Thermodynamics of Xenon Binding to Cryptophane in Water and Human Plasma

Xenon-129 biosensors offer an attractive alternative to conventional MRI contrast agents due to the chemical shift sensitivity and large nuclear magnetic resonance signal of hyperpolarized 129 Xe. Here we report the use of fluorescence spectroscopy and isothermal titration calorimetry (ITC) to determine xenon binding affinity and thermodynamics with a water-soluble triacid-cryptophane-A ( 1 ). 1 was synthesized in 10 steps with a 4% overall yield. Fluorescence spectroscopy measured an association constant of (1.7 ± 0.2) × 10 4 M -1 in phosphate buffer at 293 K. ITC measurements at 293 and 310 K yielded association constants of (1.73 ± 0.17) × 10 4 and (3.01 ± 0.26) × 10 4 M -1 and indicated a large entropic contribution to xenon binding in water. On the basis of these data, cryptophane 1 showed roughly 2-fold higher affinity for xenon than any previously measured compound. Remarkably, ITC measurements in human plasma at 310 K gave a similar binding constant, K A = (2.19 ± 0.22) × 10 4 M -1, which supports the development of 129 Xe NMR biosensors for biological applications.