STRUCTURES OF BACITRACIN-A AND ISOLATED CONGENERS - SEQUENCING OF CYCLIC-PEPTIDES WITH BLOCKED LINEAR SIDE-CHAINS BY ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

The bacitracin antibiotic complex consists principally of bacitracin A, a peptide antibiotic containing seven amino acid residues in a ring and five amino acid residues in a blacked side chain, together with a mixture of minor components presumably related but of unknown structures. A preparative high-performance liquid chromatographic method was developed for isolating the minor components A(2), B-1 and B-2 which were then characterized by amino acid analysis, exact mass fast atom bombardment (FAB) mass spectrometry, FAB tandem mass spectrometry (MS/MS) and electrospray ionization (ESI) mass spectrometry. For bacitracins A (MW 1421), A(2) (MW 1421), B-1a (MW 1407), B-1b (MW 1407), B-2 (MW 1407) and F (MW 1419), the side chain sequences were determined by ESI MS/MS and ESI nozzle-skimmer collision-induced dissociation (CID) mass spectrometry and the ring sequences elucidated by ESI nozzle-skimmer CID MS/MS. Relative to bacitracin A, bacitracin A(2a) has the modified isoelucine residue at position 1 replaced by a modified allo-isoleucine residue, bacitracin B-1a has the isoelucine residue at position 8 replaced by a valine residue, bacitracin B-1b has the isoleucine residue at position 5 replaced by a valine residue and bacitracin B-2 has the modified isoleucine residue at position 1 replaced by a modified valine residue. FAB tandem mass spectra were shown to be consistent with the above structural assignments for the isolated bacitracin components. Structures were also proposed for the trace bacitracin components C-1 (MW 1393) and D, (MW 1379) using ESI MS/MS data obtained from the analysis of the bacitracin complex without isolation.