1,1,2,2-tetrachloroethane-d2 is investigated by 129Xe and 1H NMR spectroscopy. Xenon is reversibly trapped into the cavity of this host to form a 1 to 1 host−guest complex with an apparent association constant K of the order of at least 3 × 103 M-1 at 278 K. The exchange between the free and bound xenon is slow on the 129Xe NMR time scale, and the bound xenon resonance is shifted by approximately 160 ppm to lower frequencies with respect to the free xenon resonance. The xenon complex is at least 4 and 20 times more stable, respectively, than the corresponding chloroform and methane complexes under the same conditions. The stability of this xenon complex appears to be much greater than that of the previously described xenon complex of α-cyclodextrin in water. This is probably due to the combination of three favorable effects: (i) good size matching between the guest and the cryptophane cavity in its most relaxed conformation, resulting in the optimization of ...

Abstract: 129Xe NMR biosensors are promising agents for early disease detection, especially when their interactions with target biomolecules can perturb 129Xe chemical shifts well beyond the typical field inhomogeneity of clinical MRI. We introduce human carbonic anhydrase (CA) as a single-binding-site enzyme for studying xenon biosensor−protein interactions. A xenon-binding cryptophane was substituted with linkers of varying lengths to p-benzenesulfonamide to yield nondiastereomeric biosensors with a single 129Xe NMR resonance. X-ray crystallography confirmed binding of the eight-bond-linked biosensor containing a single xenon atom in the CAII active site. Biosensor dissociation constants (Kd = 20−110 nM) were determined by isothermal titration calorimetry (ITC) for isozymes CA I and II. The biosensor−CA complexes yielded “bound” hyperpolarized 129Xe NMR resonances of narrow line width that were shifted by 3.0−7.5 ppm downfield, signifying much larger shifts than seen previously. Moreover, isozyme-specific chemica...

Abstract: Metalation of the exterior arene faces of the molecular capsule (±)-cryptophane-E with [Cp*Ru]+ moieties results in a π-acidic cavity capable of encapsulating anions. The [CF3SO3]- and [SbF6]- salts have been crystallographically characterized and demonstrate the encapsulation of these anions by the metalated cryptophane. 1H and 19F NMR spectroscopy establish the binding of anions in NO2CD3 solution and reveal the relative affinity of the cavity for different anions (KX−/KOTf−): [BF4]- ≈ 0, [PF6]- = 1.18, [CF3SO3]- ≡ 1, [SbF6]- = 0.30. Variable temperature rate studies reveal the activation barrier for triflate encapsulation to be ΔG⧧298K = 18.0(8) kcal·mol-1 (ΔH⧧ = 17.5(4) kcal·mol-1 and ΔS⧧ = 2(1) cal·mol-1·K-1).