Counterion Optimization Dramatically Improves Selectivity for Phosphopeptides and Glycopeptides in Electrostatic Repulsion-Hydrophilic Interaction Chromatography (ERLIC)

A well-hydrated counterion can selectively and dramatically increase retention of a charged analyte in hydrophilic interaction chromatography (HILIC). The effect is enhanced if the column is charged, as in electrostatic repulsion-hydrophilic interaction chromatography (ERLIC). This combination was exploited in proteomics for the isolation of peptides with certain post-translational modifications (PTMs). The best salt additive examined was magnesium trifluoroacetate. The well-hydrated Mg+2 ion promoted retention of peptides with functional groups that retained negative charge at low pH, while the poorly-hydrated trifluoroacetate counterion tuned down the retention due to the basic residues. The result was an enhancement in selectivity between 6- to 66-fold. These conditions were applied to a tryptic digest of mouse cortex. Gradient elution produced fractions enriched in peptides with phosphate, mannose-6-phosphate, N- and O-linked glycans. The numbers of such peptides identified either equaled or exceeded the numbers afforded by the best alternative methods. This method is a productive and convenient way to isolate peptides simultaneously that contain a number of different PTMs, facilitating study of proteins with “crosstalk” modifications. The fractions from the ERLIC column were desalted prior to C-18-reversed phase (RP) LC-MS/MS analysis. Between 47-100% of the peptides with more than one phosphate or sialyl- residue or with a mannose-6 phosphate group were not retained by a C-18 cartridge but were retained by a cartridge of porous graphitic carbon. This finding implies that the abundance of such peptides may have been significantly underestimated in some past studies.