Revealing the unexpected: formal charge exchanges in collisional activation of cereulide ionized molecular species
Résumé
Under positive electrospray desorption/ionization mass spectrometry (ESI-MS) conditions, a bacterial cyclodepsipeptide toxin, cereulide [ (D) Ala- (L)O Val- (L) Val- (D)O Leu]3, is mainly desorbed as ammonium adduct and alkalicationized species. In addition to the naturally occurring alkali cation (Li + /Na + /K + ), Rb + can be attached to the molecule after addition of its chlorine salt (RbCl) solution. Under non-resonant collision-induced dissociations (non-resonant CID) using a quadrupole time-of-flight (Qq/TOF) instrument, bare alkali cations were recently detected in non-resonant CID spectra. Investigation using energy-resolved mass spectrometry (ERMS) of cationized cereulide revealed coexistence of charge-solvated (CS) and protonated salt (PS) tautomers. Infusion of cereulide solution containing RbCl salt into the ion source led to the unexpected detection of 85 Rb + and 87 Rb + within their natural abundances when Rb-free species ([M+NH4] + , [M+Na] + or [M+K] + ) were selected for subsequent fragmentation. To elucidate the origin of these unexpected cations, ERMS of the various ionized cereulide species was carried out using an Orbitrap-based instrument. Curiously, in addition to the unexpected Rb + isotopique cations, Rb2Cl + isotopologues were abundantly detected in nonresonant CID spectra of [M+NH4] + , [M+Na] + , and [M+K] + , but not of [M+ 85 Rb] + . Additional calculations and ERMS logarithmic plots made it possible to rule out a possible endogenous origin of the Rb2Cl + and Rb + isotopologues. They are produced by ion/molecule reactions in the higher-energy collisional dissociation (HCD) cell between ionized cereulide (and b(12-n) ions), relaxed by the buffer gas, and neutral RbCl isotopologue salts present in gas phase. The latter are likely generated either during preliminary studies of RbnCl(n-1) + cluster dissociation in smaller cluster ions and neutral RbCl loss, or from the HCD cell surface by collisions with the buffer gas. Consequently, Rb2Cl + isotopologues are not precursors of 85 Rb + plus 87 Rb + .
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