COMPARING MASS-SPECTROMETRIC CHARACTERISTICS OF PEPTIDES AND PEPTOIDS

The collision-induced dissociation (CID) spectra of the [M + H](+) ion s of a pentapeptide and the corresponding peptoid and retropeptoid hav e been compared, The spectra of the peptide and peptoid both exhibit B - and Y-n-type sequence ions at identical m/z values. In contrast to t he peptide, the [M + H](+) ion of the peptoid and all sequence ions co ntaining an N-substituted glycine derivative corresponding to a tyrosi ne amino acid residue can easily lose a C7H6O molecule in a charge-rem ote fragmentation process, The presence of N-substituted glycine resid ues in a peptoid is further apparent from the presence of N-substitute d immonium ions, which differ significantly in their fragmentation beh aviour from the corresponding immonium ions observed in the spectra of common oligopeptides. Loss of the CH2=NH imine molecule is the domina nt fragmentation reaction in the CID spectra of all peptoid immonium i ons investigated in this study. The elimination of the CH=NH2 ylide an alogue from common peptide immonium ions is energetically less favoura ble as shown by ab initio calculations. The relative heat of formation of the CH=NH2 ylide neutral appeared to be 168 kJ mol(-1) more than t hat of the CH2=NH inine molecule.