Peptide cation radicals of the z-type were produced by electron transfer dissociation (ETD) of peptide dications and studied by UV-Vis photodissociation (UVPD) action spectroscopy. Cation radicals containing the Asp (D)， Asn (N)， Glu (E)， and Gln (Q) residues were found to spontaneously isomerize by hydrogen atom migrations upon ETD. Canonical N-terminal [z + H] fragment ion-radicals of the R-CH-CONH- type， initially formed by N-C bond cleavage， were found to be minor components of the stable ion fraction. Vibronically broadened UV-Vis absorption spectra were calculated by time-dependent density functional theory for several [DAAR + H] isomers and used to assign structures to the action spectra. The potential energy surface of [DAAR + H] isomers was mapped by ab initio and density functional theory calculations that revealed multiple isomerization pathways by hydrogen atom migrations. The transition-state energies for the isomerizations were found to be lower than the dissociation thresholds， accounting for the isomerization in non-dissociating ions. The facile isomerization in [XAAR + H] ions (X = D， N， E， and Q) was attributed to low-energy intermediates having the radical defect in the side chain that can promote hydrogen migration along backbone C positions. A similar side-chain mediated mechanism is suggested for the facile intermolecular hydrogen migration between the c- and [z + H]-ETD fragments containing Asp， Asn， Glu， and Gln residues. Graphical Abstract ᅟ.