PATHWAYS TO IMMONIUM IONS IN THE FRAGMENTATION OF PROTONATED PEPTIDES

The pathways leading to the formation of immonium (A(n)) ions in the f ragmentation of protonated peptides were investigated using metastable ion studies, including kinetic energy release measurements, and low-e nergy collision-induced dissociation studies. In addition to the estab lished pathway B-n-->A(n)+CO, it is shown that B-2 ions, in suitable c ircumstances, fragment directly to A(1) ions. In addition, metastable ion studies show that A(2) ions can be formed directly from protonated di- and tripeptides most likely by concerted elimination of CO and an amino acid or smaller peptide. A, ions can be formed directly from pr otonated dipeptides in part through the sequential loss of H2O+CO, alt hough kinetic energy release measurements suggest direct elimination o f HCOOH also may be occurring. Internal immonium ions are shown to ori ginate by further fragmentation of A(n) ions and by further fragmentat ion of Y-n '' ions. (C) 1997 by John Wiley & Sons, Ltd.