Photodissociation of protonated leucine-enkephalin in the VUV range of 8-40 eV.

Until now, photodissociation studies on free complex protonated peptides were limited to the UV wavelength range accessible by intense lasers. We have studied photodissociation of gas-phase protonated leucine-enkephalin cations for vacuum ultraviolet (VUV) photons energies ranging from 8 to 40 eV. We report time-of-flight mass spectra of the photofragments and various photofragment-yields as a function of photon energy. For sub-ionization energies our results are in line with existing studies on UV photodissociation of leucine-enkephalin. For photon energies exceeding 10 eV we could identify a new dissociation scheme in which photoabsorption leads to a fast loss of the tyrosine side chain. This loss process leads to the formation of a residual peptide that is remarkably cold internally.