Scheme 1.

Summary of oxidative pathways available to a doubly protonated peptide upon ion/ion reaction with each of the reagents derived from persulfate and subsequent MSⁿ (CID indicated by Δ) of the complexes. Black arrows indicate an ion/ion reaction, the orange arrow indicates the pathway to create a radical peptide cation, the blue arrow indicates the pathway to form the hydrogen-deficient [M-H]⁺ species, red arrows indicate oxygen transfer pathways to form the [M+H+O]⁺ species with the dashed arrow indicating a pathway that results in a structure different than that obtained via solid-arrow pathways, and the purple arrow indicates the generation of a species capable of proceeding via either the oxygen transfer or the hydrogen abstraction pathway.