title: Collision-induced dissociation of fatty acid [M - 2H + Na]- ions: charge-directed fragmentation and assignment of double bond position.
abstract: 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: ∆(9,12,15)18:3, ∆(6,9,12)18:3, and ∆(5,8,11)18:3. 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.
authors.affiliation: Independent Marine Biochemistry Research, Moreton Bay Research Station, Dunwich, QLD, 4183, Australia, Michaelt293@gmail.com.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Thomas
authors.forename: Jens
authors.initials: J
authors.lastname: Altvater
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Gallagher
authors.forename: Geoffrey W
authors.initials: GW
authors.lastname: Nette
mesh_terms: D005227:Fatty Acids / Q000737:chemistry*; D007477:Ions / Q000737:chemistry*; D013058:Mass Spectrometry / Q000379:methods*; D008958:Models, Molecular
chemical_list: D005227:Fatty Acids; D007477:Ions