Complex Formation of Crown Ethers with the Cucurbit[6]uril–Spermidine and Cucurbit[6]uril–Spermine Complex in Aqueous Solution

Cucurbit[6]uril is an ultrasensitive (129)Xe NMR contrast agent

A lack of molecular contrast agents has slowed the application of ultrasensitive hyperpolarized (129)Xe NMR methods. Here, we report that commercially available cucurbit[6]uril (CB[6]) undergoes rapid xenon exchange kinetics at 300 K, and is detectable by Hyper-CEST NMR at 1.8 pM in PBS and at 1 μM in human plasma where many molecules, including polyamines, can compete with xenon for CB[6] binding.