Aspartocin cyclic lipopeptide antibiotics: mass spectral structural confirmations and the diagnostic role played by the α,β-diaminobutyric acid residue
✍ Scribed by Marshall M. Siegel; Fangming Kong; Guy T. Carter
- Publisher
- John Wiley and Sons
- Year
- 2010
- Tongue
- English
- Weight
- 195 KB
- Volume
- 45
- Category
- Article
- ISSN
- 1076-5174
- DOI
- 10.1002/jms.1755
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✦ Synopsis
Aspartocin cyclic lipopeptide antibiotics: mass spectral structural confirmations and the diagnostic role played by the α,β-diaminobutyric acid residue
Recently, the proposed structures for the aspartocins A, B and C (MWs 1318, 1318 and 1304, respectively) were characterized by performing ESI-MSMS in the positive ionization mode on the doubly charged parent ions and ESI-Nozzle Skimmer (NS)-MSMS on selected singly charged fragment ions (m/z 1078 for aspartocins A and B, m/z 1064 for aspartocin C). [1] In this report, we demonstrate that ESI-NS-MSMS performed on three other singly charged fragment ions (m/z 981 of Aspartocin B, m/z 963/949 of Aspartocin A, B/C and m/z 741 of Aspartocin A, B) are consistent with the proposed structures. As a result of these studies, the α,β-diaminobutyric acid residue (Dab) was found to behave uniquely and is now explicitly emphasized. The fragmentation properties of α,β-Dab follow from the lability of the α,β carbon-carbon bond in the Dab residue which can readily cleave under the elevated NS voltage conditions used in the ESI-NS-MS experiments and the elevated NS voltage/collision energy conditions used in the ESI-NS-MSMS experiments. When the α,β-Dab residue is structurally involved as a linear peptide, cleavage of the α,β C-C bond releases neutral ethanimine [43 Da, NH CH(CH 3 )] and the original α,β-Dab residue (100 Da) is effectively converted to a glycine residue (57 Da). On the other hand, when α,β-Dab is involved in cyclizing aspartocin, cleavage of the α,β C-C bond is diagnostic for determining whether the α or β amino group of Dab is involved in the cyclization of the aspartocins (fragment CC, Fig. 3; Ref. [1]).