COMPARING MASS-SPECTROMETRIC CHARACTERISTICS OF PEPTIDES AND PEPTOIDS.2.

The high-energy collision-induced dissociation (CID) spectra of the [M + H](+) and [M - H](-) ions of substance P fragment 6-11, the retrope ptide and the corresponding peptoid and retropeptoid were compared, Th e CID spectra of the [M + H](+) ion of the corresponding (retro)peptid e and (retro)peptoid exhibit both B- and Y ''-type sequence ions at id entical mit-values, The differences in the relative abundances of thes e sequence ions, however, can be related to structural characteristics of the compounds, The fragmentation behaviour of the N-substituted im monium ions differs from that of the immonium ions derived from common amino acids by showing a preferential loss of a CH2=NH imine molecule . This dominant fragmentation reaction is suppressed in the CID spectr um of the N-substituted glutamine immonium ion in favour of a less ene rgy demanding cyclization reaction involving the loss of NH3. The CID spectra of the [M - H](-) ions of both peptides and peptoids show C-ty pe ions, which appeared to be more abundant in the spectra of the pept ides than in those of the peptoids. C-Terminal '' Z- and Y-type ions a re characteristic of the peptides, whereas the peptoid spectra show re latively abundant N-terminal '' B-type ions, Both [M + H](+) and [M - H](-) ion CID spectra show loss of amino acid-specific side-chains, wh ich occurs as radical loss in the case of peptides and as molecular lo ss in peptoids, (C) 1997 by John Wiley & Sons, Ltd.