Mass spectrometric determination of molecular formulas for membrane-modifying antibiotics

Abstract

THE peptide antibiotics alamethicin and suzukacillin, together with gramicidin A are of considerable biological interest as they facilitate ion transport across membranes by a mechanism involving pore or channel formation in the membranes^1–3. In discussing their mode of action it is, of course, necessary to consider the antibiotics' structures and such discussions have been hampered, especially for alamethicin^1,4–7, by incorrect structures originally proposed for the antibiotics. Thus, the earlier structures proposed for valine-gramicidin A and isoleucme-gramicidin A^8,9 were revised to 1 and 2 (Fig. 1) following the detection of N-terminal formyl and C-terminal ethanolamine groups¹⁰, and earlier structures^11,12 proposed for alamethicins I and II have recently been revised to 3 and 4 (ref. 13) following the discovery of N-terminal acetyl and C-terminal phenylalaninol groups^14,15. (The partial structure proposed for suzukacillin¹⁶ will probably also be revised in view of the newly reported structure for alamethicin¹³.) Much of the difficulty in assigning structures to these antibiotics has derived from an inability to determine accurate molecular formulas. We have until recently been hampered by the same problem in our studies of the structures of antiamoebin I (5) (ref. 17) and emerimicins III and IV (6 and 7) (ref. 18), new peptaibophol antibiotics [peptide antibiotics containing several moles of α-aminoisobutyric acid (Aib) and a C-terminal phenylalaninol (Phol) unit] which have been shown to have similar membrane-modifying properties (P. Mueller, personal communication). We have developed, and describe here, a new technique, a modification of field desorption mass spectrometry^19,20 involving cation exchange, which has allowed us to assign molecular formulas directly to these peptide antibiotics and their derivatives in the 1,500–2,100 molecular weight range.