Renaissance of Cation-Radicals in Mass Spectrometry

Abstract

This brief overview addresses the topic that was presented in the Thomson Medal Award session at the 19th International Mass Spectrometry Conference in Kyoto, Japan. Mass spectrometry of cation-radicals has enjoyed a remarkable renaissance thanks to the development of new methods for electron attachment to multiply charged peptide ions. The charge-reduced ions that are odd-electron species exhibit interesting reactivity that is useful for peptide and protein sequencing. The paper briefly reviews the fundamental aspects of the formation, energetics, and backbone dissociations of peptide cation-radicals.

Benchmarking Electronic Excitation Energies and Transitions in Peptide Radicals

“…The LC-BLYP excitation energies were most sensitive to the radical geometry, giving root-mean square deviations that ranged from 0.04 eV for 2a to 0.33 eV for 4. Inspection by the structure type indicated that the phenoxy (11) and dihydrophenol radicals (12,13) were least affected by variations in their optimized geometries. The ωB97X-D excitation energies appeared to be least sensitive to the radical geometry, showing only 0.02−0.08 eV rmsd when calculated with B3LYP/6-31G+(d,p) and fully optimized geometries.…”

“…These reactive intermediates play an important role in protein reactions catalyzed by several enzymes, , as well as in radical-induced protein degradation. , The formation of biological radical intermediates has been investigated using electron spin resonance, , and their thermochemistry has been addressed by ab initio calculations. − In the past few years, a remarkable progress has been made in generating biomolecular radicals derived from gas-phase peptide ions to study their physical properties and reactivity as models of biochemical systems. This renaissance has been driven by new experimental methods of generation peptide and protein cation-radicals, , as well as by interest in their unusual electronic properties. − Among the different types of radicals, hydrogen-rich peptide radicals and cation-radicals are produced by electron attachment to protonated peptides in the gas phase. Hydrogen-rich peptide cation-radicals undergo homolytic bond cleavages forming closed-shell and radical backbone fragments that are used to provide amino acid sequence information. , Other types of open-shell species, which are called hydrogen-deficient peptide cation radicals, can be produced by several methods, involving electron induced dissociation of peptide ions, photolysis, or collision-induced dissociation , of suitably derivatized peptides, and intramolecular electron transfer oxidation in peptide–transition-metal complexes with auxiliary ligands. − …”

“…7−12 In the past few years, a remarkable progress has been made in generating biomolecular radicals derived from gasphase peptide ions to study their physical properties and reactivity as models of biochemical systems. This renaissance has been driven by new experimental methods of generation peptide and protein cation-radicals, 13,14 as well as by interest in their unusual electronic properties. 15−19 Among the different types of radicals, hydrogen-rich peptide radicals and cationradicals are produced by electron attachment to protonated peptides in the gas phase.…”

Competitive hydrogen atom migrations accompanying cascade dissociations of peptide cation-radicals of the + type

“…Electron attachment to multiply charge peptide ions is a highly exothermic process that fundamentally changes the gas-phase peptide ion chemistry by converting even electron ions to cation-radicals [1,2]. This, and the internal excitation of the charge-reduced peptide cation-radicals, results in extensive dissociation that is utilized for peptide sequencing [3].…”

Fragmentation and Ion Desorption from Condensed Pyrimidine by Electron Impact: Implications for Cometary and Interstellar Heterocyclic Chemistry

“…Indeed, such dependence is related to the production of even-electron ions occurring at odd numbered m / z values. The formation of a more stable fragment with all electrons paired up to yield closed-shell ions is a more efficient process in comparison to the production of even-mass (odd-electron numbered) ions. , An odd-electron cation radical, like C 2 H 2 + , is expected to be very reactive, lowering its desorption probability. On the other hand, the even-mass fragment m / z = 28 (HCNH + ) is seen as the third most intense peak of group B.…”