Observation of phosphorylation site-specific dissociation of singly protonated phosphopeptides

In ultraviolet photodissociation of phosphopeptide ions with a basic residue (arginine, lysine, or histidine) at the N-terminus, intense a(n) - 97 peaks were observed. These ions were formed by cleavage at phosphorylated residues only. For multiply phosphorylated peptides, this site-specific cleavage occurred at every phosphorylated residue. H/D exchange studies showed that a(n) - 97 was formed by H(3)PO(4) loss from a(n) + 1 radical cations. The site-specificity of phosphate loss observed here is in contrast to the nonspecific phosphate loss from b(n) and y(n) reported previously. Characteristics of the reaction and its potential utility for phosphopeptide analysis are discussed.