Mass spectrometric investigation of Maltacines E1a and E1b—two members of the Maltacine family of peptide antibiotics

We have recently described the discovery of the Maltacines—a new family of cyclic peptide antibiotics from Bacillus subtilis . In this paper the mass spectrometric characterisation of two of the members is reported. A chemoselective ring opening with base to give the linear peptides was necessary before mass spectrometric characterisation could be performed. MS n of the singly and doubly charged protonated molecules gave uninterrupted series of Bn and Y″n ions that allowed determination of the amino acid sequence. By using a combination of derivatisation with phenylisothiocyanate (PITC), high‐resolution mass spectrometry and H/D exchange, the identities of three unknown residues were determined. The nature and position of the cyclic structure were disclosed by a chemoselective ring opening with Na 18 OH and it was found to be a lactone formed between a hydroxyl of residue number 4 and the C‐terminal amino acid number 12. Peptides with different combinations of P/Q and P/K at the N‐terminus were synthesised to verify the sequence of the N‐terminal B 2 ion. The structure of the two peptides is proposed to be: E1a: cyclo‐4,12(P‐Q‐Y‐Adip‐V‐E‐T‐Y‐Orn‐S‐Y‐I‐OH) and E1b: cyclo‐4,12(P‐Q‐Y‐Adip‐V‐E‐T‐Y‐K‐S‐Y‐I‐OH). Adip = (2‐amino‐4,5‐dihydroxypentanoic acid). Copyright © 2005 John Wiley & Sons, Ltd.