Charge-solvated versus protonated salt forms of cyclodepsipeptide toxins in electrospray: Dissociation of alkali-cationized forms enables straightforward sequencing of cereulide

Bacillus cereus is responsible for foodborne outbreaks worldwide. Among the produced toxins, cereulide induces nausea and vomiting after 30 min to 6 h following the consumption of contaminated foods. Cereulide, a cyclodepsipeptide, is an ionophore selective to K⁺ in solution. In electrospray, the selectivity is reduced as [M + Li]⁺; [M + Na]⁺ and [M + NH₄]⁺ can also be detected without adding corresponding salts. Two forms are possible for alkali-cationized ions: charge-solvated (CS) that exclusively dissociates by releasing a bare alkali ion and protonated salt (PS), yielding alkali product ions by covalent bond cleavages (CBC) promoted by mobile proton. Based on a modified peptide cleavage nomenclature, the PS product ion series (b, a, [b + H₂O] and [b + C_nH_2nO] [n = 4, 5]) are produced by Na⁺/Li⁺/K⁺-cationized cereulide species that specifically open at ester linkages followed by proton mobilization promoting competitive ester CBC as evidenced under resonant collision activation. What is more, unlike the sodiated or lithiated cereulide, which regenerates little or no alkali cation, the potassiated forms lead to an abundant K⁺ regeneration. This occurs by splitting of (i) the potassiated CS forms with an appearance threshold close to that of the PS first fragment ion generation and (ii) eight to four potassiated residue product ions from the PS forms. Since from Na⁺/Li⁺-cationized cereulide, (i) the negligible Na⁺/Li⁺ regeneration results in a higher sensibility than that of potassiated forms that abundantly releasing K⁺, and (ii) a better sequence recovering, the use of Na⁺ (or Li⁺) should be more pertinent to sequence isocereulides and other cyclodepsipeptides.