Gas phase structures of alkali-cationized cereulide responsible for regioselective dissociation by collisional activation, effect of the missed ions

S. Liuu, K. Trinh, E. Darii, C. Cao, A. Damont, J.-C. Tabet, Y. Gimbert, F. Fenaille, Y. Makni, C. Inthavong, G. Lavison Bompard, J.-A. Hennekinne, O. Firmesse

Abstract

Emetic toxin is increasingly reported worldwide for its involvement in Bacillus cereus food poisoning outbreaks. The preformed toxin in food called cereulide is a heat-stable cyclodepsipeptide, [(D)Ala-O(L)Val-(L)Val-O(D)Leu]3 and an ionophore selective to K+ in solution. In electrospray (ESI), the K+ selectivity is reduced since the Li+, Na+ , NH4+ adduct ions are also detected in competition without adding salt to introduced sample solutions.Here, by infusing synthetic cereulide into ESI positive ion source, Li+ /Na+/K+ cationized cereulide coexists in two forms: charge-solvated (CS) form and protonated salt (PS). Various high resolution tandem instruments based on resonant (LTQ/Orbitrap) and non-resonant (Qq/TOF, Qq/Orbitrap) excitations were used. Energy resolved mass spectrometry breakdowns (ERMS, 0 eV to 150 eV) were compared for different involved alkali cation ions to highlight coexisting forms. The [M+Li]+ , [M+Na]+ and [M+K]+ ions within PS form dissociate through covalent bond cleavages resulting in alkali retention in the a, b ion series accompanied by exotic product ions, detected with tandem based on TOF and Orbitrap analyzers. From [M+K]+ , a lot of product ions are lost from the latter analyzer compared to those detected using the former. Indeed, the lost ions are essentially K+ (for Orbitrap, m/z scale is ≥ m/z 50). This behavior differs to that observed for [M+Li]+ and [M+Na]+ , which does not display Li+ and abundant Na+using Qq/TOFMS. The direct K+ loss occurs only from the cationized cereulide within CS form. This is confirmed by the sequential MS3 experiments on the [M+K]+ , [M+Li]+ , and [M+Na]+ product ions that display exclusive 2ndgeneration product ions with K+ /Li+ /Na+ retention. This suggests that the CS form is favored for [M+K]+ and conversely for [M+Li]+ and [M+Na]+ . The existence of the missed alkali cation (vide supra) is highlighted by the loss of the total ionic current compared to that observed during the dissociation of [M+NH4]+. Finally, consideration of the various currents (total ion, product ion, alkali cation) based on the absolute abundances [1] will be discussed as well as the different product ion series.

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IMEKO-TC23-2023-072.pdf

DOI

10.21014/tc23-2023.072

IMEKO TC

TC23 - Metrology in Food and Nutrition

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