A visible-light activated [2 + 2] cycloaddition reaction enables pinpointing carbon-carbon double bonds in lipids

Three-Layer Electrospray Constructing Charged Microdroplets for Online Derivatization and Position Identification of Lipid C═C Bonds

Numerous studies have demonstrated that charged microdroplets can significantly accelerate chemical reactions. In this work, we developed a novel three-layer electrospray ionization source (TL-ESI). This source generated charged microdroplets capable of facilitating the rapid epoxidation of unsaturated lipid C═C bonds with the derivatization reagent 3-chloroperbenzoic acid (m-CPBA) in an online process, achieving a conversion rate of up to 92.8%─a performance not achievable with conventional electrospray ionization source (ESI). Compared to conventional ESI, the TL-ESI effectively compartmentalized reactants, enabled precise control over the reaction process, and supported rapid online derivatization of lipid C═C bonds with m-CPBA. When integrated with tandem mass spectrometry (MS/MS), this source further enabled the localization of lipid C═C bond positions. The TL-ESI is characterized by its simplicity in design, ease of operation, and seamless integration with mass spectrometry (MS). These advantages make it an efficient and practical tool for locating C═C bond positions in unsaturated lipids, even within complex sample matrices. Additionally, this work highlights the potential of the TL-ESI as an innovative platform for accelerating chemical reactions via charged microdroplets, offering a valuable addition to the toolbox of modern analytical chemistry.

Determination of double bond positions in unsaturated fatty acids by pre-column derivatization with dimethyl and dipyridyl disulfide followed by LC-SWATH-MS analysis

Comprehensive in-depth structural characterization of free mono-unsaturated and polyunsaturated fatty acids often requires the determination of carbon-carbon double bond positions due to their impact on physiological properties and relevance in biological samples or during impurity profiling of pharmaceuticals. In this research, we report on the evaluation of disulfides as suitable derivatization reagents for the determination of carbon-carbon double bond positions of unsaturated free fatty acids by UHPLC-ESI-QTOF-MS/MS analysis and SWATH (sequential windowed acquisition of all theoretical mass spectra) acquisition. Iodine-catalyzed derivatization of C = C double bonds with dimethyl disulfide (DMDS) enabled detection of characteristic carboxy-terminal MS2 fragments for various fatty acids in ESI negative mode. The determination of double bond positions of fatty acids with up to three double bonds, the transfer of the method to plasma samples, and its limitations have been shown. To achieve charge-switching for positive ion mode MS-detection, derivatization with 2,2'-dipyridyldisulfide (DPDS) was investigated. It enabled detection of both corresponding characteristic omega-end- and carboxy-end-fragments for fatty acids with up to two double bonds in positive ion mode. It provides a straightforward strategy for designing MRM transitions for targeted LC-MS/MS assays. Both derivatization techniques represent a simple and inexpensive way for the determination of double bond positions in fatty acids with low number of double bonds. No adaptation of MS hardware is required and the specific isotopic pattern of resulting sulfur-containing products provides additional structural confirmation. This reaction scheme opens up the avenue of structural tuning of disulfide reagents beyond DMDS and DPDS using reagents like cystine and analogs to achieve enhanced performance and sensitivity.

Stereoselective Reaction Enabling Simultaneous Analysis of Carbon-Carbon Double-Bond Configuration and the Position of Monounsaturated Fatty Acids through UHPLC-ESI-MRM-MS

Monounsaturated fatty acids (MUFA) are an important class of nutrients and are involved in lipid metabolism. The positions of the C=C bond and cis-trans isomerism have a significant influence on their physiological activity. However, simultaneously detecting these two structural properties has been challenging due to multiple isomers of MUFA. In this study, we developed a method that involved an N-aminophthalimide (PhthNH2) derivatization of the C=C bond in MUFA, followed by analysis using ultrahigh-performance liquid chromatography-electrospray ionization-multiple reaction monitoring mass spectrometry (UHPLC-ESI-MRM-MS) technology to achieve analysis of cis-trans isomers and positional isomers of the C=C bond. The derivatives of cis-trans isomers of the C=C bond were well separated in UHPLC, and their corresponding C=C bond positions were deduced from characteristic fragment ions in tandem mass spectrometry. With our method, we found MUFA with different double-bond positions and cis-trans isomers in several samples, including mouse kidney, butter, etc., achieving qualitative analysis and relative quantitation. This method is expected to be applied by more researchers in lipidomics studies in the future.

Rapid evaluation of vegetable oil varieties and geographical origins by ambient corona discharge ionization mass spectrometry

The composition and ratio of unsaturated fatty acids in vegetable oils play a crucial role in determining their overall quality. In this study, we present a corona discharge ionization mass spectrometry (MS) method for the rapid differentiation of vegetable oil varieties and their geographical origins under environmental conditions. Abundant water dimer radical cations, (H2O)2+•, were generated by the ionization setup, which effectively activated carbon‑carbon double bonds (C=C) to form epoxidized products. These epoxidation products were analyzed using tandem MS, generating diagnostic fragment ions that precisely identified CC bond positions. Statistical analysis models were subsequently developed using the resulting MS fingerprint data, revealing significant differences between various vegetable oils and olive oils from different origins. Key advantages of this method include minimal sample preparation, rapid analysis, and easily interpretable spectra. This study provides a new MS-based strategy for food quality assessment and offers a promising tool for identifying CC positional isomers in complex systems.

Tracking the Geometric and Positional Isomerization of Lipid C═C Bonds in the Bacterial Stress Responses by Mass Spectrometry

The position and configuration of the C═C bond have a significant impact on the spatial conformation of unsaturated lipids, which subsequently affects their biological functions. Double bond isomerization of lipids is an important mechanism of bacterial stress response, but its in-depth mechanistic study still lacks effective analytical tools. Here, we developed a visible-light-activated dual-pathway reaction system that enables simultaneous [2 + 2] cycloaddition and catalytic cis-trans isomerization of the C═C bond of unsaturated lipids via directly excited anthraquinone radicals. Density functional theory calculations revealed the oxygen radical addition transition state and the addition-elimination isomerization mechanism of the reaction. A full-dimensional resolution method for C═C bond position and configuration was developed based on the bifunctional reaction and liquid chromatography-mass spectrometry. This method was then applied to the study of bacterial environmental stress response mechanisms. The C═C bond cis-trans and positional isomerization patterns of Pseudomonas membrane lipids under temperature stress were discovered, and the effect of temperature stress on fatty acid biosynthesis was also revealed. This study not only provides an effective tool and key information for the study of bacterial stress response mechanisms, but also enriches the toolbox of visible light chemical reactions.

Sample preparation for fatty acid analysis in biological samples with mass spectrometry-based strategies

Fatty acids (FAs) have attracted many interests for their pivotal roles in many biological processes. Imbalance of FAs is related to a variety of diseases, which makes the measurement of them important in biological samples. Over the past two decades, mass spectrometry (MS) has become an indispensable technique for the analysis of FAs owing to its high sensitivity and precision. Due to complex matrix effect of biological samples and inherent poor ionization efficiency of FAs in MS, sample preparation including extraction and chemical derivatization prior to analysis are often employed. Here, we describe an updated overview of FA extraction techniques, as well as representative derivatization methods utilized in different MS platforms including gas chromatography-MS, liquid chromatography-MS, and mass spectrometry imaging based on different chain lengths of FAs. Derivatization strategies for the identification of double bond location in unsaturated FAs are also summarized and highlighted. The advantages, disadvantages, and prospects of these methods are compared and discussed. This review provides the development and valuable information for sample pretreatment approaches and qualitative and quantitative analysis of interested FAs using different MS-based platforms in complex biological matrices. Finally, the challenges of FA analysis are summarized and the future perspectives are prospected.

Stable-Isotope N-Me Aziridination Enables Accurate Quantitative C═C Isomeric Lipidomics

Accurate lipid quantification is essential to revealing their roles in physiological and pathological processes. However, difficulties in the structural resolution of lipid isomers hinder their further accurate quantification. To address this challenge, we developed a novel stable-isotope N-Me aziridination strategy that enables simultaneous qualification and quantification of unsaturated lipid isomers. The one-step introduction of the 1-methylaziridine structure not only serves as an activating group for the C═C bond to facilitate positional identification but also as an isotopic inserter to achieve accurate relative quantification. The high performance of this reaction for the identification of unsaturated lipids was verified by large-scale resolution of the C═C positions of 468 lipids in serum. More importantly, by using this bifunctional duplex labeling method, various unsaturated lipids such as fatty acids, phospholipids, glycerides, and cholesterol ester were accurately and individually quantified at the C═C bond isomeric level during the mouse brain ischemia. This study provides a new approach to quantitative structural lipidomics.

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.

Development of an Efficient On-Tissue Epoxidation Reaction Mediated by Urea Hydrogen Peroxide for MALDI MS/MS Imaging of Lipid C═C Location Isomers

Unsaturated lipids containing different numbers and locations of C═C bonds are significantly associated with a variety of cellular and metabolic functions. Although matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) has been used to visualize the spatial distribution patterns of various lipids in biological tissues, in situ identification, discrimination, and visualization of lipid C═C location isomers remain challenging. Herein, an efficient and fast on-tissue chemical derivatization (OTCD) approach was developed to pinpoint the locations of C═C bonds in complex lipids in situ via methyltrioxorhenium (MTO)-catalyzed epoxidation of C═C with a urea hydrogen peroxide (UHP)/hexafluoroisopropanol (HFIP) system. The efficiency of OTCD could reach 100% via one-step spray deposition of the solution mixture of MTO/UHP/HFIP at room temperature. The developed OTCD method provided rich structural information on lipid C═C location isomers, and their accurate spatial distribution patterns were resolved in mouse brain tissues. Tissue-specific distributions and changes of lipid C═C location isomers in the liver sections of obese ob/ob and diabetic db/db mice were further investigated, and their correlation in two animal models was revealed. The simplicity and high efficiency of the OTCD method developed for MALDI tandem MSI of lipid C═C location isomers possess great potential for functional spatial lipidomics.

In situ droplet-based on-tissue chemical derivatization for lipid isomer characterization using LESA

In this work, we present an in situ droplet-based derivatization method for fast tissue lipid profiling at multiple isomer levels. On-tissue derivatization for isomer characterization was achieved in a droplet delivered by the TriVersa NanoMate LESA pipette. The derivatized lipids were then extracted and analyzed by the automated chip-based liquid extraction surface analysis (LESA) mass spectrometry (MS) followed by tandem MS to produce diagnostic fragment ions to reveal the lipid isomer structures. Three reactions, i.e., mCPBA epoxidation, photocycloaddition catalyzed by the photocatalyst Ir[dF(CF₃)ppy]₂(dtbbpy)PF₆, and Mn(II) lipid adduction, were applied using the droplet-based derivatization to provide lipid characterization at carbon-carbon double-bond positional isomer and sn-positional isomer levels. Relative quantitation of both types of lipid isomers was also achieved based on diagnostic ion intensities. This method provides the flexibility of performing multiple derivatizations at different spots in the same functional region of an organ for orthogonal lipid isomer analysis using a single tissue slide. Lipid isomers were profiled in the cortex, cerebellum, thalamus, hippocampus, and midbrain of the mouse brain and 24 double-bond positional isomers and 16 sn-positional isomers showed various distributions in those regions. This droplet-based derivatization of tissue lipids allows fast profiling of multi-level isomer identification and quantitation and has great potential in tissue lipid studies requiring rapid sample-to-result turnovers.

Chiral derivatization-enabled discrimination and on-tissue detection of proteinogenic amino acids by ion mobility mass spectrometry

The importance of chiral amino acids (AAs) in living organisms has been widely recognized since the discovery of endogenous d-AAs as potential biomarkers in several metabolic disorders. Chiral analysis by ion mobility spectrometry-mass spectrometry (IMS-MS) has the advantages of high speed and sensitivity but is still in its infancy. Here, an N α-(2,4-dinitro-5-fluorophenyl)-l-alaninamide (FDAA) derivatization is combined with trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) for chiral AA analysis. For the first time, we demonstrate the simultaneous separation of 19 pairs of chiral proteinogenic AAs in a single fixed condition TIMS-MS run. The utility of this approach is presented for mouse brain extracts by direct-infusion TIMS-MS. The robust separation ability in complex biological samples was proven in matrix-assisted laser desorption/ionization (MALDI) TIMS mass spectrometry imaging (MSI) as well by directly depositing 19 pairs of chiral AAs on a tissue slide following on-tissue derivatization. In addition, endogenous chiral amino acids were also detected and distinguished. The developed methods show compelling application prospects in biomarker discovery and biological research.

Novel Aza-Paternò-Büchi Reaction Allows Pinpointing Carbon-Carbon Double Bonds in Unsaturated Lipids by Higher Collisional Dissociation

The evaluation of double bond positions in fatty acyl chains has always been of great concern given their significance in the chemical and biochemical role of lipids. Despite being the foremost technique for lipidomics, it is difficult in practice to obtain identification beyond the fatty acyl level by the sole high-resolution mass spectrometry. Paternò–Büchi reactions of fatty acids (FAs) with ketones have been successfully proposed for pinpointing double bonds in FAs in combination with the collision-induced fragmentation technique. In the present paper, an aza-Paternò–Büchi (aPB) reaction of lipids with 6-azauracil (6-AU) was proposed for the first time for the determination of carbon–carbon double bonds in fatty acyl chains using higher collisional dissociation in the negative ion mode. The method was optimized using free FA and phospholipid analytical standards and compared to the standard Paternò–Büchi reaction with acetone. The introduction of the 6-AU moiety allowed enhancing the ionization efficiency of the FA precursor and diagnostic product ions, thanks to the presence of ionizable sites on the derivatizing agent. Moreover, the aPB derivatization allowed the obtention of deprotonated ions of phosphatidylcholines, thanks to an intramolecular methyl transfer from the phosphocholine polar heads during ionization. The workflow was finally applied for pinpointing carbon–carbon double bonds in 77 polar lipids from an yeast (Saccharomyces cerevisiae) extract.

Dual-resolving of positional and geometric isomers of C=C bonds via bifunctional photocycloaddition-photoisomerization reaction system

The biological functions of lipids largely depend on their chemical structures. The position and configuration of C=C bonds are two of the essential attributes that determine the structures of unsaturated lipids. However, simultaneous identification of both attributes remains challenging. Here, we develop a bifunctional visible-light-activated photocycloaddition-photoisomerization reaction system, which enables the dual-resolving of the positional and geometric isomerism of C=C bonds in lipids when combines with liquid chromatography-mass spectrometry. The dual-pathway reaction mechanism is demonstrated by experiments and density functional theory calculations. Based on this bifunctional reaction system, a workflow of deep structural lipidomics is established, and allows the revealing of unique patterns of cis-trans-isomers in bacteria, as well as the tracking of C=C positional isomers changes in mouse brain ischemia. This study not only offers a powerful tool for deep lipid structural biology, but also provides a paradigm for developing the multifunctional visible-light-induced reaction.

The simultaneous identification of position and configuration of double bonds in unsaturated lipids is challenging. Here, the authors develop a workflow for deep structural lipidomics to address this issue using a bifunctional reaction system combined with liquid chromatography-mass spectrometry, revealing double bond patterns in bacteria and in mouse brain ischemia.

Enthalpic and entropic contributions to interleaflet coupling drive domain registration and antiregistration in biological membrane

Biological membrane is a complex self-assembly of lipids, sterols, and proteins organized as a fluid bilayer of two closely stacked lipid leaflets. Differential molecular interactions among its diverse constituents give rise to heterogeneities in the membrane lateral organization. Under certain conditions, heterogeneities in the two leaflets can be spatially synchronized and exist as registered domains across the bilayer. Several contrasting theories behind mechanisms that induce registration of nanoscale domains have been suggested. Following a recent study showing the effect of position of lipid tail unsaturation on domain registration behavior, we decided to develop an analytical theory to elucidate the driving forces that create and maintain domain registry across leaflets. Towards this, we formulated a Hamiltonian for a stacked lattice system where site variables capture the lipid molecular properties such as the position of unsaturation and various other interactions that could drive phase separation and interleaflet coupling. We solve the Hamiltonian using Monte Carlo simulations and create a complete phase diagram that reports the presence or absence of registered domains as a function of various Hamiltonian parameters. We find that the interleaflet coupling should be described as a competing enthalpic contribution due to interaction of lipid tail termini, primarily due to saturated-saturated interactions, and an interleaflet entropic contribution from overlap of unsaturated tail termini. A higher position of unsaturation is seen to provide weaker interleaflet coupling. Thermodynamically stable nanodomains could also be observed for certain points in the parameter space in our bilayer model, which were further verified by carrying out extended Monte Carlo simulations. These persistent noncoalescing registered nanodomains close to the lower end of the accepted nanodomain size range also point towards a possible "nanoscale" emulsion description of lateral heterogeneities in biological membrane leaflets.

Site-Specific Photochemical Reaction for Improved C=C Location Analysis of Unsaturated Lipids by Ultraviolet Photodissociation

Unraveling the complexity of the lipidome requires the development of novel approaches to facilitate structural identification and characterization of lipid species with isomer-level discrimination. Ultraviolet photodissociation tandem mass spectrometry (UVPD MS/MS) is a promising tool for structure determination of lipids. The sensitivity of UVPD for lipid analysis however is limited mainly due to weak absorption of UV photons by a C=C. Herein, a C=C site-specific derivatization, the Paternò-Büchi (PB) reaction, was used to incorporate a chromophore to the C=C moiety in fatty acyls, leading to significantly improved UVPD efficiency and sensitivity for pinpointing C=C locations. The wavelength-dependent photodissociation of the PB products demonstrated 4-CF₃-benzophenone as the best reagent for UVPD in terms of the efficiency of generating C=C diagnostic fragments and simplicity for C=C location assignments. We demonstrated the effectiveness of this approach for the shotgun profiling of C=C location isomers in different lipid classes from complex lipid extracts, highlighting its potential to advancing the identification of the C=C bond locations in unsaturated lipids.

Pinpointing Alkane Chain Length, Saturation, and Double Bond Regio- and Stereoisomers by Liquid Interfacial Plasmonic Enhanced Raman Spectroscopy

The lipids with a rich diversity of isomers face a formidable challenge in comprehensive structural analysis. The commonly used mass spectrometry-based techniques usually require a considerable number of molecules with sophisticated chemical derivatization or ion mobility separation, but the co-existing of structurally similar isomers often makes the distinction impossible. Here, we develop an alternative powerful liquid/liquid interfacial surface-enhanced Raman spectroscopy (SERS) strategy at normal temperature and pressure without any sources of ionization or radiation. This strategy generates high-resolution fingerprints in molecular chain length, C═C position, saturation, and regio- and stereoisomers of both glycerides and fatty acids and requires only trace amounts of molecules down to 1 ppb to achieve discrimination and exhibits great potentials to push the identification capability to trace levels or even the single-molecule level. According to experimental data and theoretical simulations, these targets have the amphiphilic and emulsifying properties, exhibit ordered molecular orientation and adsorption patterns, promote the co-assembly with plasmonic nanoarrays at the immiscible liquid/liquid interface, and consequently amplify the detection sensitivity. As a contrast, the typical SERS based on solid/air interfacial plasmonic nanoarrays faces the intrinsic bottleneck of extremely weak intensity and indistinguishable spectral fingerprints of lipid molecules. The vibrational fingerprints exhibit a rich range of well-resolved absorption features that are clearly diagnostic for fine structural changes and pave a new way for straightforward measurement without laborsome sample purification, enrichment, or complex derivatization. Although challenging, its unprecedented resolving power expands the potentials of SERS, serving as an ultimate analytical method to provide insights into the detailed structural features of other lipids under facile conditions in the future.

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.

Multi-wavelength visible-light induced [2+2] cycloaddition for identification of lipid isomers in biological samples

Ultraviolet (UV) light-catalyzed Paternò-Büchi (PB) reaction has been developed as an efficient lipid C=C double bond (DB) derivatization strategy, which can accurately assign the position of C=C bond in unsaturated lipids when coupled with tandem mass spectrometry (MS/MS). Inspired by this, here we proposed a novel visible-light induced [2+2] cycloaddition reaction combined with ESI-MS/MS and MALDI-MS/MS to identify lipid C=C position isomers. Benz[g]isoquinoline-5,10-dione (BIQD) and 6,9-difluorobenzo[g]isoquinoline-5,10-dione (DF-BIQD) were developed as a new type of [2+2] cycloaddition reagent, which can not only react with C=C bond under 254 nm UV light irradiation, but also quickly combine with lipid C=C bond under the irradiation of 405 nm visible-light and > 400 nm compact fluorescent lamp visible-light. High cycloaddition reaction conversion efficiency can be achieved by irradiating under compact fluorescent lamp light for 2 min. Moreover, we discovered that 437 nm, 489 nm, 545 nm, 581 nm, and 613 nm monochromatic light appearing in compact fluorescent lamp can individually induce the [2 + 2] cycloaddition reaction between DF-BIQD and unsaturated lipids. Using this method, we found that the expressions of lipid DB-positional isomers in rat heart, brain, lung, spleen, thymus, kidney, liver and plasma vary greatly. The relative content of FA-18:1 (Δ9) in rat heart is only 1.49 times that of FA-18:1 (Δ11), while the relative content of FA-18:1 (Δ9) in rat plasma is 5.20 times that of FA-18:1 (Δ11). The above results offer new insight into the development of photocatalytic reagent for visible-light induced [2+2] cycloaddition and structural lipidomic studies.

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.

Recent strategies used in the synthesis of saturated four-membered heterocycles

The importance and prevalance of O-, N-, and S-atom containing saturated four-membered ring motifs in biologically active molecules and potential therapeutics continues to drive efforts in their efficient synthetic preparation. In this review, general and recent strategies for the synthesis of these heterocycles are presented. Due to the limited potential bond disconnections, retrosynthetic strategies are broadly limited to cyclizations and cycloadditions. Nonetheless, diverse approaches for accessing cyclization precursors have been developed, ranging from nucleophilic substitution to C-H functionalization. Innovative methods for substrate activation have been developed for cycloadditions under photochemical and thermal conditions. Advances in accessing oxetanes, azetidines, and thietanes remain active areas of research with continued breakthroughs anticipated to enable future applications.

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.

A liquid chromatography-mass spectrometry workflow for in-depth quantitation of fatty acid double bond location isomers

Tracing compositional changes of fatty acids (FAs) is frequently used as a means of monitoring metabolic alterations in perturbed biological states. Given that more than half of FAs in the mammalian lipidome are unsaturated, quantitation of FAs at a carbon-carbon double bond (C=C) location level is necessary. The use of 2-acetylpiridine (2-acpy) as the charge-tagging PB reagent led to a limit of identification in the subnanomolar range for mono- and polyunsaturated as well as conjugated FAs. Conjugated free FAs of low abundance such as FA 18:2 (n-7, n-9) and FA 18:2 (n-6, n-8) were quantified at concentrations of 0.61 ± 0.05 and 0.05 ± 0.01 mg per 100 g in yak milk powder, respectively. This workflow also enabled deep profiling of eight saturated and 37 unsaturated total FAs across a span of four orders of magnitude in concentration, including ten groups of C=C location isomers in pooled human plasma. A pilot survey on total FAs in plasma from patients with type 2 diabetes revealed that the relative compositions of FA 16:1 (n-10) and FA 18:1 (n-10) were significantly elevated compared with that of normal controls. In this work, we have developed a workflow for global quantitation of FAs, including C=C location isomers, via charge-tagging Paternò-Büchi (PB) derivatization and liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Minimizing transient microenvironment-associated variability for analysis of environmental anthropogenic contaminants via ambient ionization

The rapid and quantitative analysis of anthropogenic contaminants in environmental matrices is crucial for regulatory testing and to elucidate the environmental fate of these pollutants. Direct ambient mass spectrometry (AMS) methodologies greatly increase sample throughput, can be adapted for onsite analysis and are often regarded as semi-quantitative by most developed protocols. One of the limitations of AMS, especially for on site analysis applications, is the irreproducibility of the measurements related to the occurrence of transient microenvironments (TME) and variable background interferences. In this work we report an effective strategy to minimize these effects by hyphenating, for the first time, solid phase microextraction (SPME) arrow to mass spectrometry via a thermal desorption unit (TDU) and direct analysis in real time (DART) source. The developed method was optimized for the extraction and analysis of pesticides and pharmaceuticals from surface water. It was demonstrated that the hyphenation of the SPME and TDU-DART resulted in reduced background contamination, indicating the suitability of the method for onsite analysis even in variable and non-ideal environments. Model analytes were quantitated in the low μg/L range with a total analysis time of less than 5 min, linear dynamic ranges (LDR) and interday reproducibility for most compounds being 2.5-500 μg/L and lower than 10%, respectively. The developed approach provides an excellent analytical tool that can be applied for the onsite high-throughput analysis of water samples as well as air and aereosols. Considering the tunability of our extraction process, time-resolved environmental monitoring can be achieved onsite within minutes.

Quantification and molecular imaging of fatty acid isomers from complex biological samples by mass spectrometry

Elucidating the isomeric structure of free fatty acids (FAs) in biological samples is essential to comprehend their biological functions in various physiological and pathological processes. Herein, we report a novel approach of using peracetic acid (PAA) induced epoxidation coupled with mass spectrometry (MS) for localization of the C[double bond, length as m-dash]C bond in unsaturated FAs, which enables both quantification and spatial visualization of FA isomers from biological samples. Abundant diagnostic fragment ions indicative of the C[double bond, length as m-dash]C positions were produced upon fragmentation of the FA epoxides derived from either in-solution or on-tissue PAA epoxidation of free FAs. The performance of the proposed approach was evaluated by analysis of FAs in human cell lines as well as mapping the FA isomers from cancer tissue samples with MALDI-TOF/TOF-MS. Merits of the newly developed method include high sensitivity, simplicity, high reaction efficiency, and capability of spatial characterization of FA isomers in tissue samples.

Screening of lipid metabolism biomarkers in patients with coronary heart disease via ultra-performance liquid chromatography-high resolution mass spectrometry

Coronary heart disease (CHD) has a high mortality worldwide. This study aimed to screen lipid metabolism biomarkers in patients with coronary heart disease via ultra-performance liquid chromatography-high resolution mass spectrometry. Extraction and reconstitution solvents, liquid chromatographic and mass spectrometry conditions were optimized to detect more plasma lipid metabolites. In this study, the chromatographic and mass spectra characteristics of lipid metabolites were summarized. A total of 316 lipid metabolites were annotated via diagnostic fragment ion filtration, nitrogen rule filtration, and neutral loss filtration. Glycerophospholipid metabolism and sphingolipid metabolism were revealed as the main lipid disorders of CHD. This study provides a novel insight for high-throughput detection of lipid metabolites in plasma and provides a further understanding of the occurrence of CHD, which can provide valuable suggestions for the prevention of CHD.

Mass spectrometric analysis of lipid A obtained from the lipopolysaccharide ofPasteurella multocida

LC/MS-based variant profiling of lipid A component of endotoxic lipopolysaccharides ofPasteurella multocidatype B:2, a causative agent of haemorrhagic septicaemia in water buffalo and cattle.