Nanomolar small-molecule detection using a genetically encoded 129 Xe NMR contrast agent.

Genetically encoded magnetic resonance imaging (MRI) contrast agents enable non-invasive detection of specific biomarkers in vivo . Here, we employed the hyper-CEST 129 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 129 Xe bound to MBP was greatly downfield-shifted (Δ δ = 95 ppm) from the 129 Xe(aq) peak, which facilitated detection in E. coli 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 129 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 129 Xe NMR/MRI.