Structural determination of modified fatty acids by collisional activation of cationized molecules

Development of an Efficient On-Tissue Epoxidation Reaction Mediated by Urea Hydrogen Peroxide for MALDI MS/MS Imaging of Lipid C═C Location Isomers

Unsaturated lipids containing different numbers and locations of C═C bonds are significantly associated with a variety of cellular and metabolic functions. Although matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) has been used to visualize the spatial distribution patterns of various lipids in biological tissues, in situ identification, discrimination, and visualization of lipid C═C location isomers remain challenging. Herein, an efficient and fast on-tissue chemical derivatization (OTCD) approach was developed to pinpoint the locations of C═C bonds in complex lipids in situ via methyltrioxorhenium (MTO)-catalyzed epoxidation of C═C with a urea hydrogen peroxide (UHP)/hexafluoroisopropanol (HFIP) system. The efficiency of OTCD could reach 100% via one-step spray deposition of the solution mixture of MTO/UHP/HFIP at room temperature. The developed OTCD method provided rich structural information on lipid C═C location isomers, and their accurate spatial distribution patterns were resolved in mouse brain tissues. Tissue-specific distributions and changes of lipid C═C location isomers in the liver sections of obese ob/ob and diabetic db/db mice were further investigated, and their correlation in two animal models was revealed. The simplicity and high efficiency of the OTCD method developed for MALDI tandem MSI of lipid C═C location isomers possess great potential for functional spatial lipidomics.

Degradation of the polar lipid and fatty acid molecular species in extra virgin olive oil during storage based on shotgun lipidomics

Among the bioactive compounds present in extra-virgin olive oil, polar lipids and free fatty acids are minor compounds with well-known nutritional values and have been studied for traceability and adulteration investigations as well. In the present paper, the simultaneous characterization of polar lipids and free fatty acids in a pool of fifteen EVOO samples was achieved by means of reversed phase C18 analysis coupled to negative polarity high-resolution mass spectrometry. A total of 24 polar lipids, comprising 19 phospholipids and 5 sulfolipids, and 27 free fatty acids were tentatively identified, including several odd-chain and very long-chain fatty acids at trace levels. Moreover, a one-month study of lipid degradation on simulated storage conditions was carried out thanks to the set-up of a dedicated approach for degradation product analysis which was implemented of Compound Discoverer software. By virtue of the customized data processing workflow, more than forty compounds were tentatively identified, including compounds deriving from hydrolysis and oxidation reactions. Finally, by analysis of peak area trends, phosphoester hydrolyses of polar heads of phospholipids emerged as the fastest reactions, followed by glycerol ester hydrolyses and oxidative processes.

Structural study of acetogenins by tandem mass spectrometry under high and low collision energy

Collision-induced dissociation experiments of seven annonaceous acetogenins were carried out under high and low collision energy conditions. Each compound was studied as protonated or deprotonated and lithium- or sodium- cationized molecules, using ElectroSpray Ionisation (ESI) with a hybrid linear trap/orbitrap mass spectrometer (LTQ-Orbitrap®). The same ion species were studied with a Matrix-Assisted Laser Desorption Ionisation (MALDI) tandem mass spectrometer in a high collision energy regime (1 or 2 keV). Although each of the techniques showed some limitations in the detection of functional groups, unambiguous structural identification of the acetogenins was obtained. MALDI ToF-ToF has the advantage over ESI-based methods to provide mass spectra rich in informative fragments which allows the confirmation of some functional groups position. By contrast, ESI-LTQ-Orbitrap® analysis has the advantage over MALDI that the mass spectra are relatively simple with only fragments close to the functional groups. However, this technique needs to carry out experiments both in negative and positive ionization modes. Copyri

Electrospray ionization with low-energy collisionally activated dissociation tandem mass spectrometry of glycerophospholipids: mechanisms of fragmentation and structural characterization

This review describes the use of low-energy collisionally activated dissociation (CAD) with both tandem quadrupole and ion-trap mass spectrometry toward structural characterization of glycerophospholipids (GPLs), including classes of glycerophosphocholine, glycerophosphoethanolamine, glycerophosphoserine, glycerophosphoglycerol glycerophosphoinositol and glycerophosphatidic acid, as well as their lyso-, plasmanyl-, and plasmenylphospholipid subclasses. The mechanisms underlying the fragmentation processes leading to structural characterization of GPLs in various ion forms desorbed by electrospray ionization in the positive-ion and negative-ion modes are also discussed. The tandem mass spectrometric approaches afford the identification of the polar head group, the fatty acid substituents and the location of the radyl groups on the glycerol backbone of all the GPLs.

Identification and relative quantification of fatty acids in Escherichia coli membranes by gas chromatography/mass spectrometry

The lipids that are essential to the functioning of the bacterial membrane exist in hundreds of different forms. The reasons for this diversity are far from clear but are presumably related to the roles of these lipids in both facilitating enzymic activities and generating proteolipid domains. A full understanding of bacterial physiology therefore requires characterization of lipids in different strains in a variety of environmental conditions. This characterization then becomes the basis for lipidomics, the lipid aspect of the growing field of metabolomics. To exploit the power of derivatization chemistry and of gas chromatography/mass spectrometry (GC/MS) and tandem mass spectrometry (MS/MS) for metabolomics studies, we report here the development of various GC/MS electron ionization (EI) and negative and positive chemical ionization (CI) methods for the identification and, for the first time, the relative quantification of fatty acids present in extracts from membranes of a laboratory strain of Escherichia coli. They consist of seven saturated fatty acids (C10:0, C12:0, C14:0, C15:0, C16:0, C17:0 and C18:0) and six unsaturated fatty acids (C16:1, cyC17:0 plus two isomers of C18:1, C18:2 and cyC19:0).

Glycerol-derived ester oligomers from cork suberin

The cork suberin polyester was partially depolymerized by a methanolysis reaction catalyzed by calcium hydroxide. The methanolisate was analysed by ESI-MS/MS in the form of [M+Li](+) adduct-ions. This reaction solubilized a mixture of monomers and oligomers, including a set of glycerol-derived dimeric and trimeric esters. Four types of glycerol esters were identified: monoacylglycerols of alpha,omega-diacids, of omega-hydroxyacids and of monoacids; diglycerol diesters of alpha,omega-diacids; diacylglycerols of alpha,omega-diacids; monoacylglycerols of linear dimeric esters of alpha,omega-diacids and omega-hydroxyacids. The alpha,omega-diacids and omega-hydroxyacids found as monomer residues in the glycerol esters are the main ones found as cork suberin monomers. It is concluded that suberin is a glycerol-derived lipid of polymeric dimensions. Due to the protective and insulating role that it plays in plants, suberin should be considered together with the other known glycerolipids that build up biological membranes.

Tandem mass spectrometry in the study of fatty acids, bile acids, and steroids

Over the last 50 years, the mass spectrometry of lipids has evolved to become one of the most mature techniques in biomolecule analysis. Many volatile and non-polar lipids are directly amenable to analysis by gas-chromatography-mass spectrometry (GC-MS), a technique that combines the unsurpassed separation properties of gas-chromatography with the sensitivity and selectivity of electron ionization mass spectrometry. Less volatile and/or thermally labile lipids can be analyzed by GC-MS, following appropriate sample derivatization. However, many complex lipids are not readily analyzed by GC-MS, and it is these molecules that are the subject of the current review. Since the early 1970s, there have been three outstanding developments in mass spectrometry that are particularly appropriate in lipid analysis; i.e., the introduction of (i) fast atom bombardment (FAB); (ii) electrospray (ES); and (iii) tandem mass spectrometry (MS/MS). The FAB and ES ionization techniques will be discussed in relation to MS/MS, and examples of their application in biochemical studies will be presented. The review will concentrate on the analysis of fatty acids, bile acids, steroid conjugates, and neutral steroids.

Pathways of epoxyeicosatrienoic acid metabolism in endothelial cells. Implications for the vascular effects of soluble epoxide hydrolase inhibition

Epoxyeicosatrienoic acids (EETs) are products of cytochrome P-450 epoxygenase that possess important vasodilating and anti-inflammatory properties. EETs are converted to the corresponding dihydroxyeicosatrienoic acid (DHET) by soluble epoxide hydrolase (sEH) in mammalian tissues, and inhibition of sEH has been proposed as a novel approach for the treatment of hypertension. We observed that sEH is present in porcine coronary endothelial cells (PCEC), and we found that low concentrations of N,N'-dicyclohexylurea (DCU), a selective sEH inhibitor, have profound effects on EET metabolism in PCEC cultures. Treatment with 3 microM DCU reduced cellular conversion of 14,15-EET to 14,15-DHET by 3-fold after 4 h of incubation, with a concomitant increase in the formation of the novel beta-oxidation products 10,11-epoxy-16:2 and 8,9-epoxy-14:1. DCU also markedly enhanced the incorporation of 14,15-EET and its metabolites into PCEC lipids. The most abundant product in DCU-treated cells was 16,17-epoxy-22:3, the elongation product of 14,15-EET. Another novel metabolite, 14,15-epoxy-20:2, was present in DCU-treated cells. DCU also caused a 4-fold increase in release of 14,15-EET when the cells were stimulated with a calcium ionophore. Furthermore, DCU decreased the conversion of [3H]11,12-EET to 11,12-DHET, increased 11,12-EET retention in PCEC lipids, and produced an accumulation of the partial beta-oxidation product 7,8-epoxy-16:2 in the medium. These findings suggest that in addition to being metabolized by sEH, EETs are substrates for beta-oxidation and chain elongation in endothelial cells and that there is considerable interaction among the three pathways. The modulation of EET metabolism by DCU provides novel insight into the mechanisms by which pharmacological or molecular inhibition of sEH effectively treats hypertension.

Structural investigation of cyclic peptidolipids from Bacillus subtilis by high-energy tandem mass spectrometry

The natural products belonging to the surfactin family are cycloheptapeptides bearing a long beta-hydroxy-fatty acyl chain at the N-terminal position. The structure of these compounds, often isolated as complex mixtures, can be elucidated by high-energy tandem mass spectrometry (MS/MS). The protonated molecules generated by cesium ion bombardment (LSIMS) undergo charge-proximate fragmentations leading to the b- and y-type ion series useful for the sequence determination. The sodium-cationised molecules show a radically different behaviour towards high-energy collisional activation. Besides the well-known charge-remote dissociation products of the alkyl side chain, complete series of d- and w-type fragments allow easy distinction between leucine and isoleucine. The complementary MS/MS data obtained from the protonated and cationised molecules prove to be of great interest for the structural characterisation of this type of compounds.

Effect of alkali metal cationization and multiple alkali metal exchange on the collision-induced dissociation of loganic acid studied by electrospray ionization tandem mass spectrometry

Under electrospray ionization conditions loganic acid undergoes alkali metal (Li, Na and K) exchange and alkali metal cationization. Multiple exchanges of up to four alkali metal ions are observed. Different populations of metal exchanged species are produced during electrospray ionization. Collision-induced dissociation of ammonium cationized species is compared with that of metal cationized species to study the effect of metal cationization. Glycosidic cleavage and ring cleavages of aglycone and sugar moieties are the major fragmentation pathways observed during collision-induced dissociation. The fragmentations of the highly metal exchanged species indicate the opening of the pyran ring. Collision-induced dissociation of the various metal exchanged and metal cationized species also reveals the nature of the different populations. Copyright 2000 John Wiley & Sons, Ltd.

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.

Novel sodium binding properties of some cyclopentapeptide endothelin A selective receptor antagonists: electrospray and fast-atom-bombardment mass spectrometric studies

Electrospray ionization and fast atom bombardment mass spectrometric methods reveal novel interactions of endothelin A selective receptor antagonists, cyclo(D-Trp-D-Asp-Pro-D-Val-Leu), cyclo(D-Trp-D-Glu-Ala-D-allo-Ile-Leu) and cyclo(D-Trp-D-Asp-Pro-D-Ile-Leu) with sodium ions. The peptides have very high intrinsic affinities for sodium ions, and form multiple sodium adducts and sandwich structures: [M + Na]+, [M + 2Na - H]+, [M + 3Na - 2H]+, [M + 4Na - 3H]+, [M + 5Na - 4H]+, [2M + Na]+, [2M + 2Na - H]+, [2M + 3Na - 2H]+, [2M + 4Na - 3H]+, [2M + 5Na - 4H]+, [2M + 6Na - 5H]+, and [2M + 7Na - 6H]+. The three cyclic peptides exhibit similar sodium binding stoichiometries despite differences in their amino acids. The observed sodium binding properties may have implications in understanding their protective effects against ischemia-induced acute renal failure. Those cyclic peptides that offer protection may be those that have high affinities for multiple sodium ions.

Observation of alkali-metal-carboxylate radical anions in fast-atom bombardment

The existence of a series of 32 molecular radical anions from carboxylic acids salts RCOO-Cat(-), where R = CH3, CH3CH2, CH3CH2CH2, (CH3)2CH, C6H5, o-CH3-C6H4, m-CH3-C6H4, and p-CH3-C6H4 and Cat = Li, Na, K, Rb, has been proven by the observation of their fragmentation in negative-ion fast-atom bombardment tandem mass spectrometry. These species occur at very low abundance and are not detected in the fast-atom bombardment spectra because they are hidden in the background. However, the collision-induced dissociation fragmentation of ions selected at the mass-to-charge ratio values that correspond to these species display characteristic signals that are completely different from the signals observed from pure matrix or after addition of corresponding metal hydroxide to the matrix. The main fragmentation observed is the loss of the neutral metal atom from RCOOCat(-·), followed by a loss of CO2 for the aromatic compounds. Neutral loss experiments also confirm the existence of these radical anions. Scans for the loss of a selected metal after addition of one of the carboxylic acid salts to the matrix display abundant peaks at mass-to-charge ratio values that correspond to the salt radical anions. Some weaker peaks appear at other mass-to-charge ratio values independent of the salt that is used and also are observed from the matrix when the corresponding metal hydroxide is added. When alkali metal salts from some deuterated acids are analyzed, the predicted shifts are observed. The loss of the neutral metal is more pronounced from RCOONa(-·), RCOOK(-·), and RCOORb(-·) than from RCOOLi(-·).

Tandem mass spectrometry of poly(ethylene glycol) lithium-attachment ions

A study of the fast-atom bombardment tandem mass spectrometry behavior of a number of ethylene glycol polymers (PEGs) has been carried out. Both linear (hydroxyl, amino, and/or alkyl end groups) and cyclic (crown ether) polymers were studied. One of the materials is a block copolymer of ethylene and propylene oxides. Collisional activation was carried out in the collision octapole of a BEoQ hybrid mass spectrometer at a translational energy of 50 eV, with collision gas air. For the linear polymers, the most intense product ions are lithiated, linear polyglycol oligomers, These ions are formed via internal hydrogen transfer reactions that are facilitated (charge-induced) by lithium. This series of product ions allows for the observation of consecutive losses of monomer units from the chain end; this is useful to determine the sequence of monomers in a copolymer. The most abundant product ions from cyclic PEGs are lithiated radical cations. An especially interesting finding in this work is the preferential loss of two internal ethylene oxide (EO) units (dioxane, 88 u) from some [M + Li](+) precursors. Factors that influence this loss include (a) the sequence length of EO repeat units in the oligomer and (b) the identity of the end groupts) on the oligomer. It is proposed that this elimination of dioxane involves a six-membered ring intermediate; this decomposition reaction is believed to be a lithium-mediated (charge-induced) rearrangement.

Charge-remote fragmentation during FAB-CAD-B/E linked-scan mass spectrometry of aminoethyl-triphenylphosphonium derivatives of fatty acids

The carboxyl group of fatty acids is derivatized by aminoethyl triphenylphosphonium (AETPP) bromide. Fast atom bombardment (FAB) collision-activated dissociation (CAD) B/E linked-scan mass spectrometry of these fixed-charge derivatives shows typical charge-remote fragmentation (CRF). Locations of various structural modifications in fatty acids can be recognized easily from CAD spectra of the AETPP derivatives. Because the triphenylphosphonium group localizes positive charge in the molecule, and because a key requirement for CRF is a tightly localized charge site, these preionized molecules fragment under FAB-CAD conditions more effectively than other derivatives that involve ionic bonding with metal cations or protonation of basic sites. Thus, CAD of AETPP derivatives is likely to produce more structurally informative spectra and provide an opportunity to gain additional under-standing of the CRF process. The most profound difference between the AETPP derivatives and other cations in positive mode FAB-CAD-B/E-MS is reflected in the substantial improvement of detection limits for the AETPP derivatives over those for the metal cation adducts. For several fatty acids (C10-C22) tested, the detectability can be enhanced by one to two orders of magnitude when the analysis is performed on the AETPP derivative. In addition, for the analysis of fatty acid mixtures, the FAB mass spectrum of AETPP derivatives produces a relative intensity of the molecular ion peak for each component of the mixture that more closely represents its mole fraction than does that of metal ion adducts.

Tandem mass spectrometry of lithium-attachment ions from polyglycols

A detailed study has been carried out of the fast atom bombardment tandem mass spectrometry (MS/MS) behavior of lithium-attachment ions from three glycol polymers: linear poly(ethylene glycol), linear poly(propylene glycol), and an ethoxylated fatty alcohol. Collisional activation was carried out in the "collision octapole" of a BEoQ hybrid mass spectrometer at a translational energy of 50 eV, with collision gas air. It was found that [M + Li](+) ions provide a number of advantages as precursors for practical MS/MS analysis as compared to the use of [M + H](+) or [M + Na](+) ions. First, [M + Li](+) ions are much more intense than the corresponding [M + H](+) ions. Second, [M + Li](+) ions dissociate to lithiated organic fragments with reasonable efficiency, which is not the case with [M + Na](+) precursors. Third, product ions are generally formed over the entire mass range for low molecular weight polyglycols. The most intense product ions are lithiated, linear polyglycol oligomers. These ions are formed via internal hydrogen transfer reactions which are facilitated by lithium (charge-induced). Two series of less intense product ions are formed via charge-remote fragmentations involving l,4-hydrogen elimination. A fourth product ion series consists of lithiated radical cations; these form via homolytic bond cleavages near chain ends. Overall, MS/MS analysis of [M + Li](+) polyglycol ions proved to be quite useful for chemical structure elucidation.

Collisional activation of a series of homoconjugated octadecadienoic acids with fast atom bombardment and tandem mass spectrometry

High-energy collisional activation (CA) of long-chain fatty acid ions induces decompositions that occur remote from the charge site. These charge-remote fragmentations (CRFs) have been shown to provide much structural information. In this report, the CRF of a continuous series of 12 homoconjugated octadecadienoic acids was studied with fast atom bombardment and tandem mass spectrometry. Each fatty acid was desorbed as the carboxylate anion, [M - H](-), the dilithiated species, [M - H+2Li](+), or the bariated species, [M - H+Ba](+), giving three ways of localizing the charge. A characteristic pattern is generated for CRF of the 1,4-diene functional group, and this allows for the rapid identification of the functional group and its location on the chain. Minor variations of this pattern are observed for the different ionic precursors and for different locations of the double bonds. Furthermore, there are a few complications from different types of charge-proximate reactions, especially of the fatty acid carboxylates.

Charge-remote fragmentation in a hybrid (BEqQ) mass spectrometer to determine isotopic purity in selectively polydeuterated surfactants

The combination of fast atom bombardment with charge-remote fragmentation using a hybrid (BEqQ) mass spectrometer was used successfully to assess and localize the extent of selective deuterium isotope labeling of tetradecyItrimethylammonium bromides. Spectral details reveal a new reaction that can give rise to ions isobaric with those formed by chargeremote fragmentation.

Characterization of epoxides of polyunsaturated fatty acids by mass spectrometry via 3-pyridinylmethyl esters

The isomeric epoxides of linoleic, arachidonic and docosahexaenoic acids were prepared by reaction with m-chloroperoxybenzoic acid and, after separation by normal-phase high-performance liquid chromatography, were esterified with 3-pyridylcarbinol via the unstable imidazolide generated by the reaction with 1,1'-carbonyldiimidazole. The electron impact mass spectra of these derivatives showed a molecular ion and a sequence of peaks with two characteristic abundant ions that resulted from formal cleavage of the carbon-carbon bonds at the oxirane ring. Both these ions retained the ester group. This fragmentation pattern allowed the unequivocal identification of the separate epoxide isomers.