Revealing the unexpected: formal charge exchanges in collisional activation of cereulide ionized molecular species

Under positive electrospray desorption/ionization mass spectrometry (ESI-MS) conditions, a bacterial cyclodepsipeptide toxin, cereulide [^(D)Ala-^(L)OVal-^(L)Val-^(D)OLeu]₃, is mainly desorbed as ammonium adduct and alkali-cationized 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 ⁸⁵Rb⁺ and ⁸⁷Rb⁺ 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 non-resonant CID spectra of [M + NH₄]⁺, [M+Na]⁺, and [M+K]⁺, but not of [M+⁸⁵Rb]⁺. 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 Rb_nCl_(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 ⁸⁵Rb⁺ plus ⁸⁷Rb⁺.