METHIONINE SPECIFIC SEQUENCE IONS FORMED BY THE DISSOCIATION OF PROTONATED PEPTIDES AT HIGH COLLISION ENERGIES

Two methionine-specific sequence ions are described that are formed up on the dissociation of protonated methionine-containing peptides at hi gh (keV) collision energies, These ions, hiherto unassigned, are denot ed a(n) + 1 -CH3S. and z(n) + 1 - CH3S.. Precursor ion scans reveal th at these species originate from the a(n) + 1 and z(n) + 1 ions, respec tively, where methionine resides at position n-1 relative to the N- or C-terminus, A comparison of the collision-induced dissociation (CID) spectra for several related peptides demonstrates that methionine resi dues are involved in the genesis of these species and mechanisms for t heir formation are proposed, Several structural features of a peptide that are important to the formation of these species are described. Th e importance of identifying these ions in order to correctly sequence a methionine-containing peptide based on its CID spectrum is illustrat ed.