LOW-MASS IONS PRODUCED FROM PEPTIDES BY HIGH-ENERGY COLLISION-INDUCEDDISSOCIATION IN TANDEM MASS-SPECTROMETRY

High-energy collision-induced dissociation (CID) mass spectrometry pro vides a rapid and sensitive means for determining the primary sequence of peptides. The low-mass region (below mass 300) of a large number o f tandem CID spectra of peptides has been analyzed. This mass region c ontains several types of informative fragment ions, including dipeptid e ions, immonium ions, and other related ions. Useful low-mass ions ar e also present in negative-ion CID spectra. Immonium ions (general str ucture [H2N=CH-R]+, where R is the amino acid side chain) and related ions characteristic of specific amino acid residues give information a s to the presence or absence of these residues in the peptide being an alyzed. Tables of observed immonium and related ions for the 20 standa rd amino acids and for a number of modified amino acids are presented. A database consisting of 228 high-energy CID spectra of peptides has been established, and the frequency of occurrence of various ions indi cative of specific amino acid residues has been determined. Two model computer-aided schemes for analysis of the amino-acid content of unkno wn peptides have been developed and tested against the database.