Characterization of moenomycin antibiotics from medicated chicken feed by ion-trap mass spectrometry with electrospray ionization

The antimicrobial moenomycin, commonly used as a growth promoter in livestock, was isolated from medicated chicken feed. The purified extract was subjected to reversed-phase liquid chromatographic separation followed by structural characterization using ion-trap mass spectrometry (ITMS), which allowed identification of five moenomycins (A, A₁₂, C₁, C₃, and C₄) as the major components. The fragmentation patterns of the protonated and deprotonated moenomycin molecules, as well as of a series of sodium adducts, were investigated using ITMS after electrospray ionization. While the protonated molecules [M+H]⁺ proved highly unstable and underwent extensive in-source fragmentation, isolation and activation of the [M-H]^- ions (m/z 1580 for moenomycin-A) yielded simple mass spectra with a dominant base peak corresponding to the loss of the carboxyglycol and the C₂₅-hydrocarbon chain (m/z 1152 for moenomycin-A). Further study of this fragment ion in an MS³ experiment gave rise to a peculiar product ion (m/z 902 for moenomycin-A) that was attributed to the expulsion of a carbohydrate moiety representing a central building block of the linear molecule. In positive ion mode the generation of the mono-sodiated adduct ions, [M+Na]⁺, was promoted by amending the mobile phase with 100 μM sodium acetate, but this also resulted in higher adducts of the type [M+2Na-H]⁺ and [M+3Na-2H]⁺ arising from the formation of the sodium salts of the phosphate acid diester and subsequently of the carboxylic acid. Substantial differences among the fragment-rich product ion profiles of the three species were observed, and could in part be traced back to the mode of complexation of the additional sodium cation(s).