Facile identification of phosphorylation sites in peptides by radical directed dissociation

Identification of phosphorylation sites is of interest due to their importance in protein regulation; however, the identification of the exact sites of this modification is not always easily obtained due to the dynamic nature of phosphorylation and the challenges faced during mass spectrometric analysis. Herein we elaborate on our previous communication (Diedrich, J. K.; Julian, R. R. J. Am. Chem. Soc. 2008, 130, 12212-12213) describing a novel technique for assignment of phosphorylation in a site-specific and facile manner. Phosphorylation sites are selectively modified through β elimination followed by Michael addition chemistry to install a photolabile group. Photodissociation with 266 nm light yields homolytic cleavage at the modification site, generating a β radical which is poised to fragment the peptide backbone. Dissociation primarily yields d-type ions at the previously phosphorylated residue, allowing facile identification. Radical directed fragmentation also occurs in smaller abundances at neighboring residues. The mechanisms behind this selective radical fragmentation are presented and the utility is discussed. Fragmentation is shown to be independent of charge state allowing analysis of a wide variety of peptide sequences including peptides with multiple phosphorylation sites. A comparison of this technique is made with collision induced dissociation (CID) and electron capture dissociation (ECD) for representative peptides.