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 129Xe. 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) × 104 M-1 in phosphate buffer at 293 K. ITC measurements at 293 and 310 K yielded association constants of (1.73 ± 0.17) × 104 and (3.01 ± 0.26) × 104 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) × 104 M-1, which supports the development of 129Xe NMR biosensors for biological applications.

Xenon-129 biosensors offer an attractive alternative to conventional MRI contrast agents due to the chemical shift sensitivity and large nuclear magnetic signal of hyperpolarized (129)Xe. Here, we report the first enzyme-responsive (129)Xe NMR biosensor. This compound was synthesized in 13 steps by attaching the consensus peptide substrate for matrix metalloproteinase-7 (MMP-7), an enzyme that is upregulated in many cancers, to the xenon-binding organic cage, cryptophane-A. The final coupling step was achieved on solid support in 80-92% yield via a copper (I)-catalyzed [3+2] cycloaddition. In vitro enzymatic cleavage assays were monitored by HPLC and fluorescence spectroscopy. The biosensor was determined to be an excellent substrate for MMP-7 (K(M) = 43 microM, V(max) = 1.3 x 10(-)(8) M s(-1), k(cat)/K(M) = 7,200 M(-1) s(-1)). Enzymatic cleavage of the tryptophan-containing peptide led to a dramatic decrease in Trp fluorescence, lambda(max) = 358 nm. Stern-Volmer analysis gave an association constant of 9000 +/- 1,000 M(-1) at 298 K between the cage and Trp-containing hexapeptide under enzymatic assay conditions. Most promisingly, (129)Xe NMR spectroscopy distinguished between the intact and cleaved biosensors with a 0.5 ppm difference in chemical shift. This difference most likely reflected a change in the electrostatic environment of (129)Xe, caused by the cleavage of three positively charged residues from the C-terminus. This work provides guidelines for the design and application of new enzyme-responsive (129)Xe NMR biosensors.

Cryptophane-A, comprised of two cyclotriguaiacylenes joined by three ethylene linkers, is a prototypal organic host molecule that binds reversibly to neutral small molecules via London forces. Of note are trifunctionalized, water-soluble cryptophane-A derivatives, which exhibit exceptional affinity for xenon in aqueous solution. In this paper, we report high-resolution X-ray structures of cryptophane-A and trifunctionalized derivatives in crown–crown and crown–saddle conformations, as well as in complexes with water, methanol, xenon or chloroform. Cryptophane internal volume varied by more than 20% across this series, which exemplifies 'induced fit' in a model host–guest system.