Xenon (atomic number 54, electron configuration [Kr]4d105s25p6) shows behaviour not observed for the lighter noble gases and is a close cousin of the lanthanides. Hyperpolarized (hp) 129Xe chemical exchange saturation transfer (Hyper-CEST) NMR methods have enabled the detection of cryptophane organic host molecules [1-3], protein vesicles, lipid vesicles, and bacterial spores [4] at sub-micromolar concentrations. In Hyper-CEST, encapsulated hp 129Xe is selectively depolarized by radiofrequency (rf) pulses, and the depolarized 129Xe rapidly exchanges with hp 129Xe to accumulate in the solvent pool, where loss of signal can be readily monitored (Figure 1). Cryptophane-A and its derivatives exhibit the highest measured affinity for xenon. However, functionalized cryptophanes require multi-step synthesis and are isolated in low yields. This has motivated our search for new molecular scaffolds for xenon, to advance applications in sensing, biomedical imaging, materials and biophysical chemistry. For example, commercially available cucurbit[6]uril (CB[6], Figure 1) can serve as an ultrasensitive 129Xe NMR contrast agent and biosensor [5]. We have learned that near-optimal xenon exchange rates can contribute even more significantly than Xe affinity to overall detection sensitivity using Hyper-CEST NMR. This concept makes accessible new classes of low-affinity Xe host molecules, including genetically encoded biosensors. References: [1] L. Schroeder, T.J. Lowery, C. Hilty, D.E. Wemmer, A. Pines, Science 314 (2006), 446-449. [2] Y. Bai, A.P. Hill, I.J. Dmochowski, Anal. Chem. 84 (2012), 9935-9941. [3] B.A. Riggle, Y. Wang, I.J. Dmochowski, J. Am. Chem. Soc., 137 (2015), 5542-5548. [4] Y. Bai, Y. Wang, A. Driks, M. Goulian, I.J. Dmochowski, Chem Sci., 5 (2014), 3197-3203. [5] Y. Wang, I.J. Dmochowski, Chem. Comm. 51 (2015), 8982-8985. Figure 1. Top: chemical structures of CB[6] and TAAC. Bottom: Hyper-CEST involving Xe-binding molecules represented by hexagons.

Xenon: New applications in materials chemistry and biosensing

251st National Meeting (2016)

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