Electron capture induced dissociation of doubly protonated pentapeptides: Dependence on molecular structure and charge separation

Photoleucine Survives Backbone Cleavage by Electron Transfer Dissociation. A Near-UV Photodissociation and Infrared Multiphoton Dissociation Action Spectroscopy Study

We report a combined experimental and computational study aimed at elucidating the structure of N-terminal fragment ions of the c type produced by electron transfer dissociation of photo-leucine (L*) peptide ions GL*GGKX. The c 4 ion from GL*GGK is found to retain an intact diazirine ring that undergoes selective photodissociation at 355 nm, followed by backbone cleavage. Infrared multiphoton dissociation action spectra point to the absence in the c 4 ion of a diazoalkane group that could be produced by thermal isomerization of vibrationally hot ions. The c 4 ion from ETD of GL*GGK is assigned an amide structure by a close match of the IRMPD action spectrum and calculated IR absorption. The energetics and kinetics of c 4 ion dissociations are discussed. Graphical Abstract ᅟ.

Gas-phase structure of amyloid-β (12-28) peptide investigated by infrared spectroscopy, electron capture dissociation and ion mobility mass spectrometry

The gas-phase structures of doubly and triply protonated Amyloid-β12-28 peptides have been investigated through the combination of ion mobility (IM), electron capture dissociation (ECD) mass spectrometry, and infrared multi-photon dissociation (IRMPD) spectroscopy together with theoretical modeling. Replica-exchange molecular dynamics simulations were conducted to explore the conformational space of these protonated peptides, from which several classes of structures were found. Among the low-lying conformers, those with predicted diffusion cross-sections consistent with the ion mobility experiment were further selected and their IR spectra simulated using a hybrid quantum mechanical/semiempirical method at the ONIOM DFT/B3LYP/6-31 g(d)/AM1 level. In ECD mass spectrometry, the c/z product ion abundance (PIA) has been analyzed for the two charge states and revealed drastic differences. For the doubly protonated species, N - Cα bond cleavage occurs only on the N and C terminal parts, while a periodic distribution of PIA is clearly observed for the triply charged peptides. These PIA distributions have been rationalized by comparison with the inverse of the distances from the protonated sites to the carbonyl oxygens for the conformations suggested from IR and IM experiments. Structural assignment for the amyloid peptide is then made possible by the combination of these three experimental techniques that provide complementary information on the possible secondary structure adopted by peptides. Although globular conformations are favored for the doubly protonated peptide, incrementing the charge state leads to a conformational transition towards extended structures with 310- and α-helix motifs.

Electron transfer dissociation of photolabeled peptides. Backbone cleavages compete with diazirine ring rearrangements

Gas-phase conformations and electron transfer dissociations of pentapeptide ions containing the photo-Leu residue (L*) were studied. Exhaustive conformational search including molecular dynamics force-field, semi-empirical, ab initio, and density functional theory calculations established that the photo-Leu residue did not alter the gas-phase conformations of (GL*GGK  +  2H)(2+) and (GL*GGK-NH2 + H)(+) ions, which showed the same conformer energy ranking as the unmodified Leu-containing ions. This finding is significant in that it simplifies conformational analysis of photo-labeled peptide ions. Electron transfer dissociation mass spectra of (GL*GGK  +  2H)(2+), (GL*GGK-NH2 + 2H)(2+),(GL*GGKK + 2H)(2+), (GL*GLK + 2H)(2+), and (GL*LGK + 2H)(2+) showed 16 %-21 % fragment ions originating by radical rearrangements and cleavages in the diazirine ring. These side-chain dissociations resulted in eliminations of N2H3, N2H4, [N2H5], and [NH4O] neutral fragments and were particularly abundant in long-lived charge-reduced cation-radicals. Deuterium labeling established that the neutral hydrazine molecules mainly contained two exchangeable and two nonexchangeable hydrogen atoms from the peptide and underwent further H/D exchange in an ion-molecule complex. Electron structure calculations on the charge-reduced ions indicated that the unpaired electron was delocalized between the diazirine and amide π* electronic systems in the low electronic states of the cation-radicals. The diazirine moiety in GL*GGK-NH2was calculated to have an intrinsic electron affinity of 1.5 eV, which was further increased by the Coulomb effect of the peptide positive charge. Mechanisms are proposed for the unusual elimination of hydrazine from the photo-labeled peptide ions.

The early life of a peptide cation-radical. Ground and excited-state trajectories of electron-based peptide dissociations during the first 330 femtoseconds

We report a new approach to investigating the mechanisms of fast peptide cation-radical dissociations based on an analysis of time-resolved reaction progress by Ehrenfest dynamics, as applied to an Ala-Arg cation-radical model system. Calculations of stationary points on the ground electronic state that were carried out with effective CCSD(T)/6-311++G(3df,2p) could not explain the experimental branching ratios for loss of a hydrogen atom, ammonia, and N-C(α) bond dissociation in (AR + 2H)(+•). The Ehrenfest dynamics results indicate that the ground and low-lying excited electronic states of (AR + 2H)(+•) follow different reaction courses in the first 330 femtoseconds after electron attachment. The ground (X) state undergoes competing loss of N-terminal ammonia and isomerization to an aminoketyl radical intermediate that depend on the vibrational energy of the charge-reduced ion. The A and B excited states involve electron capture in the Arg guanidine and carboxyl groups and are non-reactive on the short time scale. The C state is dissociative and progresses to a fast loss of an H atom from the Arg guanidine group. Analogous results were obtained by using the B3LYP and CAM-B3LYP density functionals for the excited state dynamics and including the universal M06-2X functional for ground electronic state calculations. The results of this Ehrenfest dynamics study indicate that reaction pathway branching into the various dissociation channels occurs in the early stages of electron attachment and is primarily determined by the electronic states being accessed. This represents a new paradigm for the discussion of peptide dissociations in electron based methods of mass spectrometry.