Location of double-bond position in tetradecenols without chemical modification by mass spectrometry

Twelve positional isomers of tetradecenol were analysed by conventional combined gas chromatography/mass Spectrometry without any chemical derivatization for the elucidation of the double-bond position. The spectra were interpreted in terms of similarity of mass spectral patterns based on a fuzzy reasoning method, in which the relative abundances of selected predominant fragment ions were chosen as standard parameters and similarity indices were devised. The method was tested with pure alcohols and enabled the double-bond position to be located with acceptable accuracy.     

A method for the identification of the double-bond position of isomeric linear tetradecenols and related compounds based on mass spectra of dimethyl disulfide derivatives.

A simple method is presented for calculation of the double-bond position in linear tetradecenols based on mass spectral data of dimethyl disulfide derivatives. In this approach, the m/z ratios of the molecular ion and of one (or both) of the two most abundant fragment ions were utilized to calculate the double-bond position, without the requirement to identify both fragment ions resulting from carbon-carbon cleavage across what was originally the double bond. The approach was tested with mass spectra of dimethyl disulfide derivatives of 12 isomeric tetradecenols, and the double-bond position in each isomer was successfully identified by this method. The method was shown to work also for the corresponding acetates.     

Double bond migration to methylidene positions during electron ionization mass spectrometry of branched monounsaturated fatty acid derivatives

Electron ionization mass spectra of several monounsaturated methyl-branched fatty acid methyl and trimethylsilyl esters were examined. These spectra exhibited some intensive fragment ions, whose formation could be explained after double-bond migration to methylidene position. This preferential migration (substantiated by deuterium labeling) acts significantly in the case of monounsaturated fatty acid methyl and trimethylsilyl esters possessing a methyl branch localized between the penultimate and the C₄ positions (relative to the ester group), whatever the position of the double-bond. Allylic cleavage and γ-hydrogen rearrangement of the ionized methylidene group thus formed afforded very interesting fragment ions, which could be particularly useful to determine branching positions of monounsaturated methyl-branched fatty acid methyl and trimethylsilyl esters without additional treatment.     

Ozone-induced dissociation on a modified tandem linear ion-trap: Observations of different reactivity for isomeric lipids

Ozone-induced dissociation (OzID) exploits the gas-phase reaction between mass-selected lipid ions and ozone vapor to determine the position(s) of unsaturation. In this contribution, we describe the modification of a tandem linear ion-trap mass spectrometer specifically for OzID analyses wherein ozone vapor is supplied to the collision cell. This instrumental configuration provides spatial separation between mass-selection, the ozonolysis reaction, and mass-analysis steps in the OzID process and thus delivers significant enhancements in speed and sensitivity (ca. 30-fold). These improvements allow spectra revealing the double-bond position(s) within unsaturated lipids to be acquired within 1 s: significantly enhancing the utility of OzID in high-throughput lipidomic protocols. The stable ozone concentration afforded by this modified instrument also allows direct comparison of relative reactivity of isomeric lipids and reveals reactivity trends related to (1) double-bond position, (2) substitution position on the glycerol backbone, and (3) stereochemistry. For cis- and trans-isomers, differences were also observed in the branching ratio of product ions arising from the gas-phase ozonolysis reaction, suggesting that relative ion abundances could be exploited as markers for double-bond geometry. Additional activation energy applied to mass-selected lipid ions during injection into the collision cell (with ozone present) was found to yield spectra containing both OzID and classical-CID fragment ions. This combination CID-OzID acquisition on an ostensibly simple monounsaturated phosphatidylcholine within a cow brain lipid extract provided evidence for up to four structurally distinct phospholipids differing in both double-bond position and sn-substitution.     

A novel formula for location of double bond in alkenyl acetates and alcohols based on mass spectral data of dimethyl disulfide derivatives

A new formula was presented for locating double bond position in dodecenols, tetrade-cenols, hexadecenols and their acetates, based on mass spectral data of dimethyl disulfide derivatives. In this procedure, molecular ion and base peak ion were utilized as characteristic parameters to identify the positional isomers. The method was tested with mass spectra of 25 dimethyl disulflde derivatives of mono-unsaturated acetates and alcohols, and the original double-bond position in each isomer was located unambiguously.     

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

Features of tandem mass spectra of dilithiated adduct ions of unsaturated fatty acids obtained by electrospray ionization mass spectrometry with low-energy collisionally activated dissociation (CAD) on a triple stage quadrupole instrument are described. These spectra distinguish among isomeric unsaturated fatty acids and permit assignment of double-bond location. Informative fragment ions reflect cleavage of bonds remote from the charge site on the dilithiated carboxylate moiety. The spectra contain radical cations reflecting cleavage of bonds between the first and second and between the second and third carbon atoms in the fatty acid chain. These ions are followed by a closed-shell ion series with members separated by 14 m/z units that reflect cleavage of bonds between the third and fourth and then between subsequent adjacent pairs of carbon atoms. This ion series terminates at the member reflecting cleavage of the carbon-carbon single bond vinylic to the first carbon-carbon double bond. Ions reflecting cleavages of bonds distal to the double bond are rarely observed for monounsaturated fatty acids and are not abundant when they occur. For polyunsaturated fatty acids that contain double bonds separated by a single methylene group, ions reflecting cleavage of carbon-carbon single bonds between double bonds are abundant, but ions reflecting cleavages distal to the final double bond are not. Cleavages between double bonds observed in these spectra can be rationalized by a scheme involving a six-membered transition state and subsequent rearrangement of a bis-allylic hydrogen atom to yield a terminally unsaturated charge-carrying fragment and elimination of a neutral alkene. The location of the beta-hydroxy-alkene moiety in ricinoleic acid can be demonstrated by similar methods. These observations offer the opportunity for laboratories that have tandem quadrupole instruments but do not have instruments with high energy CAD capabilities to assign double bond location in unsaturated free fatty acids by mass spectrometric methods without derivatization.     

Identification of triacylglycerols containing two short-chain fatty acids at sn-2 and sn-3 positions from bovine udder by fast atom bombardment tandem mass spectrometry

Several triacylglycerol (TAG) molecular species, that contain two short-chain fatty acids (C4 to C8) at the sn-2 and sn-3 positions of the glycerol backbone, were isolated from bovine udder by using solvent extraction and silica gel column chromatography. Their structures were identified by fast atom bombardment (FAB) tandem mass spectrometry (MS/MS), based on the information obtained from collision-induced dissociation (CID) spectra of sodium-adducted molecules ([M + Na](+)) of model TAG compounds which had been synthesized from glycerol and appropriate fatty acids. For each species, the relative positions of the three fatty acids on the glycerol backbone, as well as fatty acid composition and double-bond position in the fatty acyl group, were determined. A majority of sodium-adducted molecules observed in the FAB mass spectrum were mixtures of at least two components that have different fatty acid composition but the same molecular mass. In addition, all the components present in mixtures of all the species contain a long-chain fatty acid (C12 to C18) at the sn-1 position, a short-chain fatty acid (C4 to C8) at the sn-2 position, and a butyric acid uniquely at the sn-3 position.     

Examining the collision-induced decomposition spectra of ammoniated triglycerides as a function of fatty acid chain length and degree of unsaturation. I. The OXO/YOY series

A series of positionally pure triglycerides (TAGs) of the form OXO and YOY, where O is the oleate moiety and X and Y are large arrays of different fatty acid moieties, was synthesized and analyzed by reversed-phase high-performance liquid chromatography/tandem mass spectrometry. The intensities of the collision-induced decomposition (CID) products of ammoniated TAGs (ammonium ion adducts) were examined as a function of chain length, degree of unsaturation, double-bond position, and cis/trans configuration of X and Y. The major CID products, the diglyceride fragment ions and the MH+ ion, were plotted as functions of chain length for the saturated and mono-unsaturated series of X and Y. Different trends for each of these series were observed. Trends in the abundances of these fragment ions were also characterized as a function of degree of unsaturation in the TAGs. In general, the fractional abundances of the MH+ ions vary linearly with degree of unsaturation. However, the presence of double bonds positioned close to the carbonyl carbon of the fatty acid chain promotes the formation of the diglyceride fragment ion corresponding to loss of that fatty acid. Mechanisms of the formation and decomposition of ammoniated TAGs are proposed that fit the trends observed in the data. Extensions of this work are described, and a vision of a derived library of CID spectra is discussed as a platform for comprehensive analysis of complex TAG mixtures.     

Structural characterization of unsaturated glycerophospholipids by multiple-stage linear ion-trap mass spectrometry with electrospray ionization

Structural elucidation of glycerophospholipids (GPLs), including the polar head group, the position of double-bond(s) along the fatty acyl substituents, and the positions of acyl groups on the glycerol backbone, using multiple-stage liner ion-trap (LIT) mass spectrometric approach is described in this paper. While the product-ion spectra from MSn (n=2, 3) on the [M+Li]+ or [M-H+2Li]+ ions of GPL are readily applicable for discerning the phospholipid classes and for identifying and locating the fatty acid substituents on the glycerol backbone, the structural information from further dissociation of the dilithiated fatty acid cations produced from MSn (n=3, 4) on the [M-H+2Li]+ ion of GPLs, as well as from further dissociation of the monolithiated fragment ion that bears the unsaturated fatty acid moiety produced from subsequent MSn (n=3,4) on the [M+Li]+ ions of GPLs, affords assignment of the position of double-bond(s) along the fatty acyl groups. The application of the present method in the structural characterization of GPL molecules from the lipid extracts of biological origin, including mixtures of phosphatidylglycerol and of phosphatidylserine without prior chromatographic separation, is also demonstrated. Since lithiated molecular species of GPL are readily formed by ESI, this multiple-stage LIT mass spectrometric approach provides a direct means for the near-complete structural characterization of all the GPLs, including the molecules in the lysophospholipid and plasmalogen subclasses.     

Examining the collision-induced decomposition spectra of ammoniated triglycerides as a function of fatty acid chain length and degree of unsaturation. II. The PXP/YPY series

A series of positionally pure triglycerides (TAGs) of the form PXP and YPY, where P is the palmitate moiety and X and Y are large arrays of different fatty acid moieties, is synthesized and analyzed by reversed-phase high-performance liquid chromatography/tandem mass spectrometry. The intensities of the collision-induced decomposition (CID) products of ammoniated TAGs were examined as a function of chain length, degree of unsaturation, double-bond position, and cis/trans configuration of X and Y. The major CID products, the diglyceride (DAG) fragment ions and the MH(+) ions, are plotted as functions of chain length for the saturated and monounsaturated series of X and Y. Different trends for each of these series are observed. Trends in the intensities of these fragment ions are also characterized as a function of degree of unsaturation in the TAGs. In general, the fractional intensities of MH(+) increase with increasing degree of unsaturation in the TAGs. MH(+) is absent in the CID spectra of triglycerides containing three saturated fatty acid moieties, suggesting that the presence of double bonds fosters the formation of MH(+). Double bonds positioned close to the carbonyl carbon along the fatty acid chain promote the formation of the DAG fragment ion corresponding to the loss of the fatty acid. The collection of PXP/YPY data produced in this work is used to test the mechanisms of the formation and decomposition of ammoniated TAGs that were previously presented. The YPY data are used to predict the intensities of the fragment ions in the CID spectra of YPX-type TAGs. The limitations of the mathematical approach used in these predictions are discussed in context of a broader plan to develop a software platform for comprehensive analysis of complex TAG mixtures.     

Structure-specific fragmentations of 2,2,4-trisubstituted 1,2-dihydroquinazolines

Fragmentations of a series of 2,2,4-trisubstituted 1,2-dihydroquinazolines were studied by electron impact (EI) ionization and positive- and negative-ion fast atom bombardment (FAB) coupled with collision-induced decomposition (CID). The EI mass spectra give information that directly indicates the specific location of each substituent. The FAB–CID spectra provide data regarding the masses of each substituent, but no definitive information regarding location.     

十二碳单烯-1-乙酸酯双键位置异构体的质谱研究

本文报道了十二碳单烯-1-乙酸酯双键位置异构体质谱的特征,应用质量分析离子动能谱(MIKES)研究离子的断裂机理,结果表明: 支配此类化合物基本断裂方式的主要是游离基中心引发的各种类型H的重排,双键位移和电荷转移, 离子在断裂前已形成结构上能快速相互转化的异构化离子的混合物, 因此这类双键位置异构体的质谱非常相似.     

Electron impact ionization tandem mass spectrometry of glycosphingolipids. Part III. Study of the fragmentation modes of selected glycosphingolipids

Results are presented from a systematic study of the high-energy tandem mass spectra (fragment ion spectra) of derivitized (permethylation followed by LiAlH₄ reduction) porcine glycosphingolipids (GSLs) using a four-sector mass spectrometer. The ions studied were the ammonium ions of the GSLs formed on loss of the sphingosine side-chain following electron impact ionization. Fragment ion spectra are shown to provide structural data useful in identifying carbohydrate sequence, the location of hexosamine residues and the identification of fatty acid chain length. Differences between the fragment ion spectra of isomers differing in carbohydrate linkage position and stereochemistry were observed, but not easily predicted.     

Examining the collision-induced decomposition spectra of ammoniated triglycerides. III. The linoleate and arachidonate series

A series of positionally pure triglycerides (TAGs) of the form LXL, YLY, AXA, and YAY was synthesized and analyzed by reversed-phase high-performance liquid chromatography/tandem mass spectrometry. L and A represent the linoleate and arachidate moieties, respectively, and X and Y represent large arrays of fatty acid moieties of various chain lengths, degree of unsaturations, double-bond positions, and cis/trans configurations. The abundances of the collision-induced decomposition (CID) products of ammoniated TAGs were examined as a function of these parameters. The major CID products, the diglyceride (DAG) product ions and the MH(+) ions, are plotted as functions of chain length for the saturated and monounsaturated series of X and Y. The following trends are observed in the data. TAGs with higher degrees of unsaturation tend to show greater relative abundances of MH(+) in the CID spectra of their ammoniated precursor ions. The position of the fatty acid constituents along the glycerol backbone also seems to influence the abundances of the MH(+) ion in the CID spectra of the ammoniated precursor ions. A fatty acid constituent with double bonds along the fatty acid chain positioned close to the carbonyl promotes the formation of the DAG product ion that corresponds to its loss upon CID of the ammoniated precursor ion. Linoleic acid substituents also seem to promote the formation of DAG product ions, but to a lesser extent. Data for the YAY TAGs are used to predict the abundances of the product ions in the CID spectra of ammoniated YAX TAGs. These data are discussed in context of a broader project to develop and validate software algorithims to support a platform for comprehensive analysis of complex mixtures of TAGs.     

Die McLafferty-Umlagerung als Hauptabbaureaktion 1,1-di-, tri-und tetra-alkylsubstituierter Äthylenderivate

In contrast to the olefins I and II, olefins alkylsubstituted at the double-bond(III to IX) suffer no isomerization by hydrogen migration before fragmentation occurs. Almost the only degradation-reaction of the molecular ions of the olefins III to VIII is the McLafferty-rearrangement. From the isomers III and IV, respectively V and VI, the same McLafferty-rearrangement product is formed, therefore the spectra are nearly identical. A general scheme for the occurrence of identical mass spectra of isomeric compounds is given. The olefin IX in which the McLafferty-rearrangement is not possible shows preferential cleavage of the ally1 bond. If the chain contains a futher alky-substituent, bond-rupture at the branched c-atom competes with the McLafferty-rearrangement.     

Evaluation of low energy CID and ECD fragmentation behavior of mono-oxidized thio-ether bonds in peptides

Thio-ether bonds in the cysteinyl side chain of peptides, formed with the most commonly used cysteine blocking reagent iodoacetamide, after conversion to sulfoxide, releases a neutral fragment mass in a low-energy MS/MS experiment in the gas phase of the mass spectrometer [6]. In this study, we show that the neutral loss fragments produced from the mono-oxidized thio-ether bonds (sulfoxide) in peptides, formed by alkyl halide or double-bond containing cysteine blocking reagents are different under low-energy MS/MS conditions. We have evaluated the low-energy fragmentation patterns of mono-oxidized modified peptides with different cysteine blocking reagents, such as iodoacetamide, 3-maleimidopropionic acid, and 4-vinylpyridine using FTICR-MS. We propose that the mechanisms of gas-phase fragmentation of mono-oxidized thio-ether bonds in the side chain of peptides, formed by iodoacetamide and double-bond containing cysteine blocking reagents, maleimide and vinylpyridine, are different because of the availability of acidic beta-hydrogens in these compounds. Moreover, we investigated the fragmentation characteristics of mono-oxidized thio-ether bonds within the peptide sequence to develop novel mass-spectrometry identifiable chemical cross-linkers. This methionine type of oxidized thio-ether bond within the peptide sequence did not show anticipated low-energy fragmentation. Electron capture dissociation (ECD) of the side chain thio-ether bond containing oxidized peptides was also studied. ECD spectra of the oxidized peptides showed a greater extent of peptide backbone cleavage, compared with CID spectra. This fragmentation information is critical to researchers for accurate data analysis of this undesired modification in proteomics research, as well as other methods that may utilize sulfoxide derivatives.     

Collision-induced dissociation pathways of anabolic steroids by electrospray ionization tandem mass spectrometry

Anabolic steroids are structurally similar compounds, and their product-ion spectra obtained by tandem mass spectrometry under electrospray ionization conditions are quite difficult to interpret because of poly-ring structures and lack of a charge-retaining center in their chemical structures. In the present study, the fragmentation of nine anabolic steroids of interest to the racing industry was investigated by using triple quadrupole mass spectrometer, Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, and a linear ion trap instrument. With the aid of an expert system software (Mass Frontier version 3.0), accurate mass measurements, and multiple stage tandem mass spectrometric (MS(n)) experiments, fragmentation pathways were elucidated for boldenone, methandrostenolone, tetrahydrogestrinone (THG), trenbolone, normethandrolone and mibolerone. Small differences in the chemical structures of the steroids, such as an additional double-bond or a methyl group, result in significantly different fragmentation pathways. The fragmentation pathways proposed in this paper allow interpretation of major product ions of other anabolic steroids reported by other researchers in a recent publication. The proposed fragmentation pathways are helpful for characterization of new steroids. The approach used in this study for elucidation of the fragmentation pathways is helpful in interpretation of complicated product-ion spectra of other compounds, drugs and their metabolites.     

Mass spectra of a series of 3-halo-2-oxazolidinones

The mass spectra of a series of 3-chloro and 3-bromo alkyl substituted 2-oxazolidinones have been investigated and compared with the mass spectra of the corresponding alkyl substituted-2-oxazolidinones. Fragmentation of the 2-oxazolidinone ring system has been found to be significantly influenced by: (1) the presence of an alkyl substituent, (2) the position, number and size of the alkyl substitutents, and (3)replacement of the N? H bond by N? CI and N? Br.     

Characteristic fragmentation behavior of the analogs of endomorphin-2 with phenylglycine in position 3 or 4 by ESI-FT tandem mass spectrometry

A series of analogs of endomorphin-2 （EM-2） with phenylglycine （Phg） in position 3 or 4 were synthesized. In electrospray ionization Fourier transform ion cyclotron resonance （ESI-Fr-ICR） MS/MS spectra of these compounds, some b, y, a, and internal ions were observed and slight mass differences between the calculated and observed results are obtained. Their sequences were derived successfully. However, the MS/MS patterns of these analogs with Dphg and Lphg were very similar. It is hard to distinguish them by MS/MS spectra. Moreover, if the third position was substituted by phenylglycine （L or D）, a rearrangement could occur in MS/MS experiment to lose proline residue.     

Automated electrospray ionization FT-ICR mass spectrometry for petroleum analysis

Analysis of petroleum samples at the molecular level by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) typically requires a prolonged accumulation of ions and/or summing up a large number of scans. Here, a chip-based micro-ESI system (Advion NanoMate, Ithaca, NY) has been successfully automated in combination with FT-ICR MS analysis of petroleum samples. A foil-sealed 96-well glass plate prevents solvent evaporation, with no visible loss of sample after 20 h of continuous operation. Mass spectra obtained from the same sample but taken from different wells after various time delays were very similar. Data from replicate samples in different wells could be combined to enhance mass spectral signal-to-noise ratio and dynamic range. Furthermore, the automated data acquisition eliminates sample carryover, and produces heteroatom class distribution, double-bond equivalents (DBE), and carbon number very similar to those from the conventional (manual) micro-ESI experiments.