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Electrospray ionization multiple-stage linear ion-trap mass spectrometry for structural elucidation of triacylglycerols: assignment of fatty acyl groups on the glycerol backbone and location of double bonds.

ABSTRACT: Linear ion-trap multiple-stage mass spectrometric approach (MS(n)) towards nearly complete structural elucidation of triacylglycerol (TAG) including (1) assignment the fatty acid substituents on the glycerol backbone and (2) location of the double bond(s) on the unsaturated fatty acyl groups is reported. The characterization is established by the findings that MS(2) on the [M + Li](+) ions of TAG yields more abundant ions reflecting losses of the outer fatty acid substituents either as free acids (i.e., [M + Li - R(1)CO(2)H](+) and [M + Li - R(3)CO(2)H](+) ions) or as lithium salts (i.e., [M + Li - R(1)CO(2)Li](+) and [M + Li - R(3)CO(2)Li](+) ions) than the ions reflecting the similar losses of the inner fatty acid substituent (i.e., [M + Li - R(2)CO(2)Li](+) and [M + Li - R(2)CO(2)Li](+) ions). Further dissociation (MS(3) of [M + Li - R(n)CO(2)H](+) (n = 1, 2, or 3) gives rise to the ion series locating the double bonds along the fatty acid chain. These ions arise from charge-remote fragmentations involving beta-cleavage with gamma-H shift, analogous to those seen for the unsaturated long-chain fatty acids characterized as initiated ions. Significant differences in abundances in the ion pairs reflecting the additional losses of the fatty acid moieties, respectively, were also seen in the MS(3) spectra of the [M + Li - R(n)CO(2)H](+) and [M + Li - R(n)CO(2)Li](+) ions, leading to confirmation of the fatty acid substituents on the glycerol backbone. MS(n) on the [M + Na](+) and [M + NH(4)](+) adduct ions also affords location of fatty acid substituents on the glycerol backbone, but not the position of the double bond(s) along the fatty acid chain. Unique ions from internal losses of the glycerol residues were seen in the MS(3) spectra of [M + Alk - R(n)CO(2)H](+) (n = 1, 2, 3) and of [M + Alk - R(n)CO(2)Alk](+) (Alk = Li, Na, NH(4); n = 1, 3). They are signature ions for glycerides and the pathways leading to their formation may involve rearrangements.

SUBMITTER: Hsu FF

PROVIDER: S-EPMC2847672 | biostudies-literature | 2010 Apr

REPOSITORIES: biostudies-literature

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Electrospray ionization multiple-stage linear ion-trap mass spectrometry for structural elucidation of triacylglycerols: assignment of fatty acyl groups on the glycerol backbone and location of double bonds.

Hsu Fong-Fu FF Turk John J

Journal of the American Society for Mass Spectrometry 20100125 4

Linear ion-trap multiple-stage mass spectrometric approach (MS(n)) towards nearly complete structural elucidation of triacylglycerol (TAG) including (1) assignment the fatty acid substituents on the glycerol backbone and (2) location of the double bond(s) on the unsaturated fatty acyl groups is reported. The characterization is established by the findings that MS(2) on the [M + Li](+) ions of TAG yields more abundant ions reflecting losses of the outer fatty acid substituents either as free acid ...[more]

PMID: 20171120

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Project description:The position(s) of carbon-carbon double bonds within lipids can dramatically affect their structure and reactivity and thus has a direct bearing on biological function. Commonly employed mass spectrometric approaches to the characterization of complex lipids, however, fail to localize sites of unsaturation within the molecular structure and thus cannot distinguish naturally occurring regioisomers. In a recent communication [Thomas, M. C.; Mitchell, T. W.; Blanksby, S. J. J. Am. Chem. Soc. 2006, 128, 58-59], we have presented a new technique for the elucidation of double bond position in glycerophospholipids using ozone-induced fragmentation within the source of a conventional electrospray ionization mass spectrometer. Here we report the on-line analysis, using ozone electrospray mass spectrometry (OzESI-MS), of a broad range of common unsaturated lipids including acidic and neutral glycerophospholipids, sphingomyelins, and triacylglycerols. All lipids analyzed are found to form a pair of chemically induced fragment ions diagnostic of the position of each double bond(s) regardless of the polarity, the number of charges, or the adduct ion (e.g., [M - H](-), [M - 2H](2-), [M + H](+), [M + Na](+), [M + NH(4)](+)). The ability of OzESI-MS to distinguish lipids that differ only in the position of the double bonds is demonstrated using the glycerophosphocholine standards, GPCho(9Z-18:1/9Z-18:1) and GPCho(6Z-18:1/6Z-18:1). While these regioisomers cannot be differentiated by their conventional tandem mass spectra, the OzESI-MS spectra reveal abundant fragment ions of distinctive mass-to-charge ratio (m/z). The approach is found to be sufficiently robust to be used in conjunction with the m/z 184 precursor ion scans commonly employed for the identification of phosphocholine-containing lipids in shotgun lipidomic analyses. This tandem OzESI-MS approach was used, in conjunction with conventional tandem mass spectral analysis, for the structural characterization of an unknown sphingolipid in a crude lipid extract obtained from a human lens. The OzESI-MS data confirm the presence of two regioisomers, namely, SM(d18:0/15Z-24:1) and SM(d18:0/17Z-24:1), and suggest the possible presence of a third isomer, SM(d18:0/19Z-24:1), in lower abundance. The data presented herein demonstrate that OzESI-MS is a broadly applicable, on-line approach for structure determination and, when used in conjunction with established tandem mass spectrometric methods, can provide near complete structural characterization of a range of important lipid classes. As such, OzESI-MS may provide important new insight into the molecular diversity of naturally occurring lipids.

Dataset Information

Electrospray ionization multiple-stage linear ion-trap mass spectrometry for structural elucidation of triacylglycerols: assignment of fatty acyl groups on the glycerol backbone and location of double bonds.

Publications

Electrospray ionization multiple-stage linear ion-trap mass spectrometry for structural elucidation of triacylglycerols: assignment of fatty acyl groups on the glycerol backbone and location of double bonds.

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