Structural characterization of triacylglycerols as lithiated adducts by electrospray ionization mass spectrometry using low-energy collisionally activated dissociation on a triple stage quadrupole instrument
We describe features of tandem mass spectra of lithiated adducts of triacylglycerol (TAG) species obtained by electrospray ionization mass spectrometry (ms) with low-energy collisionally activated dissociation (CAD) on a triple stage quadrupole instrument. The spectra distinguish isomeric triacylglycerol species and permit assignment of the mass of each fatty acid substituent and positions on the glycerol backbone to which substituents are esterified. Source CAD-MS2 experiments permit assignment of double bond locations in polyunsaturated fatty acid substituents. The ESI/MS/MS spectra contain [M+Li−(R n CO2H)]+, [M+Li−(R n CO2Li)]+, and R n CO+ ions, among others, that permit assignment of the masses of fatty acid substituents. Relative abundances of these ions reflect positions on the glycerol backbone to which substituents are esterified. The tandem spectra also contain ions reflecting combined elimination of two adjacent fatty acid residues, one of which is eliminated as a free fatty acid and the other as an α,β-unsaturated fatty acid. Such combined losses always involve the sn-2 substituent, and this feature provides a robust means to identify that substituent. Fragment ions reflecting combined losses of both sn-1 and sn-3 substituents without loss of the sn-2 substituent are not observed. Schemes are proposed to rationalize formation of major fragment ions in tandem mass spectra of lithiated TAG that are supported by studies with deuterium-labeled TAG and by source CAD-MS2 experiments. These schemes involve initial elimination of a free fatty acid in concert with a hydrogen atom abstracted from the α-methylene group of an adjacent fatty acid, followed by formation of a cyclic intermediate that decomposes to yield other characteristic fragment ions. Determination of double bond location in polyunsaturated fatty acid substituents of TAG is achieved by source CAD experiments in which dilithiated adducts of fatty acid substituents are produced in the ion source and subjected to CAD in the collision cell. Product ions are analyzed in the final quadrupole to yield information on double bond location.
KeywordsTandem Mass Spectrum Collisionally Activate Dissocia Glycerol Backbone Lithium Salt Fatty Acid Substituent
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- 7.Ramanadham, S.; Hsu, F.-F.; Bohrer, A.; Nowatzke, W.; Ma, Z.; Turk, J. Electrospray ionization mass spectrometric analysis of phospholipids from rat and human pancreatic islets and subcellular membranes. Comparison to other tissues and implications for membrane fusion in insulin exocytosis. Biochemistry 1998, 37, 4533–4567.CrossRefGoogle Scholar
- 8.Hsu, F.-F.; Bohrer, A.; Turk, J. Electrospray ionization tandem mass spectrometric analysis of sulfatide. Determination of fragmentation patterns and characterization of molecular species expressed in brain and in pancreatic islets. Biochim. Biophys. Acta 1998, 1392, 202–216.Google Scholar
- 9.Murphy, R. C. Mass Spectrometry of Lipids. In Handbook of Lipid Research; Snyder, F., Ed.; Plenum: New York, 1993; Vol. 7, pp 213–243.Google Scholar
- 19.Kallio, H.; Currie, G. Analysis of low erucic acid turnip rapeseed oils (Brassica campestris) by negative ion chemical ionization tandem mass spectrometry. A method giving information on the fatty acid composition in positions sn-2 and sn-1/3 of triacylglycerols. Lipids 1993, 28, 207–215.CrossRefGoogle Scholar
- 25.Evans, N.; Games, D. E.; Harwood, J. L.; Jackson, A. H. Field desorption mass spectrometry of triglycerides and phosphoglycerides. Biochem. Soc. Trans. 1974, 2, 1091–1092.Google Scholar
- 31.Evans, C.; Traldi, P.; Bambigiotti-Alberti, M.; Gianelli, v.; Coran, S. A.; Vincieri, F. F. Positive and negative fast atom bombardment mass spectrometry and collision spectroscopy in the structural characterization of mono-, di-, and triglycerides. Biol. Mass Spectrom. 1991, 20, 351–356.CrossRefGoogle Scholar
- 37.Pittenauer, E.; Aichinger, T.; de Hueber, K.; Bailer, J. Characterization of seed oils for potential technical use by HPLC. Proceedings of the 44th ASMS Conference on Mass Spectrometry and Allied Topics; Portland, OR, 1996; p 928.Google Scholar
- 42.McIntyre, D.; Fisher, S. The characterization of di- and triglycerides in oils and fats by API LC/MS. Proceedings of the 44th ASMS Conference on Mass Spectrometry and Allied Topics; Portland, OR, 1996; p 289.Google Scholar
- 46.Hsu, F.-F.; Turk, J. Distinction among isomeric unsaturated fatty acids as lithiated adducts by electrospray ionization mass spectrometry using low energy collisionally activated dissociation on a triple stage quadrupole instrument. J. Am. Soc. Mass Spectrom., submitted.Google Scholar