Fully Automatic Separation and Identification of Phosphopeptides by Continuous pH-Gradient Anion Exchange Online Coupled with Reversed-Phase Liquid Chromatography Mass Spectrometry

Most current technologies for the enrichment of phosphopeptides rely on a tandem combination of different chromatography modes. Here, a fully automatic two-dimensional liquid chromatography mass spectrometry method was developed for global phosphopeptide identification. The peptide mixtures were loaded on a strong anion exchange (SAX) column under basic pH conditions and eluted with a continuous gradient to pH 2.0. This SAX system could be coupled online with reversed-phase liquid chromatography mass spectrometry (RP-LC-MS/MS). For peptide digests from a standard protein mixture spiked with synthesized phosphopeptides, most of the nonphosphorylated peptides were eluted in more basic pH than phosphopeptides, and the phosphopeptides were focused to acidic pH ranges and gradually eluted according to the phosphorylated states of peptides. Compared with the pH step elution method, the continuous gradient method displayed better repeatability and less peptide cross-overlap between fractions. This system provided a robust and fully automatic approach to large-scale phosphoproteomic profiling. For protein tryptic digests from HeLa cells, 1833 nonredundant phosphorylation sites were identified based on this two-phase separation. Compared with the method combining cation exchange and titanium dioxide, this anion-exchange based system preferred to identify more acidic and multiphosphorylated peptides. It also covered a more complete series of phosphorylation states of peptides.