Rapid determination of double bond configuration and position along the hydrocarbon chain in cyclic fatty acid monomers

Lipidomics era: accomplishments and challenges

Lipid mediators participate in signal transduction pathways, proliferation, apoptosis, and membrane trafficking in the cell. Lipids are highly complex and diverse owing to the various combinations of polar headgroups, fatty acyl chains, and backbone structures. This structural diversity continues to pose a challenge for lipid analysis. Here we review the current state of the art in lipidomics research and discuss the challenges facing this field. The latest technological developments in mass spectrometry, the role of bioinformatics, and the applications of lipidomics in lipid metabolism and cellular physiology and pathology are also discussed.

Discrimination among geometrical isomers of alpha-linolenic acid methyl ester using low energy electron ionization mass spectrometry

There is a consensus that electron impact ionization mass spectrometry is not capable of discriminating among geometrical isomers of unsaturated fatty acid methyl esters (and in general olefinic compounds). In this paper, we report the identification of all eight geometrical isomers of alpha-linolenic acid, one of the few essential omega-3 fatty acids that has attracted great attention, using low-energy electron ionization mass spectrometry. Three electron energies 70, 50, and 30 eV were studied and the mass spectrum of each isomer was obtained from the analysis of different concentrations of a standard mixture of alpha-linolenic acid methyl ester geometrical isomers to ensure the robustness of the method. Principal component analysis was employed to model the complex variation of m/z intensities across the isomers. Only using the data of 30 eV energy was complete differentiation among geometrical isomers observed. The unique cleavage pattern of the alpha-linolenic acid methyl ester isomers leading to a benzenium ion structure is discussed and general fragmentation rules are derived using the mass spectra of over 300 compounds with different kinds and levels of unsaturation. Application of the proposed method is not limited to alpha-linolenic acid. It can potentially be used to identify the geometrical isomers of any compounds with an olefinic chain.

Isolation of 2-methyl branched unsaturated very long fatty acids from marine sponge Halichondria panicea and identification of them by GC–MS and NMR

In order to study the biomarker fatty acids of symbionts in the marine sponge Halichondria panicea, purification and structural identification of two new 2-methyl branched monoenoic very long fatty acids (2-Me-24:1 n-7 and 2-Me-26:1 n-9) were performed for the first time. These acids amounted to 7.1% of total sponge FAs, but our attempts to determine their structures by one-step GC-MS analysis were unsuccessful because of low yields of the correspondent N-acyl pyrrolidide derivatives. Silver-ion thin-layer chromatography isolated enriched fractions of monoenoic fatty acids extracted from the sponge. Further purification of unknown fatty acid methyl esters was carried out by reversed-phase high-performance liquid chromatography. Determination of the chain length, degree and position of unsaturations was achieved by gas chromatography-mass spectrometry on methyl esters and dimethyldisulfide adducts. Structures, position of methyl substitution, and double bonds cis isomery were confirmed by 1H nuclear magnetic resonance.

Acetonitrile chemical ionization tandem mass spectrometry to locate double bonds in polyunsaturated fatty acid methyl esters

A rapid method is presented for determining the location of double bonds in polyunsaturated fatty acid methyl esters (FAME) using an ion-trap mass spectrometer. The mass spectrum of the chemical ionization reagent acetonitrile in an ion trap includes a m/z 54 ion, identified previously as 1-methyleneimino-1-ethenylium ion. We show that it reacts with double bonds of polyunsaturated FAME to yield a series of covalent product ions all appearing at (M + 54)+. Collisional dissociation of these ions yields diagnostic fragments, permitting unambiguous localization of double bonds. For methylene-interrupted and conjugated FAME, one of these fragments results from loss of the hydrocarbon end of the chain, while the other involves loss of the methyl ester. Major diagnostic-fragment ions for monoene and diene FAME occur as a result of cleavage adjacent to either allylic sites or double bonds in the original analyte and appear at one mass unit above the mass expected for homolytic cleavage. Fragmentation of polyene FAME yields major diagnostic ions resulting from cleavage between double bonds that appear one mass unit lower. The method is shown to produce highly characteristic spectra for FAME with 1 to 6 double bonds. Identification of double-bond position in highly unsaturated fatty acids is demonstrated in a mixture of unknown polyunsaturated FAME from an extract of cultured Y79 human retinoblastoma cells.

Gas chromatography-mass spectrometry methods for structural analysis of fatty acids

Procedures for structural analysis of fatty acids are reviewed. The emphasis is on methods that involve gas chromatography-mass spectrometry and, in particular, the use of picolinyl ester and dimethyloxazoline derivatives. These should be considered as complementing each other, not simply as alternatives. However, additional derivatization procedures can be of value, including hydrogenation and deuteration, and preparation of dimethyl disulfide and 4-methyl-1,2,4-triazoline-3,5-dione adducts. Sometimes complex mixtures must be separated into simpler fractions prior to analysis by gas chromatography-mass spectrometry. Silver ion and reversed-phase high-performance liquid chromatography are then of special value. In particular, a novel application of the latter technique, involving a base-deactivated stationary phase and acetonitrile as mobile phase, is described that is suited to the separation of fatty acids in the form of picolinyl ester and dimethyloxazoline derivatives, as well as methyl esters.

The effects of cyclic fatty acid monomers on cultured porcine endothelial cells

The popularity of polyunsaturated oils used in food applications and preparation continues to appreciate as a result of positive health claims. With polyunsaturated oils inherently more susceptible to oxidative and thermal degradation, the formation of new fatty acid species increases considerably. The presence of one species known as cyclic fatty acid monomers (CFAM) has been detected as a component of many oils subjected to various thermal processes including deep-fat frying. The effect of CFAM on metabolic processes has not been fully characterized. In this study, confluent porcine aortic endothelial cells incorporated CFAM into their polar and nonpolar lipid fractions following a 48-h exposure to 31 and 62 ppm CFAM in the culture medium. Subsequently, the influence of CFAM incorporation on various membrane-dependent physical properties and biochemical processes was investigated. CFAM decreased the lipid packing order of the membrane bilayer core but did not alter the lipid packing order of lipid chain segments at or near the lipid-water interface of the membrane. CFAM led to significant reductions in Ca2+ ATPase activity and monolayer integrity while eliciting a significant increase of prostacyclin synthesis and secretion.