Ozone-Induced Dissociation of Conjugated Lipids Reveals Significant Reaction Rate Enhancements and Characteristic Odd-El
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J. Am. Soc. Mass Spectrom. (2013) 24:286Y296 DOI: 10.1007/s13361-012-0521-9
RESEARCH ARTICLE
Ozone-Induced Dissociation of Conjugated Lipids Reveals Significant Reaction Rate Enhancements and Characteristic Odd-Electron Product Ions Huong T. Pham,1 Alan T. Maccarone,1 J. Larry Campbell,2 Todd W. Mitchell,3 Stephen J. Blanksby1 1
School of Chemistry, University of Wollongong, Wollongong, NSW, Australia AB SCIEX, Concord, ON, Canada 3 School of Health Sciences, University of Wollongong, Wollongong, NSW, Australia 2
Abstract. Ozone-induced dissociation (OzID) is an alternative ion activation method that relies on the gas phase ion-molecule reaction between a massselected target ion and ozone in an ion trap mass spectrometer. Herein, we evaluated the performance of OzID for both the structural elucidation and selective detection of conjugated carbon-carbon double bond motifs within lipids. + The relative reactivity trends for [M + X] ions (where X 0 Li, Na, K) formed via electrospray ionization (ESI) of conjugated versus nonconjugated fatty acid methyl esters (FAMEs) were examined using two different OzID-enabled linear ion-trap mass spectrometers. Compared with nonconjugated analogues, FAMEs derived from conjugated linoleic acids were found to react up to 200 times faster and to yield characteristic radical cations. The significantly enhanced reactivity of conjugated isomers means that OzID product ions can be observed without invoking a reaction delay in the experimental sequence (i.e., trapping of ions in the presence of ozone is not required). This possibility has been exploited to undertake neutral-loss scans on a triple quadrupole mass spectrometer targeting characteristic OzID transitions. Such analyses reveal the presence of conjugated double bonds in lipids extracted from selected foodstuffs. Finally, by benchmarking of the absolute ozone concentration inside the ion trap, second order rate constants for the gas phase reactions between unsaturated organic ions and ozone were obtained. These results demonstrate a significant influence of the adducting metal on reaction rate constants in the fashion Li 9 Na 9 K. Key words: Ozone-induced dissociation, Conjugated linoleic acid, Ion-molecule reactions, Lipidomics, Rate constants, Mass spectrometry Received: 9 August 2012/Revised: 10 October 2012/Accepted: 10 October 2012/Published online: 5 January 2013
Introduction
W
hile unsaturated lipids containing conjugated carboncarbon double bonds are less common in nature than nonconjugated variants [1], they are known to profoundly influence the physiology of plants and animals [2–4]. One of the better known classes of conjugated lipids is conjugated linoleic acids (CLA). These octadecadienoic acids (18:2) possess two carbon-carbon double bonds in conjugation and give rise to variation from the differing positions and stereochemistry of these bonds. CLAs are reported to have
Electronic supplementary material The online version of this article (doi:10.1007/s13361-012-0521-9) contains supplementary material, which i
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