Development of online high-/low-pH reversed-phase-reversed-phase two-dimensional liquid chromatography for shotgun proteomics: a reversed-phase-strong cation exchange-reversed-phase approach.

Previously, we described an online high-/low-pH RP-RP LC system exhibiting high-throughput, automatability, and performance comparable with that of SCX-RP. Herein, we report a variant of the RP-RP platform, RP-SCX-RP, featuring an additional SCX trap column between the two LC dimensions. The SCX column in combination with the second-dimension RP can be used as an SCX-RP biphasic column for trapping peptides in the eluent from the first RP column. We evaluated the performance of the new platform through proteomic analysis of Arabidopsis thaliana chloroplast samples and mouse embryonic mouse fibroblast STO cell lysate at low-microgram levels. In general, RP-SCX-RP enhanced protein identification by allowing the detection of a larger number of hydrophilic peptides. Furthermore, the platform was useful for the quantitative analyses of crude chloroplast samples for iTRAQ applications at low-microgram levels. In addition, it allowed the online removal of sodium dodecyl sulfate and other chemicals used in excess in iTRAQ reactions, avoiding the need for time-consuming offline SCX clean-up prior to RP-RP separation. Relative to the RP-RP system, our newly developed RP-SCX-RP platform allowed the detection of a larger number of differentially expressed proteins in a crude iTRAQ-labeled chloroplast protein sample.