CHARACTERIZATION OF UNDERIVATIZED TETRAPEPTIDES BY NEGATIVE-ION FAST-ATOM-BOMBARDMENT MASS-SPECTROMETRY

The [M - H](-) ions derived from tetrapeptides generally show two diff erent collision-induced backbone cleavages which allow the determinati on of the amino acid sequence of the peptide. The first of these invol ves the formation of the carboxylate anions of either constituent amin o acids or fragment peptides. In the second, amino acids or fragment p eptides are eliminated as neutrals. There are a number of residues whi ch undergo characteristic side-chain fragmentations irrespective of th eir position in the tetrapeptide, e.g. Ser, Thr, Cys, Met, Phe and Tyr . However, there are also some residues which, when situated at the C- terminal end of the peptide, promote pronounced fragmentation at the C -terminal position which occurs to the exclusion of the normal backbon e cleavages. We conclude that the data obtained from these negative-io n cleavages are analytically useful, and complement those provided by the cognate positive-ion technique.