Near-UV photodissociation of phosphopeptide cation-radicals

Laser photodissociation at 355 nm (near-UVPD) of phosphopeptide cation-radicals provides information on the position of the radical center and the phosphorylated serine residue. Cation-radicals of the ztype are produced by electron transfer dissociation of peptide dications from pSAAAR, ApSAAR, AApSAR, and AAApSR and found to absorb light at 355 nm to undergo photodissociation. Near-UVPD can induce backbone dissociation of the CONH bond next to the radical center on its N-terminal side concurrent with only minor radical scrambling and loss of the post-translational modification. The pSer residue effectively blocks backbone cleavage when the radical site is in its closest vicinity. Near-UVPD of loss-of-ammonia fragment ions produced by ETD results in backbone dissociations that distinguish isomeric ions differing in the site of ammonia loss. The experimental data are interpreted with the help of ab initio and density functional theory calculations of ion structures, transition states, and excitation energies.