Structural significance of CnH2nO losses from sodiated cereulide

Identification of the bacterial Bacillus cereus toxin, cereulide, is mainly based on LC-MS/MS analysis. In electrospray, cereulide is desorbed as a mixture of ammonium adduct and cationized species. The highly specific fragmentation of sodium-cationized cereulide has been previously explored by non-resonant collision-induced-dissociation (nrCID) and is of particular interest for toxin identification in food matrices. Here, we present a detailed study of sodium-cationized cereulide fragmentation by resonant collision-induced-dissociation (CID) in sequential high resolution sequential MS 2 /MS 3 experiments. We focused on specific ion series, initiated by the main pathway based on specific aldehyde losses, possible only from the cleaved ester bond in open-ring species with very likely protonated oxazolone and cationized alkoxide ends. These initial aldehyde losses resulting from surprising hydride transfer combined with Na + migration give rise to two main ion series: one expected in a counterclockwise direction and the other unexpected in a clockwise direction . Combining the data from these ion series allows the complete sequence to be established. The mechanisms explaining these fragmentation pathways are proposed. This approach should be useful for the identification of cereulide isoforms (isocereulides), also often present in food matrices implicated in emetic Bacillus cereus foodborne outbreaks, in a single LCMS analysis using the sequential MS 2 and MS 3 experiments.