Abstract
VUV action spectroscopy has recently gained interest for the study of peptides and proteins. However, numerous aspects of the fundamental processes involved in the photodissociation are yet to be understood. It can, for example, be expected that sulfur-containing amino-acid residues have a significant impact on the dissociation processes following photoionization because of their potential involvement in the transport of electron holes in proteins. In order to investigate the influence of the sulfur-containing methionine residue on the VUV photodissociation of a small peptide a VUV action spectroscopy study of gas-phase protonated methionine-enkephalin and leucine-enkephalin in the photon energy range of 6–35 eV was performed. The results show that upon non-ionizing photoexcitation, the fragmentation patterns of the two peptides are nearly identical, whereas significant differences were observed upon photoionization. The differences between the fragment yields and the identified specific dissociation channels for methionine-enkephalin could be explained by the high electron hole affinity of sulfur, which efficiently directs the radical to the methionine side chain. Additionally, for both peptides the presence of the intact photoionized precursor ions was confirmed by their isotopic patterns and the stability of these species could be evaluated.Graphic abstract
Highlights
VUV action spectroscopy has recently gained interest for the study of peptides and proteins
In order to investigate the influence of the sulfur-containing methionine residue on the VUV photodissociation of a small peptide a VUV action spectroscopy study of gas-phase protonated methionine-enkephalin and leucine-enkephalin in the photon energy range of 6–35 eV was performed
In order to investigate the extent of the effects of the methionine residue on the dissociation mechanism of small protonated peptides following valence shell photoexcitation and photoionization, we performed a VUV action spectroscopy study on gas-phase protonated LeuEnk and MetEnk covering the photon energy range of 6–35 eV
Summary
Action spectroscopy in the VUV range, commonly with photon energies ranging from 5 to 40 eV, enables, with increasing photon energy, probing of photoexcitations and photoionizations from outer down to inner valence levels. The electronic energy may be quickly converted into vibrational energy by intramolecular vibrational energy redistribution (IVR), leading, in most cases, to statistical fragmentation While large systems, such as proteins, can withstand the increased charge and stay intact following ionization by distributing the internal energy over their many degrees of freedom, smaller peptides undergo extensive fragmentation into small internal fragments, immonium ions and side-chain fragments [8]. We want to investigate the influence of a sulfur-containing residue on specific bond cleavage in a peptide upon VUV photoabsorption These amino acids, namely cysteine and methionine, are known for participating in electron transfer reactions within proteins thanks to their low ionization potentials and their ability to interact with aromatic side chains [13,14,15]. We discuss the influence of the methionine residue on the dissociation mechanisms of MetEnk based on the experimental partial ion yield spectra of several photoproducts and the comparison with collision-induced dissociation results
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