Use of 157-nm Photodissociation to Probe Structures of y- and b-Type Ions Produced in Collision-Induced Dissociation of Peptide Ions

y- and b-type fragment ions produced in the collisional dissociation of arginine-terminated peptide ions are photodissociated with 157-nm light in a linear trap. y-type ions are shown to have the same structure as that of intact peptides of the same sequence with the ionizing proton located at the most basic residue(s). For generic b-type ions, the ionizing proton is shown to be sequestered at the N-terminal arginine, which is consistent with the proposed oxazolone structure.