Loss Of Ketene Research Articles

PE, or not PE, that is the question: The case of overlooked lyso-N-acylphosphatidylethanolamines.

Lyso derivatives of N-acyl-1,2-diacylglycero-3-phosphoethanolamines (L-NAPEs) are a lipid class mostly expressed in vegetables during stress and tissue damage that is involved in the synthesis of the lipid mediator N-acylethanolamines. L-NAPEs can be challenging to distinguish from isomeric phosphatidylethanolamines (PEs), especially in extracted complex samples where they could be confused with abundant PEs. In this study, hydrophilic interaction liquid chromatography with electrospray ionization hyphenated with (tandem) mass spectrometry (MS) was proposed to distinguish L-NAPEs and PEs as deprotonated molecules, [M - H]─ , using both high-resolution/accuracy Fourier transform MS and low-resolution linear ion trap (LIT) mass analyzers. MS3 experiments of [M - H - KE]─ as precursor ions (KE, ketene loss) using the LIT instrument allowed us to distinguish between isomeric L-NAPE and PE species. Regiochemical rules were proposed working on enzymatically synthesized L-NAPEs. A few key differences in MS/MS spectra, including abnormal intensity of acyl chain losses as fatty acids, the presence of N-acylphosphoethanolamine ions, and diagnostic ions of the polar head, were disclosed. Additionally, MS3 spectra of [M - H - KE]─ as precursor ions allowed us to confirm the identification of L-NAPE species. The proposed rules were applied to samples extracted from tomato by-products including stems and leaves. Overall, our methodology is demonstrated as a robust approach to recognizing L-NAPEs in complex samples. L-NAPEs 18:2-N-18:2, 18:2-N-18:3, 18:3-N-18:2, and 18:2-N-18:1 were the prevailing compounds in the analyzed tomato samples, accounting for more than 90%. In summary, a reliable method for identifying L-NAPEs in complex samples is described. The proposed method could prevent overlooking L-NAPEs and overestimating isomeric PE species in future lipid analyses.

Manual mass spectrometry de novo sequencing of the anionic host defense peptides of the Cuban Treefrog Osteopilus septentrionalis.

Host defense peptides accumulated in the skin glands of the animals constitute the basis of the adaptive and immune system of amphibians. The peptidome of the Cuban frog Osteopilus septentrionalis was established using tandem mass spectrometry as the best analytical tool to elucidate the sequence of these peptides. Manual interpretation of complementary collision-induced dissociation (CID), higher energy collision-induced dissociation (HCD), and electron transfer dissociation (ETD) tandem mass spectra recorded with an Orbitrap Elite mass spectrometer in liquid chromatography/mass spectrometry (LC/MS) mode was used to sequence the peptide components of the frog skin secretion, obtained by mild electrostimulation. Although the vast majority of amphibian peptides discovered so far are cationic, surprisingly only anionic peptides were identified in the skin secretion of the Cuban frog Osteopilus septentrionalis. Mass spectrometry allowed the sequences to be established of 16 representatives of new peptide families: septenins 1 and septenins 2. The highest sequence coverage when dealing with these anionic peptides was obtained with CID normalized collision energy 35 and HCD normalized collision energy 28. Mirror-symmetrical peptides are sequenced using N-terminal acetylation. Acetylated Ser is reliably distinguished from isomeric Glu by the loss of ketene from b-ions containing the corresponding residue. Calculations of the physicochemical and structural properties of the discovered anionic septenins 1 and 2 allowed the mechanism of their interaction with microbe cells to be postulated.

Tandem mass spectrometry of nitric oxide and hydrogen sulfide releasing aspirins: a hint into activity behavior.

Aspirin (acetylsalicylic acid, ASA) is the most popular non-steroidal anti-inflammatory drug. However, due to its action on cyclooxygenase and its acid nature, aspirin is associated with adverse gastrointestinal effects. In an effort to minimize these side effects, NO-donor and H2S-donor ASA co-drugs have been designed and tested. Their mass spectrometric behavior is now analyzed and reported. Positive ions were obtained by electrospray ionization involving protonation or alkali metal attachment. Their dissociation processes have been studied by collision induced dissociation in a triple quadrupole instrument. High mass accuracy measurements have been recorded on a Fourier transform ion cyclotron resonance mass spectrometer. The protonated molecules dissociate by an exclusive or largely prevailing path leading to acetyloxy-substituted benzoyl cation, namely an ASA unit. The process is reminiscent of the enzymatic hydrolysis, releasing intact ASA to a large extent. Only at higher collision energy does the formal ketene loss disrupt the ASA moiety. The gas phase chemistry of protonated ASA-releasing drugs develops along elementary dissociation steps analogous to the reactive processes in complex biological environments. This notion may provide a tool for preliminary testing of new compounds.

Intriguing roles of reactive intermediates in dissociation chemistry of N-phenylcinnamides

In mass spectrometry of protonated N-phenylcinnamides, the carbonyl oxygen is the thermodynamically most favorable protonation site and the added proton is initially localized on it. Upon collisional activation, the proton transfers from the carbonyl oxygen to the dissociative protonation site at the amide nitrogen atom or the α-carbon atom, leading to the formation of important reactive intermediates. When the amide nitrogen atom is protonated, the amide bond is facile to rupture to form ion/neutral complex 1, [RC(6)H(4)CH=CHCO(+)/aniline]. Besides the dissociation of the complex, proton transfer reaction from the α-carbon atom to the nitrogen atom within the complex takes place, leading to the formation of protonated aniline. The presence of electron-withdrawing groups favored the proton transfer reaction, whereas electron-donating groups strongly favored the dissociation (aniline loss). When the proton transfers from the carbonyl oxygen to the α-carbon atom, the cleavage of the C(α)-CONHPh bond results in another ion/neutral complex 2, [PhNHCO(+)/RC(6)H(4)CH=CH(2)]. However, in this case, electron-donating groups expedited the proton transfer reaction from the charged to the neutral partner to eliminate phenyl isocyanate. Besides the cleavage of the C(α)-CONHPh bond, intramolecular nucleophilic substitution (a nucleophilic attack of the nitrogen atom at the β-carbon) and stepwise proton transfer reactions (two 1,2-H shifts) also take place when the α-carbon atom is protonated, resulting in the loss of ketene and RC(6)H(5), respectively. In addition, the H/D exchanges between the external deuterium and the amide hydrogen, vinyl hydrogens and the hydrogens of the phenyl rings were discovered by D-labeling experiments. Density functional theory-based (DFT) calculations were performed to shed light on the mechanisms for these reactions.

Mechanism Studies of LCP Synthesis

The LCP (Liquid Crystal Polymer) known as Vectra is synthesized by acidolysis of 4-hydroxybenzoic acid with 6-hydroxy-2-naphthoic acid. The apparently simple acidolysis mechanism for LCP polycondensation is in fact a complex blend of mechanisms. Kinetics of model reactions and of actual polycondensations followed second-order kinetics and their rate constants were comparable. In the latter stages, ketene loss leads to phenolic ends, while decarboxylation provides phenyl ester ends. Accordingly, the mechanism changes to phenolysis. A quinone methide intermediate may also intervene, as revealed by kinetics studies and MALDI-TOF spectroscopy. Tailor-made matrices and synthesis of alternating well-defined oligomers assisted our studies. Nucleophilic aromatic substitutions may play a role, and we speculate on possible chain polycondensation. Esterolysis may be a useful alternative to LCP synthesis. Complications caused by ketene loss can be averted by the use of methoxycarbonyloxy monomers.

Gas‐phase fragmentation of γ‐lactone derivatives by electrospray ionization tandem mass spectrometry

Fragmentation reactions of beta-hydroxymethyl-, beta-acetoxymethyl- and beta-benzyloxymethyl-butenolides and the corresponding gamma-butyrolactones were investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS) using collision-induced dissociation (CID). This study revealed that loss of H(2)O [M+H-8](+) is the main fragmentation process for beta-hydroxymethylbutenolide (1) and beta-hydroxymethyl-gamma-butyrolactone (2). Loss of ketene ([M+H-42](+)) is the major fragmentation process for protonated beta-acetoxymethyl-gamma-butyrolactone (4), but not for beta-acetoxymethylbutenolide (3). The benzyl cation (m/z 91) is the major ion in the ESI-MS/MS spectra of beta-benzyloxymethylbutenolide (5) and beta-benzyloxymethyl-gamma-butyrolactone (6). The different side chain at the beta-position and the double bond presence afforded some product ions that can be important for the structural identification of each compound. The energetic aspects involved in the protonation and gas-phase fragmentation processes were interpreted on the basis of thermochemical data obtained by computational quantum chemistry.

Gas-Phase Nazarov Cyclization of Protonated 2-Methoxy and 2-Hydroxychalcone: An Example of Intramolecular Proton-Transport Catalysis

Upon CA, ESI generated [M + H](+) ions of chalcone (benzalacetophenone) and 3-phenyl-indanone both undergo losses of H(2)O, CO, and the elements of benzene. CA of the [M + H](+) ions of 2-methoxy and 2-hydroxychalcone, however, prompts instead a dominant loss of ketene. In addition, CA of the [M + H](+) ions of 2-methoxy-beta-methylchalcone produces an analogous loss of methylketene instead. Furthermore, the [M + D](+) ion of 2-methoxychalcone upon CA eliminates only unlabeled ketene, and the resultant product, the [M + D - ketene](+) ion, yields only the benzyl-d(1) cation upon CA. We propose that the 2-methoxy and 2-hydroxy (ortho) substituents facilitate a Nazarov cyclization to the corresponding protonated 3-aryl-indanones by mediating a critical proton transfer. The resultant protonated indanones then undergo a second proton transport catalysis facilitated by the same ortho substituents producing intermediates that eliminate ketene to yield 2-methoxy- or 2-hydroxyphenyl-phenyl-methylcarbocations, respectively. The basicity of the ortho substituent is important; for example, replacement of the ortho function with a chloro substituent does not provide an efficient catalyst for the proton transports. The Nazarov cyclization must compete with an alternate cyclization, driven by the protonated carbonyl group of the chalcone that results in losses of H(2)O and CO. The assisted proton transfer mediated by the ortho substituent shifts the competition in favor of the Nazarov cyclization. The proposed mechanisms for cyclization and fragmentation are supported by high-mass resolving power data, tandem mass spectra, deuterium labeling, and molecular orbital calculations.

Use of diagnostic neutral losses for structural information on unknown aromatic metabolites: an experimental and theoretical study

This work presents the use of neutral losses (NL) for the identification of compounds related to the metabolism of tyrosine. The mass spectra of all the studied compounds, recorded at several collision energies, are compared. The fragmentation mechanism of protonated molecules, MH(+), is explained by combining collision-induced dissociation (CID) mass spectra and density functional theory (DFT) calculations. The results show that the first fragmentation is the elimination from MH(+) of a neutral molecule including a functional group of the linear chain. Three primary neutral losses are observed: 17 u (NH(3)), 18 u (H(2)O) and 46 u (H(2)O+CO) characterizing amino, hydroxyl and carboxylic functions on the linear chain. The presence and abundance of ions corresponding to these losses are dependent on (i) the position of the functional group on the linear chain, (ii) the initial localisation of the protonating hydrogen, and (iii) the substitution of the aromatic ring. For compounds including a functional group on the benzylic carbon atom, the investigation of the other functions requires the knowledge of secondary fragmentations. Among these secondary fragmentations we have retained the loss of NH(3) from [MH-18u](+) and the loss of ketene from [MH-17u](+). Experimentally these fragmentations are detected using losses of 35 u and 59/73 u. In other words, NL35 identifies hydroxy and amino compounds and NL 46 and/or NL59/73 identify carboxylic acids. The search for characteristic neutral losses is used for the analysis of compounds in a mixture and the analysis of biological fluid. We show that selective search of several neutral losses allows also the unambiguous differentiation of isomers and gives the opportunity to identify compounds in biological fluids.

Electrospray Ionization Multiple Stage Quadrupole Ion-Trap and Tandem Quadrupole Mass Spectrometric Studies on Phosphatidylglycerol from Arabidopsis Leaves

Phosphatidylglycerol (PG) is the major phospholipid of plant chloroplasts. PG from Arabidopsis thaliana has an unusual fatty acyl chain, 3-trans-hexadecenoyl (Delta(3)16:1) in the sn-2 position of the major 18:3/Delta(3)16:1-PG species, as well as in 18:2/Delta(3)16:1-PG and 16:0/Delta(3)16:1-PG. Upon low-energy collisionally activated dissociation (CAD) in a tandem quadrupole or in an ion-trap mass spectrometer, the [M - H]- ions of the PG molecules containing Delta(3)16:1 give product-ion spectra that are readily distinguishable from those arising from PGs without the Delta(3)16:1 species. The Delta(3)16:1-fatty acyl-containing PGs are characterized by MS(2) product-ion mass spectra that contain predominant [M - H - 236]- ions arising from loss of the Delta(3)16:1-fatty acyl substituent as a ketene. This is attributable to the fact that the alpha-hydrogen of the Delta(3)16:1-fatty acid substituent involved in the ketene loss is an allylic hydrogen, which is very labile. This leads to preferential neutral loss of 236 and drastic decline in the neutral loss of 254 (i.e., loss as a fatty acid), the unique features that signify the presence of Delta(3)16:1-fatty acyl containing PGs. The neutral loss scan of 236, thus, provides a sensitive tandem quadrupole mass spectrometric means to identify Delta(3)16:1-containing PG species in lipid mixtures. This low-energy tandem mass spectrometric approach also permits the structures of the Arabidopsis PGs that consist of two isomeric structures to be unveiled.

A fragmentation study of isoflavones in negative electrospray ionization by MSn ion trap mass spectrometry and triple quadrupole mass spectrometry

This study has elucidated the fragmentation pathway for deprotonated isoflavones in electrospray ionization using MS(n) ion trap mass spectrometry and triple quadrupole mass spectrometry. Genistein-d(4) and daidzein-d(3) were used as references for the clarification of fragment structures. To confirm the relationship between precursor and product ions, some fragments were traced from MS(2) to MS(5). The previous literature for the structurally related flavones and flavanones located the loss of ketene (C(2)H(2)O) to ring C, whereas the present fragmentation study for isoflavones has shown that the loss of ketene occurs at ring A. In the further fragmentation of the [M-H-CH(3)](-*) radical anion of methoxylated isoflavones, loss of a hydrogen atom was commonly found. [M-H-CH(3)-CO-B-ring](-) is a characteristic fragment ion of glycitein and can be used to differentiate glycitein from its isomers. Neutral losses of CO and CO(2) were prominent in the fragmentation of deprotonated anions in ion trap mass spectrometry, whereas recyclization cleavage accounted for a very small proportion. In comparison with triple quadrupole mass spectrometry, ion trap MS(n) mass spectrometry has the advantage of better elucidation of the relationship between precursor and product ions.

The fragmentation of protonated tyrosine and iodotyrosines: The effect of substituents on the losses of NH 3 and of H 2O and CO

The gas-phase dissociation chemistries of protonated 3-iodo- l-tyrosine, 3,5-diiodo- l-tyrosine and 3,3′,5,5′-tetraiodo-thyronine (thyroxine) have been examined using a combination of tandem mass spectrometry and density functional theory (DFT) calculations. It was found that, at low collision energy, all protonated tyrosines exhibit common fragmentation pathways, including the competitive eliminations of NH 3 and the concomitant loss of H 2O and CO, but there are significant differences in relative abundances, depending on the combined electron-donating abilities of the substituents in the phenyl ring. The ions initially formed by loss of NH 3 are phenonium ions, but subsequent fragmentation is most easily understood in terms of the isomeric benzyl cation structures. These [ M + H − NH 3] + ions fragment at relatively low collision energies, mainly by loss of ketene; by contrast, the [ M + H − H 2O − CO] + ions are more stable towards dissociation. At higher collision energies, losses of one, two and even three iodine atoms were observed. DFT calculations (at the B3LYP/DZVP level of theory) were performed on protonated 3-iodotyrosine to compare the reaction profiles for the fragmentation mechanisms. The iodo-substituent in the 3-position is weakly electron-withdrawing and this results in a barrier (27.5 kcal/mol at 0 K) that is slightly higher than that for protonated tyrosine (26.8 kcal/mol). The phenoxy group PhO– is a weaker electron-donor than HO– and protonated 3,5-diiodo-4-phenoxytyrosine has an even higher barrier (31.1 kcal/mol) to NH 3 loss than protonated 3,5-diiodotyrosine (28.8 kcal/mol). Linear free energy plots for Δ H 0 ° ‡ and Δ G 298 ° ‡ against σ + for the four protonated tyrosine derivatives show good correlations. More importantly, as the products of the dissociation are higher in energy than the transition states to their formation, the plots of Δ H 0 ° and Δ G 298 ° for the overall reaction for NH 3 loss also correlate very well with σ + (correlation coefficients of 0.99 and 0.98, respectively). The positive slopes of these Hammett plots show that the barriers to the loss of NH 3 by the neighboring-group mechanism are increased by the presence of electron-withdrawing groups in the phenyl rings.

Modeling the Competitive Dissociation of Protonated 2,3-Butanedione. The Enthalpy of Formation of Methylhydroxycarbene

The enthalpy of formation of methylhydroxycarbene, CH(3)COH, has been determined from measurements of the threshold energy for collision-induced dissociation of protonated 2,3-butanedione in a flowing afterglow-triple quadrupole mass spectrometer and found to be 16 +/- 4 kcal/mol, 57 +/- 4 kcal/mol higher than that of acetaldehyde. From the measured enthalpy of formation, the difference between the first and second C-H BDEs in ethanol is found to be 17 kcal/mol, which implies a singlet-triplet splitting of 28 kcal/mol in the carbene. The activation energies for loss of ketene and carbon monoxide from protonated butanedione are found to be 60 +/- 4 and 50 +/- 4 kcal/mol, respectively. On the basis of experimental and computational results, the loss of carbon monoxide is proposed to proceed through a tight transition state. Although calculations also suggest a tight transition state for loss of ketene, the experimental data indicate that it occurs via a loose transition state, possibly forming by proton transfer along the direct dissociation pathway.

Proton migration and its effect on the MS fragmentation of N-acetyl OMe proline: MS/MS experiments and ab initio and density functional calculations

The comparison of low energy gas-phase collision-induced dissociation (eV CID) and low energy surface-induced dissociation (eV SID) of protonated N-acetyl O-methoxy ( N-acetyl OMe) proline shows that this ion has two main fragmentation pathways. One of them is the loss of methanol that occurs at low internal energy. The other one is a higher energy process, the loss of ketene from the protonated molecule. Theoretical calculations at three different levels provide further support for the energy requirements of these fragmentation reactions. These calculations also suggest that several protonated forms may coexist together after the ion activation and several fragmentation pathways may be operative, especially for the ketene loss.

Laser pyrolysis studies of β-diketonate chemical vapour deposition precursors. Part 1: β-diketones

The thermal decomposition of the β-diketones pentane-2,4-dione, 1,1,1,5,5,5-hexafluoropentane-2,4-dione, and 2,2,6,6-tetramethylheptane-3,5-dione has been studied using the technique of IR laser-powered homogeneous pyrolysis combined with the ab initio calculation of activation energies of possible reaction pathways. Pentane-2,4-dione decomposes by two parallel molecular elimination pathways, the keto tautomer affording acetone and ketene via a 6-electron retroene reaction and the enol tautomer 2-methylfuran via H2O elimination, followed by cyclisation. 1,1,1,5,5,5-Hexafluoropentane-2,4-dione eliminates HF with ring closure to yield 2,2-difluoro-5-(trifluoromethyl)furan-3(2H)-one. There is also a minor route of loss of one or two CF3 radicals, followed by H-abstraction to yield 3,3-difluoroacryloylfluoride and carbon suboxide, respectively. 2,2,6,6-Tetramethylheptane-3,5-dione decomposes via central C–CO bond homolysis, followed by CO or ketene loss from the resultant radicals and subsequent disproportionation of t-Bu radicals to produce 2-methylpropene and 2-methylpropane.

Tandem (quadrupole‐time‐of‐flight) electrospray mass spectrometry of oligo(vinyl acetates) with isopropylol or (1‐hydroxyethyl)‐2‐oxyisopropanyl end groups

Poly(vinyl acetates) with either isopropylol or (1-hydroxyethyl)-2-oxyisopropanyl end groups were analysed by tandem mass spectrometry using a quadrupole-time-of-flight (Q-TOF) instrument. Random scission along the polymer backbone was not observed. Instead the initial scissions were derived from the cleavage of end-group species. The resultant macrocations were subject to further elimination reactions that eventually produced polyene macrocations. A smaller fraction of the initial macrocations also reacted by loss of ketene and this reaction produced macrocations containing vinyl alcohol units.

Reactions of gaseous acylium ions with 1,3-dienes: further evidence for polar [4 + 2+] Diels-Alder cycloaddition.

A novel reaction of acylium and thioacylium ions, polar [4 + 2(+)] Diels-Alder cycloaddition with 1,3-dienes and O-heterodienes, has been systematically investigated in the gas phase (Eberlin MN, Cooks RG. J. Am. Chem. Soc. 1993; 115: 9226). This polar cycloaddition, yet without precedent in solution, likely forms cyclic 2,5-dihydropyrylium ions. Here we report the reactions of gaseous acylium ions [(CH(3))(2)N-C(+)=O, Ph-C(+)=O, (CH(3))(2)N-C(+)=S, CH(3)-C(+)=O, CH(3)CH(2)-C(+)=O, and CH(2)=CH-C(+)=O] with several 1-oxy-substituted 1,3-dienes of the general formula RO-CH=CH-C(R(1))=CH(2), which were performed to collect further evidence for cycloaddition. In reactions with 1-methoxy and 1-(trimethylsilyloxy)-1,3-butadiene, adducts are formed to a great extent, but upon collision activation they mainly undergo structurally unspecific retro-addition dissociation. In reactions with Danishefsky's diene (trans-1-methoxy-3-(trimethylsilyloxy)-1,3-butadiene), adducts are also formed to great extents, but retro-addition is no longer their major dissociation; the ions dissociate instead mainly to a common fragment, the methoxyacryl cation of m/z 85. This fragment ion is most likely formed with the intermediacy of the acyclic adduct, which isomerizes prior to dissociation by a trimethylsilyl cation shift. Theoretical calculations predict that meta cycloadducts bearing 1-methoxy and 1-trimethylsilyloxy substituents are unstable, undergoing barrierless ring opening induced by the charge-stabilizing effect of the 1-oxy substituents. In contrast, for the reactions with 1-acetoxy-1,3-butadiene, both the experimental results and theoretical calculations point to the formation of intrinsically stable cycloadducts, but the intact cycloadducts are either not observed or observed in low abundances. Both the isomeric ortho and meta cycloadducts are likely formed, but the nascent ions dissociate to great extents owing to excess internal energy. The ortho cycloadducts dissociate by ketene loss; the meta cycloadducts undergo intramolecular proton transfer to the acetoxy group followed by dissociation by acetic acid loss to yield aromatic pyrylium ions. Either or both of these dissociations, ketene and/or acetic acid loss, dominate over the otherwise favored retro-Diels-Alder alternative. The pyrylium ion products therefore constitute compelling evidence for polar [4 + 2(+)] cycloaddition since their formation can only be rationalized with the intermediacy of cyclic adducts.

Collision-induced dissociation of glycero phospholipids using electrospray ion-trap mass spectrometry.

Characterisation of phospholipids was achieved using collision-induced dissociation (CID) with an ion-trap mass spectrometer. The product ions were compared with those obtained with a triple quadrupole mass spectrometer. In the negative ion mode the product ions were mainly sn-1 and sn-2 lyso-phospholipids with neutral loss of ketene in combination with neutral loss of the polar head group. Less abundant product ions were sn-1 and sn-2 carboxylate anions. CID using a triple quadrupole mass spectrometer, however, gave primarily the sn-1 and sn-2 carboxylate anions together with lyso-phosphatidic acid with neutral loss of water. For the ion trap a charge-remote-type mechanism is proposed for formation of the lyso-phospholipid product ions by loss of alpha-hydrogen on the fatty acid moiety, electron rearrangement and neutral loss of ketene. A second mechanism involves nucleophilic attack of the phosphate oxygen on the sn-1 and sn-2 glycerol backbone to form carboxylate anions with neutral loss of cyclo lyso-phospholipids. CID (MS(3) and MS(4)) of the lyso-phospholipids using the ion-trap gave the same carboxylate anions as those obtained with a triple quadrupole instrument where multiple collisions in the collision cell are expected to occur. The data demonstrate that phospholipid species determination can be performed by using LC/MS(n) with an ion-trap mass spectrometer with detection of the lyso-phospholipid anions. The ion-trap showed no loss in sensitivity in full scan MS(n) compared to multiple reaction monitoring data acquisition. In combination with on-line liquid chromatography this feature makes the ion-trap useful in the scanning modes for rapid screening of low concentrations of phospholipid species in biological samples as recently described (Uran S, Larsen A, Jacobsen PB, Skotland T. J. Chromatogr. B 2001; 758: 265).

A Diruthenium μ-Methylene Complex

The reaction of [Ru2(μ-CO)(CO)4(μ-dppm)2] (1; dppm = Ph2PCH2PPh2) with CH2N2 gives the μ-methylene complex [Ru2(μ-CH2)(CO)4(μ-dppm)2] (2), and complex 2 reacts with CO to regenerate complex 1 with loss of ketene. Complex 2 reacts with HBF4 or CF3SO3H at low temperature to form the fluxional μ-methyl complex [Ru2(μ-CH3)(CO)4(μ-dppm)2]+ (3). Variable-temperature 1H, 13C, and 31P NMR studies establish that the μ-CH3 group has an unsymmetrical coordination mode with an agostic hydrogen and is fluxional. At room temperature, the reaction of 2 with formic acid gives an equimolar mixture of complex 1 and [Ru2(μ-H)(H)(μ-CO)(CO)2(μ-dppm)2] (4), which is an active catalyst for the decomposition of formic acid to hydrogen and carbon dioxide, and the reaction is shown to occur via the intermediate complexes 3 and [Ru2(μ-H){μ-C(O)Me}(HCOO)(CO)3(μ-dppm)2]+ (5). The reaction of 2 with acetic acid at room temperature gives in sequence the complexes [Ru2{μ-C(O)Me}(OAc)(CO)3(μ-dppm)2] (6) and [Ru2(μ-H){μ-C(O)Me}(OAc)(CO)3(μ-dppm)2]+ (7) before loss of methane occurs with formation of [Ru2(μ-OAc)(CO)4(μ-dppm)2]+ (8). Complex 2 reacts with methyl triflate to give ethylene and [Ru2(μ-H)(μ-CO)(CO)3(μ-dppm)2]+ (9), as the triflate salt, probably via an intermediate with an ethylruthenium group. In the presence of HBF4, complex 2 is an efficient precatalyst for the ring-opening polymerization of norbornene.

Charge-remote and charge-driven fragmentation processes in diacyl glycerophosphoethanolamine upon low-energy collisional activation: a mechanistic proposal

A mechanistic study of diacyl glycerophosphoethanolamine fragmentation under low energy collision-activated dissociation with electrospray ionization tandem mass spectrometry is reported. The fragmentation pathways leading to the formation of carboxylate anions (R x CO 2 −) ( x = 1, 2) and the formation of the ions representing neutral loss of ketene ([M − H − R ′ x HCO] −) are charge-driven processes, which are governed by the gas-phase basicity and the steric configuration of the molecules. The fragmentation pathway for the formation of the [M − H − R x CO 2H] − ions, reflecting neutral loss of fatty acid, is a charge-remote process, which involves the participation of the hydrogens at C-1 and C-2 of the glycerol, resulting in [M − H − R 2CO 2H] − > [M − H − R 1CO 2H] −. The preferential formations of R 2CO 2 − > R 1CO 2 −, and of [M − H − R ′ 2CHCO] − > [M − H − R ′ 1CHCO] − are attributed to the findings that charge-driven processes are sterically more favorable at sn-2. The observation of the abundance of [M − H − R ′ x CHCO] − > [M − H − R x CO 2H] − is attributed to the fact that the [M − H] − ions of GPE are basic precursor ions, which undergo preferential loss of ketene than loss of acid. The major pathway for the formation of R x CO 2 − ions arises from the nucleophilic attack of the anionic charge site of the phosphate on the C-1 or C-2 of the glycerol to render a charge transfer. The sterically more favorable attack on the C-2 than C-2 of the glycerol results in the abundance of R 2CO 2 − > R 1CO 2 −. These features of tandem spectra readily identify and locate the fatty acid substituents of GPE in the glycerol backbone.

Charge-driven fragmentation processes in diacyl glycerophosphatidic acids upon low-energy collisional activation. A mechanistic proposal

A mechanistic study of diacyl glycerophosphatidic acid (GPA) under low energy collisionally activated decomposition (CAD) with electrospray ionization tandem mass spectrometry is reported. The fragmentation pathways leading to the formation of carboxylate anions [R x CO 2 −], ( x = 1, 2) and the formation of the ions representing neutral loss of fatty acid ([M–H–R x CO 2H] −) and neutral loss of ketene ([M–H–R ′ x CHCO] −) (R x R ′ x CH 2) are charge-driven processes that are governed by the gas-phase basicity and the steric configuration of the molecules. The preferential formation of the ions of [M–H–R 2CO 2H] − > [M–H–R 1CO 2H] − and [M–H–R ′ 2CHCO] − > [M–H–R ′ 1CHCO] − are attributed to the fact that loss of fatty acid and loss of ketene are sterically more favorable at sn-2. While the observation of the abundance of [M–H–R x CO 2H] − > [M–H–R ′ x CHCO] − is attributed to the acidity of the gas phase ion of GPA, which undergoes a more facile neutral loss of acid than loss of ketene. The major pathway leading to the formation of R x CO 2 − ion under low energy CAD arises from further fragmentation of the [M–H–R x CO 2H] − ions by neutral loss of 136, resulting in an abundance of R 1CO 2 − > R 2CO 2 −. The differential formation of the carboxylate anions permits accurate assignment of the regiospecificity of the fatty acid substituents of GPA molecules by tandem mass spectrometry.