Formation and Fragmentation of Unsaturated Fatty Acid [M – 2H + Na]^– Ions: Stabilized Carbanions for Charge-Directed Fragmentation

Abstract

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), 237-247. Formation and fragmentation of unsaturated fatty acid [m -2h + na] -ions: stabilized carbanions for charge-directed fragmentation AbstractFatty 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 + FeIICl]-. 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.