Design of Highly Fluorinated Peptides for Cell-based 19 F NMR.

The design of imaging agents with high fluorine content is essential for overcoming the challenges associated with signal detection limits in 19 F MRI-based molecular imaging. In addition to perfluorocarbon and fluorinated polymers, fluorinated peptides offer an additional strategy for creating sequence-defined 19 F magnetic resonance imaging (MRI) imaging agents with a high fluorine signal. Our previously reported unstructured trifluoroacetyllysine-based peptides possessed good physiochemical properties and could be imaged at high magnetic field strength. However, the low detection limit motivated further improvements in the fluorine content of the peptides as well as removal of nonspecific cellular interactions. This research characterizes several new highly fluorinated synthetic peptides composed of highly fluorinated amino acids. 19 F NMR analysis of peptides TB-1 and TB-9 led to highly overlapping, intense fluorine resonances and acceptable aqueous solubility. Flow cytometry analysis and fluorescence microscopy further showed nonspecific binding could be removed in the case of TB-9 . As a preliminary experiment toward developing molecular imaging agents, a fluorinated EGFR-targeting peptide ( KKK FF KK- β A-YHWYGYTPENVI ) and an EGFR-targeting protein complex E 1 -DD bioconjugated to TB-9 were prepared. Both bioconjugates maintained good 19 F NMR performance in aqueous solution. While the E 1 -DD-based imaging agent will require further engineering, the success of cell-based 19 F NMR of the EGFR-targeting peptide in A431 cells supports the potential use of fluorinated peptides for molecular imaging.