Charging and Charge Switching of Unsaturated Lipids and Apolar Compounds Using Paternò-Büchi Reactions

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.

Paternò-Büchi Reaction Mass Spectrometry Enables Positional Assignment of Polymethylene-Interrupted Double Bonds in Food-Derived Lipids

Fatty acids (FAs) containing polymethylene-interrupted (PMI) double bonds are a component of human foods; however, they present a significant analytical challenge for de novo identification. Covalent adduct chemical ionization and ozone-induced dissociation mass spectrometry (MS) methods enable unambiguous assignment of PMI-FA double bond positions, however, no method has been reported with electrospray ionization (ESI) platform using off-the-shelf systems. In the current work, we studied the Paternò-Büchi (PB) fragmentation patterns of PMI-FA and triacylglycerol (TG) by analyzing several known food sources. PB-MS/MS and MS3 enabled complete double bond location assignments, including the isolated double bond in PMI-FA and triacylglycerols. Sea urchin ("uni"), oyster, pine nut, and ginkgo nut were characterized for their signature PMI-FA, 20:2(5Z,11Z), 22:2(7Z, 15Z), 18:3(5Z,9Z,12Z), and 20:3(5Z,11Z,14Z), respectively. Quantitative analyses of the relative abundance of these PMI-FA led to results similar to reference methods. 18:3(5Z,9Z,12Z) was enriched at the sn-1/sn-3 position in pine nut major TG.

Fatty acid isomerism: analysis and selected biological functions

The biological functions of fatty acids and the lipids in which they are esterified are determined by their chain length, double bond position and geometry and other structural motifs such as the presence of methyl branches. Unusual isomeric features in fatty acids of human foods such as conjugated double bonds or chain branching found in dairy products, some seeds and nuts, and marine foods potentially have important effects on human health. Recent advancements in identifying fatty acids with unusual double bond positions and pinpointing the position of methyl branches have empowered the study of their biological functions. We present recent advances in fatty acid structural elucidation by mass spectrometry in comparison with the more traditional methods. The double bond position can be determined by purely instrumental methods, specifically solvent-mediated covalent adduct chemical ionization (SM-CACI) and ozone induced dissociation (OzID), with charge inversion methods showing promise. Prior derivatization using the Paternò-Büchi (PB) reaction to yield stable structures that, upon collisional activation, yield the double bond position has emerged. The chemical ionization (CI) based three ion monitoring (MRM) method has been developed to simultaneously identify and quantify low-level branched chain fatty acids (BCFAs), unattainable by electron ionization (EI) based methods. Accurate identification and quantification of unusual fatty acid isomers has led to research progress in the discovery of biomarkers for cancer, diabetes, nonalcoholic fatty liver disease (NAFLD) and atherosclerosis. Modulation of eicosanoids, weight loss and the health significance of BCFAs are also presented. This review clearly shows that the improvement of analytical capacity is critical in the study of fatty acid biological functions, and stronger coupling of the methods discussed here with fatty acid mechanistic research is promising in generating more refined outcomes.

Charge inversion under plasma-nanodroplet reaction conditions excludes Fischer esterification for unsaturated fatty acids: a chemical approach for type II isobaric overlap

Direct infusion ionization methods provide the highest throughput strategy for mass spectrometry (MS) analysis of low-volume samples. But the trade-off includes matrix effects, which can significantly reduce analytical performance. Herein, we present a novel chemical approach to tackle a special type of matrix effect, namely type II isobaric overlap. We focus on detailed investigation of a nanodroplet-based esterification chemistry for differentiating isotopologue [M + 2] signal due to unsaturated fatty acid (FA) from the monoisotopic signal from a saturated FA. The method developed involves the online fusion of nonthermal plasma with charged nanodroplets, enabling selective esterification of saturated FAs. We discovered that unsaturated FAs undergo spontaneous intramolecular reaction via a novel mechanism based on a carbocation intermediate to afford a protonated lactone moiety (resonance stabilized cyclic carbonium ion), whose mass is the same as the original protonated unsaturated FA. Therefore, the monoisotopic signal from any saturated FA can be selectively shifted away from the mass-to-charge position where the isobaric interference occurs to enable effective characterization by MS. The mechanism governing the spontaneous intramolecular reactions for unsaturated FAs was validated with DFT calculations, experimentation with standards, and isotope labeling. This novel insight achieved via the ultrafast plasma-nanodroplet reaction environment provides a potentially useful synthetic pathway to achieve catalyst-free lactone preparation. Analytically, we believe the performance of direct infusion MS can be greatly enhanced by combining our approach with prior sample enrichment steps for applications in biomedicine and food safety. Also, combination with portable mass spectrometers can improve the efficiency of field studies since front-end separation is not possible under such conditions.

Permethylation and Metal Adduction: A Toolbox for the Improved Characterization of Glycolipids with Cyclic Ion Mobility Separations Coupled to Mass Spectrometry

Lipids are an important class of molecules involved in various biological functions but remain difficult to characterize through mass-spectrometry-based methods because of their many possible isomers. Glycolipids, specifically, play important roles in cell signaling but display an even greater level of isomeric heterogeneity as compared to other lipid classes stemming from the introduction of a carbohydrate and its corresponding linkage position and α/β anomericity at the headgroup. While liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) remains the gold standard technique in lipidomics, it is still unable to characterize all isomeric species, thus presenting the need for new, orthogonal, methodologies. Ion mobility spectrometry-mass spectrometry (IMS-MS) can provide an additional dimension of information that supplements LC-MS/MS workflows, but has seen little use for glycolipid analyses. Herein, we present an analytical toolbox that enables the characterization of various glycolipid isomer sets using high-resolution cyclic ion mobility separations coupled with mass spectrometry (cIMS-MS). Specifically, we utilized a combination of both permethylation and metal adduction to fully resolve isomeric sphingolipids and ceramides with our cIMS-MS platform. We also introduce a new metric that can enable comparing peak-to-peak resolution across varying cIMS-MS pathlengths. Overall, we envision that our presented methodologies are highly amenable to existing LC-MS/MS-based workflows and can also have broad utility toward other omics-based analyses.

Formation and Tandem Mass Spectrometry of Doubly Charged Lipid-Metal Ion Complexes

Phospholipids are major components of most eukaryotic cell membranes. Changes in metabolic states are often accompanied by phospholipid structure variations. The structural changes of phospholipids are the hallmark of disease states, or specific lipid structures have been associated with distinct organisms. Prime examples are microorganisms that synthesize phospholipids with, for example, different branched chain fatty acids. Assignment and relative quantitation of structural isomers of phospholipids that arise from attachment of different fatty acids to the glycerophospholipid backbone are difficult with routine tandem mass spectrometry or with liquid chromatography without authentic standards. In this work, we report on the observation that all investigated phospholipid classes form doubly charged lipid-metal ion complexes during electrospray ionization (ESI) and show that these complexes can be used to assign lipid classes and fatty acid moieties, distinguish isomers of branched chain fatty acids, and relatively quantify these isomers in positive-ion mode. Use of water free methanol and addition of divalent metal salts (100 mol %) to ESI spray solutions afford highly abundant doubly charged lipid-metal ion complexes (up to 70 times of protonated compounds). Higher-energy collisional dissociation and collision-induced dissociation of doubly charged complexes yield a diverse set of lipid class-dependent fragment ions. In common for all lipid classes is the liberation of fatty acid-metal adducts that yield fragment ions from the fatty acid hydrocarbon chain upon activation. This ability is used to pinpoint sites of branching in saturated fatty acids and is showcased for free fatty acids as well as glycerophospholipids. The analytical utility of doubly charged phospholipid-metal ion complexes is demonstrated by distinguishing fatty acid branching-site isomers in phospholipid mixtures and relatively quantifying the corresponding isomeric compounds.

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.

Isobaric 6-plex and tosyl dual tagging for the determination of positional isomers and quantitation of monounsaturated fatty acids using rapid UHPLC-MS/MS

Isobaric labelling of fatty acids is complicated by chromatographic co-elution of double bond isomers. This produces contaminated spectra which can mask important biological changes. Here two derivatization strategies are combined to improve throughput and produce MS² reporters which change mass depending on double bond position. A 6-plex isobaric tag is attached to the acid group, followed by the tosylation of the double bond using chloramine-T. These two derivatizations allowed for the chromatographic resolution of nearly all investigated isomers using a 3.5 minute ultrafast method. Further isomer differentiation is achieved upon fragmentation as reporter masses scale with the double bond location. This occurs by a dual-fragmentation route which reveals the isobaric labelling and fragments along the double bond of each analyte. These unique fragments allowed for accurate quantitation of co-isolated double bond isomers where traditional isobaric tags would experience ratio distortion. Saturated and monounsaturated fatty acids were characterized by this rapid 6-plex method and produced an average signal RSD of 9.3% and R² of 0.99. The method was then used to characterize fatty acid dysregulation upon inhibition of stearoyl CoA desaturase with CAY10566.

Quantitative Analysis and Structural Elucidation of Fatty Acids by Isobaric Multiplex Labeling Reagents for Carbonyl-Containing Compound (SUGAR) Tags and m-CPBA Epoxidation

In this study, a novel analytical method was developed to investigate fatty acids (FAs) for relative quantification, carbon-carbon double-bond localization, and cis-/trans-geometry differentiation by isobaric multiplex labeling reagents for carbonyl-containing compound (SUGAR) tag conjugation and meta-chloroperoxybenzoic acid (m-CPBA) epoxidation. FAs are essential components of cells and have diverse functions in energy storage and as complex lipid constituents. It has been reported that FAs play different roles in various biological processes such as the functional development of the brain. The comprehensive characterization and quantification of FAs are crucial to further elucidate their biological roles. However, it is challenging to perform relative quantification and structural elucidation of FAs using integrated mass spectrometry (MS)-based methods. Recently, our group developed isobaric multiplex SUGAR tags for quantitative glycomics. Besides aldehyde/ketone groups on glycans, hydrazide groups also possess reactivity toward carboxylic acids on FAs. In this study, we extended SUGAR tag labeling with FAs for the quantitative analysis by liquid chromatography (LC)-MS/MS in the positive ion mode and applied this strategy for the comparative analysis of FAs hydrolyzed from oil samples. In addition, to comprehensively elucidate the structures of unsaturated FAs, epoxidation by m-CPBA was performed before SUGAR tag labeling to enable carbon-carbon double-bond localization. Moreover, the cis- and trans-geometries of carbon-carbon double bonds in multiple pairs of monounsaturated FAs could also be differentiated in higher-energy collisional dissociation (HCD)-MS/MS. This study developed a high-throughput comprehensive FA analysis platform, which could be widely applied and utilized in biological and clinical studies.

Liquid Chromatography-Mass Spectrometry (LC-MS) Derivatization-Based Methods for the Determination of Fatty Acids in Biological Samples

Fatty acids (FAs) play pleiotropic roles in living organisms, acting as signaling molecules and gene regulators. They are present in plants and foods and may affect human health by food ingestion. As a consequence, analytical methods for their determination in biological fluids, plants and foods have attracted high interest. Undoubtedly, mass spectrometry (MS) has become an indispensable technique for the analysis of FAs. Due to the inherent poor ionization efficiency of FAs, their chemical derivatization prior to analysis is often employed. Usually, the derivatization of the FA carboxyl group aims to charge reversal, allowing detection and quantification in positive ion mode, thus, resulting in an increase in sensitivity in determination. Another approach is the derivatization of the double bond of unsaturated FAs, which aims to identify the double bond location. The present review summarizes the various classes of reagents developed for FA derivatization and discusses their applications in the liquid chromatography-MS (LC-MS) analysis of FAs in various matrices, including plasma and feces. In addition, applications for the determination of eicosanoids and fatty acid esters of hydroxy fatty acids (FAHFAs) are discussed.

An enzymatic tandem reaction to produce odor-active fatty aldehydes

Abstract

Aldehydes represent a versatile and favored class of flavoring substances. A biocatalytic access to odor-active aldehydes was developed by conversion of fatty acids with two enzymes of the α-dioxygenase pathway. The recombinant enzymes α-dioxygenase (α-DOX) originating from Crocosphaera subtropica and fatty aldehyde dehydrogenase (FALDH) from Vibrio harveyi were heterologously expressed in E. coli, purified, and applied in a coupled (tandem) repetitive reaction. The concept was optimized in terms of number of reaction cycles and production yields. Up to five cycles and aldehyde yields of up to 26% were achieved. Afterward, the approach was applied to sea buckthorn pulp oil as raw material for the enzyme catalyzed production of flavoring/fragrance ingredients based on complex aldehyde mixtures. The most abundant fatty acids in sea buckthorn pulp oil, namely palmitic, palmitoleic, oleic, and linoleic acid, were used as substrates for further biotransformation experiments. Various aldehydes were identified, semi-quantified, and sensorially characterized by means of headspace–solid phase microextraction–gas chromatography–mass spectrometry–olfactometry (HS–SPME–GC–MS–O). Structural validation of unsaturated aldehydes in terms of double-bond positions was performed by multidimensional high-resolution mass spectrometry experiments of their Paternò–Büchi (PB) photoproducts. Retention indices and odor impressions of inter alia (Z,Z)-5,8-tetradecadienal (Z,Z)-6,9-pentadecadienal, (Z)-8-pentadecenal, (Z)-4-tridecenal, (Z)-6-pentadecenal, and (Z)-8-heptadecenal were determined for the first time.

Key points

• Coupled reaction of Csα-DOX and VhFALDH yields chain-shortened fatty aldehydes.

• Odors of several Z-unsaturated fatty aldehydes are described for the first time.

• Potential for industrial production of aldehyde-based odorants from natural sources.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-12134-3.

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.

Chloramine-T-Enabled Mass Spectrometric Analysis of C═C Isomers of Unsaturated Fatty Acids and Phosphatidylcholines in Human Thyroids

Specific locations of carbon-carbon double bonds (C═C) in lipids often play an essential role in biological processes, and there has been a booming development in C═C composition analysis by mass spectrometry. However, a universal derivatization and fragmentation pattern for the annotation of C═C positions in lipids is still challenging and attractive. To expand this field in lipidomics, a flexible and convenient N-tosylaziridination method was developed, with high derivatization efficiency, sensitivity, and specificity. The derivatization was very fast (15 s), and C═C numbers as well as locations could be pinpointed specifically in tandem mass spectra. By qualitative and quantitative studies of paratumor and tumor thyroid tissues of human beings, the total content of unsaturated fatty acids was suggested to be increased in tumor tissues, and good correlations in and between lysophosphatidylcholines and phosphatidylcholines were revealed by Spearman analysis. Further studies of C═C isomers showed that n-6/n-3 ratios were closely associated with human thyroid tumorigenesis, and high ratios of n-6/n-3 isomers seemed to suffer a high risk of carcinogenesis. Other isomers were not very representative; however, C═C in n-9/n-7 could also be significant for oncology research. Generally, it is supposed that both total amounts and C═C isomer ratios were related to cancer, and N-tosylaziridine derivatization could provide an alternative strategy for the C═C isomer study of disease models.

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.

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.

Regio-distribution and double bond locations of unsaturated fatty acids in phospholipids of bovine milk

Hundreds of phospholipid (PL) species with defined fatty acid (FA) composition have been identified previously in bovine milk using liquid chromatography tandem mass spectrometry (LC-MS/MS). Paterno-Buchi photochemical reaction coupled with LC-MS/MS was applied in this study to further unravel the regio-distribution and double bond (DB) locations of FAs. Using SPE-purified PLs and 2-acetylpyridine as the photochemical derivatization reagent, we were able to reveal the non-specific regio-distribution of unsaturated FAs and the widespread occurrence of regioisomers in milk PLs. Although Δ9 and Δ9,12 were found to be the predominant DB location(s) for C18:1 and C18:2 respectively, other DB positional isomers such as C18:1Δ11, C18:1Δ12 and C18:1Δ13 and C18:2Δ9,11 were widely detected in PL structures, implying that the minor isomers of C18:1 and C18:2 equally participate in the synthesis of PLs. Our study provides novel information on the fine structure of milk PLs and further underlines the complexity of milk lipid composition.

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).

Pseudotargeted Lipidomics Strategy Enabling Comprehensive Profiling and Precise Lipid Structural Elucidation of Polyunsaturated Lipid-Rich Echium Oil

Echium oil has great nutritional value as a result of its high content of α-linolenic acid (ALA, 18:3ω-3) and stearidonic acid (SDA, 18:4ω-3). However, the comprehensive lipid profiling and exact structural characterization of bioactive polyunsaturated lipids in echium oil have not yet been obtained. In this study, we developed a novel pseudotargeted lipidomics strategy for comprehensive profiling and lipid structural elucidation of polyunsaturated lipid-rich echium oil. Our approach integrated untargeted lipidomics analysis with a targeted lipidomics strategy based on Paternò-Büchi (PB)-tandem mass spectrometry (MS/MS) using 2-acetylpyridine (2-AP) as the reaction reagent, allowing for high-coverage lipid profiling and simultaneous determination of C═C locations in triacylglycerols (TGs), diacylglycerols (DGs), free fatty acids (FFAs), and sterol esters (SEs) in echium oil. A total of 209 lipid species were profiled, among which 162 unsaturated lipids were identified with C═C location assignment and 42 groups of ω-3 and ω-6 C═C location isomers were discovered. In addition, relative isomer ratios of certain groups of lipid C═C location isomers were revealed. This pseudotargeted lipidomics strategy described in this study is expected to provide new insight into structural characterization of distinctive bioactive lipids in food.

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.

Deep Structural Annotation of Glycerolipids by the Charge-Tagging Paterno-Büchi Reaction and Supercritical Fluid Chromatography-Ion Mobility Mass Spectrometry

Glycerolipids (GLs) are essential for cellular lipid homeostasis, while dysregulation in GL metabolism is often associated with the onset or progression of human-related metabolic diseases. The profile of GLs is thus frequently used as a molecular readout for disease phenotyping. Although mass spectrometry (MS) is the method of choice for GL profiling, the current MS methods are unable to differentiate two major types of structural isomers due to the fact that fatty acyls can be linked to different positions on the glycerol backbone (sn-positions) and the site(s) of unsaturation in acyl chains. Herein, by utilizing charge-tagging Paterno-Büchi (PB) derivatization of carbon-carbon double bond (C═C), supercritical fluid chromatography (SFC), and mobility aligned tandem mass spectrometry (MS/MS), a workflow has been developed for the sensitive and structurally informative analysis of GLs. SFC allows fast separation (within 25 min) of sn-isomers of diacylglycerols (DGs) and separation of triacylglycerols (TGs) of different chain lengths and degrees of unsaturation. Time-aligned parallel fragmentation enables multiple-stage MS/MS of the PB-derivatized lipids in a high-throughput fashion and allows pinpointing C═C location to a specific fatty acyl chain. This workflow reveals the presence of more than 500 molecular structures of neutral lipids from pooled human plasma. A comparison of human plasma samples between type 2 diabetes (N = 7) and control (N = 7) shows significant changes in isomer compositions (C18:1 Δ9 vs Δ11) from nine groups of TG and DG. These findings suggest that the developed workflow can be potentially applied to lipid marker discovery for disease monitoring or diagnosis.

Single-cell lipidomics with high structural specificity by mass spectrometry

Single-cell analysis is critical to revealing cell-to-cell heterogeneity that would otherwise be lost in ensemble analysis. Detailed lipidome characterization for single cells is still far from mature, especially when considering the highly complex structural diversity of lipids and the limited sample amounts available from a single cell. We report the development of a general strategy enabling single-cell lipidomic analysis with high structural specificity. Cell fixation is applied to retain lipids in the cell during batch treatments prior to single-cell analysis. In addition to tandem mass spectrometry analysis revealing the class and fatty acyl-chain for lipids, batch photochemical derivatization and single-cell droplet treatment are performed to identify the C=C locations and sn-positions of lipids, respectively. Electro-migration combined with droplet-assisted electrospray ionization enables single-cell mass spectrometry analysis with easy operation but high efficiency in sample usage. Four subtypes of human breast cancer cells are correctly classified through quantitative analysis of lipid C=C location or sn-position isomers in ~160 cells. Most importantly, the single-cell deep lipidomics strategy successfully discriminates gefitinib-resistant cells from a population of wild-type human lung cancer cells (HCC827), highlighting its unique capability to promote precision medicine.

Structural elucidation of triacylglycerol using online acetone Paternò-Büchi reaction coupled with reversed-phase liquid chromatography mass spectrometry

Triacylglycerol (TG) is a class of lipids that is responsible for energy storage and cell metabolism in biological systems; it is found in relatively high abundances in biological fluids such as human plasma. Due to structural complexity, analyzing TGs using shotgun lipidomic approaches is challenging because of the presence of multiple fatty acyl compositional isomers. In this work, reversed-phase liquid chromatography (RPLC) was used for separation of TG species due to the capability of separating lipids based on fatty acyl chain lengths and degrees of unsaturation. RPLC alone does not provide structurally informative information for the location of carbon-carbon double-bonds (C[double bond, length as m-dash]Cs) without using synthesized standards that correspond to each species analyzed. The Paternò-Büchi (PB) reaction was employed online to confidently characterize the location of C[double bond, length as m-dash]Cs within lipid species via photo-initiated modification of the alkene group with acetone, which was later subjected to electrospray ionization (ESI) and tandem mass spectrometry (MS/MS) to form signature fragmentation peaks. This online RPLC-PB-MS/MS system was able to distinguish fatty acyl level and C[double bond, length as m-dash]C level isomeric species. The systems allowed for the identification of 46 TG molecular species in human plasma with confident C[double bond, length as m-dash]C location assignment in fatty acyls at a limit of identification of 50 nM.

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.

Shedding light on isomeric FAHFA lipid structures using 213 nm ultraviolet photodissociation mass spectrometry

Fatty Acid Esters of Hydroxy Fatty Acids (FAHFAs) are a recently discovered class of biological active lipids with anti-diabetic and anti-inflammatory functions. Given that structure and function are intimately related, we report here the use of direct infusion multi-stage hybrid tandem mass spectrometry involving sequential Collisional Activated Dissociation (CAD) and 213 nm UltraViolet PhotoDissociation (UVPD), as a novel technique for the unambiguous denovo identification and detailed structural characterisation of FAHFA lipid ions, including determination of the esterified fatty acid identity, the hydroxy fatty acid identity and position of esterification, and localization of the site(s) of endogenous unsaturations, without need for chromatographic separation or authentic reference standards. The utility of this approach is demonstrated for the identification of individual FAHFA lipids introduced to the mass spectrometer in positive ionization mode as their lithiated adducts, as well as from mixtures containing isomeric FAHFA species with differing esterification sites, including those that are not resolved by current liquid chromatography methods.

Multifunctional Reactive MALDI Matrix Enabling High-Lateral Resolution Dual Polarity MS Imaging and Lipid C═C Position-Resolved MS2 Imaging

Local lipid variations in tissues are readily revealed with mass spectrometry imaging (MSI) methods, and the resulting lipid distributions serve as bioanalytical signatures to reveal cell- or tissue-specific lipids. Comprehensive MSI lipid mapping requires measurements in both ion polarities. Additionally, structural lipid characterization is necessary to link the lipid structure to lipid function. Whereas some structural elements of lipids are readily derived from high-resolution mass spectrometry (MS) and tandem-MS (MSⁿ), the localization of C═C double bonds (DBs) requires specialized fragmentation and/or functionalization methods. In this work, we identify a multifunctional matrix-assisted laser desorption/ionization (MALDI) matrix for spatially resolved lipidomics investigations that reacts with lipids in Paternò-Büchi (PB) reactions during laser irradiation facilitating DB-position assignment and allows dual-polarity high-resolution MALDI-MSI and MALDI MS²I studies. By screening 12 compounds for improved ionization efficiency in positive-/negative-ion mode and the functionalization yield compared to the previously introduced reactive MALDI matrix benzophenone, 2-benzoylpyridine (BzPy) is identified as the best candidate. The new matrix enables DB localization of authentic standards belonging to 12 lipid classes and helps to assign 133/58 lipid features in positive-/negative-ion mode from mouse cerebellum tissue. The analytical capabilities of BzPy as a multifunctional MALDI-MSI matrix are demonstrated by imaging endogenous and PB-functionalized lipids in mouse kidney sections with 7 μm lateral resolution in both ion modes. Tracking diagnostic lipid DB-position fragment ions in mouse pancreatic tissue with down to 10 μm pixel size allows us to identify the islets of Langerhans associated with lipid isomer upregulation and depletion.

Double bond localization in unsaturated rhamnolipid precursors 3-(3-hydroxyalkanoyloxy)alkanoic acids by liquid chromatography-mass spectrometry applying online Paternò-Büchi reaction

Lipids are biomolecules with a broad variety of chemical structures, which renders them essential not only for various biological functions but also interestingly for biotechnological applications. Rhamnolipids are microbial glycolipids with surface-active properties and are widely used biosurfactants. They are composed of one or two L-rhamnoses and up to three hydroxy fatty acids. Their biosynthetic precursors are 3-hydroxy(alkanoyloxy)alkanoic acids (HAAs). The latter are also present in cell supernatants as complex mixtures and are extensively studied for their potential to replace synthetically derived surfactants. The carbon chain lengths of HAAs determine their physical properties, such as their abilities to foam and emulsify, and their critical micelle concentration. Despite growing biotechnological interest, methods for structural elucidation are limited and often rely on hydrolysis and analysis of free hydroxy fatty acids losing the connectivity information. Therefore, a high-performance liquid chromatography-mass spectrometry method was developed for comprehensive structural characterization of intact HAAs. Information is provided on chain length and number of double bonds in each hydroxy fatty acid and their linkage by tandem mass spectrometry (MS/MS). Post-column photochemical derivatization by online Paternὸ-Büchi reaction and MS/MS fragmentation experiments generated diagnostic fragments allowing structural characterization down to the double bond position level. Furthermore, the presented experiments demonstrate a powerful approach for structure elucidation of complex lipids by tailored fragmentation.

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.

Structural elucidation of hydroxy fatty acids by photodissociation mass spectrometry with photolabile derivatives

RATIONALE

Eicosanoids are short-lived bio-responsive lipids produced locally from oxidation of polyunsaturated fatty acids (FAs) via a cascade of enzymatic or free radical reactions. Alterations in the composition and concentration of eicosanoids are indicative of inflammation responses and there is strong interest in developing analytical methods for the sensitive and selective detection of these lipids in biological mixtures. Most eicosanoids are hydroxy FAs (HFAs), which present a particular analytical challenge due to the presence of regioisomers arising from differing locations of hydroxylation and unsaturation within their structures.

METHODS

In this study, the recently developed derivatization reagent 1-(3-(aminomethyl)-4-iodophenyl)pyridin-1-ium (4-I-AMPP⁺ ) was applied to a representative set of HFAs including bioactive eicosanoids. Photodissociation (PD) mass spectra obtained at 266 nm of 4-I-AMPP⁺ -modified HFAs exhibit abundant product ions arising from photolysis of the aryl-iodide bond within the derivative with subsequent migration of the radical to the hydroxyl group promoting fragmentation of the FA chain and facilitating structural assignment.

RESULTS

Representative polyunsaturated HFAs (from the hydroxyeicosatetraenoic acid and hydroxyeicosapentaenoic acid families) were derivatized with 4-I-AMPP⁺ and subjected to a reversed-phase liquid chromatography workflow that afforded chromatographic resolution of isomers in conjunction with structurally diagnostic PD mass spectra.

CONCLUSIONS

PD of these complex HFAs was found to be sensitive to the locations of hydroxyl groups and carbon-carbon double bonds, which are structural properties strongly associated with the biosynthetic origins of these lipid mediators.

Revelation of Acyl Double Bond Positions on Fatty Acyl Coenzyme A Esters by MALDI/TOF Mass Spectrometry

Fatty acyl coenzyme A esters (FA-CoAs) are important crossroad intermediates in lipid catabolism and anabolism, and the structures are complicated. Several mass spectrometric approaches have been previously described to elucidate their structures. However, a direct mass spectrometric approach toward a complete identification of the molecule, including the location of unsaturated bond(s) in the fatty acid chain has not been reported. In this study, we applied a simple MALDI/TOF mass spectrometric method to a near-complete characterization of long-chain FA-CoAs, including the location(s) of the double bond in the fatty acyl chain, and the common structural features that recognize FA-CoAs. Negative ion mass spectra of saturated, monounsaturated, and polyunsaturated FA-CoAs were acquired with a MALDI/TOF mass spectrometer using 2,5-dihydroxybenzoic acid as the matrix and ionized with a laser fluence two folds of the threshold to induce the in-source fragmentation (ISF) of the analytes. The resulting ISF spectra contained fragment ions arising from specific cleavages of the C-C bond immediate adjacent to the acyl double-bond. This structural feature affords locating the double-bond position(s) of the fatty acyl substituent. Thereby, positional isomer such as 18:3(n - 3) and 18:3(n - 6) FA-CoA can be differentiated without applying tandem mass spectrometry.

Visible-Light-Driven [2 + 2] Photocycloadditions between Benzophenone and C═C Bonds in Unsaturated Lipids

The [2 + 2] photocycloaddition of alkenes and carbonyls is of fundamental interest and practical importance, as this process is extensively involved in oxetane-ring constructions. Although individual carbonyl group or alkene moiety has been utilized as photoactive species for oxetane formations upon ultraviolet photoexcitation, direct excitation of the entire noncovalent complex involving alkene and carbonyl substrates to achieve [2 + 2] photocycloadditions is rarely addressed. Herein, complexes with noncovalent interactions between benzophenone and C═C bonds in unsaturated lipids have been successfully characterized, and for the first time a [2 + 2] cycloaddition leading to the formation of oxetanes has been identified under visible-light irradiation. The mechanism of this reaction is distinctly different from the well-studied Paternò-Büchi reaction. The entire complexes characterized as dimeric proton-bonded alkene and carbonyl substrates can be excited under visible light, leading to electron transfer from the alkene moiety in fatty acyls to the carbonyl group within the complex. These results provide new insight into utilizing noncovalent complexes for the synthesis of oxetanes in which the excitation wavelength becomes independent of each individual substrate.

Investigating C[double bond, length as m-dash]C positions and hydroxylation sites in lipids using Paternò-Büchi functionalization mass spectrometry

Lipid oxidation plays a major role in biochemical processes and nutrition. Structural changes during oxidation can lead to alterations of lipid functions. Rancidification and production of secondary lipid messengers are well-known examples for the impact of oxidation on lipid function. Especially lipids with a high degree of unsaturation are prone to oxidize. In order to investigate structural changes of lipids upon oxidation, we here introduce a photochemical Paternò-Büchi functionalization workflow and subsequent mass spectrometric analysis for analysis of unsaturated, oxidized lipids. Results for hydroxylated fatty acids and triglycerides containing isolated and conjugated C[double bond, length as m-dash]C bonds will be presented making use of 3-acetylpyridine as a photochemically active compound. Photochemical derivatization is performed in nano-electrospray emitter tips in 30 s resulting in the formation of oxetanes without inducing light-triggered oxidation of analytes. Collisional-activation of photoproducts facilitates selective cleavage of oxetane moieties. Resulting fragment ions not only allow the determination of C[double bond, length as m-dash]C bond locations for isolated and conjugated C[double bond, length as m-dash]C bonds but also restrict the site of oxidation. By registering the mass shift in some fragment ions of +15.99 Da due to hydroxylation, the oxidized sections of lipids can be identified. In order to demonstrate its analytical robustness, the method is applied to determine the structural impact of non-selective ambient oxidation on fatty acids, triglycerides and complex triglyceride mixtures obtained from Sacha inchi oil.

Large-scale lipid analysis with C=C location and sn-position isomer resolving power

Lipids play a pivotal role in biological processes and lipid analysis by mass spectrometry (MS) has significantly advanced lipidomic studies. While the structure specificity of lipid analysis proves to be critical for studying the biological functions of lipids, current mainstream methods for large-scale lipid analysis can only identify the lipid classes and fatty acyl chains, leaving the C=C location and sn-position unidentified. In this study, combining photochemistry and tandem MS we develop a simple but effective workflow to enable large-scale and near-complete lipid structure characterization with a powerful capability of identifying C=C location(s) and sn-position(s) simultaneously. Quantitation of lipid structure isomers at multiple levels of specificity is achieved and different subtypes of human breast cancer cells are successfully discriminated. Remarkably, human lung cancer tissues can only be distinguished from adjacent normal tissues using quantitative results of both lipid C=C location and sn-position isomers.

Industrial Riboflavin Fermentation Broths Represent a Diverse Source of Natural Saturated and Unsaturated Lactones

Fermentation broths of Ashbya gossypii from the industrial production of riboflavin emit an intense floral, fruity, and nutty smell. Typical Ehrlich pathway products, such as 2-phenylethan-1-ol and 2-/3-methylbutan-1-ol, were detected in large amounts as well as some intensely smelling saturated and unsaturated lactones, e.g., γ-decalactone and γ-(Z)-dodec-6-enlactone. An aroma extract dilution analysis identified 2-phenylethan-1-ol and γ-(Z)-dodec-6-enlactone as the main contributors to the overall aroma, with flavor dilution factors of 32 768. The position of the double bonds of unsaturated lactones was determined by the Paternò-Büchi reaction, and reference compounds that were not available commercially were synthesized to elucidate the structures of the uncommon lactones. The absolute configuration and enantiomeric excess values of the lactones were determined by converting the lactones to their corresponding Mosher's esters. In addition, the odor impressions and odor thresholds in air were determined.

Olefinic reagents tested for peptide derivatization with switchable properties: Stable upon collision induced dissociation and cleavable by in-source Paternò-Büchi reactions

This contribution is part of our ongoing efforts to develop innovative cross-linking (XL) reagents and protocols for facilitated peptide mixture analysis and efficient assignment of cross-linked peptide products. In this report, we combine in-source Paternò-Büchi (PB) photo-chemistry with a tandem mass spectrometry approach to selectively address the fragmentation of a tailor-made cross-linking reagent. The PB photochemistry, so far exclusively used for the identification of unsaturation sites in lipids and in lipidomics, is now introduced to the field of chemical cross-linking. Based on trans-3-hexenedioic acid, an olefinic homo bifunctional amine reactive XL reagent was designed and synthesized for this proof-of-principle study. Condensation products of the olefinic reagent with a set of exemplary peptides are used to test the feasibility of the concept. Benzophenone is photochemically reacted in the nano-electrospray ion source and forms oxetane PB reaction products. Subsequent CID-MS triggered retro-PB reaction of the respective isobaric oxetane molecular ions and delivers reliably and predictably two sets of characteristic fragment ions of the cross-linker. Based on these signature ion sets, a straightforward identification of covalently interconnected peptides in complex digests is proposed. Furthermore, CID-MSⁿ experiments of the retro-PB reaction products deliver peptide backbone characteristic fragment ions. Additionally, the olefinic XL reagents exhibit a pronounced robustness upon CID-activation, without previous UV-excitation. These experiments document that a complete backbone fragmentation is possible, while the linker-moiety remains intact. This feature renders the new olefinic linkers switchable between a stable, noncleavable cross-linking mode and an in-source PB cleavable mode.

Lipid Alterations during Zebrafish Embryogenesis Revealed by Dynamic Mass Spectrometry Profiling with C=C Specificity

Lipids exert substantial influences on vertebrate embryogenesis, but their metabolic dynamics at detailed structural levels remains elusive, primarily owing to the lack of a tool capable of resolving their huge structural diversity. Herein, we present the first large-scale and spatiotemporal monitoring of unsaturated lipids with C=C specificity in single developing zebrafish embryos enabled by photochemical derivatization and tandem mass spectrometry (MS). The lipid isomer composition was found extremely stable in yolk throughout embryogenesis, while notable differences in ratios of C=C location (e.g., PC 16:0_16:1 (7) vs. 16:0_16:1 (9)) and fatty acyl composition isomers (e.g., PC 16:1_18:1 vs. 16:0_18:2) were unveiled between blastomeres and yolk from zygote to 4 h post fertilization (hpf). From 24 hpf onwards, lipid isomer compositions in embryo head and tail evolved distinctively with development, suggesting a meticulously regulated lipid remodeling essential for cell division and differentiation. This work has laid the foundation for functional studies of structurally defined lipids in vertebrate embryology.

Mapping Lipid C=C Location Isomers in Organ Tissues by Coupling Photochemical Derivatization and Rapid Extractive Mass Spectrometry

Lipid desaturation plays important roles in biological processes and the disease states. Here, we report a simple but efficient method for mapping unsaturated phospholipids including the spatial distribution of lipid C=C location isomers in animal organs by coupling the C=C specific derivatization with direct analysis mass spectrometry (MS). Lipids are sampled directly by a stainless-steel wire from rat brain or kidney, extracted, and derivatized via the Paternò-Büchi reaction in a glass emitter of the nanoelectrospray ionization (nanoESI) source. Subsequent analysis by nanoESI-tandem mass spectrometry reveals C=C locations and relative quantities of lipid C=C location isomers. Unsaturated lipids, such as phospholipids and free fatty acids, have been identified with ion intensities spanning two orders of magnitude in rat brain. Typical sample consumption is less than 10 μg/measurement and the time for each analysis is about 3 min. This method should serve as a complementary method to high spatial resolution mass spectrometry imaging techniques, because it offers a streamlined experimental workflow for rapid profiling of lipids with C=C specificity to enable such applications as point-of-care disease diagnostics.

Reactive Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging Using an Intrinsically Photoreactive Paternò-Büchi Matrix for Double-Bond Localization in Isomeric Phospholipids

The location and identity of phospholipids (PLs) within tissues can serve as diagnostic markers for tissue types or diseases. Whereas mass spectrometry imaging (MSI) has emerged as a powerful bioanalytical tool to visualize PL distributions, inferring PL identities from MSI experiments is challenging. Especially, C═C double-bond (DB) positions are not identifiable in most MSI experiments. Herein, we introduce benzophenone (BPh) as a novel reactive matrix for matrix-assisted laser desorption/ionization (MALDI). BPh promotes desorption/ionization and simultaneously serves as derivatization reagent that allows functionalization of unsaturated PLs during the MALDI process via a laser-light driven Paternò-Büchi (PB) reaction without the need for additional equipment. Using BPh, PB product ions of numerous PL classes are readily generated to pinpoint the location of DBs. High lateral resolution MSI results of DB-position isomers are presented, highlighting the capabilities of BPh as a PB-reactive MALDI matrix to potentially unveil the impact of DB-position isomers in PL metabolism.

Reshaping Lipid Biochemistry by Pushing Barriers in Structural Lipidomics

Lipidomics is a rapidly growing field with numerous examples showing the importance of lipid molecules throughout biology. It has also shed light onto the vast and complex functions performed by many lipids that possess an immense diversity in molecular structures. Mass spectrometry (MS) is the tool of choice for analyzing lipids and has been the key catalyst driving the field forward. However, MS does not yet permit true molecular lipidomics wherein the identification and quantification of lipids having defined molecular structures can be routinely achieved. Here we describe recent advances in MS-based lipidomics that allow access to higher levels of molecular information in lipidomics experiments. These advances will form a key piece of the puzzle as the field moves towards systems characterization of lipids at the molecular level.