Formation and Fragmentation of Unsaturated Fatty Acid M-2H+Na (-) Ions: Stabilized Carbanions for Charge-Directed Fragmentation
Thomas, M. C., Kirk, B. B., Altvater, J., Blanksby, S. J., & Nette, G. W. (2014) Formation and Fragmentation of Unsaturated Fatty Acid M-2H+Na (-) Ions: Stabilized Carbanions for Charge-Directed Fragmentation. Journal of the American Society for Mass Spectrometry, 25(2), pp. 237-247.
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 + (FeCl)-Cl-II](-). 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.
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|Item Type:||Journal Article|
|Additional Information:||Also associated with ARC grant CE0561607|
|Keywords:||Fatty acid, Anionic complexes, Collision-induced dissociation, [M-2H+Na](-) ions, Carbanion formation, tandem mass-spectrometry, earth metal-ions, collision-induced, dissociation, gas-phase fragmentations, double-bond position, anionic, complexes, electrospray-ionization, regiospecific decompositions, remote, fragmentation, peptides, ARC|
|Subjects:||Australian and New Zealand Standard Research Classification > CHEMICAL SCIENCE (030000)|
|Divisions:||Current > QUT Faculties and Divisions > Science & Engineering Faculty|
|Deposited On:||19 Mar 2014 23:31|
|Last Modified:||19 Jan 2016 16:00|
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