title: Phosphopeptide analysis by matrix-assisted laser desorption time-of-flight mass spectrometry.
abstract: In this paper we present methods for identifying and sequencing phosphopeptides in simple mixtures, such as HPLC fractions, at the subpicomole level by (+) ion matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-MS). Data are presented which indicate that when a reflectron time-of-flight mass spectrometer is used, MALDI can distinguish tyrosine phosphorylation from serine and threonine phosphorylation for peptides containing a single phosphate group. Phosphopeptides are identified in the (+) ion MALDI reflector spectrum by the presence of [MH-H3PO4]+ and [MH-HPO3]+ fragment ions formed by metastable decomposition. An abundant [MH-H3PO4]+ ion, accompanied by a weaker [MH-HPO3]+ ion indicates that the peptide is most likely phosphorylated on serine or threonine. In contrast, phosphotyrosine-containing peptides generally exhibit [MH-HPO3]+ fragment ions and little, if any [MH-H3PO4]+. Ambiguities do arise, most often with phosphopeptides that contain residues which readily lose water (such as unmodified serine), but these can often be resolved by recording a complete metastable fragment ion (postsource decay) spectrum. Postsource decay is shown here to be a viable technique for sequencing phosphopeptides. It can be used to distinguish between serine/ threonine and tyrosine phosphorylation and in many cases can be used to determine the exact site of phosphorylation in a peptide sequence. Nearly complete sequence coverage and phosphorylation site mapping is generally possible using approximately 300 fmol of peptide.
authors.affiliation: Department of Physical and Structural Chemistry, SmithKline Beecham Pharmaceuticals, King of Prussia, Pennsylvania 19406, USA. Roland_S_Annan@SBPHRD.com
authors.forename: R S
authors.initials: RS
authors.lastname: Annan
authors.forename: S A
authors.initials: SA
authors.lastname: Carr
mesh_terms: D002851:Chromatography, High Pressure Liquid; D010748:Phosphopeptides / Q000032:analysis*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization / Q000379:methods*
chemical_list: D010748:Phosphopeptides