Structure characterization of lipocyclopeptide antibiotics, aspartocins A, B & C, by ESI-MSMS and ESI-nozzle-skimmer-MSMS.

Three lipocyclopeptide antibiotics, aspartocins A (1), B (2), and C (3), were obtained from the aspartocin complex by HPLC separation methodology. Their structures were elucidated using previously published chemical degradation results coupled with spectroscopic studies including ESI-MS, ESI-Nozzle Skimmer-MSMS and NMR. All three aspartocin compounds share the same cyclic decapeptide core of cyclo [Dab2 (Asp1-FA)-Pip3-MeAsp4-Asp5-Gly6-Asp7-Gly8-Dab9-Val10-Pro11]. They differ only in the fatty acid side chain moiety (FA) corresponding to (Z)-13-methyltetradec-3-ene-carbonyl, (+,Z)-12-methyltetradec-3-ene-carbonyl and (Z)-12-methyltridec-3-ene-carbonyl for aspartocins A (1), B (2), and C (3), respectively. All of the sequence ions were observed by ESI-MSMS of the doubly charged parent ions. However, a number of the sequence ions observed were of low abundance. To fully sequence the lipocyclopeptide antibiotic structures, these low abundance sequence ions together with complementary sequence ions were confirmed by ESI-Nozzle-Skimmer-MSMS of the singly charged linear peptide parent fragment ions H-Asp5-Gly6-Asp7-Gly8-Dab9-Val10-Pro11-Dab2(1+)-Asp1-FA. Cyclization of the aspartocins was demonstrated to occur via the beta-amino group of Dab2 from ions of moderate intensity in the ESI-MSMS spectra. As the fatty acid moieties do not undergo internal fragmentations under the experimental ESI mass spectral conditions used, the 14 Da mass difference between the fatty acid moieties of aspartocins A (1) and B (2) versus aspartocin C (3) was used as an internal mass tag to differentiate fragment ions containing fatty acid moieties and those not containing the fatty acid moieties. The most numerous and abundant fragment ions observed in the tandem mass spectra are due to the cleavage of the tertiary nitrogen amide of the pipecolic acid residue-3 (16 fragment ions) and the proline residue-11 (7 fragment ions). In addition, the neutral loss of ethanimine from alpha,beta-diaminobutyric acid residue 9 was observed for the parent molecular ion and for 7 fragment ions.