Nanomolar small-molecule detection using a genetically encoded Xe NMR contrast agent.

Genetically encoded magnetic resonance imaging (MRI) contrast agents enable non-invasive detection of specific biomarkers . Here, we employed the hyper-CEST Xe NMR technique to quantify maltose (32 nM to 1 mM) through its modulation of conformational change and xenon exchange in maltose binding protein (MBP). Remarkably, no hyper-CEST signal was observed for MBP in the absence of maltose, making MBP an ultrasensitive "smart" contrast agent. The resonance frequency of Xe bound to MBP was greatly downfield-shifted (Δ = 95 ppm) from the Xe peak, which facilitated detection in as well as multiplexing with TEM-1 β-lactamase. Finally, a Val to Ala mutation at the MBP-Xe binding site yielded 34% more contrast than WT, with Xe resonance frequency shifted 59 ppm upfield from WT. We conclude that engineered MBPs constitute a new class of genetically encoded, analyte-sensitive molecular imaging agents detectable by Xe NMR/MRI.