The synthesis of Tfm for site-directed editing of C-H→C-F in Met is described in the Supporting Information. TEM samples were prepared by crushing powders onto standard TEM grids coated with lacy carbon. EELS data were collected at 60 keV using a Nion UltraSTEM100MC 'Hermes' scanning transmission electron microscope, at the SuperSTEM National Research Facility for Advanced Electron Microscopy. FTIR spectra were recorded with a Bruker Tensor 27 FT at the University of Oxford, Department of Chemistry. DFT calculations for the assignment of FTIR data were performed with the B3LYP functional and the cc-pVTZ basis set. Further details of the EELS and FTIR data collection and processing are reported in the Supplementary Information, and raw The acceleration voltage used was 60 kV and the probe convergence semi-angle was 21 mrad, with a probe current of approximately 3 pA after monochromation. The spectrometer acceptance semi-angle was 24 mrad. The nominal energy resolution was determined to be ~8 meV from the zero-loss peak full width at half-maximum (FWHM), at a spectral sampling of 0.002 eV/channel, and recorded in vacuum far away from any sample using aligned series of short 1ms acquisitions.