Enhanced Phospholipid Isomer Analysis by Online Photochemical Derivatization and RPLC-MS

Combination of Metabolomics, Lipidomics, and Molecular Biology for the Investigation of the Metabolic Disturbance of Short-Term Administration of Emodin

Emodin, a natural anthraquinone derivative, is an active ingredient in many Chinese traditional herbs. Interestingly, although it is generally considered to possess hepatoprotective activity, some studies have also reported that it has a certain degree of hepatotoxicity. Additionally, the underlying metabolic regulation of emodin remains uncertain. Therefore, we conducted a nontargeted metabolomic study based on UHPLC/Q-Orbitrap-MS and NMR. Data are available via ProteomeXchange with the identifier PXD055000. The results indicated a close association between the short-term administration of emodin and lipid metabolism. Moreover, a lipidomics investigation utilizing QTRAP 6500+ UHPLC-MS/MS was conducted, with a focus on determining the position of C═C double bonds in unsaturated lipids based on Paternò-Büchi (PB) reaction to discover the metabolic disturbance more precisely. Specifically, lipidomics revealed elevated levels of free fatty acids (FFA) alongside notable reductions in sphingomyelin (SM) and triacylglycerol (TAG) levels. Furthermore, the combination of PB reaction and molecular biology results indicated that short-term administration of emodin may lead to the accumulation of n-6 polyunsaturated fatty acids by up-regulating the expression of FASN, stearyl CoA desaturase 1 (SCD1), and cytosolic phospholipase A 2 (cPLA2). Simultaneously, up-regulation of cyclooxygenase-2 (Cox-2) expression was observed, potentially fostering the production of prostaglandin E2 (PGE2) and subsequent inflammation.

Shotgun Lipidomic Profiling of Sebum Lipids via Photocatalyzed Paternò-Büchi Reaction and Ion Mobility-Mass Spectrometry

Sebum lipids are composed of nonpolar lipids, and they pose challenges for mass spectrometry-based analysis due to low ionization efficiency and the existence of numerous isomers and isobars. To address these challenges, we have developed ethyl 2-oxo-2-(pyridine-3-yacetate as a charge-tagging Paternò-Büchi reagent and Michler's ketone as a highly efficient photocatalyst, achieving ∼90% conversion for C═C derivatization under 440 nm LED irradiation. This derivatization, when coupled with electrospray ionization-tandem mass spectrometry, boosts the detection of sebum lipids and pinpoints C═C location in a chain-specific fashion. Identification and quantitation of isomers are readily achieved for wax esters, a class of underexplored sebum lipids, which have C═C bonds distributed in fatty alcohol and fatty acyl chains. A shotgun analysis workflow has been developed by pairing the offline PB derivatization with cyclic ion mobility spectrometry-mass spectrometry. Besides the dominant n-10 C═C location in unsaturated wax esters, profiling of low abundance isomers, including the rarely reported n-7 and n-13 locations, is greatly enhanced due to separations of C═C diagnostic ions by ion mobility. Over 900 distinct lipid structures from human sebum lipid extract have been profiled at the chain-specific C═C level, including wax esters (500), glycerolipids (393), and cholesterol esters (22), far more exceeding previous reports. Overall, we have developed a fast and comprehensive lipidomic profiling tool for sebum samples, a type of noninvasive biofluids holding potential for the discovery of disease markers in distal organs.

Analysis of Covarine Particle in Toothpaste Through Microfluidic Simulation, Experimental Validation, and Electrical Impedance Spectroscopy

Covarine, copper phthalocyanine, a novel tooth whitening ingredient, has been incorporated into various toothpaste formulations using diverse technologies such as larger flakes, two-phase pastes, and microbeads. In this study, we investigated the behavior of covarine microbeads (200 μm) in Colgate advanced white toothpaste when mixed with artificial and real saliva. Our analysis utilized a custom-designed microfluidic mixer with 400 μm wide channels arranged in serpentine patterns, featuring a Y-shaped design for saliva and toothpaste flow. The mixer, fabricated using stereolithography 3D printing technology, incorporated a flexible transparent resin (Formlabs' Flexible 80A resin) and PMMA layers. COMSOL simulations were performed by utilizing parameters extracted from toothpaste and saliva datasheets, supplemented by laboratory measurements, to enhance simulation accuracy. Experimental assessments encompassing the behavior of covarine particles were conducted using an optical profilometer. Viscosity tests and electrical impedance spectroscopy employing recently developed all-carbon electrodes were employed to analyze different toothpaste dilutions. The integration of experimental data from microfluidic chips with computational simulations offers thorough insights into the interactions of covarine particles with saliva and the formation of microfilms on enamel surfaces.

Mass filtering combined with photochemical derivatization enables high throughput mass spectrometric analysis of unsaturated phosphatidylcholine isomers

Phosphatidylcholines (PCs) are closely related to coronary heart disease, such as myocardial infarction. The analysis of the deep structure of PCs is of great significance for exploring the effects of exercise rehabilitation and lipid metabolism. Here, we present a mass filtering combined with photochemical derivatization method for rapid screening and accurate identification of the CC position and sn-location isomer of PCs. This method is simple to execute and easily implementable for routine analysis. The accurate qualitative and quantitative analysis of PCs and isomers facilitates the discovery of biomarkers for exercise rehabilitation of patients with myocardial infarction.

Unraveling the Double Bond Position of Fatty Acids by GC-MS Using Electron Capture APCI and In-Source Fragmentation Patterns

The position of double bonds in unsaturated fatty acids is strongly connected to their biological effects, but their analytical characterization is still challenging. However, the ionization of unsaturated fatty acids by a GC-APCI leads to regiospecific in-source fragment ions, which can be used to identify the double bond position. The fragment ions are oxidized species that occur mostly at the double bond closest to the carboxylic acid group. This effect can be further promoted by using benzaldehyde as a gas-phase reactant. This allows the identification of the Δ-notation of the fatty acid, and based on additional information such as m/z and retention time, it is possible to annotate the corresponding fatty acid. The developed method also enables the quantification of fatty acids in one step with high selectivity and sensitivity. Moreover, rare fatty acids can be identified in suspected target approaches that are often not available as standards. This was demonstrated by analyzing fish oil samples that provide a complex mixture of highly unsaturated fatty acids and by identifying rare fatty acids such as hexadecatetraenoic acid (FA 16:4 Δ6).

Selective Characterization of Olefins by Paternò-Büchi Reaction with Ultrahigh Resolution Mass Spectrometry

Petroleum olefins play important roles in various secondary processing procedures and are important feedstocks for the modern organic chemical industry. It is quite challenging to analyze petroleum olefins beyond the gas chromatography (GC)-able range using mass spectrometry (MS) due to the difficulty of soft ionization and the matrix complexity. In this work, a Paternò-Büchi (PB) reaction combined with atmospheric pressure chemical ionization and ultrahigh resolution mass spectrometry (APCI-UHRMS) was developed for selective analysis of olefins. Through the PB reaction, C═C bonds were transformed into four-membered rings of oxetane with improved polarity so that soft ionization of olefins could be achieved. The systematic optimization of PB reaction conditions, as well as MS ionization conditions, ensured a high reaction yield and a satisfied MS response. Furthermore, a sound scheme was set up to discriminate the coexisting unsaturated alkanes in complex petroleum, including linear olefins, nonlinear olefins, cycloalkanes, and aromatics, making use of their different behaviors during the PB reaction and chemical ionization. The developed strategy was successfully applied to the analysis of olefins in fluid catalytic cracking oil slurry, a complex heavy oil sample. This method extended the characterization of petroleum olefins from lower to higher with high efficiency and selectivity to provide a comprehensive molecular library for heavy petroleum samples and process optimization.

Deep-profiling of phospholipidome via rapid orthogonal separations and isomer-resolved mass spectrometry

A lipidome comprises thousands of lipid species, many of which are isomers and isobars. Liquid chromatography-tandem mass spectrometry (LC-MS/MS), although widely used for lipidomic profiling, faces challenges in differentiating lipid isomers. Herein, we address this issue by leveraging the orthogonal separation capabilities of hydrophilic interaction liquid chromatography (HILIC) and trapped ion mobility spectrometry (TIMS). We further integrate isomer-resolved MS/MS methods onto HILIC-TIMS, which enable pinpointing double bond locations in phospholipids and sn-positions in phosphatidylcholine. This system profiles phospholipids at multiple structural levels with short analysis time (<10 min per LC run), high sensitivity (nM detection limit), and wide coverage, while data analysis is streamlined using a home-developed software, LipidNovelist. Notably, compared to our previous report, the system doubles the coverage of phospholipids in bovine liver and reveals uncanonical desaturation pathways in RAW 264.7 macrophages. Relative quantitation of the double bond location isomers of phospholipids and the sn-position isomers of phosphatidylcholine enables the phenotyping of human bladder cancer tissue relative to normal control, which would be otherwise indistinguishable by traditional profiling methods. Our research offers a comprehensive solution for lipidomic profiling and highlights the critical role of isomer analysis in studying lipid metabolism in both healthy and diseased states.

Enhancing the Signal-to-Noise of Diagnostic Fragment Ions of Unsaturated Glycerophospholipids via Precursor Exclusion Ultraviolet Photodissociation Mass Spectrometry (PEx-UVPD-MS)

Understanding and elucidating the diverse structures and functions of lipids has motivated the development of many innovative tandem mass spectrometry (MS/MS) strategies. Higher-energy activation methods, such as ultraviolet photodissociation (UVPD), generate unique fragment ions from glycerophospholipids that can be used to perform in-depth structural analysis and facilitate the deconvolution of isomeric lipid structures in complex samples. Although detailed characterization is central to the correlation of lipid structure to biological function, it is often impeded by the lack of sufficient instrument sensitivity for highly bioactive but low-abundance phospholipids. Here, we present precursor exclusion (PEx) UVPD, a simple yet powerful technique to enhance the signal-to-noise (S/N) of informative low-abundance fragment ions produced from UVPD of glycerophospholipids. Through the exclusion of the large population of undissociated precursor ions with an MS3 strategy, the S/N of diagnostic fragment ions from PC 18:0/18:2(9Z, 12Z) increased up to an average of 13x for PEx-UVPD compared to UVPD alone. These enhancements were extended to complex mixtures of lipids from bovine liver extract to confidently identify 35 unique structures using liquid chromatography PEx-UVPD. This methodology has the potential to advance lipidomics research by offering deeper structure elucidation and confident identification of biologically active lipids.

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.

Referenced Kendrick Mass Defect Annotation and Class-Based Filtering of Imaging MS Lipidomics Experiments

Because of their diverse functionalities in cells, lipids are of primary importance when characterizing molecular profiles of physiological and disease states. Imaging mass spectrometry (IMS) provides the spatial distributions of lipid populations in tissues. Referenced Kendrick mass defect (RKMD) analysis is an effective mass spectrometry (MS) data analysis tool for classification and annotation of lipids. Herein, we extend the capabilities of RKMD analysis and demonstrate an integrated method for lipid annotation and chemical structure-based filtering for IMS datasets. Annotation of lipid features with lipid molecular class, radyl carbon chain length, and degree of unsaturation allows image reconstruction and visualization based on each structural characteristic. We show a proof-of-concept application of the method to a computationally generated IMS dataset and validate that the RKMD method is highly specific for lipid components in the presence of confounding background ions. Moreover, we demonstrate an application of the RKMD-based annotation and filtering to matrix-assisted laser desorption/ionization (MALDI) IMS lipidomic data from human kidney tissue analysis.

The foundations and development of lipidomics

For over a century, the importance of lipid metabolism in biology was recognized but difficult to mechanistically understand due to the lack of sensitive and robust technologies for identification and quantification of lipid molecular species. The enabling technological breakthroughs emerged in the 1980s with the development of soft ionization methods (Electrospray Ionization and Matrix Assisted Laser Desorption/Ionization) that could identify and quantify intact individual lipid molecular species. These soft ionization technologies laid the foundations for what was to be later named the field of lipidomics. Further innovative advances in multistage fragmentation, dramatic improvements in resolution and mass accuracy, and multiplexed sample analysis fueled the early growth of lipidomics through the early 1990s. The field exponentially grew through the use of a variety of strategic approaches, which included direct infusion, chromatographic separation, and charge-switch derivatization, which facilitated access to the low abundance species of the lipidome. In this Thematic Review, we provide a broad perspective of the foundations, enabling advances, and predicted future directions of growth of the lipidomics field.

Mass Spectrometry as a Crucial Analytical Basis for Omics Sciences

This review is devoted to the consideration of mass spectrometric platforms as applied to omics sciences. The most significant attention is paid to omics related to life sciences (genomics, proteomics, meta-bolomics, lipidomics, glycomics, plantomics, etc.). Mass spectrometric approaches to solving the problems of petroleomics, polymeromics, foodomics, humeomics, and exosomics, related to inorganic sciences, are also discussed. The review comparatively presents the advantages of various principles of separation and mass spectral techniques, complementary derivatization, used to obtain large arrays of various structural and quantitative information in the mentioned omics sciences.

Deep Profiling of Aminophospholipids Reveals a Dysregulated Desaturation Pattern in Breast Cancer Cell Lines

Phosphatidylethanolamines (PEs), ether-PEs, and phosphatidylserines (PSs) are glycerophospholipids harboring a primary amino group in their headgroups. They are key components of mammalian cell membranes and play pivotal roles in cell signaling and apoptosis. In this study, a liquid chromatography-mass spectrometry (LC-MS) workflow for deep profiling of PEs, ether-PEs, and PSs has been developed by integrating two orthogonal derivatizations: (1) derivatization of the primary amino group by 4-trimethylammoniumbutyryl-N-hydroxysuccinimide (TMAB-NHS) for enhanced LC separation and MS detection and (2) the Paternò-Büchi (PB) reaction for carbon-carbon double bond (C═C) derivatization and localization. Significant improvement of the limit of identification down to the C═C location has been achieved for the standards of PSs (3 nM) and ether-PEs (20 nM). This workflow facilitates an identification of more than 200 molecular species of aminophospholipids in the porcine brain, two times more than those identified without TMAB-NHS derivatization. Importantly, we discovered that the n-10 isomers in C16:1 and C18:1 of aminophospholipids showed elevated contribution among other isomers, which correlated well with an increased transcription of the corresponding desaturase (FADS2) in the human breast cancer cell line (MDA-MB-231) relative to that in the normal cell line (HMEC). The abovementioned data suggest that lipid reprograming via forming different C═C location isomers might be an alternative mechanism in cancer cells.

Polymerized phosphonium ionic liquid functionalized silica microspheres as mixed-mode stationary phase for liquid chromatographic separation of phospholipids

There is a large and growing demand for the vigorous development of new high performance liquid chromatography stationary phases in order to solve complex phospholipids separation. Herein, phosphonium-based ionic liquid trioctyl(allyl)phosphonium bromide ([P_888Allyl]Br) was first synthesized with trioctylphosphine and allyl bromide. With [P_888Allyl]Br as the polymerizable monomer, polymerized phosphonium ionic liquid functionalized silica microsphere (PIL@SiO₂) was further synthetized via click chemistry reaction. Significantly, based on the inherent amphiphilic nature of the introduced [P_888Allyl]Br, the packed PIL@SiO₂ column displayed hydrophilic/hydrophobic mixed-mode retention mechanisms. The PIL@SiO₂ column can achieve separation of nucleic acid bases and nucleosides, sulfonamides, amides and anilines with excellent selectivity in a shorter separation time. The column efficiency reached 109,700 N/m for 2-iodoacetamide. One of the important characteristics of the PIL@SiO₂ column is that both phospholipid classes and species can be efficiently separated via the same column, outperforming that of the commercial amino column. Furthermore, the application potential of the PIL@SiO₂ column was further verified via separation of phospholipids extracted from soy lecithin. The proposed PIL@SiO₂ column provides a promising candidate for separation of complex phospholipid samples.

Mass spectrometry-based lipid analysis and imaging

Mass spectrometry imaging (MSI) is a powerful tool for in situ mapping of analytes across a sample. With growing interest in lipid biochemistry, the ability to perform such mapping without antibodies has opened many opportunities for MSI and lipid analysis. Herein, we discuss the basics of MSI with particular emphasis on MALDI mass spectrometry and lipid analysis. A discussion of critical advancements as well as protocol details are provided to the reader. In addition, strategies for improving the detection of lipids, as well as applications in biomedical research, are presented.

Review of Recent Advances in Lipid Analysis of Biological Samples via Ambient Ionization Mass Spectrometry

The rapid and direct structural characterization of lipids proves to be critical for studying the functional roles of lipids in many biological processes. Among numerous analytical techniques, ambient ionization mass spectrometry (AIMS) allows for a direct molecular characterization of lipids from various complex biological samples with no/minimal sample pretreatment. Over the recent years, researchers have expanded the applications of the AIMS techniques to lipid structural elucidation via a combination with a series of derivatization strategies (e.g., the Paternò–Büchi (PB) reaction, ozone-induced dissociation (OzID), and epoxidation reaction), including carbon–carbon double bond (C=C) locations and sn-positions isomers. Herein, this review summarizes the reaction mechanisms of various derivatization strategies for C=C bond analysis, typical instrumental setup, and applications of AIMS in the structural elucidation of lipids from various biological samples (e.g., tissues, cells, and biofluids). In addition, future directions of AIMS for lipid structural elucidation are discussed.

Enabling High Structural Specificity to Lipidomics by Coupling Photochemical Derivatization with Tandem Mass Spectrometry

Lipids have pivotal roles in many biological processes, including energy storage, signal transduction, and plasma membrane formation. A disruption of lipid homeostasis is found to be associated with a range of diseases, such as cardiovascular diseases, diabetes, and cancer. Fundamental lipid biology and disease diagnostics can benefit from monitoring lipid changes in cells, tissues, organs, or the whole biological system. Therefore, it is important to develop lipid analysis tools to achieve comprehensive lipid characterization and quantitation. Over the past two decades, mass spectrometry (MS) has become the method of choice for qualitative and quantitative analyses of lipids, owing to its high sensitivity, multiplexed analysis, and soft ionization features. With the rapid development and adoption of ultrahigh-resolution MS, isobaric lipids can now be routinely resolved. By contrast, the structural characterization and quantitation of isomeric lipids remain an analytical challenge. Although some lipid C═C location or sn-isomers can be resolved by chromatography, ion mobility, or selective ionization approaches, a detailed structural characterization on the lipidome-wide level needs to be achieved.Over the past six years, we have successfully combined the Paternò-Büchi (PB) reaction, which is a UV-promoted photocycloaddition reaction specific to the C═C, with tandem MS (MS/MS) to locate the C═C in lipids and quantify lipid C═C location isomers. The PB reactions have analytical advantages such as a simple experimental setup, rapid lipid C═C derivatization, and highly specific C═C cleavage during PB-MS/MS to produce abundant diagnostic ions. More importantly, without a need of isomer separation or a comparison to authentic standards, PB-MS/MS can be directly applied to identify and quantify a mixture of lipid C═C location isomers, often coexisting with molar ratios sensitive to the biological state of the system. The PB-MS/MS method is compatible with conventional shotgun lipidomics employing a nanoelectrospray ionization or a large-sale lipid structural analysis via liquid chromatography (LC) coupled to any mass spectrometer with tandem MS capability. The PB-MS/MS method is highly versatile, as a variety of PB reagents can be tailored to a broad range of applications. Besides UV-promoted PB reactions, visible-light PB reactions have also been developed to offer more flexibility for a lipid analysis. By using selected PB reagents, the sn-positions of fatty acyls can be resolved together with C═C locations in phospholipids. This method has been used in lipidomic analyses of tissue, blood, and plasma from animal models and clinical samples, demonstrating the potential of using lipid C═C or sn-location isomer ratios for phenotyping and disease diagnostics. Lipid isomer-resolving MS imagings of tissues and single-cell lipid analysis have also been demonstrated by a proper implementation of PB-MS/MS.

Advanced tandem mass spectrometry in metabolomics and lipidomics-methods and applications

Metabolomics and lipidomics are new drivers of the omics era as molecular signatures and selected analytes allow phenotypic characterization and serve as biomarkers, respectively. The growing capabilities of untargeted and targeted workflows, which primarily rely on mass spectrometric platforms, enable extensive charting or identification of bioactive metabolites and lipids. Structural annotation of these compounds is key in order to link specific molecular entities to defined biochemical functions or phenotypes. Tandem mass spectrometry (MS), first and foremost collision-induced dissociation (CID), is the method of choice to unveil structural details of metabolites and lipids. But CID fragment ions are often not sufficient to fully characterize analytes. Therefore, recent years have seen a surge in alternative tandem MS methodologies that aim to offer full structural characterization of metabolites and lipids. In this article, principles, capabilities, drawbacks, and first applications of these "advanced tandem mass spectrometry" strategies will be critically reviewed. This includes tandem MS methods that are based on electrons, photons, and ion/molecule, as well as ion/ion reactions, combining tandem MS with concepts from optical spectroscopy and making use of derivatization strategies. In the final sections of this review, the first applications of these methodologies in combination with liquid chromatography or mass spectrometry imaging are highlighted and future perspectives for research in metabolomics and lipidomics are discussed.

Relative Quantitation of Unsaturated Phosphatidylcholines Using 193 nm Ultraviolet Photodissociation Parallel Reaction Monitoring Mass Spectrometry

Structural characterization of glycerophospholipids beyond the fatty acid level has become a major endeavor in lipidomics, presenting an opportunity to advance the understanding of the intricate relationship between lipid metabolism and disease state. Distinguishing subtle lipid structural features, however, remains a major challenge for high-throughput workflows that implement traditional tandem mass spectrometry (MS/MS) techniques, stunting the molecular depth of quantitative strategies. Here, reversed phase liquid chromatography is coupled to parallel reaction mass spectrometry utilizing the double bond localization capabilities of ultraviolet photodissociation (UVPD) mass spectrometry to produce double bond isomer specific responses that are leveraged for relative quantitation. The strategy provides lipidomic characterization at the double bond level for phosphatidylcholine phospholipids from biological extracts. In addition to quantifying monounsaturated lipids, quantitation of phospholipids incorporating isomeric polyunsaturated fatty acids is also achieved. Using this technique, phosphatidylcholine isomer ratios are compared across human normal and tumor breast tissue to reveal significant structural alterations related to disease state.

Mapping the distribution of double bond location isomers in lipids across mouse tissues

Lipids are highly diverse and essential biomolecules in all living systems. As lipid homeostasis is often perturbed in metabolic diseases, these molecules can serve as both biomarkers and drug targets. The development of modern mass spectrometry (MS) provided the platform for large-scale lipidomic studies at the level of molecular species. Traditionally, more detailed structural information, such as the C[double bond, length as m-dash]C location, was mostly assumed instead of properly measured, though the specific isomers were indicated as potential biomarkers of cancers or cardiovascular diseases. Recent C[double bond, length as m-dash]C localization methods, including the Paternò-Büchi (PB) reaction, have shown the prevalent and heterogeneous distribution of C[double bond, length as m-dash]C location in lipids across tissues. Mapping the lipidome of model animals at the level of C[double bond, length as m-dash]C position would increase the understanding of the metabolism and function of lipid isomers, facilitating clinical research. In this study, we employed an online PB reaction on a liquid chromatography-high resolution MS platform to map C[double bond, length as m-dash]C location isomers in five different murine tissues. We analyzed phosphatidylcholines, phosphatidylethanolamines, and sphingomyelins; we relatively quantified and mapped the distribution of ∼30 groups of co-existing isomers, characterized by different chain lengths and degrees of unsaturation. More specifically, we performed relative quantitation of four isomers of the C16:1 fatty acyl, which included rarely reported n-10 and n-5 species besides n-9 and n-7 isomers. We showed a small variation of the isomers' relative composition among individual animals (<20%) but significant differences across different lipid species and mouse tissues. Our results provided an initial database to map alternative lipid metabolic pathways at the tissue level.

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.

High-coverage lipidomics for functional lipid and pathway analyses

Rapid advances in front-end separation approaches and analytical technologies have accelerated the development of lipidomics, particularly in terms of increasing analytical coverage to encompass an expanding repertoire of lipids within a single analytical approach. Developments in lipid pathway analysis, however, have somewhat lingered behind, primarily due to (1) the lack of coherent alignment between lipid identifiers in common databases versus that generated from experiments, owing to the differing structural resolution of lipids at molecular level that is specific to the analytical approaches adopted by various laboratories; (2) the immense complexity of lipid metabolic relationships that may entail head group changes, fatty acyls modifications of various forms (e.g. elongation, desaturation, oxidation), as well as active remodeling that demands a multidimensional, panoramic view to take into account all possibilities in lipid pathway analyses. Herein, we discuss current efforts undertaken to address these challenges, as well as alternative form of "pathway analyses" that may be particularly useful for uncovering functional lipid interactions under different biological contexts. Consolidating lipid pathway analyses will be indispensable in facilitating the transition of lipidomics from its prior role of phenotype validation to a hypothesis-generating tool that uncovers novel molecular targets to drive downstream mechanistic pursuits under biomedical settings.

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.