Photoionization and excitation processes in proteins and peptides

We have performed systematic studies of molecular fragmentation patterns of peptides and proteins and their non-dissociative ionization yields as a function of molecular size and conformation. The molecular ionization and fragmentation processes have been triggered by absorption of energetic photons.

We presented the results of near-edge X-ray absorption mass-spectrometry of peptides and proteins of various masses. The molecular masses ran from 500 Da to more than 12 kDa. For the smaller molecules up to approximately 2kDa the fragment mass spectra were dominated by small fragments while for larger molecules with masses exceeding ~8 kDa the prominent mass-spectral features were due to non-dissociative ionization.

For 20 eV VUV photons the transition from the extensive fragmentation to the non-dissociative ionization regime occured at much smaller molecule size compared to soft X-ray. Additionally, the influence of the photon energy on the photofragmentation patterns was studied.

In the final we performed a soft X-ray absorption spectroscopy case study for the melittin peptide. We observed a clear decline of non-dissociative double ionization below the ionization threshold with increasing melittin protonation state. This was explained by the lower probability of secondary electron ionization by the Auger electrons due to the less compact structure of melittin for higher charge states. In addition to non-dissociative ionization, we studied small neutral molecule loss and formation of ions due to backbone scission.