¹²⁹Xe and ¹H NMR Study of the Reversible Trapping of Xenon by Cryptophane-A in Organic Solution

Abstract

The interaction of xenon with cryptophane-A in 1,1,2,2-tetrachloroethane-d 2 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 the London forces between the highly polarizable guest and the electron rich aromatic rings of the host (enthalpic stabilization); (ii) no rotational or vibrational entropy loss of the monatomic guest in the cryptophane cavity; and (iii) no (or little) entropy loss due to reduction of the conformational freedom of the host. Analysis of the line widths of the signals corresponding to the free and bound xenon as a function of the relative xenon/cryptophane ratio suggests that the incoming xenon atom must displace the departing one to enter the cryptophane cavity, and that the empty cryptophane is not involved in the complexation equilibrium.

Development of a Functionalized Xenon Biosensor

“…65 In organic solvents at room temperature, cryptophane-A-bound xenon shows exchanged-broadened linewidths of hundreds of Hertz, 31 suggesting exchange rates more than an order of magnitude higher than those measured here.…”

“…31 Similarly, the xenon spectrum for a racemic mixture of the allyl-substituted cryptophane-A cage enantiomers (15) shows only one resonance (Figure 3a). However, the spectra of the xenon biosensor exhibit multiple peaks rather than a single resonance (Figure 3c).…”

Temperature‐Controlled Molecular Depolarization Gates in Nuclear Magnetic Resonance

“…Cryptophane cages temporarily encapsulate xenon (6) and facilitate functionalized molecular biosensors that combine high specificity in detecting biomolecules and high sensitivity of lp 129 Xe (7). The spins of bound nuclei can be selectively depolarized with rf pulses because of the unique chemical environment offered by the cage.…”

“…When using temperature to control depolarization transfer, three effects should be considered: First, increasing temperature increases the exchange rate of xenon with the cage molecules (6,9,10). Second, the binding constant of the cage-xenon complex tends to increase as temperature increases (11), making more xenon susceptible to selective saturation.…”

“…Biosensor detection now includes features of MRI thermometry (13)-which could be used to monitor hyperthermia in oncologic therapy (14)-but the concept of a transpletor also illustrates increased sensitivity at body temperature. Moreover, this concept can be applied to other problems that rely on exchangeable NMR-detected guests to reveal properties of the host structure or indirect detection of competing guests (6,10,15,16). Such experiments would facilitate studies of the exchange dynamics of other nanostructure hosts (alternative cages, carbon nanotubes, zeolites (17)).…”