Vibrational Electron Energy Loss Spectroscopy for Zero-Size Labeling of Amino Acids

Vibrational electron energy-loss spectroscopy (vibEELS) measures the bonding environment of molecules with high spatial resolution. Here, we demonstrate that vibEELS can be used to identify site-specific vibrational signatures of selectively labelled functional groups in unnatural amino acids. We have examined the utility of trifluorination of methionine, obtained by site-directed conversion of carbon-hydrogen (C–H) bonds to carbon–fluorine (C–F) bonds. Fluorine is considered to be a ‘zero-size’ label and can be used for minimally invasive chemical alterations of proteins. The vibEEL spectra of trifluoromethionine (Tfm) directly records the C–F stretching mode, characteristic of the trifluoromethyl (CF3) functional group, for comparison with infrared spectra collected from the same sample. However, the EELS data have superior spatial resolution and as the vibrational response is localised at the nanometer level, large, macroscopic volumes of samples are not required for measurements. For these reasons the use of vibEELS with site-selective incorporation enables the possibility of spatially tracking selectively modified protein labels in the electron microscope for the study of biological pathways and protein structure dynamics with high spatial resolution.