Collision-Induced Dissociation of Fatty Acid [M – 2H + Na]– Ions: Charge-Directed Fragmentation and Assignment of Double Bond Position

The collision-induced dissociation (CID) of cationic fatty acid–metal ion complexes has been extensively studied and, in general, provides rich structural information. In particular, charge-remote fragmentation processes are commonly observed allowing the assignment of double bond position. In a previous manuscript, we presented two methods to doubly deprotonate polyunsaturated fatty acids to form anionic fatty acid–sodium ion complexes, referred to as [M – 2H + Na] – ions. In the current manuscript, the CID behavior of these [M – 2H + Na] – ions is investigated for the first time. Significantly, we also present a deuterium-labeling experiment, which excludes the possibility that deprotonation occurs predominately at the α-carbon in the formation of fatty acid [M – H + NaF]– ions. This supports our original proposal where deprotonation occurs at the bis-allylic positions of polyunsaturated fatty acids. CID spectra of polyunsaturated fatty acid [M – 2H + Na]– ions display abundant product ions arising from acyl chain cleavages. Through the examination of fatty acid isomers, it is demonstrated that double bond position may be unequivocally determined for methylene-interrupted polyunsaturated fatty acids with three or more carbon–carbon double bonds. In addition, CID of [M – 2H + Na]– ions was applied to 18:3 isomers of Nannochloropsis oculata and three isomers were tentatively identified: ∆9,12,1518:3, ∆6,9,1218:3, and ∆5,8,1118:3. We propose that structurally-informative product ions are formed via charge-driven fragmentation processes at the site of the resonance-stabilized carbanion as opposed to charge-remote fragmentation processes, which could be inferred if deprotonation occurred predominately at the α-carbon.