Generation and Characterization of Gas-Phase Doubly Charged Biradical Peptide Ions (M^2+••)

The gas-phase chemistry of peptide radical ions is attracting considerable interest in the fields of biology and mass spectrometry owing to its capability to provide sequence information on peptides and proteins. In this study, we observed that doubly charged peptide ions (M²⁺) can be produced from the collision-induced dissociation (CID) of Hg(II)-adducted peptide ions. The chemical nature and, thus, the dissociation pathways of this hydrogen-deficient biradical M²⁺ species is intriguing. We investigated the generation and dissociation behavior of this M²⁺ species under electron-capture dissociation (ECD) and CID conditions. The side-chain loss in the CID of the charge-reduced M^+• ions formed by single-electron capture suggested that M²⁺ existed as a biradical ion. This ion underwent the combination of the two radical sites and conversion to hydrogen surplus species through structural rearrangement with increased energies. This study demonstrated a promising method to generate reactive doubly charged biradical precursor ions and, thus, help characterize novel biomolecules.