PhosTShunter:  A Fast and Reliable Tool to Detect Phosphorylated Peptides in Liquid Chromatography Fourier Transform Tandem Mass Spectrometry Data Sets

A database independent search algorithm for the detection of phosphopeptides is described. The program interrogates the tandem mass spectra of LC−MS/MS data sets regarding the presence of phosphorylation specific signatures. To achieve maximum informational content, the complementary fragmentation techniques electron capture dissociation (ECD) and collisionally activated dissociation (CAD) are used independently for peptide fragmentation. Several criteria characteristic for peptides phosphorylated on either serine or threonine residues were evaluated. The final algorithm searches for product ions generated by either the neutral loss of phosphoric acid or the combined neutral loss of phosphoric acid and water. Various peptide mixtures were used to evaluate the program. False positive results were not observed because the program utilizes the parts-per-million mass accuracy of Fourier transform ion cyclotron resonance mass spectrometry. Additionally, false negative results were not generated owing to the high sensitivity of the chosen criteria. The limitations of database dependent data interpretation tools are discussed and the potential of the novel algorithm to overcome these limitations is illustrated. Keywords: phosphorylation • mass spectrometry • phosphopeptides • identification • FT-ICR