title: Formation and fragmentation of unsaturated fatty acid [M - 2H + Na]- ions: stabilized carbanions for charge-directed fragmentation.
abstract: Fatty acids are long-chain carboxylic acids that readily produce [M - H](-) ions upon negative ion electrospray ionization (ESI) and cationic complexes with alkali, alkaline earth, and transition metals in positive ion ESI. In contrast, only one anionic monomeric fatty acid-metal ion complex has been reported in the literature, namely [M - 2H  +  Fe(II)Cl](-). In this manuscript, we present two methods to form anionic unsaturated fatty acid-sodium ion complexes (i.e., [M - 2H  +  Na](-)). We find that these ions may be generated efficiently by two distinct methods: (1) negative ion ESI of a methanolic solution containing the fatty acid and sodium fluoride forming an [M - H  +  NaF](-) ion. Subsequent collision-induced dissociation (CID) results in the desired [M - 2H  +  Na](-) ion via the neutral loss of HF. (2) Direct formation of the [M - 2H  +  Na](-) ion by negative ion ESI of a methanolic solution containing the fatty acid and sodium hydroxide or bicarbonate. In addition to deprotonation of the carboxylic acid moiety, formation of [M - 2H  +  Na](-) ions requires the removal of a proton from the fatty acid acyl chain. We propose that this deprotonation occurs at the bis-allylic position(s) of polyunsaturated fatty acids resulting in the formation of a resonance-stabilized carbanion. This proposal is supported by ab initio calculations, which reveal that removal of a proton from the bis-allylic position, followed by neutral loss of HX (where X = F(-) and (-)OH), is the lowest energy dissociation pathway.
authors: Independent Marine Biochemistry Research, Moreton Bay Research Station, Dunwich, Qld, 4183, Australia, Michaelt293@gmail.com.
authors: Michael C
authors: MC
authors: Thomas
authors: Benjamin B
authors: BB
authors: Kirk
authors: Jens
authors: J
authors: Altvater
authors: Stephen J
authors: SJ
authors: Blanksby
authors: Geoffrey W
authors: GW
authors: Nette
mesh_terms: D000838:Anions / Q000737:chemistry*; D005231:Fatty Acids, Unsaturated / Q000737:chemistry*; D012964:Sodium / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization
chemical_list: D000838:Anions; D005231:Fatty Acids, Unsaturated; D012964:Sodium