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In combustion, acetylene is a key species in molecular-weight growth reactions that form polycyclic aromatic hydrocarbons (PAHs) and ultimately soot particles. Radical addition to acetylene generates a vinyl radical intermediate, which has both trans and cis isomers. This isomerism can lead to profound changes in product distributions that are as yet insufficiently investigated. Herein, we explore acetylene addition to substituted trans -vinyl radicals, including potential rearrangement to cis structures, for eight combustion-related hydrocarbon radicals calculated using a composite method (G3X-K). Of these eight systems, the phenyl trans - and cis -Bittner-Howard HACA (hydrogen abstraction, C 2 H 2 Addition) process, where acetylene successively adds to a phenyl radical via a β-styryl intermediate, is simulated using a unified Master Equation model. Including the trans -Bittner-Howard pathway changes the products significantly at all simulated temperatures (550-1800 K) and pressures (5.33 × 10 2 -10 7 Pa), relative to a cis -only model. Typically, naphthalene remains the dominant product, but its abundance decreases at higher temperatures and pressures. For example, at 1200 K and 10 5 Pa, its branching ratio decreases from 78.5% to 62.9% when the trans pathway is included. At higher temperatures this decrease corresponds to the formation of alternative C 10 H 8 isomers, including the cis product benzofulvene with 8% maximum abundance at 1200 K and 5.33 × 10 2 Pa, and E -2-ethynyl-1-phenylethylene, a trans product with 26% maximum abundance at 1800 K, with little pressure dependence. At higher pressures, our model predicts a range of C 10 H 9 radicals, including resonance-stabilised radicals (RSRs). The impact of trans -vinyl radical chemistry in reactive environments means that they are essential to accurately describe combustion reactions and inhibit soot formation.

Specific lipid isomers are functionally critical, but their structural rigidity and usually minute geometry differences make separating them harder than other biomolecules. Such separations by ion mobility spectrometry (IMS) were recently enabled by new high-definition methods using dynamic electric fields, but major resolution gains are needed. Another problem of identifying many isomers with no unique fragments in ergodic collision-induced dissociation (CID) was partly addressed by the direct ozone-induced dissociation (OzID) that localizes the double bonds, but a low reaction efficiency has limited the sensitivity, dynamic range, throughput, and compatibility with other tools. Typically lipids are analyzed by MS as singly charged protonated, deprotonated, or ammoniated ions. Here, we explore the differential IMS (FAIMS) separations with OzID for exemplary lipids cationized by polyvalent metals. These multiply charged adducts have much greater FAIMS compensation voltages ( U C ) than the 1+ ions, with up to 10-fold resolution gain enabling baseline isomer separations even at a moderate resolving power of the SelexION stage. Concomitantly OzID speeds up by many orders of magnitude, producing a high yield of diagnostic fragments already in 1 ms. These capabilities can be ported to the superior high-definition FAIMS and high-pressure OzID systems to take lipidomic analyses to the next level.

Polystyrene (PS) is a thermoplastic polymer commonly used in various applications due to its bulk properties. Designing functional polystyrenes with well-defined structures for targeted applications is of significant interest due to the rigid and apolar nature of the polymer chain. Progress is hindered to date by the limitations of current analytical methods in defining the atomistic-level folding of the polymer chain. The integration of ion mobility spectrometry and molecular dynamics simulations is beneficial in addressing these challenges. However, data on gas-phase polystyrene ions are rarely reported in the literature. We herein investigate the gas phase structure of polystyrene ions with different end groups to establish how the nature and the rigidity of the monomer unit affect the charge stabilization. We find that, in contrast to polar polymers in which the charges are located deep in the ionic globules, the charges in the PS ions are rather located at the periphery of the polymer backbone, leading to singly and doubly charged PS ions adopting dense elliptic-shaped structures. Molecular dynamics (MD) simulations indicate that the folding of the PS rigid chain is controlled by phenyl ring interactions with the charge ultimately remaining excluded from the core of the globular ions, whereas the folding of polyether ions is initiated by the folding of the flexible polyether chain around the sodium ion that remains deeply enclosed in the core of the ions.

Nicotiana benthamiana is predominantly distributed in arid habitats across northern Australia. However, none of six geographically isolated accessions shows obvious xerophytic morphological features. To investigate how these tender-looking plants withstand drought, we examined their responses to water deprivation, assessed phenotypic, physiological, and cellular responses, and analysed cuticular wax composition and wax biosynthesis gene expression profiles. Results showed that the Central Australia (CA) accession, globally known as a research tool, has evolved a drought escape strategy with early vigour, short life cycle, and weak, water loss-limiting responses. By contrast, a northern Queensland (NQ) accession responded to drought by slowing growth, inhibiting flowering, increasing leaf cuticle thickness, and altering cuticular wax composition. Under water stress, NQ increased the heat stability and water impermeability of its cuticle by extending the carbon backbone of cuticular long-chain alkanes from c. 25 to 33. This correlated with rapid upregulation of at least five wax biosynthesis genes. In CA, the alkane chain lengths (c. 25) and gene expression profiles remained largely unaltered. This study highlights complex genetic and environmental control over cuticle composition and provides evidence for divergence into at least two fundamentally different drought response strategies within the N. benthamiana species in < 1 million years., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

Acquired drug resistance is the major cause for disease recurrence in cancer patients, and this is particularly true for patients with metastatic melanoma that carry a BRAF V600E mutation. To address this problem, we investigated cyclic membrane-active peptides as an alternative therapeutic modality to kill drug-tolerant and resistant melanoma cells to avoid acquired drug resistance. We selected two stable cyclic peptides (cTI and cGm), previously shown to have anti-melanoma properties, and compared them with dabrafenib, a drug used to treat cancer patients with the BRAF V600E mutation. The peptides act via a fast membrane-permeabilizing mechanism and kill metastatic melanoma cells that are sensitive, tolerant, or resistant to dabrafenib. Melanoma cells do not become resistant to long-term treatment with cTI, nor do they evolve their lipid membrane composition, as measured by lipidomic and proteomic studies. In vivo studies in mice demonstrated that the combination treatment of cTI and dabrafenib resulted in fewer metastases and improved overall survival. Such cyclic membrane-active peptides are thus well suited as templates to design new anticancer therapeutic strategies., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The onset and progression of cancer is associated with changes in the composition of the lipidome. Therefore, better understanding of the molecular mechanisms of these disease states requires detailed structural characterization of the individual lipids within the complex cellular milieu. Recently, changes in the unsaturation profile of membrane lipids have been observed in cancer cells and tissues, but assigning the position(s) of carbon-carbon double bonds in fatty acyl chains carried by membrane phospholipids, including the resolution of lipid regioisomers, has proven analytically challenging. Conventional tandem mass spectrometry approaches based on collision-induced dissociation of ionized glycerophospholipids do not yield spectra that are indicative of the location(s) of carbon-carbon double bonds. Ozone-induced dissociation (OzID) and ultraviolet photodissociation (UVPD) have emerged as alternative ion activation modalities wherein diagnostic product ions can enable de novo assignment of position(s) of unsaturation based on predictable fragmentation behaviors. Here, for the first time, OzID and UVPD (193 nm) mass spectra are acquired on the same mass spectrometer to evaluate the relative performance of the two modalities for lipid identification and to interrogate the respective fragmentation pathways under comparable conditions. Based on investigations of lipid standards, fragmentation rules for each technique are expanded to increase confidence in structural assignments and exclude potential false positives. Parallel application of both methods to unsaturated phosphatidylcholines extracted from isogenic colorectal cancer cell lines provides high confidence in the assignment of multiple double bond isomers in these samples and cross-validates relative changes in isomer abundance.

Proton-transfer reactions are commonplace during electrospray ionization (ESI) mass spectrometry experiments and are often responsible for imparting charge to analyte molecules. Multiple protonation-site isomers (protomers) can arise for polyfunctional molecules and these isomers can interconvert via solvent-mediated proton transfer reactions during various stages of the ESI process. Studying the populations and interconversion of protonation isomers provides key insight into the ESI process, ion-molecule interactions, and ion dissociation mechanisms. An archetype molecule to study protomer interconversion fundamentals in this context is para -aminobenzoic acid ( p ABA), where both the amino and carboxylic acid protomers are typically formed under ESI and the mechanisms for interconversion are still under refinement. Using ion-trap mass spectrometry reaction kinetics (2.5 mTorr, 300 K), this study examines gas-phase interconversion catalysis of p ABA protomers by seven neutral species, which are commen solvents and additives used for ESI: water, formic acid, methanol, ethanol, propanol, ammonia, and acetonitrile. Three distinct reaction cases are reported: (i) formic acid, methanol, ethanol, propanol, and ammonia each catalyze the interconversion between the amino and carboxylic acid protomers via a n = 1 solvent-molecule vehicle mechanism; (ii) for water, however, a n = 6 adduct complex is detected and this suggests that the observed protomer interconversion occurs through a Grotthuss mechanism, in accord with literature reports; (iii) acetonitrile inhibits proton transfer by the formation of particularly stable n = 1 and 2 adduct complexes. The second-order rate constants for the protomer interconversion are observed to increase in the following order: H 2 O < HCO 2 H < MeOH < EtOH < PrOH < NH 3 . Potential energy schemes are reported for all neutral-catalyzed proton transfer reactions using the DSD-PBEP86-D3(BJ)/aug-cc-pVDZ level of theory. A central transition state, which connects the protonation site adducts, is shown to be the key rate-limiting step. The energy of this transition state is sensitive to the proton affinity of the neutral solvent, and this is supported by the correlation between the reaction rate and the solvent proton affinity.

In recent years there has been a significant interest in the development of innovative lipidomics techniques capable of resolving lipid isomers. To date, methods applied to resolving sn-isomers have resolved only a limited number of species. We report a workflow based on ozone-induced dissociation for untargeted characterisation of hundreds of sn-resolved glycerophospholipid isomers from biological extracts in under 20 min, coupled with an automated data analysis pipeline. It provides an order of magnitude increase in the number of sn-isomer pairs identified as compared to previous reports and reveals that sn-isomer populations are tightly regulated and significantly different between cell lines. The sensitivity of this method and potential for de novo molecular discovery is further demonstrated by the identification of unexpected lipids containing ultra-long monounsaturated acyl chains at the sn-1 position., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

In the gas-phase chemistry of the atmosphere and automotive fuel combustion, peroxyl radical intermediates are formed following O 2 addition to carbon-centered radicals which then initiate a complex network of radical reactions that govern the oxidative processing of hydrocarbons. The rapid association of the phenyl radical-a fundamental radical related to benzene-with O 2 has hitherto been modeled as a barrierless process, a common assumption for peroxyl radical formation. Here, we provide an alternate explanation for the kinetics of this reaction by deploying double-hybrid density functional theory (DFT), at the DSD-PBEP86-D3(BJ)/aug-cc-pVTZ level of theory, and locate a submerged adiabatic transition state connected to a prereaction complex along the reaction entrance pathway. Using this potential energy scheme, experimental rate coefficients k ( T ) for the addition of O 2 to the phenyl radical are accurately reproduced within a microcanonical kinetic model. This work highlights that purportedly barrierless radical oxidation reactions may instead be modeled using stationary points, which in turn provides insight into pressure and temperature dependence.

Homocubane, a highly strained cage hydrocarbon, contains two very different positions for the introduction of a nitrogen atom into the skeleton, e. g., a position 1 exchange results in a tertiary amine whereas position 9 yields a secondary amine. Herein reported is the synthesis of 9-azahomocubane along with associated structural characterization, physical property analysis and chemical reactivity. Not only is 9-azahomocubane readily synthesized, and found to be stable as predicted, the basicity of the secondary amine was observed to be significantly lower than the structurally related azabicyclo[2.2.1]heptane, although similar to 1-azahomocubane., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Many families of lipid isomers remain unresolved by contemporary liquid chromatography-mass spectrometry approaches, leading to a significant underestimation of the structural diversity within the lipidome. While ion mobility coupled to mass spectrometry has provided an additional dimension of lipid isomer resolution, some isomers require a resolving power beyond the capabilities of conventional platforms. Here, we present the application of high-resolution traveling-wave ion mobility for the separation of lipid isomers that differ in (i) the location of a single carbon-carbon double bond, (ii) the stereochemistry of the double bond ( cis or trans ), or, for glycerolipids, (iii) the relative substitution of acyl chains on the glycerol backbone ( sn -position). Collisional activation following mobility separation allowed identification of the carbon-carbon double-bond position and sn -position, enabling confident interpretation of variations in mobility peak abundance. To demonstrate the applicability of this method, double-bond and sn -position isomers of an abundant phosphatidylcholine composition were resolved in extracts from a prostate cancer cell line and identified by comparison to pure isomer reference standards, revealing the presence of up to six isomers. These findings suggest that ultrahigh-resolution ion mobility has broad potential for isomer-resolved lipidomics and is attractive to consider for future integration with other modes of ion activation, thereby bringing together advanced orthogonal separations and structure elucidation to provide a more complete picture of the lipidome.

The biological functions of lipids are entirely dependent on their molecular structures with even small changes in structure─such as different sites of unsaturation─providing critical markers for changes in the underlying metabolism. Conventional mass spectrometry imaging (MSI) approaches, however, face the twin challenges of mixture and structural complexity and are typically unable to differentiate lipid isomers that differ only in the position(s) of carbon-carbon double bonds. Recent coupling of ozone-induced dissociation (OzID) with matrix-assisted laser desorption/ionization (MALDI)-MSI has demonstrated the potential to map changes in individual double-bond isomers, thus enabling visualization of the modulation in lipid desaturation in adjacent tissue types. This has, to date, only been performed in positive-ion mode due to a generally higher abundance of phosphatidylcholines (PC) in mammalian tissues and the efficient desorption/ionization of this lipid subclass. Many other glycerophospholipids (GPLs), however, are better detected in negative-ion mode as deprotonated anions. Recently, OzID has been implemented on a traveling-wave ion-mobility mass spectrometer (Waters, SYNAPT G2-S i ) that provides a 50-fold increase in the rate of the gas-phase reaction between ionized lipids and ozone and a commensurate increase in sensitivity for isomer-resolved mass spectrometry. These gains are exploited here to interrogate the distributions of anionic GPL isomers in biological tissues, covering the subclasses phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylinositol (PI), phosphatidylglycerol (PG), and phosphatidic acid (PA). Exploiting both ozone- and collision-induced dissociation in a single acquisition simultaneously identifies sites of unsaturation and acyl chain composition from the same mass spectrum.

Background: Enzymes are central components of many physiological processes, and changes in enzyme activity are linked to numerous disease states, including osteoarthritis (OA). Assessing changes in enzyme function can be challenging because of difficulties in separating affected tissue areas that result in the homogenisation of healthy and diseased cells. Direct correlation between spatially-resolved enzyme distribution(s) and diseased cells/tissues can thus lead to advances in our understanding of OA pathophysiology. Herein, we present a method that uses mass spectrometry imaging (MSI) to visualise the distribution of lipase enzymes and their downstream lipid products in fresh bone and cartilage tissue sections. Immunohistostaining of adjacent tissue sections was then used to identify OA cells/tissues, which were then statistically correlated with molecular-level images. Methods: MSI was used to image lipase enzymes, their substrates, and their metabolic products to validate enzymatic activity and correlate to OA regions determined by immunohistochemistry (IHC). Based on the modified Mankin score, six non-OA and OA patient-matched osteochondral samples were analysed by matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI-MSI). Due to the involvement of phospholipase A2 (PLA 2 ) in inflammatory pathways, explant tissues were treated with IL-1β to mimic inflammation observed in OA. Bovine explant tissues were then subject to MSI methods to observe the spatial distribution of PLA 2 . Results: Compared with non-OA samples, OA samples showed an elevated level of multiple arachidonic acid (AA)-containing phospholipids ( P < 0.001), in which the elevation in the surface and deep layer cartilage of OA tissues is correlated to elevated PLA 2 activity ( P < 0.001). Bovine explant tissues treated with IL-1β to mimic OA pathophysiology validated these results and displayed elevated PLA 2 levels in OA mimic samples relative to the controls ( P < 0.001). It was established that the PLA 2 G 2 A isoform specifically was responsible for PLA 2 enzyme activity changes in OA tissues ( P < 0.001). Conclusion: Our results present a reliable method for imaging enzyme dynamics in OA cartilage, which sets up the foundation for future spatial enzyme dynamics in the OA field. We demonstrated that OA patients exhibit increased expression of PLA 2 G 2 A at the superficial and deep cartilage zone that degrades cartilage differently at the spatial level. A tissue-specific PLA 2 G 2 A precision inhibition may be the potential target for OA., Competing Interests: Competing Interests: The authors have declared that no competing interest exists., (© The author(s).)

Electrospray ionization (ESI) is used to deliver analytes for mass analysis across a huge range of mass spectrometry applications. Despite its ubiquitous application and many mechanistic investigations, it remains that a fundamental understanding of ESI processes is not complete. In particular, all the factors that influence the populations of protonation isomers are elusive such that it remains a challenge to optimize experimental conditions to favor one isomer over another. The molecule para -aminobenzoic acid has emerged as an archetype for the study of protonation isomers, with both amino and carboxylic acid protonation site isomers (protomers) typically formed upon ESI, with the isomer ratio shown to be sensitive to several physical and chemical parameters. Here we report an ion-trap mass spectrometry study of the time-resolved methanol-catalyzed proton transfer between the amine and carboxylic acid moieties of para -aminobenzoic acid. The experimental and computational results presented are consistent with a bimolecular mechanism where isomerization is mediated by a single methanol rather than a multimolecular Grotthuss proton transfer process. Pseudo-first-order rate constants for protomer specific product ions are reported and confirm the depletion of the amino protomer is correlated to the growth of the carboxylic acid protomer. Under the controlled conditions of a low-pressure ion-trap mass spectrometer (2.5 mTorr, 300 K), the number of methanol molecules required to isomerize para -aminobenzoic acid is determined to be one, and the second-order rate constant for methanol-catalyzed isomerization is (1.9 ± 0.1) × 10 -11 cm 3 molecule -1 s -1 . The para -aminobenzoic acid vehicle mechanism is explored computationally at the DSD-PBEP86-D3BJ/aug-cc-pVDZ level of theory and reveals that the transition state for proton transfer is submerged (-10 kJ mol -1 ) relative to the separated reactant energies. The findings from this paper show that single-solvent catalyzed intramolecular proton transfer reactions are possible and must be considered during the late stages of ESI to predict the site(s) of protonation and the ion's stability in the presence of solvent molecules.

Fatty acid isomers are responsible for an under-reported lipidome diversity across all kingdoms of life. Isomers of unsaturated fatty acids are often masked in contemporary analysis by incomplete separation and the absence of sufficiently diagnostic methods for structure elucidation. Here, we introduce a comprehensive workflow, to discover unsaturated fatty acids through coupling liquid chromatography and mass spectrometry with gas-phase ozonolysis of double bonds. The workflow encompasses semi-automated data analysis and enables de novo identification in complex media including human plasma, cancer cell lines and vernix caseosa. The targeted analysis including ozonolysis enables structural assignment over a dynamic range of five orders of magnitude, even in instances of incomplete chromatographic separation. Thereby we expand the number of identified plasma fatty acids two-fold, including non-methylene-interrupted fatty acids. Detection, without prior knowledge, allows discovery of non-canonical double bond positions. Changes in relative isomer abundances reflect underlying perturbations in lipid metabolism., (© 2023. The Author(s).)

Coordination cages can be used for enantio- and regioselective catalysis and for the selective sensing and separation of isomeric guest molecules. Here, stereoisomers of a family of coordination cages are resolved using ultra-high-resolution cyclic ion-mobility mass spectrometry (cIM-MS). The observed ratio of diastereomers is dependent on both the metal ion and counter ion. Moreover, the point groups can be assigned through complementary NMR experiments. This method enables the identification and interrogation of the individual isomers in complex mixtures of cages which cannot be performed in solution. Furthermore, these techniques allow the stability of individual isomers within the mixture to be probed, with the T-symmetric isomers in this case shown to be more robust than the C 3 and S 4 analogues., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Triacylglycerol estolides (TG-EST) are biologically active lipids extensively studied for their anti-inflammatory and anti-diabetic properties. In this work, eight standards of TG-EST were synthesized and systematically investigated by nanoelectrospray tandem mass spectrometry. Mass spectra of synthetic TG-EST were studied with the purpose of enabling the unambiguous identification of these lipids in biological samples. TG-EST glycerol sn -regioisomers and isomers with the fatty acid ester of hydroxy fatty acid (FAHFA) subunit branched in the ω-, α-, or 10-position were used. Ammonium, lithium, and sodium adducts of TG-EST formed by nanoelectrospray ionization were subjected to collision-induced dissociation (CID) and higher-energy collisional dissociation (HCD). Product ion spectra allowed for identification of fatty acid (FA) and FAHFA subunits originally linked to the glycerol backbone and distinguished the α-branching site of the FAHFA from other estolide-branching isomers. The ω- and 10-branching sites were determined by combining CID with ozone-induced dissociation (OzID). Lithium adducts provided the most informative product ions, enabling characterization of FA, hydroxy fatty acid (HFA), and FAHFA subunits. Glycerol sn -regioisomers were distinguished based on the relative abundance of product ions and unambiguously identified using CID/OzID of lithium and sodium adducts.

Highly strained cage hydrocarbons have long stood as fundamental molecules to explore the limits of chemical stability and reactivity, probe physical properties, and more recently as bioactive molecules and in materials discovery. Interestingly, the nitrogenous congeners have attracted much less attention. Previously absent from the literature, azahomocubanes, offer an opportunity to investigate the effects of a nitrogen atom when incorporated into a highly constrained polycyclic environment. Herein disclosed is the synthesis of 1-azahomocubane, accompanied by comprehensive structural characterization, physical property analysis and chemical reactivity. These data support the conclusion that nitrogen is remarkably well tolerated in a highly strained environment., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Fatty acids are an abundant class of lipids that are characterised by wide structural variation including isomeric diversity arising from the position and configuration of functional groups. Traditional approaches to fatty acid characterisation have combined chromatography and mass spectrometry for a description of the composition of individual fatty acids while infrared (IR) spectroscopy has provided insights into the functional groups and bond configurations at the bulk level. Here we exploit universal 3-pyridylcarbinol ester derivatization of fatty acids to acquire IR spectra of individual lipids as mass-selected gas-phase ions. Intramolecular interactions between the protonated pyridine moiety and carbon-carbon double bonds present highly sensitive probes for regiochemistry and configuration through promotion of strong and predictable shifts in IR resonances. Gas-phase IR spectra obtained from unsaturated fatty acids are shown to discriminate between isomers and enable the first unambiguous structural assignment of 6 Z -octadecenoic acid in human-derived cell lines. Compatibility of 3-pyridylcarbinol ester derivatization with conventional chromatography-mass spectrometry and now gas-phase IR spectroscopy paves the way for comprehensive structure elucidation of fatty acids that is sensitive to regio- and stereochemical variations and with the potential to uncover new pathways in lipid metabolism., Competing Interests: SJB holds patents on ozone-induced dissociation technology (A method for the determination of the position of unsaturation in a compound, US8242439 and US7771943). The work presented in this publication was supported in-part through funding from Waters Corporation and the Australian Research Council through the Linkage Partnership Scheme (LP18010023)., (This journal is © The Royal Society of Chemistry.)

Separation and identification of fatty acid (FA) isomers in biological samples represents a challenging problem for lipid chemists. Notably, FA regio- and stereo-isomers differing in the location or ( cis/trans ) geometry of carbon-carbon double bonds are often incompletely separated and ambiguously assigned in conventional chromatography-mass spectrometry analyses. To address this challenge, FAs have been derivatized with the charge-switch derivatization reagents N -methyl-pyridinium-3-methanamine and N -(4-aminomethylphenyl)pyridinium and subjected to reversed-phase liquid chromatography-tandem mass spectrometry. Charge-remote fragmentation of the fixed-charge derivatives leads to characteristic product ions arising from dissociation at allylic positions that enable assignment of position(s) of unsaturation, while a newly discovered dihydrogen neutral loss was found to be dominant for double bonds with cis -stereochemistry. The structure of the [M - 2] + product ions was probed by gas-phase ozonolysis revealing the presence of two new carbon-carbon bonds on either side of the initial position of unsaturation consistent with an electrocyclic mechanism of 1,4-dihydrogen elimination. Charge-remote fragmentation pathways diagnostic of double bond position and stereochemistry were found to be generalized for FAs of different carbon-chain lengths, double bond positions, and degrees of unsaturation and were effective in the unequivocal assignment of the FA structure in complex mixtures of FA isomers, including bovine milk powder.

Coordination cages with well-defined cavities show great promise in the field of catalysis on account of their unique combination of molecular confinement effects and transition-metal redox chemistry. Here, three coordination cages are reduced from their native 16 + oxidation state to the 2 + state in the gas phase without observable structural degradation. Using this method, the reaction rate constants for each reduction step were determined, with no noticeable differences arising following either the incorporation of a C 60 -fullerene guest or alteration of the cage chemical structure. The reactivity of highly reduced cage species toward molecular oxygen is "switched-on" after a threshold number of reduction steps, which is influenced by guest molecules and the structure of cage components. These new experimental approaches provide a unique window to explore the chemistry of highly-reduced cage species that can be modulated by altering their structures and encapsulated guest species., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

While various mass spectrometric approaches have been applied to lipid analysis, unraveling the extensive structural diversity of lipids remains a significant challenge. Notably, these approaches often fail to differentiate between isomeric lipids─a challenge that is particularly acute for branched-chain fatty acids (FAs) that often share similar (or identical) mass spectra to their straight-chain isomers. Here, we utilize charge-switching strategies that combine ligated magnesium dications with deprotonated fatty acid anions. Subsequent activation of these charge inverted anions yields mass spectra that differentiate anteiso -branched- from straight-chain and iso -branched-chain FA isomers with the predictable fragmentation enabling de novo assignment of anteiso branch points. The application of these charge-inversion chemistries in both gas- and solution-phase modalities is demonstrated to assign the position of anteiso -methyl branch-points in FAs and, with the aid of liquid chromatography, can be extended to de novo assignment of additional branching sites via predictable fragmentation patterns as methyl branching site(s) move closer to the carboxyl carbon. The gas-phase approach is shown to be compatible with top-down structure elucidation of complex lipids such as phosphatidylcholines, while the integration of solution-phase charge-inversion with reversed phase liquid chromatography enables separation and unambiguous identification of FA structures within isomeric mixtures. Taken together, the presented charge-switching MS-based technique, in combination with liquid chromatography, enables the structural identification of branched-chain FA without the requirement of authentic methyl-branched FA reference standards.

Gas phase ion-molecule reactions are central to chemical processes across many environments. A feature of many of these reactions is an inverse relationship between temperature and reaction rate arising from a submerged barrier (an early reaction barrier that is below the energy of the separated reactants), which often arises due to a stable pre-reactive complex. While the thermodynamics and kinetics of many ion-molecule reactions have been extensively modelled, the reaction kinetics of ion-molecule reactions involving radical ions are less explored. In this investigation, the target reactions involve distonic radical ions, where the charge and radical moieties are separated within the molecular structure. Experimental rate coefficients for the reaction of either C 2 H 2 or C 2 H 4 with a suite of eighteen distonic radical ions are reported. Rate coefficients are modelled using potential energy schemes combined with a statistical reaction-rate (RRKM-ME) model. Second-order rate coefficients are in good agreement with experimental values with an average RMS deviation of 37% across three orders of magnitude. These predictions are generally sensitive to the relative energetics of the pre-reactive complex forward transition state but are relatively insensitive to the overall exothermicity of the covalent-addition product.

Disorder of lipid homeostasis is closely associated with a variety of diseases. Although mass spectrometry (MS) approaches have been well developed for the characterization of lipids, it still lacks an integrated and compact MS system that is capable of rapid and detailed lipid structural characterization and can be conveniently transferred into different laboratories. In this work, we describe a novel miniature MS system with the capability of both ozone-induced dissociation (OzID) and collision-induced dissociation (CID) for the assignment of sites of unsaturation and sn- positions in glycerolipids. A miniature ozone generator was developed, which can be operated at a relatively high pressure. By maintaining high-concentration ozone inside the linear ion trap, OzID efficiency was significantly improved for the identification of C═C locations in unsaturated lipids, with reaction times as short as 10 ms. Finally, the miniature OzID MS system was applied to the analysis of C═C locations and sn- positions of lipids from biological samples. Direct sampling and fast detection of changes in phospholipid isomers were demonstrated for the rapid discrimination of breast cancer tissue samples, showing the potential of the miniature OzID MS system for point-of-care analysis of lipid isomer biomarkers in complex samples.

The first ππ* transition for protonated 2-, 3-, and 4-formylpyridine (FPH + ) (m/z 108) is investigated by mass spectrometry coupled with photodissociation action spectroscopy at room temperature and 10 K. The photoproduct ions are detected over 35 000-43 000 cm -1 , and the major product channel for 3-FPH + and 4-FPH + is the loss of CO forming protonated pyridine at m/z 80. For 2-FPH + , the CO loss product is present but a more abundant photoproduct arises from the loss of CH 2 O to form m/z 78. Plausible potential energy pathways that lead to dissociation are mapped out and comparisons are made to products arising from collision-induced dissociation. Although, in all cases, the elimination of CO is the overwhelming thermodynamically preferred pathway, the protonated 2-FPH + results suggest that the CH 2 O product is kinetically driven and competitive with CO loss. In addition, for each isomer, radical photoproduct ions are detected at lower abundances. SCS-CC2/aug-cc-pVTZ Franck-Condon simulations assist with the assignment of vibrionic structure and adiabatic energies (0-0) for 2-FPH + at 36 560 cm -1 , 37 430 cm -1 for 3-FPH + , and 36 140 cm -1 for 4-FPH + , yielding an accurate prediction, on average, within 620 cm -1 .

The cellular energy and biomass demands of cancer drive a complex dynamic between uptake of extracellular FAs and their de novo synthesis. Given that oxidation of de novo synthesized FAs for energy would result in net-energy loss, there is an implication that FAs from these two sources must have distinct metabolic fates; however, hitherto, all FAs have been considered part of a common pool. To probe potential metabolic partitioning of cellular FAs, cancer cells were supplemented with stable isotope-labeled FAs. Structural analysis of the resulting glycerophospholipids revealed that labeled FAs from uptake were largely incorporated to canonical (sn-) positions on the glycerol backbone. Surprisingly, labeled FA uptake also disrupted canonical isomer patterns of the unlabeled lipidome and induced repartitioning of n-3 and n-6 PUFAs into glycerophospholipid classes. These structural changes support the existence of differences in the metabolic fates of FAs derived from uptake or de novo sources and demonstrate unique signaling and remodeling behaviors usually hidden from conventional lipidomics., Competing Interests: Conflict of interest S. J. B. holds patents on ozone-induced dissociation technology (A method for the determination of the position of unsaturation in a compound, US8242439 and US7771943). All other authors declare that they have no conflicts of interest with the contents of this article., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Gas-phase ion-molecule reactions provide structural insights across a range of analytical applications. A hindrance to the wider use of ion-molecule reactions is that they are relatively slow compared to other ion activation modalities and can thereby impose a bottleneck on the time required to analyze each sample. Here we describe a method for accelerating the rate of ion-molecule reactions involving ozone, implemented by supplementary RF-activation of mass-selected ions within a linear ion trap. Reaction rate accelerations between 15-fold (for ozonolysis of alkenes in ionised lipids) and 90-fold (for ozonation of halide anions) are observed compared to thermal conditions. These enhanced reaction rates with ozone increase sample throughput, aligning the reaction time with the overall duty cycle of the mass spectrometer. We demonstrate that the acceleration is due to the supplementary RF-activation surmounting the activation barrier energy of the entrance channel of the ion-molecule reaction. This rate acceleration is subsequently shown to aid identification of new, low abundance lipid isomers and enables an equivalent increase in the number of lipid species that can be analyzed.

Understanding how neutral molecules become protonated during positive-ion electrospray ionization (ESI) mass spectrometry is critically important to ensure analytes can be efficiently ionized, detected, and unambiguously identified. The ESI solvent is one of several parameters that can alter the dominant site of protonation in polyfunctional molecules and thus, in turn, can significantly change the collision-induced dissociation (CID) mass spectra relied upon for compound identification. Ciprofloxacin─a common fluoroquinolone antibiotic─is one such example whereby positive-ion ESI can result in gas-phase [M + H] + ions protonated at either the keto-oxygen or the piperazine-nitrogen. Here, we demonstrate that these protonation isomers (or protomers ) of ciprofloxacin can be resolved by differential ion mobility spectrometry and give rise to distinctive CID mass spectra following both charge-directed and charge-remote mechanisms. Interaction of mobility-selected protomers with methanol vapor (added via the throttle gas supply) was found to irreversibly convert the piperazine N -protomer to the keto- O -protomer. This methanol-mediated proton-transport catalysis is driven by the overall exothermicity of the reaction, which is computed to favor the O -protomer by 93 kJ mol -1 (in the gas phase). Conversely, gas phase interactions of mobility-selected ions with acetonitrile vapor selectively depletes the N -protomer ion signal as formation of stable [M + H + CH 3 CN] + cluster ions skews the apparent protomer population ratio, as the O -protomer is unaffected. These findings provide a mechanistic basis for tuning protomer populations to ensure faithful characterization of multifunctional molecules by tandem mass spectrometry.

Iodinate anions are important in the chemistry of the atmosphere where they are implicated in ozone depletion and particle formation. The atmospheric chemistry of iodine is a complex overlay of neutral-neutral, ion-neutral, and photochemical processes, where many of the reactions and intermediates remain poorly characterized. This study targets the visible spectroscopy and photostability of the gas-phase hypoiodite anion (IO - ), the initial product of the I - + O 3 reaction, by mass spectrometry equipped with resonance-enhanced photodissociation and total ion-loss action spectroscopies. It is shown that IO - undergoes photodissociation to I - + O ( 3 P) over 637-459 nm (15700-21800 cm -1 ) because of excitation to the bound first singlet excited state. Electron photodetachment competes with photodissociation above the electron detachment threshold of IO - at 521 nm (19200 cm -1 ) with peaks corresponding to resonant autodetachment involving the singlet excited state and the ground state of neutral IO possibly mediated by a dipole-bound state.

The preponderance and functional importance of isomeric biomolecules have become topical in biochemistry. Therefore, one must distinguish and identify all such forms across compound classes, over a wide dynamic range as minor species often have critical activities. With all the power of modern mass spectrometry for compositional assignments by accurate mass, the identical precursor and often fragment ion masses render this task a steep challenge. This is recognized in proteomics and epigenetics, where proteoforms are disentangled and characterized employing novel separations and non-ergodic dissociation mechanisms. This issue is equally pertinent to lipidomics, where the lack of isomeric depth has thwarted the deciphering of functional networks. Here we introduce a new platform, where the isomeric lipids separated by high-resolution differential ion mobility spectrometry (FAIMS) are identified using ozone-induced dissociation (OzID). Cationization by metals (here K + , Ag + , and especially Cu + ) broadly improves the FAIMS resolution of isomers with alternative C═C double bond (DB) positions or stereochemistry, presumably via metal attaching to the DB and reshaping the ion around it. However, the OzID yield diminishes for Ag + and vanishes for Cu + adducts. Argentination still strikes the best compromise between efficient separation and diagnostic fragmentation for optimal FAIMS/OzID performance.

Prostate cancer is the fourth most common cancer worldwide with definitive diagnosis reliant on biopsy and human-graded histopathology. As with other pathologies, grading based on classical haematoxylin and eosin (H&E) staining of formalin fixed paraffin-embedded material can be prone to variation between pathologists, prompting investigation of biomolecular markers. Comprising around 50% of cellular mass, and with known metabolic variations in cancer, lipids provide a promising target for molecular pathology. Here we apply isomer-resolved lipidomics in combination with imaging mass spectrometry to interrogate tissue sections from radical prostatectomy specimens. Guided by the histopathological assessment of adjacent tissue sections, regions of interest are investigated for molecular signatures associated with lipid metabolism, especially desaturation and elongation pathways. Monitoring one of the most abundant cellular membrane lipids within these tissues, phosphatidylcholine (PC) 34:1, high positive correlation was observed between the n-9 isomer (site of unsaturation 9-carbons from the methyl terminus) and epithelial cells from potential pre-malignant lesions, while the n-7 isomer abundance was observed to correlate with immune cell infiltration and inflammation. The correlation of lipid isomer signatures with human disease states in tissue suggests a future role for isomer-resolved mass spectrometry imaging in assisting pathologists with prostate cancer diagnoses and patient stratification., Competing Interests: S.J.B. holds patents on ozone-induced dissociation technology (A method for the determination of the position of unsaturation in a compound, US8242439 and US7771943). The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Young, Claes, Bowman, Williams, Shepherd, Perren, Poad, Ellis, Heeren, Sadowski and Blanksby.)

Electrospray ionization-mass spectrometry (ESI-MS) of low-charging synthetic polymers typically produces mass spectra exhibiting a bias toward the low-mass region of the polymer mass distribution. To examine the origin(s) of this ionization bias, narrow dispersity polystyrene polymers ( Đ < 1.10) were prepared with ionizable carboxylic acid end-groups at one or both chain termini. The mixture complexity was further reduced through preparative size-exclusion chromatography (SEC), and these well-defined polymers were subjected to negative ion ESI-MS on a high-resolution instrument with a mass-to-charge ( m / z ) range up to 8000. Incorporation of one carboxylic acid end-group facilitated the generation of singly charged [M - H] - ions across the entire range of the mass analyzer. The comparison of mass spectra with size-exclusion chromatograms of the same polymer revealed an ionization bias toward lower masses, which was partially overcome through fractionation, modification of electrospray solvent, and increased declustering potentials. Incorporation of a second ionizable moiety within polymers of equivalent size facilitated multiply charged [M - 2H] 2- ion formation with significantly improved ionization efficiency, spectral coverage of the molar mass distribution, and minimal cluster ion formation. These findings indicate that increased charging of polymers through multiple, well-defined sites of ionization can enhance volatilization and ionization of higher-mass polymers. Generation of higher-molecular-weight polymers in low-charge states-while possible under ideal conditions-competes ineffectively with either nonspecific, multiple-charging of similar sized polymers or ionization of the smaller polymers in the distribution.

Mass spectrometry imaging (MSI) of lipids within tissues has significant potential for both biomolecular discovery and histopathological applications. Conventional MSI technologies are, however, challenged by the prevalence of phospholipid regioisomers that differ only in the location(s) of carbon-carbon double bonds and/or the relative position of fatty acyl attachment to the glycerol backbone ( i.e ., sn position). The inability to resolve isomeric lipids within imaging experiments masks underlying complexity, resulting in a critical loss of metabolic information. Herein, ozone-induced dissociation (OzID) is implemented on a mobility-enabled quadrupole time-of-flight (Q-TOF) mass spectrometer capable of matrix-assisted laser desorption/ionization (MALDI). Exploiting the ion mobility region in the Q-TOF, high number densities of ozone were accessed, leading to ∼1000-fold enhancement in the abundance of OzID product ions compared to earlier MALDI-OzID implementations. Translation of this uplift into imaging resulted in a 50-fold improvement in acquisition rate, facilitating large-area mapping with resolution of phospholipid isomers. Mapping isomer distributions across rat brain sections revealed distinct distributions of lipid isomer populations with region-specific associations of isomers differing in double bond and sn positions. Moreover, product ions arising from sequential ozone- and collision-induced dissociation enabled double bond assignments in unsaturated fatty acyl chains esterified at the noncanonical sn -1 position.

Gas-phase reactions of the o-methylphenyl (o-CH3C6H4) radical with the C3H4 isomers allene (H2C[double bond, length as m-dash]C[double bond, length as m-dash]CH2) and propyne (HC[triple bond, length as m-dash]C-CH3) are studied at 600 K and 4 Torr (533 Pa) using VUV synchrotron photoionisation mass spectrometry, quantum chemical calculations and RRKM modelling. Two major dissociation product ions arise following C3H4 addition: m/z 116 (CH3 loss) and 130 (H loss). These products correspond to small polycyclic aromatic hydrocarbons (PAHs). The m/z 116 signal for both reactions is conclusively assigned to indene (C9H8) and is the dominant product for the propyne reaction. Signal at m/z 130 for the propyne case is attributed to isomers of bicyclic methylindene (C10H10) + H, which contains a newly-formed methylated five-membered ring. The m/z 130 signal for allene, however, is dominated by the 1,2-dihydronaphthalene isomer arising from a newly created six-membered ring. Our results show that new ring formation from C3H4 addition to the methylphenyl radical requires an ortho-CH3 group - similar to o-methylphenyl radical oxidation. These reactions characteristically lead to bicyclic aromatic products, but the structure of the C3H4 co-reactant dictates the structure of the PAH product, with allene preferentially leading to the formation of two six-membered ring bicyclics and propyne resulting in the formation of six and five-membered bicyclic structures.

The nitrile imine-mediated tetrazole-ene cycloaddition is a widely used class of photoligation. Optimizing the reaction outcome requires detailed knowledge of the tetrazole photoactivation profile, which can only partially be ascertained from absorption spectroscopy, or otherwise involves laborious reaction monitoring in solution. Photodissociation action spectroscopy (PDAS) combines the advantages of optical spectroscopy and mass spectrometry in that only absorption events resulting in a mass change are recorded, thus revealing the desired wavelength dependence of product formation. Moreover, the sensitivity and selectivity afforded by the mass spectrometer enable reliable assessment of the photodissociation profile even on small amounts of crude material, thus accelerating the design and synthesis of next-generation substrates. Using this workflow, we demonstrate that the photodissociation onset for nitrile imine formation is red-shifted by ca. 50 nm with a novel N -ethylcarbazole derivative relative to a phenyl-substituted archetype. Benchmarked against solution-phase tunable laser experiments and supported by quantum chemical calculations, these discoveries demonstrate that PDAS is a powerful tool for rapidly screening the efficacy of new substrates in the quest toward efficient visible light-triggered ligation for biological applications.

Canonical fatty acid metabolism describes specific enzyme-substrate interactions that result in products with well-defined chain lengths, degree(s), and positions of unsaturation. Deep profiling of lipids across a range of prostate cancer cell lines reveals a variety of fatty acids with unusual site(s) of unsaturation that are not described by canonical pathways. The structure and abundance of these unusual lipids correlate with changes in desaturase expression and are strong indicators of cellular phenotype. Gene silencing and stable isotope tracing demonstrate that direct Δ6 and Δ8 desaturation of 14:0 (myristic), 16:0 (palmitic), and 18:0 (stearic) acids by FADS2 generate new families of unsaturated fatty acids (including n-8, n-10, and n-12) that have rarely-if ever-been reported in human-derived cells. Isomer-resolved lipidomics reveals the selective incorporation of these unusual fatty acids into complex structural lipids and identifies their presence in cancer tissues, indicating functional roles in membrane structure and signaling., Competing Interests: Declaration of interests MALDI-MSI OzID technology was developed through an industry linkage supported by Waters Corporation and the Australian Research Council (LP180100238). S.J.B. holds patents on ozone-induced dissociation technology (“A method for the determination of the position of unsaturation in a compound,” US8242439 and US7771943)., (Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.)

Phosphatidylethanol (PEth) in human blood samples is a marker for alcohol usage. Typically, PEth is detected by reversed-phase liquid chromatography coupled with negative ion tandem mass spectrometry, investigating the fatty acyl anions released from the precursor ion upon collision-induced dissociation (CID). It has been established that in other classes of asymmetric glycerophospholipids, the unimolecular fragmentation upon CID is biased depending on the relative position (known as sn-position) of each fatty acyl chain on the glycerol backbone. As such, the use of product ions in selected-reaction-monitoring (SRM) transitions could be prone to variability if more than one regioisomer is present in either the reference materials or the sample. Here, we have investigated the regioisomeric purity of three reference materials supplied by different vendors, labeled as PEth 16:0/18:1. Using CID coupled with ozone-induced dissociation, the regioisomeric purity (% 16:0 at sn-1) was determined to be 76, 80 and 99%. The parallel investigation of the negative ion CID mass spectra of standards revealed differences in product ion ratios for both fatty acyl chain product ions and ketene neutral loss product ions. Furthermore, investigation of the product ion abundances in CID spectra of PEth within authentic blood samples appears to indicate a limited natural variation in isomer populations between samples, with the cannonical, PEth 16:0/18:1 (16:0 at sn-1) predominant in all cases. Different reference material isomer distributions led to variation in fully automated quantification of PEth in 56 authentic dried blood spot (DBS) samples when a single quantifier ion was used. Our results suggest caution in ensuring that the regioisomeric compositions of reference materials are well-matched with those of the authentic blood samples., (© The Author(s) 2020. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Cardiolipins (CLs) are comprised of two phosphatic acid moieties bound to a central glycerol backbone and are substituted with four acyl chains. Consequently, a vast number of distinct CL structures are possible in different biological contexts, representing a significant analytical challenge. Electrospray ionization tandem mass spectrometry (ESI-MS/MS) has become a widely used approach for the detection, characterization, and quantitation of complex lipids, including CLs. Central to this approach is fragmentation of the [CLs - H] - or [CL - 2H] 2- anions by collision-induced dissociation (CID). Product ions in the resulting tandem mass spectra confirm the CL subclass assignment and reveal the numbers of carbons and degrees of unsaturation in each of the acyl chains. Conventional CID, however, affords limited structural elucidation of the fatty acyl chains, failing to discriminate isomers arising from different site(s) of unsaturation or cyclopropanation and potentially obscuring their metabolic origins. Here, we report the application of charge inversion ion/ion chemistry in the gas phase to enhance the structural elucidation of CLs. Briefly, CID of [CL - H] 2- anions generated via negative ion ESI allowed for the assignment of individual fatty acyl substituents and phosphatidic acid moieties. Next, gas-phase derivatization of the resulting CL product ions, including fatty acyl carboxylate anions, was effected with gas-phase ion/ion charge inversion reactions with tris-phenanthroline magnesium reagent dications. Subsequent isolation and activation of the charge-inverted fatty acyl complex cations permitted the localization of both carbon-carbon double bond and cyclopropane motifs within each of the four acyl chains of CLs. This approach was applied to the de novo elucidation of unknown CLs in a biological extract revealing distinct isomeric populations and regiochemical relationships between double bonds and carbocyles.

A key step in gas-phase polycyclic aromatic hydrocarbon (PAH) formation involves the addition of acetylene (or other alkyne) to σ-type aromatic radicals, with successive additions yielding more complex PAHs. A similar process can happen for N-containing aromatics. In cold diffuse environments, such as the interstellar medium, rates of radical addition may be enhanced when the σ-type radical is charged. This paper investigates the gas-phase ion-molecule reactions of acetylene with nine aromatic distonic σ-type radical cations derived from pyridinium (Pyr), anilinium (Anl), and benzonitrilium (Bzn) ions. Three isomers are studied in each case (radical sites at the ortho , meta , and para positions). Using a room temperature ion trap, second-order rate coefficients, product branching ratios, and reaction efficiencies are measured. The rate coefficients increase from para to ortho positions. The second-order rate coefficients can be sorted into three groups: low, between 1 and 3 × 10 -12 cm 3 molecule -1 s -1 (3Anl and 4Anl); intermediate, between 5 and 15 × 10 -12 cm 3 molecule -1 s -1 (2Bzn, 3Bzn, and 4Bzn); and high, between 8 and 31 × 10 -11 cm 3 molecule -1 s -1 (2Anl, 2Pyr, 3Pyr, and 4Pyr); and 2Anl is the only radical cation with a rate coefficient distinctly different from its isomers. Quantum chemical calculations, using M06-2X-D3(0)/6-31++G(2df,p) geometries and DSD-PBEP86-NL/aug-cc-pVQZ energies, are deployed to rationalize reactivity trends based on the stability of prereactive complexes. The G3X-K method guides the assignment of product ions following adduct formation. The rate coefficient trend can be rationalized by a simple model based on the prereactive complex forward barrier height.

Ultraviolet-photodissociation (UVPD) mass spectrometry is an emerging analytical tool for structural elucidation of biomolecules including lipids. Gas phase UVPD of ionised fatty acids (FAs) can promote fragmentation that is diagnostic for molecular structure including the regiochemistry of carbon-carbon double bonds and methyl branching position(s). Typically, however, lipids exhibit poor conversion to photoproducts under UVPD and thus require longer integration times to achieve the signal-to-noise required for structural assignments. Consequently, the integration of UVPD into liquid-chromatography mass spectrometry (LC-MS) workflows for FAs has been limited. To enhance photofragmentation efficiency, an alternative strategy has been devised using wet-chemical derivatization of FAs to explicitly incorporate photolabile groups. FA derivatives that include an aryl-iodide motif have photodissociation conversions of up to 28% when activated by a single 266 nm photon. The radical-directed dissociation product ions resulting from UVPD of these derivatives provide key details of molecular structure and discriminate between lipid isomers. Herein, we describe the structure-activity guided development of new FA derivatives capable of photoproduct yields of up to 97%. UVPD-action spectroscopy demonstrates that photodissociation for FAs derivatized with N-(2-aminoethyl)-4-iodobenzamide (NIBA) is maximised near 266 nm and highlights the key role of the 4-iodobenzamide motif in the efficient formation of [M - I]˙+ radical cations (and diagnostic secondary product ions). The high photodissociation yield of NIBA-derivatized lipids is maintained across 37 commonly observed FAs with the resulting UVPD mass spectra shown to be effective in the discrimination of isomeric FAs that differ in the position(s) of carbon-carbon double bonds. Integration of this strategy with reversed-phase LC-MS workflows is confirmed with high-quality UVPD mass spectra acquired across each chromatographic peak.

We present the first application of electrochemical reduction in an ion trap mass spectrometer as a dual-function tool to synthesise and probe the reactivity of interlocked viologen-based complexes. Compared with non-complexed archetypes, electron-donating macrocyclic porphyrin ethers retard electron transfer reaction rates and stabilise intact structures in low oxidation states.

Heightened awareness regarding the implication of disturbances in lipid metabolism with respect to prevalent human-related pathologies demands analytical techniques that provide unambiguous structural characterization and accurate quantitation of lipids in complex biological samples. The diversity in molecular structures of lipids along with their wide range of concentrations in biological matrices present formidable analytical challenges. Modern mass spectrometry (MS) offers an unprecedented level of analytical power in lipid analysis, as many advancements in the field of lipidomics have been facilitated through novel applications of and developments in electrospray ionization tandem mass spectrometry (ESI-MS/MS). ESI allows for the formation of intact lipid ions with little to no fragmentation and has become widely used in contemporary lipidomics experiments due to its sensitivity, reproducibility, and compatibility with condensed-phase modes of separation, such as liquid chromatography (LC). Owing to variations in lipid functional groups, ESI enables partial chemical separation of the lipidome, yet the preferred ion-type is not always formed, impacting lipid detection, characterization, and quantitation. Moreover, conventional ESI-MS/MS approaches often fail to expose diverse subtle structural features like the sites of unsaturation in fatty acyl constituents or acyl chain regiochemistry along the glycerol backbone, representing a significant challenge for ESI-MS/MS. To overcome these shortcomings, various charge manipulation strategies, including charge-switching, have been developed to transform ion-type and charge state, with aims of increasing sensitivity and selectivity of ESI-MS/MS approaches. Importantly, charge manipulation approaches afford enhanced ionization efficiency, improved mixture analysis performance, and access to informative fragmentation channels. Herein, we present a critical review of the current suite of solution-based and gas-phase strategies for the manipulation of lipid ion charge and type relevant to ESI-MS/MS., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Branched fatty acid esters of hydroxy fatty acids (FAHFAs) are a recently discovered class of endogenous bioactive lipids with anti-diabetic and anti-inflammatory effects. Identification of FAHFAs is challenging due to both the relatively low abundance of these metabolites in most biological samples and the significant structural diversity arising from the co-occurrence of numerous regioisomers. Ultimately, development of sensitive analytical techniques that enable rapid and unambiguous identification of FAHFAs is integral to understanding their diverse physiological functions in health and disease. While a battery of mass spectrometry (MS) based methods for complex lipid analysis has been developed, FAHFA identification presents specific challenges to conventional approaches. Notably, while the MS 2 product ion spectra of [FAHFA - H]¯ anions afford the assignment of fatty acid (FA) and hydroxy fatty acid (HFA) constituents, FAHFA regioisomers are usually indistinguishable by this approach. Here, we report the development of a novel MS-based technique employing charge inversion ion/ion reactions with tris-phenanthroline magnesium complex dications, Mg(Phen) 3 2+ , to selectively and efficiently derivatize [FAHFA - H]¯ anions in the gas phase, yielding fixed-charge cations. Subsequent activation of [FAHFA - H + MgPhen 2 ] + cations yield product ions that facilitate the assignment of FA and HFA constituents, pinpoints unsaturation sites within the FA moiety, and elucidates ester linkage regiochemistry. Collectively, the presented approach represents a rapid, entirely gas-phase method for near-complete FAHFA structural elucidation and confident isomer discrimination without the requirement for authentic FAHFA standards., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)