Monensins are a series of polyether ionophore antibiotic factors produced by Streptomyces cinnamonensis. Three monensin factors and four metabolites of monensin A isolated from cattle feces were investigated by electrospray ionization mass spectrometry (ESI-MS). In-source collision-induced dissociation (CID) of the alkali metal and ammonium molecular ion adducts of monensin produce strikingly different mass spectra, with the alkali metal complexes exhibiting little dissociation and the ammoniated forms characterized by extensive fragmentation of the polyether skeletal structure with the production of several structurally diagnostic ions. The observed fragmentation of ammoniated monensin primarily involves opening of the cyclic ether rings and consecutive H2O losses. The propensity of ammoniated monensin derivatives toward skeletal fragmentation may involve the complexation of the thermally labile NH4+ ion through multiple hydrogen-bonding interactions with the polyether complexing oxygens resulting in decreased stability of the resultant complex. Comparison of the in-source CID spectra associated with the structurally similar compounds evaluated provides for the proposal of a general scheme for the fragmentation of ammoniated monensins and related compounds, the consistency of which indicates the usefulness of ESI-MS with in-source CID in ionophore structure elucidation.
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Kiehl et al. (1998) studied this question.