NMR-Based Biosensing with Optimized Delivery of Polarized ¹²⁹Xe to Solutions

Abstract

Laser-enhanced (LE) 129Xe nuclear magnetic resonance (NMR) is an exceptional tool for sensing extremely small physical and chemical changes; however, the difficult mechanics of bringing polarized xenon and samples of interest together have limited applications, particularly to biological molecules. Here we present a method for accomplishing solution 129Xe biosensing based on flow (bubbling) of LE 129Xe gas through a solution in situ in the NMR probe, with pauses for data acquisition. This overcomes fundamental limitations of conventional solution-state LE 129Xe NMR, e.g., the difficulty in transferring hydrophobic xenon into aqueous environments, and the need to handle the sample to refresh LE 129Xe after an observation pulse depletes polarization. With this new method, we gained a factor of >100 in sensitivity due to improved xenon transfer to the solution and the ability to signal average by renewing the polarized xenon. Polarized xenon in biosensors was detected at very low concentrations, </=250 nanomolar, while retaining all the usual information from NMR. This approach can be used to simultaneously detect multiple sensors with different chemical shifts and is also capable of detecting signals from opaque, heterogeneous samples, which is a unique advantage over optical methods. This general approach is adaptable for sensing minute quantities of xenon in heterogeneous in vitro samples, in miniaturized devices and should be applicable to certain in-vivo environments.

Spectrally Resolved Magnetic Resonance Imaging of a Xenon Biosensor

“…Water flowed from the liquid supply (1000 ml) to the bubbling cell (5 ml), where it was saturated with hyperpolarized xenon (4.4 mM·atm -1 ) [1,6] (Amersham Health, Durham, NC), then through the sample to the "liquid out" port. Hyperpolarized xenon gas mixture (1% Xe at natural isotope abundance, polarized to ~3%, mixed with 10% N 2 , 89% He; Airgas, Radnor, PA) was supplied with a pressure of 6 atm to the "gas in" port, and flowed through the bubbling cell to the "gas out" port.…”

“…Chemical structure of the functionalized xenon biosensor; [2,6] NMR spectrum of xenon biosensor bound to avidin containing agarose beads. …”

“…Recent work has shown the possibility of using the xenon biosensor to detect small amounts of a substance in a heterogeneous environment by NMR. [6] Here, we demonstrate that magnetic resonance (MR) provides the capability to obtain spectrally and spatially resolved images of the distribution of 1 immobilized biosensor, opening the possibility for using the xenon biosensor for targeted imaging.…”

“…The downfield peaks at 193.6 and 192.5 ppm are from dissolved xenon in agarose beads and water, respectively, [6] and the 0 ppm peak is from xenon gas. The xenon resonance from the immobilized biosensor is broadened when compared to that for solution-free biosensor (~20 Hz), [2,7] presumably due to motional hindrance of the cage.…”

Time-of-Flight Flow Imaging Using NMR Remote Detection

“…The experimental setup is detailed elsewhere [21], and only a brief description is given here. Two independent RF probes were used for encoding and detection.…”