Dipole-Guided Electron Capture Causes Abnormal Dissociations of Phosphorylated Pentapeptides

Abstract

Electron transfer and capture mass spectra of a series of doubly charged ions that were phosphorylated pentapeptides of a tryptic type (pS,A,A,A,R) showed conspicuous differences in dissociations of charge-reduced ions. Electron transfer from both gaseous cesium atoms at 100 keV kinetic energies and fluoranthene anion radicals in an ion trap resulted in the loss of a hydrogen atom, ammonia, and backbone cleavages forming complete series of sequence z ions. Elimination of phosphoric acid was negligible. In contrast, capture of lowenergy electrons by doubly charged ions in a Penning ion trap induced loss of a hydrogen atom followed by elimination of phosphoric acid as the dominant dissociation channel. Backbone dissociations of charge-reduced ions also occurred but were accompanied by extensive fragmentation of the primary products. z-Ions that were terminated with a deaminated phosphoserine radical competitively eliminated phosphoric acid and H 2 PO 4 radicals. A mechanism is proposed for this novel dissociation on the basis of a computational analysis of reaction pathways and transition states. Electronic structure theory calculations in combination with extensive molecular dynamics mapping of the potential energy surface provided structures for the precursor phosphopeptide dications. Electron attachment produces a multitude of low lying electronic states in charge-reduced ions that determine their reactivity in backbone dissociations and H-atom loss. The predominant loss of H atoms in ECD is explained by a distortion of the Rydberg orbital space by the strong dipolar field of the peptide dication framework. The dipolar field steers the incoming electron to preferentially attach to the positively charged arginine side chain to form guanidinium radicals and trigger their dissociations.

The Early Life of a Peptide Cation-Radical. Ground and Excited-State Trajectories of Electron-Based Peptide Dissociations During the First 330 Femtoseconds

“…Molecular dynamics (MD) search of the conformational space for the (AR + 2H) 2+ precursor dication was performed using a previously reported scheme [43]. This consists of replica exchange molecular dynamics (REMD) [44] calculations on the peptide ion.…”

“…RRKM calculations on the CCSD(T) ground state potential energy surface indicate unimolecular rate constants for the Referring to the experimental data [41], resonant electron transfer from a Cs atom (IE=3.894 eV) to the (AR + 2H) 2+ ion having vertical recombination energies of 5.03-5.07 eV (ref. [41]) is estimated to occur at a collisional cross section of 5×10 -14 cm 2 [43]. This would deposit 242 kJ mol -1 in 2 and promote N-C α bond cleavage with k=4.7×10 10 s -1 ( Figure S2).…”

Electron Transfer Reduction of the Diazirine Ring in Gas-Phase Peptide Ions. On the Peculiar Loss of [NH4O] from Photoleucine

“…First, a full conformational search was carried out using the ConformSearch engine [21,22] for analogous (GLGGR +2H) 2+ ions containing standard amino acid residues for which there are molecular dynamics parameters. The procedure consists of molecular dynamics mapping of the conformational space in a replica-exchange format [23] using the NAMD program [24] and the CHARMM force field [25].…”

Ground and Excited-Electronic-State Dissociations of Hydrogen-Rich and Hydrogen-Deficient Tyrosine Peptide Cation Radicals

“…Selected peptide cation radicals of either type have been studied with the aim of determining their structure and elucidating dissociation mechanisms [22][23][24][25][26][27][28][29][30][31][32]. However, despite there being a large body of data on peptide cation radicals and substantial progress in understanding their chemistry, there has not been a study exploring side by side the properties of both hydrogen-deficient and hydrogen-rich peptide cation radicals derived from the same peptide molecules.…”