Amnon Bar-Shir1,
2, Yajie Liang1, 2, Assaf A. Gilad1,
2, Jeff W.M. Bulte1, 2
1Department
of Radiology, Johns Hopkins University, Baltimore, MD, United States; 2Institute
for Cell Engineering, Johns Hopkins University, Baltimore, MD, United States
Positively charged amino acids (mostly lysine and arginine) in peptide and proteins enable their use as CEST-based contrast agents or reporter genes. We show here that superpositively-charged mutants of the green fluorescent protein (GFP) reporter gene can generate superior CEST contrast. The additional, water-exposed, lysine and arginine amino acids of these mutants were found to increase the CEST contrast obtained from the baseline guanidine (arginine) and amide (lysine and arginine) exchangeable protons. These mutant GFPs retained their fluorescence thus making it a bimodal reporter gene.