Formation of peptide radical ions through dissociative electron transfer in ternary metal-ligand-peptide complexes

The formation and fragmentation of odd-electron ions of peptides and proteins is of interest to applications in biological mass spectrometry. Gas-phase redox chemistry occurring during collision-induced dissociation of ternary metal-ligand-peptide complexes enables the formation of a variety of peptide radicals, including the canonical radical cations, M(+•), radical dications, [M+H](2+•), radical anions, [M-2H](-•) and phosphorylated radical cations. In addition, odd-electron peptide ions with well-defined initial location of the radical site are produced through side-chain losses from the radical ions. Subsequent fragmentation of these species provides information regarding the role of charge and location of the radical site on the competition between radical-induced and proton-driven fragmentation of odd-electron peptide ions. This account summarizes current understanding of the factors that control the efficiency of the intramolecular electron transfer (ET) in ternary metal-ligand-peptide complexes resulting in formation of odd-electron peptide ions. Specifically, we discuss the effect of the metal center, the ligand and the peptide structure on the competition between the ET, proton transfer (PT) and loss of neutral peptide and neutral peptide fragments from the complex. Fundamental studies of the structures, stabilities and the energetics and dynamics of fragmentation of these complexes are also important for detailed molecular-level understanding of photosynthesis and respiration in biological systems.