COLLISION-INDUCED DISSOCIATIONS OF DEPROTONATED DIPEPTIDE METHYL-ESTERS CONTAINING SERINE OR THREONINE

The collision-induced mass spectra (MS/MS) of (M - H)- ions derived fr om dipeptide methyl esters containing serine or threonine lack the cha racteristic backbone cleavage of the underivatized peptides (which pro vide primary sequencing data). Instead, competitive fragmentation occu rs through the ester and alpha-side chain functions. For example, Ser methyl esters lose both CH2O (from the side chain) and MeOH (MeO comes from the methyl ester). Isomeric dipeptides may be differentiated by competitive fragmentations; for example [Gly Ser(OMe) - H]- fragments first by loss of CH2O, while [Ser Gly(OMe) - H]-, in contrast, shows i nitial elimination of MeOH. The structures of the product ions in thes e spectra have been probed by deuterium labelling and MS/MS/MS studies .