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

Abstract

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.

Figure 1

HP ¹²⁹Xe NMR spectrum with 5 mM CB[6] dissolved in pH 7.2 PBS at 300 K. A 30 degree pulse was used and signal averaged over 8 scans. Fourier-transformed spectra were processed with zero-filling and Lorentzian line-broadening of 20 Hz. Peak width (FWHM) was 463 Hz for ¹²⁹Xe-aq peak, and 570 Hz for ¹²⁹Xe-CB[6] peak.

Figure 2

Hyper-CEST frequency-scan profile of 0.8 μM CB[6] in pH 7.2 PBS at 300 K. When saturation rf pulse was positioned at 121 ppm (-72 ppm from ¹²⁹Xe-aq peak), encapsulated ¹²⁹Xe was depolarized and exchange caused rapid decrease in ¹²⁹Xe-aq signal. The black squares show the experimental data, and the lines show the exponential Lorentzian fits.

Figure 3

Representative Hyper-CEST profile of 1.8 pM CB[6] in pH 7.2 PBS at 300 K. Saturation frequencies of Dsnob-shaped pulses were positioned at 122.3 ppm (193.5 – 71.2 ppm) and 264.7 ppm (193.5 + 71.2 ppm), for on- and off-resonance. Pulse length, τ_pulse = 1.05 ms; field strength, B_1,max = 279 μT.

Figure 4

Hyper-CEST spectra shown for 1 μM CB[6] in PBS (black), in blood plasma (blue), and in blood plasma with 10 μM putrescine (red); all data collected at 300 K.

Scheme 1

Top: Chemical structures of CB[6] and TAAC. Bottom: Hyper-CEST mechanism involving xenon-binding molecules represented by hexagons.