Determination Of Double Bond Position Research Articles

Evaluating Software Tools for Lipid Identification from Ion Mobility Spectrometry-Mass Spectrometry Lipidomics Data.

The unambiguous identification of lipids is a critical component of lipidomics studies and greatly impacts the interpretation and significance of analyses as well as the ultimate biological understandings derived from measurements. The level of structural detail that is available for lipid identifications is largely determined by the analytical platform being used. Mass spectrometry (MS) coupled with liquid chromatography (LC) is the predominant combination of analytical techniques used for lipidomics studies, and these methods can provide fairly detailed lipid identification. More recently, ion mobility spectrometry (IMS) has begun to see greater adoption in lipidomics studies thanks to the additional dimension of separation that it provides and the added structural information that can support lipid identification. At present, relatively few software tools are available for IMS-MS lipidomics data analysis, which reflects the still limited adoption of IMS as well as the limited software support. This fact is even more pronounced for isomer identifications, such as the determination of double bond positions or integration with MS-based imaging. In this review, we survey the landscape of software tools that are available for the analysis of IMS-MS-based lipidomics data and we evaluate lipid identifications produced by these tools using open-access data sourced from the peer-reviewed lipidomics literature.

Determination of double bond positions in methyl ketones by gas chromatography-mass spectrometry using dimethyl disulfide derivatives.

Methyl ketones are of interest for the application as biofuels. The fatty acid metabolism of different microbes has been rearranged to achieve a sustainable production of methyl ketones. The biofuel properties and possible further chemical modifications of these methyl ketones are influenced by their chain length, as well as their degree of saturation and the corresponding double bond position. A method based on gas chromatography-electron ionization; mass spectrometry (GC-EI-MS) was used to determine the double bond position of methyl ketones. Derivatization using dimethyl disulfide (DMDS) and an iodine catalyst enabled the activation of the double bonds for selective fragmentation during electron ionization. The cleavage led to key fragments in the Orbitrap high-resolution mass spectrum and allowed the unequivocal localization of the double bond position of the respective monounsaturated methyl ketone. The double bond position of several medium chain length methyl ketones originating from the gram-negative bacterium Pseudomonas taiwanensis (P. taiwanensis) VLB120 was determined. The DMDS derivatives of methyl ketones can yield isobaric fragment ions for different possible double bond positions, which can be distinguished only using high-resolution MS. The double bond position of all methyl ketones deriving from P. taiwanensis VLB120 was the same, counting from the end of the aliphatic chain, and was determined as ω-7. The derivatization of medium chain length monounsaturated methyl ketones with DMDS allowed the determination of the corresponding double bond position via GC-EI-MS. High-resolution MS is needed to differentiate possible double bond positions that yield isobaric fragment ions.

Lipid Analysis by Gas Chromatography and Gas Chromatography-Mass Spectrometry.

Gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) represent powerful tools for the quantitative and structural analysis of plant lipids. Here, we outline protocols for the isolation, separation, and derivatization of plant lipids for subsequent GC and GC-MS analysis. Plant lipids are extracted with organic solvents and separated according to their polarity by thin-layer chromatography or solid phase extraction. As most lipids are not volatile, the analytes are derivatized by transmethylation or trimethylsilylation to enable the transition of the molecules into the gas phase. After separation on the polymer matrix of the GC column, the analytes are detected by flame ionization or mass spectrometry. This chapter includes methods suitable for the analysis of lipid-bound or free fatty acids, long chain alcohols, and monoacylglycerols and for the determination of double bond positions in fatty acids.

Determination of Double Bond Positions in Polyunsaturated Fatty Acids Using the Photochemical Paternò-Büchi Reaction with Acetone and Tandem Mass Spectrometry.

The positions of double bonds along the carbon chain of methylene interrupted polyunsaturated fatty acids are unique identifiers of specific fatty acids derived from biochemical reactions that occur in cells. It is possible to obtain direct structural information as to these double bond positions using tandem mass spectrometry after collisional activation of the carboxylate anions of an acetone adduct at each of the double bond positions formed by the photochemical Paternò-Büchi reaction with acetone. This reaction can be carried out by exposing a small portion of an inline fused silica capillary to UV photons from a mercury vapor lamp as the sample is infused into the electrospray ion source of a mass spectrometer. Collisional activation of [M - H]- yields a series of reverse Paternò-Büchi reaction product ions that essentially are derived from cleavage of the original carbon-carbon double bonds that yield an isopropenyl carboxylate anion corresponding to each double bond location. Aldehydic reverse Paternò-Büchi product ions are much less abundant as the carbon chain length and number of double bonds increase. The use of a mixture of D0/D6-acetone facilitates identification of these double bonds indicating product ions as shown for arachidonic acid. If oxygen is present in the solvent stream undergoing UV photoactivation, ozone cleavage ions are also observed without prior collisional activation. This reaction was used to determine the double bond positions in a 20:3 fatty acid that accumulated in phospholipids of RAW 264.7 cells cultured for 3 days.

Investigation and Applications of In-Source Oxidation in Liquid Sampling-Atmospheric Pressure Afterglow Microplasma Ionization (LS-APAG) Source

A liquid sampling-atmospheric pressure afterglow microplasma ionization (LS-APAG) source is presented for the first time, which is embedded with both electrospray ionization (ESI) and atmospheric pressure afterglow microplasma ionization (APAG) techniques. This ion source is capable of analyzing compounds with diverse molecule weights and polarities. An unseparated mixture sample was detected as a proof-of-concept, giving complementary information (both polarities and non-polarities) with the two ionization modes. It should also be noted that molecular mass can be quickly identified by ESI with clean and simple spectra, while the structure can be directly studied using APAG with in-source oxidation. The ionization/oxidation mechanism and applications of the LS-APAG source have been further explored in the analysis of nonpolar alkanes and unsaturated fatty acids/esters. A unique [M + O - 3H]+ was observed in the case of individual alkanes (C5-C19) and complex hydrocarbons mixture under optimized conditions. Moreover, branched alkanes generated significant in-source fragments, which could be further applied to the discrimination of isomeric alkanes. The technique also facilitates facile determination of double bond positions in unsaturated fatty acids/esters due to diagnostic fragments (the acid/ester-containing aldehyde and acid oxidation products) generated by on-line ozonolysis in APAG mode. Finally, some examples of in situ APAG analysis by gas sampling and surface sampling were given as well. Graphical Abstract ᅟ.

Determination of Double Bond Positions and Geometry of Methyl Linoleate Isomers with Dimethyl Disulfide Adducts by GC/MS.

The dimethyl disulfide (DMDS) adduct method is one of the convenient and effective methods for determining double bond positions of unsaturated fatty acid methyl esters (FAME) except conjugated FAME. When analyzed using gas chromatography/electron ionization-mass spectrometry (GC/EI-MS), unsaturated FAME with DMDS added to the double bonds yields high intensity MS spectra of characteristic ions. The MS spectra of characteristic ions can then be used to easily confirm double bond positions. Here we explore the GC/EI-MS analysis of the DMDS adducts of methyl linoleate geometrical isomers isolated by high performance liquid chromatography (HPLC) with a silver nitrate column. For C18:2-c9, c12 and C18:2-t9, t12, DMDS randomly formed adducts with double bonds at either carbon 9-10 or carbon 12-13, but not both at the same time due to steric hindrance. For C18:2-c9, t12 and C18:2-t9, c12, however, DMDS only formed adducts with the double bond in the cis configuration. Consequently, when analyzing fatty acids with methylene interrupted double bonds, with one double bond in the cis and one in the trans configuration, double bond positions cannot be completely confirmed.

Electron-induced dissociation (EID) for structure characterization of glycerophosphatidylcholine: determination of double-bond positions and localization of acyl chains.

Glycerophospholipids are a highly abundant and diverse collection of biologically relevant lipids, and distinction between isomeric and isobaric species is a fundamental aspect for confident identification. The ability to confidently assign a unique structure to a glycerophospholipid of interest is dependent on determining the number and location of the points of unsaturation and assignment of acyl chain position. The use of high-energy electrons (>20 eV) to induce gas-phase dissociation of intact precursor ions results in diagnostic product ions for localizing double-bond positions and determining acyl chain assignment. We describe a high-resolution, tandem mass spectrometry method for structure characterization of glycerophospholipids using electron-induced dissociation (EID). Furthermore, the inclusion of nomenclature to systematically assign bond cleavage sites with acyl chain position and double-bond location enables a uniform platform for lipid identification. The EID methodology detailed here combines novel application of an electron-based dissociation technique with high-resolution mass spectrometry that facilitates a new experimental approach for lipid biomarker discovery and validation.

The direct determination of double bond positions in lipid mixtures by liquid chromatography/in-line ozonolysis/mass spectrometry

The direct determination of double bond positions in unsaturated lipids using in-line ozonolysis-mass spectrometry (O3-MS) is described. In this experiment, ozone penetrates through the semi-permeable Teflon AF-2400 tubing containing a flow of a solution of fatty acid methyl esters (FAME). Unsaturated FAME are thus oxidized by the ozone and cleaved at the double bond positions. The ozonolysis products then flow directly into the atmospheric pressure photoionization (APPI) source of a mass spectrometer for analysis. Aldehyde products retaining the methyl ester group are indicative of the double bond positions in unsaturated FAME. For the first time, O3-MS is able to couple directly to high performance liquid chromatography (HPLC), making the double bond localization in lipid mixtures possible. The application of LC/O3-MS has been demonstrated for a fat sample from bovine adipose tissue. A total of 9 unsaturated FAME including 6 positional isomers were identified unambiguously, without comparison to standards. The in-line ozonolysis reaction apparatus is applicable to most mass spectrometers without instrumental modification; it is also directly compatible with various LC columns. The LC/O3-MS method described here is thus a practical, versatile and easy to use new approach to the direct determination of double bond positions in lipids, even in complex mixtures.

Development of a three-steps derivatization assay for the localization of double bond in monounsaturated monomers of poly-beta-hydroxyalkanoates by GC–MS

A new gas chromatography–mass spectrometry (GC–MS) method for the localization of double bond in monounsaturated 3-hydroxyalkenoic acids monomers has been developed. A three steps derivation assay was used including a methanolysis, then acetylation and dimethyldisulfide (DMDS) addition to alkene groups. Electron impact GC–MS analysis of such derivatives offers characteristic fragments allowing the unambiguous determination of double bond position in side chain. This novel method is well-suited for the routine analysis of poly-beta-hydroxyalkanoates (PHAs), and was used to characterize monounsaturated monomers in both 3-hydroxyalkenoic acids standards as well as in mcl-PHAs and poly(3-hydroxyoctanoate-co-3-hydroxyundecenoate) (PHOU) produced by bacterial strain Pseudomonas guezennei from glucose or a mixture of sodium octanoate plus 10-undecenoic acid, respectively.

Facile Determination of Double Bond Position in Unsaturated Fatty Acids and Esters by Low Temperature Plasma Ionization Mass Spectrometry

Unsaturated fatty acids and esters can be oxidized in situ during ionization using a low temperature plasma (LTP) probe. The discharge generates ozone from air that reacts with and cleaves olefins. The molecular ions of the resulting acid/ester oxidation products are present in the full scan mass spectra and are confirmed by exact mass measurements. The fragmentation information can be used to assign double bond positions. We have successfully applied this strategy to a range of mono-/polyunsaturated fatty acids and fatty acid methyl/ethyl esters to assign their double bond locations. The procedure allows rapid and direct identification of double bond positions in situ at atmospheric pressure without sample preparation prior to mass spectrometric analysis. Microbial fatty acid ethyl ester (FAEE) mixtures from complex bacterial samples were directly analyzed by this method. Structural confirmation of their diagnostic ions by using exact mass measurements and tandem mass spectrometry confirms double bond positions in unsaturated bacterial FAEEs.

Long chain alkyl and alkenyl phenols from the roots of Ozoroa insignis

Bioassay-guided fractionation of the ethanolic root extract of Ozoroa insignis, collected in Guinea-Bissau, led to the isolation of a 41-member library of alkyl and alkenylphenols, whose structures were determined by ¹H and 13C NMR, and GCMS. Determination of double-bond positions in the side chains of alkenylphenols were established by methylthiolation-GCMS.

Avian Exocrine Secretions. I. Chemical Characterization of the Volatile Fraction of the Uropygial Secretion of the Green Woodhoopoe, Phoeniculus purpureus

Using gas chromatography-mass spectrometry in conjunction with auxiliary techniques such as solid phase microextraction and determination of double bond positions by means of dimethyl disulfide derivatization, 45 constituents of the uropygial secretion of the green woodhoopoe, Phoeniculus purpureus, have been identified. The majority of these constituents are long-chain branched and unbranched alkanes, and (Z)-alkenes such as (Z)-9-tricosene, and a number of unidentified wax esters. The more volatile fraction of the secretion contained short-chain fatty acids, aldehydes, aliphatic and heterocyclic aromatic amines, ketones, and dimethyl sulfides. This group of volatile compounds is responsible for the obnoxious odor of the secretion and also for its defensive action against predators.

Determination of trans double bonds in polyunsaturated fatty acid methyl esters from their electron impact mass spectra

AbstractThe geometrical isomers of unsaturated fatty acid methyl esters were analysed by gas chromatography — electron impact mass spectrometry. Distinct differences in the electron impact mass spectra of methylene‐interrupted tri‐ and tetra‐unsaturated fatty acid methyl esters were observed. The differences were particularly related to the geometry of the central double bond in trienes or the two central double bonds in tetraenes. Trans geometry in these positions led to a significant decrease of m/z 79 [C6H7]+, which is the base peak in the all‐cis isomers. Disappearance or significant reductions of ions that are used for the determination of double bond positions in the corresponding cis isomers were also observed in the same spectra. The observed differences may be of help in the identification of double bond geometry in fatty acid isomers.

Determination of Double Bond Positions in Triunsaturated Compounds by Means of Gas Chromatography/Mass Spectrometry of Dimethyl Disulfide Derivatives

The determination of the positions of double bonds after dimethyl disulfide (DMDS) addition is described for triunsaturated compounds (alkatrienes and an ester). Different DMDS derivatives were studied according to the numbers, n1 and n2, of methylene groups separating the double bonds. When n1 and n2 are smaller than four, a bicyclic thioether is formed and a predictable fragmentation pathway is observed in each case after electron ionization, allowing the determination of the positions of the three double bonds and the molecular structure of the compounds. © 1997 John Wiley & Sons, Ltd.

ChemInform Abstract: Identification of the Sex Pheromone of Scrobipalpula absoluta; Determination of Double Bond Positions in Triple Unsaturated Straight Chain Molecules by Means of Dimethyl Disulfide Derivatization.

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Identification of the sex pheromone of Scrobipalpula absoluta; determination of double bond positions in triple unsaturated straight chain molecules by means of dimethyl disulphide derivatization

The sex pheromone of Scrobipalpula absoluta (Meyrick) (Lepidoptera: Gelechiidae) was identified as a 92:8 mixture of (3 E,8 Z,11 Z)-3,8,11-tetradecatrienyl acetate ( 1) and (3 E,8 Z)-3,8-tetradecadienyl acetate ( 2) through mass spectrometric investigation of the dimethyl disulphide derivatives of excised sex pheromone glands. It is the first time that this method was used for triple unsaturated straight chain molecules. Compound ( 2) was identified as a new pheromone component. A synthetic mixture of the two identified compounds proved to be attractive in wind tunnel experiments.

Utility of nicotinoyl derivatives in structural studies of mono‐ and diacylglycerols by gas chromatography/mass spectrometry. Part 2

AbstractThe positional isomers of mono[(Z)‐9‐octadecenoyl] glycerol, di[(Z)‐12‐octadecadienoyl] glycerol were derivatized with nicotinic acid chloride hydrochloride. The resulting nicotinoyl derivatives were examined by gas chromatography/mass spectrometry. In their electron impact mass spectra, each methylene group and double bond of the alkyl chain is reflected by a fragmentation pattern in the high mass region. This is caused by radical‐induced cleavage of the alkyl chains following random hydrogen abstraction by the pyridine nucleus. An accurate determination of double bond positions in mono‐ and diacylglycerols is possible by characteristic spacings between abundant diagnostic ions in this fragmentation pattern.

Letter: Determination of double bond position in di-unsaturated alkadienes by means of mass spectrometry of dimethyl di-sulphide derivatives

Letter: Determination of double bond position in di-unsaturated alkadienes by means of mass spectrometry of dimethyl di-sulphide derivatives

Determination of double-bond position in some unsaturated terpenes and other branched compounds by alkylthiolation.

The electron-impact mass spectra of alpha, beta-bis(methylthio) derivatives of certain terpenes and other compounds with branched alkenyl groups contain diagnostic peaks that can be used for locating the position of the double bond in the parent compound. In contrast to the spectra of dimethyl disulfide (DMDS) derivatives obtained from compounds containing CH=CH type double bonds, which show two predominant fragment ions, most of the compounds examined in the current investigation showed only one predominant fragment ion, arising from that part of the molecule which possesses the more substituted carbon of the original double bond. For example, all the derivatives from compounds with an isopropylidene moiety showed a base peak at m/z89 which can be attributed to the formation of a [(CH3)2C=SCH3]+ fragment. The application of the DMDS procedure to naturally occurring terpenes is discussed.

Accurate determination of double bond position in mono-unsaturated straight-chain fatty acid ethyl esters from conventional electron impact mass spectra by quantitative spectrum—structure modelling

Brakstad, F., 1993. Accurate determination of double bond position in mono-unsaturated straight-chain fatty acid ethyl esters from conventional electron impact mass spectra by quantitative spectrum—structure modelling. Chemometrics and Intelligent Laboratory Systems, 19: 87–100. An objective approach is given for selecting the most significant fragments for mass spectrum—structure correlation. The method is based on partial least-squares regression (PLSR) on normalised spectral intensities as independent variables, and the carbon—carbon double bond position as response. Such a correlation between a structural descriptor and the mass spectra of pure compounds has not hitherto been reported in the literature. The procedure is demonstrated using conventional electron impact (70 eV)mass spectra of twelve straight-chain mono-unsaturated fatty acid ethyl esters, ranging in carbon—carbon double bond positions from 5 to 11 (except 7), incorporating both cis- and trans-isomers and chain lengths from C 16 to C 20. On the basis of the relative intensity of thirteen fragments, a quantitative model relating these to the response was developed. At a 95% confidence interval, all the residuals from the cross-validation fall within the confidence interval given by [true value ± 0.46] carbon position units. Thus, the model practically always gives an unambiguous estimate of the carbon—carbon double bond position. Possible further developments of this method are discussed for the use of gas chromatography—mass spectrometry and gas chromatography—Fourier transform infrared spectroscopy in combination.