Large-shift, Rapid Exchange Endogenous CEST Contrast for Reporter Gene Product Design

Motivation: Endogenous CEST contrast from reporter gene products would benefit from exchangeable protons resonating above 3.5 ppm and exchanging rapidly. Goal(s): We sought to characterize high-shift CEST contrast in tryptophan-enriched peptide sequences to design a highly specific and selective reporter gene protein product. Approach: We performed CEST z-spectroscopy and QUESP analysis on several tryptophan-containing peptide sequences with variations on a WDWEQ motif. Results: We identified a CEST z-peak at 5.5 ppm exchanging at 250-350 s-1. Surprisingly, we also discovered a new fast-exchanging (ksw ~ 1800 s-1) CEST resonance at 4.4 ppm in one peptide. Both improve our ability to generate unique CEST contrast. Impact: Developing selective and specific MRI-detectable CEST contrast will greatly benefit noninvasive assessment of viral and cell based therapies. Our work in high-shift CEST contrast shows great potential to improve our ability to reliably monitor these therapies.