Near-edge X-ray absorption fine structure spectra of some peptide model molecules were measured utilizing an OA-TOF mass spectrometer and were calculated with time-dependent density functional theory. Excitation source such as soft X-ray photons were from synchrotron radiation at TLS. The intensity and branching ratio as functions of photon energy were obtained for all ionic products. Based on our theoretical calculation, the absorption peaks were assigned as core excitations to various destination virtual orbitals. To verify what the dissociation paths are, isotopic molecules were utilized to reveal the identities of the enhanced ionic fragments. It is interesting to note that most of the enhanced products were produced via breaking the peptide bond (including hydrogen atom(s) elimination) no matter the initial excited atom was carbon, nitrogen or oxygen. The branching ratios of breaking peptide bonds could be as high as 71% from specific core excited N-methylformamide. Adding a phenyl group on the peptide model, the branching ratios decrease dramatically to about 35% which is possibly due to the mixed destination molecular orbital. The specific dissociation also occurs on all peptide bonds while a molecule contains more than one peptide-bond. For the models composed of more than one peptide bonds, the ionic products from core-excited molecules and their branching ratios as functions of excitation energy show branching ratios 42% - 75% of dissociation of the peptide bond, especially at resonant excitations. One example of mass spectra and fragment assignments is shown in Figure 1. The major destination orbitals assigned at these excitation energies show significant antibonding character along peptide bonds, which causes large probabilities of bond cleavage.

NEXAFS spectra and specific dissociation of oligo-peptide model molecules

2019 Fall National Meeting

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