Analysis of peptides with unusual structural features by fast atom bombardment mass spectrometry: Protease inhibitors

Positive ion and negative ion fast atom bombardment mass spectra of several natural and synthetic enzyme inhibitors are presented. It is shown that the fast atom bombardment method is well suited to the analysis of these unusual peptides. Complete amino acid sequences can be deduced from the mass spectra in most cases. Structural features typical of the inhibitors include (a) C‐terminal aldehyde groups (e.g. leupeptin), (b) urea linkages in the peptide backbone (e.g. antipain, chymostatin), (c) prevalence of arginyl or α‐(2‐iminohexahydropyrimidyl)glycl residues, and (d) presence of an N‐(glycosylphosphoryl) group (e.g. phosphoramidon). Several examples of synthetic peptides with common protecting groups are also presented. It is concluded that positive ion and negative ion fast atom bombardment mass spectra provide complementary information, the two techniques being sensitive to entirely different structural aspects of the inhibitors.