Gas-Phase Fragmentation of Coenzyme Q10 Radical Anion Generated by APCI: A Study by High/Low-Resolution Tandem/Sequential Mass Spectrometry

Coenzyme Q10 (CoQ10) and closely related compounds with varying isoprenoid tail lengths (CoQn, n = 6-9) are biochemical cofactors involved in many physiological processes, playing important roles in cellular respiration and energy production. Liquid chromatography (LC) coupled with single or tandem mass spectrometry (MS) using electrospray (ESI) or atmospheric pressure chemical ionization (APCI) is considered the gold standard for the identification and quantification of CoQ10 in food and biological samples. However, the characteristic fragmentation exhibited by the CoQ10 radical anion ([M]•-, m/z 862.684), the prevailing ion generated by APCI in negative polarity, has not been studied in detail. In this work, a systematic study was carried out to clarify this issue, using higher collisional energy dissociation (HCD) with high-resolution tandem FTMS and collision-induced dissociation-low-resolution sequential mass spectrometry (CID-MSn, n = 2-4). Various fragmentation pathways were successfully interpreted, with some structures proposed for product ions checked using density functional theory (DFT) calculations. Besides the already-known detachments of methyl radicals occurring directly from the CoQ10 radical anion and leading to ions like [M - CH3]- and [M - 2CH3]•-, the homolytic cleavage of C-C bonds along the oligo-isoprenoid side chain was tentatively proposed to explain some of the observed fragmentations. As a result, the generation of uncommon yet potentially stable distonic biradical anions was hypothesized, with some of them likely undergoing intramolecular cyclization to generate ions without unpaired electrons. Diagnostic product ions emerged from the fragmentation processes of CoQ10 and were found to be common also to the radical anions of other CoQn derivatives (n = 7-9), facilitating their identification in extracts of edible Brassicaceae plant microgreens by reversed-phase liquid chromatography (RPLC)-APCI-FTMS.