In the case of peptides, the subsequent CID of the isolated radical anions results in intense backbone cleavages. The oxidized species can be isolated and further activated by collisions (activated-EPD). This results in the production of radical species with low excess internal energy. For peptides that contain natural aromatic amino-acids (i.e., tryptophan, tyrosine and, in a lesser extent, phenylalanine), absorption at ~260 nm is mainly due to a resonant electronic excitation (π–π * transition), followed by electronic reorganization and electron emission. For peptide and protein polyanions, electron photodetachment is the main relaxation channel observed following UV excitation. This led to the development of negative electron transfer dissociation (NETD), electron detachment dissociation (EDD), and electron photodetachment dissociation (EPD). Most of photodissociation experiments were first performed on peptide cations, while the interest in negative ions is growing due to the need of techniques suitable for acidic proteins, nucleic acids, or acidic sugars. What is going on at much higher energy is an unexplored field. That is, both yield mostly C α–C bond cleavage for singly protonated species, but both generate very abundant b/y-type ions for multiply-charged precursors 157 nm generally produces higher abundance x-ions compared with 193 nm, but both methods yield similar a-, v-, and w- products. Both 193 (6.4 eV) and 157 nm (7.9 eV) VUVPD methods are charge states dependent. In this aim, ArF (193 nm) and F 2 (157 nm) excimer lasers, which provide high photon fluences, short pulses duration, and high repetition rates, have been coupled to mass spectrometers. Another approach, which can be generalized to any peptide whatever its sequence, is to use higher photon energy (in the VUV range) to excite not only the aromatic amino acids but also the peptide backbone amides (below 200 nm). The above mentioned UV excitations rely on the existence of aromatic amino acids in the peptide sequence. In particular, in UVPD, specific fragmentation channels are detected, namely formation of radical cations following a hydrogen atom loss and C α–C β bond cleavage fragments. After excitation, direct dissociation in excited states competes with internal conversion to the electronic ground state and with radiative de-excitation. In UV photodissociation (UVPD, 200 nm < λ < 400 nm) and vacuum UV photodissociation (VUVPD, λ < 200 nm), the photon induces an electronic excitation of the peptide. In particular, a better understanding of the different mechanisms involved between the initial excitation event and the experimentally observed fragmentation might allow the development of new strategies for peptide analysis and protein identification. Optical properties of proteins and radical proteins and their relaxation pathways in the VUV range is of fundamental interest to understand the response of biomolecules to radiation and oxidative stresses, and may also have important applications in analytical sciences. Spectra for radical and non-radical ions are quite similar in terms of observed bands however, the VUV fragmentation yield is enhanced by the presence of a radical in caerulein peptides. The detachment yield increases monotonically with the energy with the appearance of several absorption bands. These latter ions are generated by electron photodetachment from 3– precursor ions. We also report photofragment yields as a function of photon energy for doubly deprotonated caerulein ions, for both closed-shell ( 2–) non-radical ions and open-shell ( 2– Thus, there is no memory effect of the initial excitation energy on the fragmentation channels of the oxidized species on the time scale of our tandem MS experiment. The branching ratios of the different fragments observed by CID as a function of the initial VUV photon energy are found to be independent of the initial photon energy. The oxidized ions, generated by electron photodetachment were further isolated and activated by collision (CID) in a MS 3 scheme. However, an increase in the fragmentation efficiency (neutral losses and peptide backbone cleavages) as a function of the energy is also observed. Electron loss is found to be the major relaxation channel at every photon energy. Caerulein is a sulphated peptide with three aromatic residues and nine amide bonds. We have studied the photodissociation of gas-phase deprotonated caerulein anions by vacuum ultraviolet (VUV) photons in the 4.5 to 20 eV range, as provided by the DESIRS beamline at the synchrotron radiation facility SOLEIL (France).
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