Determination of the relative energies of activation for the dissociation of aromatic versus aliphatic phosphopeptides by ESI-FTICR-MS and IRMPD.

Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FTICR-MS) coupled with infrared multiphoton dissociation (IRMPD) is potentially a powerful method for rapid phosphopeptide mapping of complex proteolytic digests. The dissociation of deprotonated phosphopeptides by IRMPD is energetically favorable over unmodified deprotonated peptides because of a lower energy of activation and a higher internal energy under identical irradiation conditions. The energies of activation for dissociation are determined for model peptides phosphorylated on an aliphatic side chain (serine) and an aromatic side chain (tyrosine). The determination of phosphorylation location provides important biochemical information identifying the kinase involved in specific phosphorylation mechanisms. The data presented in this manuscript also support the theory that for phosphopeptides, the phosphate moiety's P-O stretch is in direct resonance with the infrared laser (10.6 microm), thus increasing the relative absorptivity of the modified species. A greater extinction coefficient affords more extensive photon absorption and subsequently a greater internal energy at the rapid exchange limit.