AN EXPANDED NOMENCLATURE SCHEME FOR LABELING PEPTIDE FRAGMENTATIONS AND ITS USE WITH AMASS, A COMPUTER-PROGRAM FOR GENERATING ALL POSSIBLE FRAGMENT ION STRUCTURES FROM KNOWN PRECURSORS

A nomenclature scheme for exhaustively labeling peptide fragment ions is proposed. The scheme is based on IUPAC nomenclature1-3 and the previously proposed Roepstorff nomenclature scheme' used to label fragment ions in linear peptides. The descriptor used is specifically defined in order to increase the number of peptide and side chain linkages to which the nomenclature scheme can be applied compared with the Roepstorff scheme. The proposed descriptor can be used unambiguously to assign all possible fragments from linear, cyclized, branched, extended (i.e beta-amino acids) and retro inverso peptides. A significant advantage of the proposed scheme is its simple interface with the currently accepted Roepstorff scheme. This nomenclature scheme is able to label all theoretical fragments generated by the computer program 'AMASS. AMASS is proposed as a means of systematically calculating the mass of all possible fragment ions from known precursor structures. The program can help determine whether a peptide fragment was derived from an internal sequence fragment or a combination of side chain and backbone cleavages. The program AMASS and the proposed nomenclature scheme are used to illustrate a procedure for identifying fragment ions in the metastable product ion spectrum of somatostatin-14. We envisage that this procedure will be useful for identifying fragment ions which are characteristic of particular structural arrangements in dicyclic and polycyclic peptides.