A new charge-associated mechanism to account for the production of fragment ions in the high-energy CID spectra of fatty acids

Abstract

A new mechanism, termed a charge-assisted process, is proposed as an additional mechanism to the charge-remote process to account for ions of the [M Ϫ C n H 2nϩ2 ] series found in the positive and negative high energy CID spectra of fatty acids and related compounds when ionized as closed-shell ( [M Ϫ H] Ϫ or [M ϩ X] ϩ ) species. The new mechanism is based on that commonly invoked to account for similar ions in the electron-impact spectra of derivatized fatty acids whereby the positive charge on the derivative abstracts a hydrogen atom from various positions of the alkyl chain to leave a radical that initiates a radical-induced cleavage of the chain. It is proposed that in the high energy CID spectra of closed-shell ions, similar hydrogen migrations occur but unpairing of electrons is avoided by charge transfer to the alkyl chain. This charge then initiates a concerted cleavage of the chain to give an allylic carbonium (positive ion spectrum) or carbanion (negative ion spectrum). The mechanism avoids the need to involve radicals or loss of hydrogen atoms from even-electron (closed shell) ions and provides a driving force for the reaction, namely, the formation of ions with a stabilized charge. An extension of the mechanism is also proposed to account for the formation of odd-electron ions from these compounds. The charge-assisted mechanism does not rule out the occurrence of other mechanisms that have been accepted for many years but provides an alternative process that can account for some spectral features which were difficult to explain earlier. show remarkable similarities with those of their oddelectron counterparts produced by EI [18,22,23,28,42]. The major fragment ions in these high energy CID spectra are even-electron (closed shell) species belonging to the [M Ϫ C n H 2nϩ2 ] series but also present, depending on the ionization conditions and internal energy of the ions, are other series, predominantly of the composition [M Ϫ C n H 2nϩ1 ] [41] that have odd numbers of electrons and must be radicals. Hydrogen atom loss has frequently been proposed to account, not only for this ion series, but to be involved in the formation of the major, closed-shell C n H 2nϩ2 ion series [23,28] even though such a mechanism involves the energetically unfavorable unpairing of electrons. Much of the fragmentation of the high energy, evenelectron CID ions has been rationalized by charge-remote mechanisms of the type first proposed by Gross and co-workers in the 1980s [33,[43][44][45]. Thus, Jensen et al.

Effect of Cysteic Acid Position on the Negative Ion Fragmentation of Proteolytic Derived Peptides

“…In an analogous experiment, fatty acid [M -H] − ions also fragmented through a charge-remote fragmentation process; i.e., a fixed negative charge site at either end of the molecule and collisional activation are required to achieve extensive decomposition and realize parallel losses of C n H 2n+2 fragments from the alkyl chain [32]. More recently, a charge-assisted fragmentation mechanism has been reported for [M -H] − ions of long-chain fatty acids that suggests the involvement of the charge site in the fragmentation reactions (series of [M -C n H 2n + 2] − ions) that occur along the fatty acid chain [33]. Therefore, we cannot completely eliminate the involvement of the negative charge site in the d i fragment ion mechanism.…”

Comparison of methods for the analysis of therapeutic immunoglobulin G Fc-glycosylation profiles—Part 2: Mass spectrometric methods

“…[16][17][18] While most of the mass spectrometric analyses of glycans and glycopeptides are performed in positive-ion mode, both negative-mode MALDI-MS and ESI-MS have recently been applied very successfully with improved ionization efficiency, reduced cation adduct formation, and highly informative fragmentation patterns including diagnostic ions valuable for structural analysis. 16,19 The analysis of sialic acid by MS is challenging because the ionization may be different compared to neutral glycans. Furthermore, sialic acid-containing glycans are even more prone to fragmentation than neutral glycans, which results in in-source or metastable decay.…”

Revisiting the Reactivity of Uracil During Collision Induced Dissociation: Tautomerism and Charge-Directed Processes

“…However, in addition to the instrumental challenges associated with acquiring high quality spectra for metabolites in urine [1], the interpretation of MS/MS data for the purpose of unknown identification remains a significant challenge. Such de novo identification aims to correlate observed spectral features in MS/MS to the structures of investigated species based on established reactivity for a class of compounds [4,[6][7][8][9]. The idea of using MS/MS as stand-alone identification tool is of increasing interest and is also being addressed using other more general approaches intended to generalize dissociation pathways [10], to automate data analysis [11], or to compile spectra in databases [12,13].…”