FRAGMENTATION EFFICIENCIES OF PEPTIDE IONS FOLLOWING LOW-ENERGY COLLISIONAL ACTIVATION

Low energy fragmentations of protonated peptides in the gas phase are generally attributed to charge-directed processes. The extent and loca tion of peptide backbone fragmentation is accordingly influenced by th e extent to which charge is sequestered on amino acid side-chains. We describe systematic studies of the efficiencies of decomposition of pe ptide ions to assess in particular the influence of the presence of ba sic amino acid residues and of the protonation state. In a set of anal ogues containing two arginine, two histidine or two lysine residues, t he extent of fragmentation of [M + 2H](2+) ions decreases with increas ed basicity, reflecting decreased backbone protonation. The collisiona lly activated dissociation of multiply protonated melittin ions shows an increase in fragmentation efficiency with higher charge state (usin g activation conditions which are similar for each charge state). For a single charge state, acetylation of primary amine groups increases f ragmentation efficiency, consistent with the reduction in basicity of lysine side-chains. Conversion of arginine residues to the less basic dimethylpyrimidylornithine, however, decreases fragmentation efficienc y, suggesting more effective sequestering of ionizing protons; the eff ect may be attributable to a disfavouring of proton-bridged structures but this hypothesis requires further study. Preliminary data for the decompositions of [M - 2H](2-) ions derived from peptides containing t wo acidic residues suggest that the sequestration of charge away from the backbone is again detrimental to efficient fragmentation. Apparent ly diagnostic cleavages adjacent to aspartic acid residues are observe d. (C) 1997 Elsevier Science B.V.