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Formation and Fragmentation of Unsaturated Fatty Acid [M - 2H + Na](-) Ions: Stabilized Carbanions for Charge-Directed Fragmentation.
MedLine Citation:
PMID:  24338213     Owner:  NLM     Status:  Publisher    
Abstract/OtherAbstract:
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:
Michael C Thomas; Benjamin B Kirk; Jens Altvater; Stephen J Blanksby; Geoffrey W Nette
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2013-12-13
Journal Detail:
Title:  Journal of the American Society for Mass Spectrometry     Volume:  -     ISSN:  1879-1123     ISO Abbreviation:  J. Am. Soc. Mass Spectrom.     Publication Date:  2013 Dec 
Date Detail:
Created Date:  2013-12-16     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  9010412     Medline TA:  J Am Soc Mass Spectrom     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
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