Analysis of unsaturated lipids by ozone-induced dissociation

Structural characterization of sphingomyelins from tissue using electron-induced dissociation

RATIONALE

Sphingomyelins (SMs) and resulting metabolic products serve important functional and cell signaling roles and can act as potential biomarkers and therapeutic targets in many pathological disorders. SMs each contain a sphingoid base, an amide-linked fatty acyl chain, and a phosphocholine headgroup. Despite these simple building blocks, variations and modifications of both the sphingoid base and the fatty acyl chain result in a diverse array of structurally complicated SM compounds. Conventional tandem mass spectrometry (MS/MS) using the collision-induced dissociation (CID) method only provides limited structural information, necessitating other tools to unravel the structural complexity of these lipids.

METHODS

We utilize electron-induced dissociation (EID) and sequential CID/EID approaches to elucidate detailed structural features of SMs. Integrating the CID/EID method into an imaging MS workflow enables accurate identification of SMs directly from kidney tissue.

RESULTS

The application of EID enables identification of SMs at the molecular species level, identifying the sphingosine base and the amide-linked fatty acyl chains. Furthermore, removal of the phosphocholine headgroup via CID followed by sequential EID in an MS3 analysis (CID/EID) enhances the structural information obtained. CID/EID provides diagnostic fragmentation patterns revealing the hydroxylation site and double bond position in both the sphingosine base and amide-linked fatty acyl chains.

CONCLUSIONS

Detailed structural information of SMs from synthetic standards and biological tissue samples is obtained using an alternative electron-based dissociation method. Accurate characterization of SMs promises to better inform studies of tissue biochemistry, lipid metabolism, and molecular pathology.

High-resolution ion mobility based on traveling wave structures for lossless ion manipulation resolves hidden lipid features

High-resolution ion mobility (resolving power > 200) coupled with mass spectrometry (MS) is a powerful analytical tool for resolving isobars and isomers in complex samples. High-resolution ion mobility is capable of discerning additional structurally distinct features, which are not observed with conventional resolving power ion mobility (IM, resolving power ~ 50) techniques such as traveling wave IM and drift tube ion mobility (DTIM). DTIM in particular is considered to be the "gold standard" IM technique since collision cross section (CCS) values are directly obtained through a first-principles relationship, whereas traveling wave IM techniques require an additional calibration strategy to determine accurate CCS values. In this study, we aim to evaluate the separation capabilities of a traveling wave ion mobility structures for lossless ion manipulation platform integrated with mass spectrometry analysis (SLIM IM-MS) for both lipid isomer standards and complex lipid samples. A cross-platform investigation of seven subclass-specific lipid extracts examined by both DTIM-MS and SLIM IM-MS showed additional features were observed for all lipid extracts when examined under high resolving power IM conditions, with the number of CCS-aligned features that resolve into additional peaks from DTIM-MS to SLIM IM-MS analysis varying between 5 and 50%, depending on the specific lipid sub-class investigated. Lipid CCS values are obtained from SLIM IM (TW(SLIM)CCS) through a two-step calibration procedure to align these measurements to within 2% average bias to reference values obtained via DTIM (DTCCS). A total of 225 lipid features from seven lipid extracts are subsequently identified in the high resolving power IM analysis by a combination of accurate mass-to-charge, CCS, retention time, and linear mobility-mass correlations to curate a high-resolution IM lipid structural atlas. These results emphasize the high isomeric complexity present in lipidomic samples and underscore the need for multiple analytical stages of separation operated at high resolution.

Triboelectric Nanogenerators for the Masses: A Low-Cost Do-It-Yourself Pulsed Ion Source for Sample-Limited Applications

Triboelectric nanogenerators (TENG) are useful devices for converting mechanical motion into electric current using readily available materials. Though the applications for these devices span across many fields, TENG can be leveraged for mass spectrometry (MS) as inexpensive and effective power supplies for pulsed nanoelectrospray ionization (nESI). The inherently discontinuous spray provided by TENG is particularly useful in scenarios where high sample economy is imperative, as in the case of ultraprecious samples. Previous work has shown the utility of TENG MS as a highly sensitive technique capable of yielding quality spectra from only a few microliters of sample at low micromolar concentrations. As the field of miniaturized, fieldable mass spectrometers grows, it remains critical to develop advanced ion sources with similarly small power requirements and footprints. Here, we present a redesigned TENG ion source with a sub-1000 USD material cost, lower power consumption, reduced footprint, and improved capabilities. We validate the performance of this new device for a diverse set of applications, including lipid double bond localization and native protein analysis.

LipiDex 2 Integrates MSn Tree-Based Fragmentation Methods and Quality Control Modules to Improve Discovery Lipidomics

As lipidomics experiments increase in scale and complexity, data processing tools must support workflows for new liquid chromatography-mass spectrometry (LC-MS) methods while simultaneously supporting quality controls to maximize the confidence in lipid identifications. LipiDex 2 improves lipidomics data processing algorithms from LipiDex 1 and introduces new tools for spectral matching and peak annotation functions, with improvements in speed and user-friendliness. In silico spectral library generation now supports tandem mass spectral (MSn) tree-based fragmentation methods, and the LipiDex 2 workflow fully integrates the fragmentation logic into the data processing steps to enable lipid identification at the appropriate level of structural resolution. Finally, LipiDex 2 features new modules for automated quality control checks that also allow users to visualize data quality in a data dashboard user interface.

Challenges and perspectives for naming lipids in the context of lipidomics

INTRODUCTION

Lipids are key compounds in the study of metabolism and are increasingly studied in biology projects. It is a very broad family that encompasses many compounds, and the name of the same compound may vary depending on the community where they are studied.

OBJECTIVES

In addition, their structures are varied and complex, which complicates their analysis. Indeed, the structural resolution does not always allow a complete level of annotation so the actual compound analysed will vary from study to study and should be clearly stated. For all these reasons the identification and naming of lipids is complicated and very variable from one study to another, it needs to be harmonized.

METHODS & RESULTS

In this position paper we will present and discuss the different way to name lipids (with chemoinformatic and semantic identifiers) and their importance to share lipidomic results.

CONCLUSION

Homogenising this identification and adopting the same rules is essential to be able to share data within the community and to map data on functional networks.

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

Multiple biological activities and biosynthesis mechanisms of specific conjugated linoleic acid isomers and analytical methods for prospective application

Conjugated linoleic acid (CLA) is a collective term for the octadecadienoic acid isomers containing conjugated double bonds. This article reviewed CLA isomers from biological activities, biosynthesis mechanisms and analytical methods. The biological activities of CLA isomers in anti-obesity, cardiovascular protection, diabetes management and anti-cancer in vitro and in vivo were mainly reviewed. More attention has been paid to the production of the specific CLA isomer due to its biological activity. The biosynthesis methods of CLA isomers, such as dietary modification in ruminants and fermentation by microorganisms & enzymes, were systematically introduced. A rapid, accurate and economic analysis method will promote the research in both biological activities and biosynthesis mechanisms of CLA isomers. The merits of UV spectrometry, GC, HPLC, MS and CE used in the analysis of CLA isomers were also compared in detail. This paper aims to put into perspective the current status and future trends on CLA isomers.

Visible-Light Paternò-Büchi Reaction for Lipidomic Profiling at Detailed Structure Levels

The Paternò-Büchi (PB) derivatization of carbon-carbon double bond (C═C) has been increasingly employed with tandem mass spectrometry to analyze unsaturated lipids. It enables the discovery of altered or uncanonical lipid desaturation metabolism, which would be otherwise undetected by conventional methods. Although highly useful, the reported PB reactions only provide moderate yield (∼30%). Herein, we aim to determine the key factors that affect the PB reactions and develop a system with improved capabilities for lipidomic analysis. An Ir(III) photocatalyst is chosen as the triplet energy donor for the PB reagent under 405 nm light irradiation, while phenylglyoxalate and its charge-tagging version, pyridylglyoxalate, are developed as the most efficient PB reagents. The above visible-light PB reaction system provides higher PB conversions than all previously reported PB reactions. Around 90% conversion can be achieved at high concentrations (>0.5 mM) for different classes of lipids but drops as the lipid concentration decreases. The visible-light PB reaction has then been integrated with shotgun and liquid chromatography-based workflows. The limits of detection for locating C═C in standard lipids of glycerophospholipids (GPLs) and triacylglycerides (TGs) are in the sub-nM to nM range. More than 600 distinct GPLs and TGs have been profiled at the C═C location level or the sn-position level from the total lipid extract of bovine liver, demonstrating that the developed method is capable of large-scale lipidomic analysis.

Recent progress in the analysis of unsaturated fatty acids in biological samples by chemical derivatization-based chromatography-mass spectrometry methods

Unsaturated fatty acids (UFAs) are essential fatty acids that execute various biological functions in the human body. Therefore, the qualitative and quantitative analysis of UFAs in biological samples can help to clarify their roles in the occurrence and development of diseases, so to reveal the mechanisms of pathogenesis and potential drug intervention strategies. Chromatography-mass spectrometry is one of the most commonly used techniques for the analysis of UFAs in biological samples. However, due to factors such as the complex structural information of UFAs (the number and specific location of CC double bonds) and the low concentration of UFAs in biological samples, it is still difficult to conduct accurate qualitative and/or quantitative studies of UFAs in complex biological samples. In recent years, the integration and application of chemical derivatization and chromatography-mass spectrometry has been widely used in the detection of UFAs. Based on this overview, we reviewed recent developments and application progress for chemical derivatization-based chromatography-mass spectrometry methods for the qualitative and/or quantitative analysis of UFAs in biological samples over the past ten years. Potential trends for the design and improvement of novel derivatization reagents were proposed.

Recent Advances in Gas-phase Ion/Ion Chemistry for Lipid Analysis

Gas-phase ion/ion reactions can be used to alter analyte ion-types for subsequent dissociation both quickly and efficiently without the need for altering analyte ionization conditions. This capability can be particularly useful when the ion-type that is most efficiently generated by the ionization method at hand does not provide the structural information of interest using available dissociation methods. This situation often arises in the analysis of lipids, which constitute a diverse array of chemical species with many possibilities for isomers. Gas-phase ion/ion reactions have been demonstrated to be capable of enhancing the ability of tandem mass spectrometry to characterize the structures of various lipid classes. This review summarizes progress to date in the application of gas-phase ion/ion reactions to lipid structural characterization.

Characterization of glycerophospholipids at multiple isomer levels via Mn(II)-catalyzed epoxidation

Characterization of glycerophospholipid isomers is of significant importance as they play different roles in physiological and pathological processes. In this work, we present a novel and bifunctional derivatization method utilizing Mn(II)-catalyzed epoxidation to simultaneously identify carbon-carbon double bond (CC bond)- and stereonumbering (sn)-positional isomers of phosphatidylcholine. Mn(II) coordinates with picolinic acid and catalyzes epoxidation of unsaturated lipids by peracetic acid. Collision-induced dissociation (CID) of the epoxides generates diagnostic ions that can be used to locate CC bond positions. Meanwhile, CID of Mn(II) ion-lipid complexes produces characteristic ions for determination of sn positions. This bifunctional derivatization takes place in seconds, and the diagnostic ions produced in CID are clear and easy to interpret. Moreover, relative quantification of CC bond-and sn-positional isomers was achieved. The capability of this method in identifying lipids at multiple isomer levels was shown using lipid standards and lipid extracts from complex biological samples.

Development of a Miniature Mass Spectrometry System for Point-of-Care Analysis of Lipid Isomers Based on Ozone-Induced Dissociation

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.

Effect of Ozone Treatment Intensity on Pasting Property, Protein Composition, and Steamed Bread Quality of Ozone-Treated Wheat Flour

Wheat flours were treated with ozone gas at low- and high-intensity conditions (0.61 and 3.82 g/h) for different durations (short: 5 min; long: 30 min), and the ozone-treated flours were evaluated in quality properties, including pH, protein component, water molecular mobility of dough, pasting property, and steamed bread quality. At both conditions, ozone treatment decreased the pH of wheat flour. Long duration of high-intensity treatment aroused significant increase in insoluble polymeric protein (IPP) content of wheat flour, but other treatments did not significantly change the IPP content. Dough of ozone-treated flour had higher water molecular mobility than that of native flour. Short duration of low-intensity treatment did not significantly change most pasting viscosity parameters of wheat flour, but other treatments increased the peak viscosity, breakdown viscosity, and setback viscosity. Steamed bread of ozone-treated flour had lower specific volume and pore uniformity than that of native flour. Long duration of high-intensity treatment of flour increased the hardness and chewiness of the steamed bread product, but other treatment showed opposite effect. Among the four ozone treatments, long duration of high-intensity treatment aroused the greatest change in pH, IPP, water molecular mobility of dough, and the quality of steamed bread, while short duration of low-intensity treatment had the minimum effect. Long duration of low-intensity treatment was close to the short duration of high-intensity treatment in quality attributes of wheat flour and the total ozone yield. These results suggested that the quality of wheat flour gradually changed with the increase of total ozone yield, and overozonization would greatly deteriorate the quality of wheat flour.

Utilizing Skyline to analyze lipidomics data containing liquid chromatography, ion mobility spectrometry and mass spectrometry dimensions

Lipidomics studies suffer from analytical and annotation challenges because of the great structural similarity of many of the lipid species. To improve lipid characterization and annotation capabilities beyond those afforded by traditional mass spectrometry (MS)-based methods, multidimensional separation methods such as those integrating liquid chromatography, ion mobility spectrometry, collision-induced dissociation and MS (LC-IMS-CID-MS) may be used. Although LC-IMS-CID-MS and other multidimensional methods offer valuable hydrophobicity, structural and mass information, the files are also complex and difficult to assess. Thus, the development of software tools to rapidly process and facilitate confident lipid annotations is essential. In this Protocol Extension, we use the freely available, vendor-neutral and open-source software Skyline to process and annotate multidimensional lipidomic data. Although Skyline ( https://skyline.ms/skyline.url ) was established for targeted processing of LC-MS-based proteomics data, it has since been extended such that it can be used to analyze small-molecule data as well as data containing the IMS dimension. This protocol uses Skyline's recently expanded capabilities, including small-molecule spectral libraries, indexed retention time and ion mobility filtering, and provides a step-by-step description for importing data, predicting retention times, validating lipid annotations, exporting results and editing our manually validated 500+ lipid library. Although the time required to complete the steps outlined here varies on the basis of multiple factors such as dataset size and familiarity with Skyline, this protocol takes ~5.5 h to complete when annotations are rigorously verified for maximum confidence.

Positional Assignment of C-C Double Bonds in Fatty Acyl Chains of Intact Arsenosugar Phospholipids Occurring in Seaweed Extracts by Epoxidation Reactions

Water-soluble diacyl arsenosugar phospholipids (As-PL) are natural products widespread in marine animals and algae, including the brown alga Undaria pinnatifida, also known as wakame. The systematic recognition of As-PL has been hampered by the lack of standard and of qualitative methods to establish the carbon-carbon double bond positions of unsaturated fatty acyl chains. Here, the epoxidation reaction of fatty acyl substituents of As-PL was carried out with high selectivity by meta-chloroperoxybenzoic acid and the C-C double bond localization was established by collision-induced dissociation of epoxidized species as deprotonated molecules, [epoM - H]-. Reversed-phase liquid chromatography (RPLC) separation and a sequential triple-stage MS (i.e., MS3) analysis of unsaturated and epoxidized As-PL were very helpful to characterize the carbon-carbon double bond locations of both sn-1 and sn-2 fatty acyl chains, starting from a diagnostic product ion pair with 16.0 Da mass difference. These results indicate that intact As-PL can be annotated in terms of fatty acyl chain composition and in terms of their C-C double bond position(s). Interestingly, hexadecenoic (16:1 Δ9) and octadecenoic (18:1 Δ9) along with octadecadienoic (18:2 Δ9,12) and octadecatrienoic (18:3 Δ9,12,15) were found to be the most abundant unsaturated fatty acyl chains of As-PL in the brown alga wakame, thus confirming it as a good source of essential fatty acids with a balanced ω6/ω3 ratio. Although the toxicity of As-including metabolites of algal As-PL is still a matter of debate and needs to be studied in more detail, the described approach can be exploited to assess if As-PL could contribute to the supply of essential fatty acids related to the use of algae as nutritious food.

Deep-lipidotyping by mass spectrometry: recent technical advances and applications

In-depth structural characterization of lipids is an essential component of lipidomics. There has been a rapid expansion of mass spectrometry methods that are capable of resolving lipid isomers at various structural levels over the past decade. These developments finally make deep-lipidotyping possible, which provides new means to study lipid metabolism and discover new lipid biomarkers. In this review, we discuss recent advancements in tandem mass spectrometry (MS/MS) methods for identification of complex lipids beyond the species (known headgroup information) and molecular species (known chain composition) levels. These include identification at the levels of carbon-carbon double bond (C=C) location and sn-position as well as characterization of acyl chain modifications. We also discuss the integration of isomer-resolving MS/MS methods with different lipid analysis workflows and their applications in lipidomics. The results showcase the distinct capabilities of deep-lipidotyping in untangling the metabolism of individual isomers and sensitive phenotyping by using relative fractional quantitation of the isomers.

Enhanced Ion Mobility Separation and Characterization of Isomeric Phosphatidylcholines Using Absorption Mode Fourier Transform Multiplexing and Ultraviolet Photodissociation Mass Spectrometry

The structural diversity of phospholipids plays a critical role in cellular membrane dynamics, energy storage, and cellular signaling. Despite its importance, the extent of this diversity has only recently come into focus, largely owing to advances in separation science and mass spectrometry methodology and instrumentation. Characterization of glycerophospholipid (GP) isomers differing only in their acyl chain configurations and locations of carbon-carbon double bonds (C═C) remains challenging due to the need for both effective separation of isomers and advanced tandem mass spectrometry (MS/MS) technologies capable of double-bond localization. Drift tube ion mobility spectrometry (DTIMS) coupled with MS can provide both fast separation and accurate determination of collision cross section (CCS) of molecules but typically lacks the resolving power needed to separate phospholipid isomers. Ultraviolet photodissociation (UVPD) can provide unambiguous double-bond localization but is challenging to implement on the timescales of modern commercial drift tube time-of-flight mass spectrometers. Here, we present a novel method for coupling DTIMS with a UVPD-enabled Orbitrap mass spectrometer using absorption mode Fourier transform multiplexing that affords simultaneous localization of double bonds and accurate CCS measurements even when isomers cannot be fully resolved in the mobility dimension. This method is demonstrated on two- and three-component mixtures and shown to provide CCS measurements that differ from those obtained by individual analysis of each component by less than 1%.

Development and Application of Multidimensional Lipid Libraries to Investigate Lipidomic Dysregulation Related to Smoke Inhalation Injury Severity

The implication of lipid dysregulation in diseases, toxic exposure outcomes, and inflammation has brought great interest to lipidomic studies. However, lipids have proven to be analytically challenging due to their highly isomeric nature and vast concentration ranges in biological matrices. Therefore, multidimensional techniques such as those integrating liquid chromatography, ion mobility spectrometry, collision-induced dissociation, and mass spectrometry (LC-IMS-CID-MS) have been implemented to separate lipid isomers as well as provide structural information and increased identification confidence. These data sets are however extremely large and complex, resulting in challenges for data processing and annotation. Here, we have overcome these challenges by developing sample-specific multidimensional lipid libraries using the freely available software Skyline. Specifically, the human plasma library developed for this work contains over 500 unique lipids and is combined with adapted Skyline functions such as indexed retention time (iRT) for retention time prediction and IMS drift time filtering for enhanced selectivity. For comparison with other studies, this database was used to annotate LC-IMS-CID-MS data from a NIST SRM 1950 extract. The same workflow was then utilized to assess plasma and bronchoalveolar lavage fluid (BALF) samples from patients with varying degrees of smoke inhalation injury to identify lipid-based patient prognostic and diagnostic markers.

Unravel the Local Complexity of Biological Environments by MALDI Mass Spectrometry Imaging

Classic metabolomic methods have proven to be very useful to study functional biology and variation in the chemical composition of different tissues. However, they do not provide any information in terms of spatial localization within fine structures. Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI MSI) does and reaches at best a spatial resolution of 0.25 μm depending on the laser setup, making it a very powerful tool to analyze the local complexity of biological samples at the cellular level. Here, we intend to give an overview of the diversity of the molecules and localizations analyzed using this method as well as to update on the latest adaptations made to circumvent the complexity of samples. MALDI MSI has been widely used in medical sciences and is now developing in research areas as diverse as entomology, microbiology, plant biology, and plant–microbe interactions, the rhizobia symbiosis being the most exhaustively described so far. Those are the fields of interest on which we will focus to demonstrate MALDI MSI strengths in characterizing the spatial distributions of metabolites, lipids, and peptides in relation to biological questions.

Location of carbon-carbon double bonds in unsaturated lipids using microdroplet mass spectrometry

An aqueous solution containing unsaturated fatty acids (100 μM) or lipids (50 μg mL-1) and chloroauric acid (HAuCl4, 10 μM) is electrosprayed (-4.5 kV for unsaturated fatty acids and +4.0 kV for lipids) from a 50 μm diameter capillary with N2 nebulizing gas (60 psi), and the resulting microdroplets enter a mass spectrometer with a flight distance of 10 mm for chemical analysis. The HAuCl4 oxidizes the C[double bond, length as m-dash]C double bond to cause the formation of an aldehyde group or a hydroxyl group on one side and a carboxyl group on the other (i.e., CHO-R-COOH or HO-R-COOH), allowing the location of the double bond to be identified. This approach was successfully applied to four unsaturated fatty acids [linoleic acid (LA), ricinoleic acid (RA), isooleic acid (IA), and nervonic acid (NA)] and two phospholipids [1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and L-α-lysophosphatidylcholine (lysoPC)]. A mechanism for this transformation is proposed, which involves epoxidation of the double bond, followed by the formation of the final products. This method has the advantages of being simple and rapid, and requiring a small amount of analyte.

HIGH RESOLUTION MASS SPECTROMETRY IN LIPIDOMICS

The boost of research output in lipidomics during the last decade is tightly linked to improved instrumentation in mass spectrometry. Associated with this trend is the shift from low resolution-toward high-resolution lipidomics platforms. This review article summarizes the state of the art in the lipidomics field with a particular focus on the merits of high mass resolution. Following some theoretical considerations on the benefits of high mass resolution in lipidomics, it starts with a historical perspective on lipid analysis by sector instruments and moves further to today's instrumental approaches, including shotgun lipidomics, liquid chromatography-mass spectrometry, matrix-assisted laser desorption ionization-time-of-flight, and imaging lipidomics. Subsequently, several data processing and data analysis software packages are critically evaluated with all their pros and cons. Finally, this article emphasizes the importance and necessity of quality standards as the field evolves from its pioneering phase into a mature and robust omics technology and lists various initiatives for improving the applicability of lipidomics. © 2020 The Authors. Mass Spectrometry Reviews published by John Wiley & Sons Ltd. Mass Spec Rev.

Perspective on Emerging Mass Spectrometry Technologies for Comprehensive Lipid Structural Elucidation

Lipids and metabolites are of interest in many clinical and research settings because it is the metabolome that is increasingly recognized as a more dynamic and sensitive molecular measure of phenotype. The enormous diversity of lipid structures and the importance of biological structure-function relationships in a wide variety of applications makes accurate identification a challenging yet crucial area of research in the lipid community. Indeed, subtle differences in the chemical structures of lipids can have important implications in cellular metabolism and many disease pathologies. The speed, sensitivity, and molecular specificity afforded by modern mass spectrometry has led to its widespread adoption in the field of lipidomics on many different instrument platforms and experimental workflows. However, unambiguous and complete structural identification of lipids by mass spectrometry remains challenging. Increasingly sophisticated tandem mass spectrometry (MS/MS) approaches are now being developed and seamlessly integrated into lipidomics workflows to meet this challenge. These approaches generally either (i) alter the type of ion that is interrogated or (ii) alter the dissociation method in order to improve the structural information obtained from the MS/MS experiment. In this Perspective, we highlight recent advances in both ion type alteration and ion dissociation methods for lipid identification by mass spectrometry. This discussion is aimed to engage investigators involved in fundamental ion chemistry and technology developments as well as practitioners of lipidomics and its many applications. The rapid rate of technology development in recent years has accelerated and strengthened the ties between these two research communities. We identify the common characteristics and practical figures of merit of these emerging approaches and discuss ways these may catalyze future directions of lipid structural elucidation research.

Imaging and Analysis of Isomeric Unsaturated Lipids through Online Photochemical Derivatization of Carbon-Carbon Double Bonds*

Unraveling the complexity of the lipidome requires the development of novel approaches for the structural characterization of lipid species with isomer-level discrimination. Herein, we introduce an online photochemical approach for lipid isomer identification through selective derivatization of double bonds by reaction with singlet oxygen. Lipid hydroperoxide products are generated promptly after laser irradiation. Fragmentation of these species in a mass spectrometer produces diagnostic fragments revealing the C=C locations in the unreacted lipids. This approach uses an inexpensive light source and photosensitizer making it easy to incorporate into any lipidomics workflow. We demonstrate the utility of this approach for the shotgun profiling of C=C locations in different lipid classes present in tissue extracts using electrospray ionization (ESI) and ambient imaging of lipid species differing only by the location of C=C bonds using nanospray desorption electrospray ionization (nano-DESI).

Localization of Carbon-Carbon Double Bond and Cyclopropane Sites in Cardiolipins via Gas-Phase Charge Inversion Reactions

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.

Shotgun lipidomics and mass spectrometry imaging unveil diversity and dynamics in Gammarus fossarum lipid composition

Sentinel species are playing an indispensable role in monitoring environmental pollution in aquatic ecosystems. Many pollutants found in water prove to be endocrine disrupting chemicals that could cause disruptions in lipid homeostasis in aquatic species. A comprehensive profiling of the lipidome of these species is thus an essential step toward understanding the mechanism of toxicity induced by pollutants. Both the composition and spatial distribution of lipids in freshwater crustacean Gammarus fossarum were extensively examined herein. The baseline lipidome of gammarids of different sex and reproductive stages was established by high throughput shotgun lipidomics. Spatial lipid mapping by high resolution mass spectrometry imaging led to the discovery of sulfate-based lipids in hepatopancreas and their accumulation in mature oocytes. A diverse and dynamic lipid composition in G. fossarum was uncovered, which deepens our understanding of the biochemical changes during development and which could serve as a reference for future ecotoxicological studies.

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Baseline lipidome profiling of G. fossarum of different sex and reproductive stages

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Spatial localization of lipids in gammarid tissue by mass spectrometry imaging

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SIMS imaging guided discovery of sulfate-based lipids in hepatopancreas epithelium

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Disclosure of a dynamic lipid composition in maturing female oocytes

Update on LIPID MAPS classification, nomenclature, and shorthand notation for MS-derived lipid structures

A comprehensive and standardized system to report lipid structures analyzed by MS is essential for the communication and storage of lipidomics data. Herein, an update on both the LIPID MAPS classification system and shorthand notation of lipid structures is presented for lipid categories Fatty Acyls (FA), Glycerolipids (GL), Glycerophospholipids (GP), Sphingolipids (SP), and Sterols (ST). With its major changes, i.e., annotation of ring double bond equivalents and number of oxygens, the updated shorthand notation facilitates reporting of newly delineated oxygenated lipid species as well. For standardized reporting in lipidomics, the hierarchical architecture of shorthand notation reflects the diverse structural resolution powers provided by mass spectrometric assays. Moreover, shorthand notation is expanded beyond mammalian phyla to lipids from plant and yeast phyla. Finally, annotation of atoms is included for the use of stable isotope-labeled compounds in metabolic labeling experiments or as internal standards. This update on lipid classification, nomenclature, and shorthand annotation for lipid mass spectra is considered a standard for lipid data presentation.

Recent advances in the mass spectrometric analysis of glycosphingolipidome - A review

Aberrant expression of glycosphingolipids has been implicated in a myriad of diseases, but our understanding of the strucural diversity, spatial distribution, and biological function of this class of biomolecules remains limited. These challenges partly stem from a lack of sensitive tools that can detect, identify, and quantify glycosphingolipids at the molecular level. Mass spectrometry has emerged as a powerful tool poised to address most of these challenges. Here, we review the recent developments in analytical glycosphingolipidomics with an emphasis on sample preparation, mass spectrometry and tandem mass spectrometry-based structural characterization, label-free and labeling-based quantification. We also discuss the nomenclature of glycosphingolipids, and emerging technologies like ion mobility spectrometry in differentiation of glycosphingolipid isomers. The intrinsic advantages and shortcomings of each method are carefully critiqued in line with an individual's research goals. Finally, future perspectives on analytical sphingolipidomics are stated, including a need for novel and more sensive methods in isomer separation, low abundance species detection, and profiling the spatial distribution of glycosphingolipid molecular species in cells and tissues using imaging mass spectrometry.

Enhancing detection and characterization of lipids using charge manipulation in electrospray ionization-tandem mass spectrometry

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.

Structure determination of conjugated linoleic and linolenic acids

Conjugated linoleic and linolenic acids (CLA and CLnA) can be found in dairy, ruminant meat and oilseeds, these types of unsaturated fatty acids consist of various positional and geometrical isomers, and have demonstrated health-promoting potential for human beings. Extensive reviews have reported the physiological effects of CLA, CLnA, while little is known regarding their isomer-specific effects. However, the isomers are difficult to identify, owing to (i) the similar retention time in common chromatographic methods; and (ii) the isomers are highly sensitive to high temperature, pH changes, and oxidation. The uncertainties in molecular structure have hindered investigations on the physiological effects of CLA and CLnA. Therefore, this review presents a summary of the currently available technologies for the structural determination of CLA and CLnA, including the presence confirmation, double bond position determination, and the potential stereo-isomer determination. Special focus has been projected to the novel techniques for structure determination of CLA and CLnA. Some possible future directions are also proposed.

A Novel Function of Sphingosine Kinase 2 in the Metabolism of Sphinga-4,14-Diene Lipids

The number, position, and configuration of double bonds in lipids affect membrane fluidity and the recruitment of signaling proteins. Studies on mammalian sphingolipids have focused on those with a saturated sphinganine or mono-unsaturated sphingosine long chain base. Using high-resolution liquid chromatography-tandem mass spectrometry (LC-MS/MS), we observed a marked accumulation of lipids containing a di-unsaturated sphingadiene base in the hippocampus of mice lacking the metabolic enzyme sphingosine kinase 2 (SphK2). The double bonds were localized to positions C4–C5 and C14–C15 of sphingadiene using ultraviolet photodissociation-tandem mass spectrometry (UVPD-MS/MS). Phosphorylation of sphingoid bases by sphingosine kinase 1 (SphK1) or SphK2 forms the penultimate step in the lysosomal catabolism of all sphingolipids. Both SphK1 and SphK2 phosphorylated sphinga-4,14-diene as efficiently as sphingosine, however deuterated tracer experiments in an oligodendrocyte cell line demonstrated that ceramides with a sphingosine base are more rapidly metabolized than those with a sphingadiene base. Since SphK2 is the dominant sphingosine kinase in brain, we propose that the accumulation of sphingadiene-based lipids in SphK2-deficient brains results from the slower catabolism of these lipids, combined with a bottleneck in the catabolic pathway created by the absence of SphK2. We have therefore uncovered a previously unappreciated role for SphK2 in lipid quality control.

Profiling of Cholesteryl Esters by Coupling Charge-Tagging Paternò-Büchi Reaction and Liquid Chromatography-Mass Spectrometry

The profile of cholesteryl esters (CEs) is increasingly used in metabolic disease monitoring due to the roles of CE in regulating the cholesterol level. While electrospray ionization-tandem mass spectrometry is routinely applied for the identification and quantitation of CE, it has a limitation of not being able to provide the location of carbon-carbon double bond (C═C) within unsaturated fatty acyls. In this study, we paired offline 2-acetylpyridine (2-AP) Paternò-Büchi (PB) reaction and reversed-phase liquid chromatography-tandem mass spectrometry to achieve highly sensitive and structural informative CE analysis from complex mixtures. The 2-AP PB reactions of CE standards provided 20-30% conversion but resulted in enhanced ion signal relative to that of intact CE detected as ammonium adduct ions. MS/MS of 2-AP derivatized CE via collision-induced dissociation produced two abundant diagnostic ions for each C═C in a fatty acyl, leading to both sensitive identification and quantitation of C═C location isomers. Twelve saturated and twenty-seven unsaturated CEs were profiled in pooled human plasma; of the latter group, relative quantitation of 6 groups of C═C location isomers was achieved. A dehydrocholesteryl ester, DHE 18:2 (Δ9,12), was confidently differentiated from coexisting compositional isomers: CE 18:3 (Δ9,12,15) and CE 18:3 (Δ6,9,12). The above results represented improved CE coverage at the C═C location level over those reported by gas chromatography MS or acetone PB-MS/MS methods.

Localization of Double Bonds in Bacterial Glycerophospholipids Using 193 nm Ultraviolet Photodissociation in the Negative Mode

The need for detailed structural characterization of glycerophospholipids (GPLs) for many types of biologically motivated applications has led to the development of novel mass spectrometry-based methodologies that utilize alternative ion activation methods. Ultraviolet photodissociation (UVPD) has shown great utility for localizing sites of unsaturation within acyl chains and to date has predominantly been used for positive mode analysis of GPLs. In the present work, UVPD is used to localize sites of unsaturation in GPL anions. Similar to UVPD mass spectra of GPL cations, UVPD of deprotonated or formate-adducted GPLs yields diagnostic fragment ions spaced 24 Da apart. This method was integrated into a liquid chromatography workflow and used to evaluate profiles of sites of unsaturation of lipids in Escherichia coli (E. coli) and Acinetobacter baumannii (A. baumannii). When assigning sites of unsaturation, E. coli was found to contain all unsaturation elements at the same position relative to the terminal methyl carbon of the acyl chain; the first carbon participating in a site of unsaturation was consistently seven carbons along the acyl chain when counting carbons from the terminal methyl carbon. GPLs from A. baumannii exhibited more variability in locations of unsaturation. For GPLs containing sites of unsaturation in both acyl chains, an MS3 method was devised to assign sites to specific acyl chains.

Coupling Headgroup and Alkene Specific Solution Modifications with Gas-Phase Ion/Ion Reactions for Sensitive Glycerophospholipid Identification and Characterization

Shotgun lipidomics provides sensitive and fast lipid identification without the need for chromatographic separation. Challenges faced by shotgun analysis of glycerophospholipids (GPs) include the lack of signal uniformity across GP classes and the inability to determine the carbon-carbon double bond (C═C) location within the fatty acyl chains of an unsaturated species. Two distinct derivatization strategies were employed to both enhance the ionization of GPs, via trimethylation enhancement using ¹³C-diazomethane (¹³C-TrEnDi), as well as determine location of double bonds within fatty acyl chains, employing an in-solution photochemical reaction with acetone (via the Paternò-Büchi reaction). The modified GPs were then subjected to positive ion mode ionization via electrospray ionization, producing uniform ionization efficiencies for different classes of GP species. The GPs were charge inverted via gas-phase ion/ion reactions and sequentially fragmented using ion trap collision-induced dissociation (CID). The CID of the species led to fragmentation producing diagnostic ions indicative of C═C bond location. The approach enabled enhanced ionization and the identification of phosphatidylcholine and phosphatidylethanolamine species at the C═C level in a bovine lipid extract.

Large-Area Triboelectric Nanogenerator Mass Spectrometry: Expanded Coverage, Double-Bond Pinpointing, and Supercharging

Efficient ionization is a necessary condition for mass spectrometric analysis, but many compounds fail to ionize well enough to yield sufficient detection limits. Triboelectric nanogenerators (TENG) coupled to nanoelectrospray ionization (nanoESI) mass spectrometry (MS) are a highly effective approach to high sensitivity MS analysis. Here, we report on new, large-area TENG that constructively leverage the relationship between electrode size, created charges, and open-circuit voltage, leading to wider chemical coverage. Large-area TENG were found to also promote electrospray gas-phase oxidation reactions that enabled double bond position pinpointing for unsaturated lipids and species-specific lipid quantitation. Furthermore, large-area TENG MS of proteins was observed to yield higher charge state distributions (i.e., supercharging) without the need for high surface tension additives.

Imaging Isomers on a Biological Surface: A Review

Mass spectrometry imaging is an imaging technology that allows the localization and identification of molecules on (biological) sample surfaces. Obtaining the localization of a compound in tissue is of great value in biological research. Yet, the identification of compounds remains a challenge. Mass spectrometry alone, even with high-mass resolution, cannot always distinguish between the subtle structural differences of isomeric compounds. This review discusses recent advances in mass spectrometry imaging of lipids, steroid hormones, amino acids and proteins that allow imaging with isomeric resolution. These improvements in detailed identification can give new insights into the local biological activity of isomers.

Lipidomics from sample preparation to data analysis: a primer

Lipids are amongst the most important organic compounds in living organisms, where they serve as building blocks for cellular membranes as well as energy storage and signaling molecules. Lipidomics is the science of the large-scale determination of individual lipid species, and the underlying analytical technology that is used to identify and quantify the lipidome is generally mass spectrometry (MS). This review article provides an overview of the crucial steps in MS-based lipidomics workflows, including sample preparation, either liquid–liquid or solid-phase extraction, derivatization, chromatography, ion-mobility spectrometry, MS, and data processing by various software packages. The associated concepts are discussed from a technical perspective as well as in terms of their application. Furthermore, this article sheds light on recent advances in the technology used in this field and its current limitations. Particular emphasis is placed on data quality assurance and adequate data reporting; some of the most common pitfalls in lipidomics are discussed, along with how to circumvent them.

Integration of a Multichannel Pulsed-Valve Inlet System to a Linear Quadrupole Ion Trap Mass Spectrometer for the Rapid Consecutive Introduction of Nine Reagents for Diagnostic Ion/Molecule Reactions

Gas-phase ion/molecule reactions have been used extensively for the structural elucidation of organic compounds in tandem mass spectrometry. Reagents for ion/molecule reactions can be introduced into a mass spectrometer via a continuous flow apparatus or through a pulsed inlet system. However, most of these approaches enable the use of only a single reagent at a time. In this work, a multichannel pulsed-valve inlet system was developed for the rapid consecutive introduction of up to nine different reagents or reagent systems into a linear quadrupole ion trap mass spectrometer for diagnostic gas-phase ion/molecule reactions. Automated triggering of the pulsed valves enabled these experiments to be performed on the high-performance liquid chromatography (HPLC) time scale. This enables high-throughput screening of several functionalities in analytes as they elute from an HPLC column.

Analytical separations for lipids in complex, nonpolar lipidomes using differential mobility spectrometry

Secretions from meibomian glands located within the eyelid (commonly known as meibum) are rich in nonpolar lipid classes incorporating very-long (22-30 carbons) and ultra-long (>30 carbons) acyl chains. The complex nature of the meibum lipidome and its preponderance of neutral, nonpolar lipid classes presents an analytical challenge, with typically poor chromatographic resolution, even between different lipid classes. To address this challenge, we have deployed differential mobility spectrometry (DMS)-MS to interrogate the human meibum lipidome and demonstrate near-baseline resolution of the two major nonpolar classes contained therein, namely wax esters and cholesteryl esters. Within these two lipid classes, we describe ion mobility behavior that is associated with the length of their acyl chains and location of unsaturation. This capability was exploited to profile the molecular speciation within each class and thus extend meibum lipidome coverage. Intriguingly, structure-mobility relationships in these nonpolar lipids show similar trends and inflections to those previously reported for other physicochemical properties of lipids (e.g., melting point and phase-transition temperatures). Taken together, these data demonstrate that differential ion mobility provides a powerful orthoganol separation technology for the analysis of neutral lipids in complex matrices, such as meibum, and may further provide a means to predict physicochemical properties of lipids that could assist in inferring their biological function(s).

Combining Charge-Switch Derivatization with Ozone-Induced Dissociation for Fatty Acid Analysis

The specific positions of carbon-carbon double bond(s) within an unsaturated fatty acid exert a significant effect on the physical and chemical properties of the lipid that ultimately inform its biological function(s). Contemporary liquid chromatography-mass spectrometry (MS) strategies based on electrospray ionization coupled to tandem MS can easily detect fatty acyl lipids but generally cannot reveal those specific site(s) of unsaturation. Herein, we describe a novel and versatile workflow whereby fatty acids are first converted to fixed charge N-(4-aminomethylphenyl)pyridinium (AMPP) derivatives and subsequently subjected to ozone-induced dissociation (OzID) on a modified triple quadrupole mass spectrometer. The AMPP modification enhances the detection of fatty acids introduced by direct infusion. Fragmentation of the derivatized fatty acids also provides diagnostic fragment ions upon collision-induced dissociation that can be targeted in precursor ion scans to subsequently trigger OzID analyses in an automated data-dependent workflow. It is these OzID analyses that provide unambiguous assignment of carbon-carbon double bond locations in the AMPP-derivatized fatty acids. The performance of this analysis pipeline is assessed in profiling the patterns of unsaturation in fatty acids within the complex biological secretion vernix caseosa. This analysis uncovers significant isomeric diversity within the fatty acid pool of this sample, including a number of hitherto unreported double bond positional isomers that hint at the activity of potentially new metabolic pathways.

Fragmentation Behavior and Gas-Phase Structures of Cationized Glycosphingolipids in Ozone-Induced Dissociation Mass Spectrometry

The role of cationization in the fragmentation behavior of glycoconjugates is amply documented in collisional activation techniques but remains less explored in ozone-induced dissociation mass spectrometry (OzID-MS). OzID-MS has been used to elucidate the location of carbon-carbon double bonds in unsaturated lipids. Previously, we demonstrated the structural analysis of unsaturated glycosphingolipids using OzID-MS by mass-selecting the [M+Na]+ adduct for fragmentation. In this work, we aimed to examine the effect of different adducts, namely [M+Na]+, [M+Li]+, and [M+H]+ on the OzID-MS fragmentation behavior of a representative unsaturated glycosphingolipid, LacCer d18:1/18:1(9Z). Our data show that [M+H]+ primarily undergoes dehydration followed by collision-induced dissociation-like loss of the headgroup, while [M+Li]+ and [M+Na]+ dissociate at the double bonds albeit with slightly different intensities of the resulting fragments. Using molecular mechanics and theoretical calculations at the semiempirical level, we report for the first time the gas-phase structure of cationized glycosphingolipids, which helps rationalize the observed bond cleavage. Our findings highlight that the type of adducts can influence gas-phase ion structure of glycosphingolipids and subsequently affect their fragmentation in OzID-MS. This study contributes to the growing body of knowledge on OzID-MS and gas-phase structures of ionized lipids and the findings have the potential to be extended to other more complex glycoconjugates.

Generating Fatty Acid Profiles in the Gas Phase: Fatty Acid Identification and Relative Quantitation Using Ion/Ion Charge Inversion Chemistry

Representing the most fundamental lipid class, fatty acids (FA) play vital biological roles serving as energy sources, cellular signaling molecules, and key architectural components of complex lipids. Direct infusion electrospray ionization spectrometry, also known as shotgun lipidomics, has emerged as a rapid and powerful toolbox for lipid analysis. While shotgun lipidomics can be a sensitive approach to FA detection, the diverse molecular structure of FA presents challenges for unambiguous identification and the relative quantification of isomeric contributors. In particular, pinpointing double bond position(s) in unsaturated FA and determining the relative contribution of double bond isomers has limited the application of the shotgun approach. Recently, we reported the use of gas-phase ion/ion reactions to facilitate the identification of FA. Briefly, singly deprotonated FA anions undergo charge inversion when reacted in the gas phase with tris-phenanthroline magnesium dications by forming [FA - H + MgPhen]⁺ complex ions. These charge-inverted FA complex cations fragment upon ion-trap collision-induced dissociation (CID) to generate product ion spectra unique to individual FA isomers. Herein, we report the development of a mass spectral library comprised of [FA - H + MgPhen]⁺ product ion spectra. The developed FA library permits confident FA identification, including polyunsaturated FA isomers. Furthermore, we demonstrate the ability to determine relative contributions of isomeric FA using multiple linear regression analysis paired with gas-phase ion/ion reactions. We successfully applied the presented method to generate a FA profile for bovine liver phospholipidome based entirely on gas-phase chemistries.

Enzymatically oxidized phospholipids assume center stage as essential regulators of innate immunity and cell death

Enzymatically oxidized phospholipids (eoxPLs) are formed through regulated processes by which eicosanoids or prostaglandins are attached to phospholipids (PLs) in immune cells. These eoxPLs comprise structurally diverse families of biomolecules with potent bioactivities, and they have important immunoregulatory roles in both health and disease. The formation of oxPLs through enzymatic pathways and their signaling capabilities are emerging concepts. This paradigm is changing our understanding of eicosanoid, prostaglandin, and PL biology in health and disease. eoxPLs have roles in cellular events such as ferroptosis, apoptosis, and blood clotting and diseases such as arthritis, diabetes, and cardiovascular disease. They are increasingly recognized as endogenous bioactive mediators and potential targets for drug development. This review will describe recent evidence that places eoxPLs and their biosynthetic pathways center stage in immunoregulation.