Action Spectroscopy of Gas-Phase Peptide Ions with Energetic Photons

Photodissociation studies on free complex protonated peptides and other biomolecular ions have long been limited to the UV wavelength range and longer wavelengths which are accessible by intense lasers. By interfacing tandem mass spectrometry with synchrotron beamlines, it is possible to overcome this limitation. We have thoroughly studied the interaction of vacuum ultraviolet (VUV) and soft X-ray photons with gas phase protonated peptides. Molecular fragmentation patterns, unobserved in conventional mass spectrometry, can be observed experimentally. Instead of relatively slow statistical fragmentation along the peptide backbone, much faster formation of fragment ions related to sidechains of aromatic amino acids is observed. The underlying process most likely involves fast charge migration. A previously unobserved dissociation scheme, in which photoabsorption leads to a fast loss of a tyrosine side chain can be observed for the VUV and soft X-ray range. This loss process leads to the formation of a residual peptide that is remarkably cold internally.