Differentiation of phosphorylated and unphosphorylated peptides by high-performance liquid chromatography-electrospray ionization-infrared multiphoton dissociation in a quadrupole ion trap

Infrared multiphoton dissociation (IRMPD) in a quadrupole ion trap coupled to high-performance liquid chromatography allows the selective dissociation of phosphorylated peptides in mixtures following chromatographic separation. This method is shown to be effective for differentiation of phosphorylated peptides from unphosphorylated ones; only the abundances of the phosphorylated species are appreciably decreased following exposure to 125 ms of 10.6-mu m radiation. This LC-IRMPD-MS strategy is demonstrated for a mock mixture of peptides and a tryptic digest of alpha(s1)-casein. The ability of this technique to differentiate peptides based on phosphorylation state is unaffected by whether the peptides are protonated or sodium-cationized.