Formation and Fragmentation of Radical Peptide Anions: Insights from Vacuum Ultra Violet Spectroscopy

Abstract

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). Caerulein is a sulphated peptide with three aromatic residues and nine amide bonds. Electron loss is found to be the major relaxation channel at every photon energy. However, an increase in the fragmentation efficiency (neutral losses and peptide backbone cleavages) as a function of the energy is also observed. The oxidized ions, generated by electron photodetachment were further isolated and activated by collision (CID) in a MS 3 scheme. 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. 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. We also report photofragment yields as a function of photon energy for doubly deprotonated caerulein ions, for both closed-shell ([M -2H] 2-) non-radical ions and open-shell ([M -3H] 2-• ) radical ions. These latter ions are generated by electron photodetachment from [M -3H] 3-precursor ions. The detachment yield increases monotonically with the energy with the appearance of several absorption bands. 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.

Photo-induced electron detachment of protein polyanions in the VUV range

“…29 To assess the optical properties of radical protein anions, we also combined a laser (Nd 3+ :YAG laser, time duration 5 ns, 4th harmonic λ = 266 nm, repetition rate 20 Hz, 20 mJ/pulse) to the synchrotron radiation in a two-color scheme. 26,27 The two photon beams were spatially combined via a drilled-coated mirror located ∼30 cm upstream of the ion trap entrance. The UV laser light was reflected at 90 • by the coated mirror and injected collinearly to the axis of the ion trap.…”

“…2 We used electron photodetachment to report the first electronic excitation spectra of polypeptides, 22,23 and of a protein in vacuo, 24 and demonstrated the feasibility of UV spectroscopy on large isolated biomolecular ions. 25 Electron emission following VUV excitation of peptides and proteins was recently reported, 26,27 opening the route for VUV spectroscopy of large proteins in the gas phase. In this paper, we explore the photo-induced electron detachment of protein polyanions in the 6-16 eV VUV range.…”

Photodissociation mass spectrometry: new tools for characterization of biological molecules

“…Ion spectroscopy (typically undertaken as a type of photodissociation action spectroscopy) has been covered in a number of excellent reviews and will not be included in depth here. 27–32 …”

Contribution of synchrotron radiation to photoactivation studies of biomolecular ions in the gas phase

“…Electron photodetachment spectroscopy has been probed further in the VUV for carulein, a sulfated peptide, melittin and insulin using the coupling of the LTQ XL and a VUV beamline described in part II.B.3.2. Electron detachment remained the main relaxation channel in the 4.5–20 eV range (Brunet et al, ). Observation of resonances on the photodetachment yields, which correspond closely to electronic transitions reported for peptides (Serrano‐Andrés & Fülscher, ), indicated that the two steps photophysical model for photodetachment was still valid in an extend wavelength range.…”

“…Observation of resonances on the photodetachment yields, which correspond closely to electronic transitions reported for peptides (Serrano‐Andrés & Fülscher, ), indicated that the two steps photophysical model for photodetachment was still valid in an extend wavelength range. Investigations on the doubly deprotonated carulein (a sulfated peptide) revealed that the main fragmentations were a 7 , a 8 , and a 9 ‐sequence ions accompanied by abundant tryptophan, CO 2 and SO 3 losses from the oxidized species (Brunet et al, ). The fragmentation pattern did not evolve much in the 5.5–20 eV range.…”