Peptide radicals play a significant role in biology as well as mass spectrometry. They can be differentiated into two groups: conventional hydrogen-deficient radicals, e.g. M(+•) as in electron ionization, and much more rare hydrogen-abundant radicals, e.g. [M+2H](+•), as in electron capture/transfer dissociation. The dissociation chemistries of these two types of radicals are vastly different. Both types tend to lose small molecules or radical groups, but the overlap between the losses from different radical types is minimal. The backbone cleavage for hydrogen-deficient radicals is dominated by Cα-C cleavage (a (•), x fragments) and for hydrogen-abundant radicals-by N-Cα cleavage (c, z (•) ions). The latter types of fragmentation produces more sequencing information than the former. Therefore, hydrogen-abundant peptide radicals are more valuable in mass spectrometry. The efficiency of the main method of their production, electron capture/transfer dissociation, is however limited by charge reduction. Alternative methods of generation of hydrogen-abundant radicals are needed to improve the sequencing capabilities of mass spectrometry.
Keywords: electron capture dissociation; electron ionization; electron transfer dissociation; hydrogen-abundant radicals; hydrogen-deficient radicals; peptide fragmentation.