Charge-remote and charge-driven fragmentation processes in diacyl glycerophosphoethanolamine upon low-energy collisional activation: a mechanistic proposal

Gas-Phase Ion/Ion Reactions to Enable Radical-Directed Dissociation of Fatty Acid Ions: Application to Localization of Methyl Branching

Methyl branching on the carbon chains of fatty acids and fatty esters is among the structural variations encountered with fatty acids and fatty esters. Branching in fatty acid/ester chains is particularly prominent in bacterial species and, for example, in vernix caseosa and sebum. The distinction of branched chains from isomeric straight-chain species and the localization of branching can be challenging to determine by mass spectrometry (MS). Condensed-phase derivatization strategies, often used in conjunction with separations, are most commonly used to address the identification and characterization of branched fatty acids. In this work, a gas-phase ion/ion strategy is presented that obviates condensed-phase derivatization and introduces a radical site into fatty acid ions to facilitate radical-directed dissociation (RDD). The gas-phase approach is also directly amenable to fatty acid anions generated via collision-induced dissociation from lipid classes that contain fatty esters. Specifically, divalent magnesium complexes bound to two terpyridine ligands that each incorporate a ((2,2,6,6-tetramethyl-1-piperidine-1-yl)oxy) (TEMPO) moiety are used to charge-invert fatty acid anions. Following the facile loss of one of the ligands and the TEMPO group of the remaining ligand, a radical site is introduced into the complex. Subsequent collision-induced dissociation (CID) of the complex exhibits preferred cleavages that localize the site(s) of branching. The approach is illustrated with iso-, anteiso-, and isoprenoid branched-chain fatty acids and an intact glycerophospholipid and is applied to a mixture of branched- and straight-chain fatty acids derived from Bacillus subtilis.

Methyl carbamates of phosphatidylethanolamines and phosphatidylserines reveal bacterial contamination in mitochondrial lipid extracts of mouse embryonic fibroblasts

The occurrence of methyl carbamates of phosphatidylethanolamines and phosphatidylserines in the lipid extract of mitochondria obtained from mouse embryonic fibroblasts was ascertained by hydrophilic interaction liquid chromatography with electrospray ionization single and multi-stage mass spectrometry, performed using sinergically a high resolution (quadrupole-Orbitrap) and a low resolution (linear ion trap) spectrometer. Two possible routes to the synthesis of methyl carbamates of phospholipids were postulated and evaluated: (i) a chemical transformation involving phosgene, occurring as a photooxidation by-product in the chloroform used for lipid extraction, and methanol, also used for the latter; (ii) an enzymatic methoxycarbonylation reaction due to an accidental bacterial contamination, that was unveiled subsequently on the murine mitochondrial sample. A specific lipid extraction performed on a couple of standard phosphatidyl-ethanolamines/-serines, based on purposely photo-oxidized chloroform and deuterated methanol, indicated route (i) as negligible in the specific case, thus highlighting the enzymatic route related to bacterial contamination as the most likely source of methyl carbamates. The unambiguous recognition of the latter might represent the starting point toward a better understanding of their generation in biological systems and a minimization of their occurrence when an artefactual formation is ascertained.

Lipidome of the Bacteroides Genus Containing New Peptidolipid and Sphingolipid Families Revealed by Multiple-Stage Mass Spectrometry

The anaerobic bacteria of the Bacteroides fragilis group including Bacteroides thetaiotaomicron, B. fragilis, Bacteroides vulgatus, and Bacteroides ovatus in genus Bacteroides are among the most commonly found human gut microbiota. They are generally commensal but are also opportunistic pathogens. Both the inner and outer membranes of the Bacteroides cell envelope contain abundant lipids with diversified structures, and dissection of the lipid composition of the inner and outer membrane fractions is important for understanding the biogenesis of this multilaminate wall structure. Here, we describe mass spectrometry-based approaches to delineate in detail the lipidome of the membrane and the outer membrane vesicle of the bacteria cells. We identified 15 lipid class/subclasses (>100 molecular species), including sphingolipid families [dihydroceramide (DHC), glycylseryl (GS) DHC, DHC-phosphoinositolphosphoryl-DHC (DHC-PIP-DHC), ethanolamine phosphorylceramide, inositol phosphorylceramide (IPC), serine phosphorylceramide, ceramide-1-phosphate, and glycosyl ceramide], phospholipids [phosphatidylethanolamine, phosphatidylinositol (PI), and phosphatidylserine], peptide lipids (GS-, S-, and G-lipids) and cholesterol sulfate, of which several have not been reported previously, or have similar structures to those found in Porphyromonas gingivalis, the periodontopathic bacterium in oral microbiota. The new DHC-PIPs-DHC lipid family is found only in B. vulgatus, which, however, lacks the PI lipid family. The galactosyl ceramide family is exclusively present in B. fragilis, which nevertheless lacks IPC and PI lipids. The lipidomes as revealed in this study demonstrate the lipid diversity among the various strains and the utility of multiple-stage mass spectrometry (MSn) with high-resolution mass spectrometry in the structural elucidation of complex lipids.

Glycerophospholipidomics of Five Edible Oleaginous Microgreens

Microgreens are a special type of vegetal product, born as a culinary novelty (traditionally used to garnish gourmet dishes) and then progressively studied for their potentially high content in nutraceuticals, like polyphenolic compounds, carotenoids, and glucosinolates, also in the perspective of implementing their cultivation in space stations/colonies. Among further potential nutraceuticals of microgreens, lipids have received very limited attention so far. Here, glycerophospholipids contained in microgreens of typical oleaginous plants, namely, soybean, chia, flax, sunflower, and rapeseed, were studied using hydrophilic interaction liquid chromatography (HILIC), coupled to high-resolution Fourier transform mass spectrometry (FTMS) or low-resolution collisionally induced dissociation tandem mass spectrometry (CID-MS²) with electrospray ionization (ESI). Specifically, this approach was employed to obtain qualitative and quantitative profiling of the four main classes of glycerophospholipids (GPL) found in the five microgreens, i.e., phosphatidylcholines (PC), phosphatidylethanolamines (PE), phosphatidylglycerols (PG), and phosphatidylinositols (PI). Saturated chains with 16 and 18 carbon atoms and unsaturated 18:X (with X = 1-3) chains emerged as the most common fatty acyl substituents of those GPL; a characteristic 16:1 chain (including a C═C bond between carbon atoms 3 and 4) was also found in some PG species. Among polyunsaturated acyl chains, the 18:3 one, likely referred mainly to α-linolenic acid, exhibited a relevant incidence, with the highest estimated amount (corresponding to 160 mg per 100 g of lyophilized vegetal tissue) found for chia. This outcome opens interesting perspectives for the use of oleaginous microgreens as additional sources of essential fatty acids, especially in vegetarian/vegan diets.

Recommendations for good practice in MS-based lipidomics

In the last 2 decades, lipidomics has become one of the fastest expanding scientific disciplines in biomedical research. With an increasing number of new research groups to the field, it is even more important to design guidelines for assuring high standards of data quality. The Lipidomics Standards Initiative is a community-based endeavor for the coordination of development of these best practice guidelines in lipidomics and is embedded within the International Lipidomics Society. It is the intention of this review to highlight the most quality-relevant aspects of the lipidomics workflow, including preanalytics, sample preparation, MS, and lipid species identification and quantitation. Furthermore, this review just does not only highlights examples of best practice but also sheds light on strengths, drawbacks, and pitfalls in the lipidomic analysis workflow. While this review is neither designed to be a step-by-step protocol by itself nor dedicated to a specific application of lipidomics, it should nevertheless provide the interested reader with links and original publications to obtain a comprehensive overview concerning the state-of-the-art practices in the field.

Study on the CID Fragmentation Pathways of Deprotonated 4'-Monophosphoryl Lipid A

Lipid A, the membrane-bound phosphoglycolipid component of bacteria, is held responsible for the clinical syndrome of gram-negative sepsis. In this study, the fragmentation behavior of a set of synthetic lipid A derivatives was studied by electrospray ionization multistage mass spectrometry (ESI-MSn), in conjunction with tandem mass spectrometry (MS/MS), using low-energy collision-induced dissociation (CID). Genealogical insight about the fragmentation pathways of the deprotonated 4'-monophosphoryl lipid A structural analogs led to proposals of a number of alternative dissociation routes that have not been reported previously. Each of the fragment ions was interpreted using various possible mechanisms, consistent with the principles of reactions described in organic chemistry. Specifically, the hypothesized mechanisms are: (i) cleavage of the C-3 primary fatty acid leaves behind an epoxide group attached to the reducing sugar; (ii) cleavage of the C-3' primary fatty acid (as an acid) generates a cyclic phosphate connected to the nonreducing sugar; (iii) cleavage of the C-2' secondary fatty acid occurs both in acid and ketene forms; iv) the C-2 and C-2' primary fatty acids are eliminated as an amide and ketene, respectively; (v) the 0,2A2 cross-ring fragment contains a four-membered ring (oxetanose); (vi) the 0,4A2 ion is consecutively formed from the 0,2A2 ion by retro-aldol, retro-cycloaddition, and transesterification; and (vii) formations of H2PO4- and PO3- are associated with the formation of sugar epoxide. An understanding of the relation between 0,2A2 and 0,4A2-type sugar fragments and the different cleavage mechanisms of the two ester-linked primary fatty acids is invaluable for distinguishing lipid A isomers with different locations of a single ester-linked fatty acid (i.e., at C-3 or C-3'). Thus, in addition to a better comprehension of lipid A fragmentation processes in mass spectrometers, our observations can be applied for a more precise elucidation of naturally occurring lipid A structures.

Direct infusion and ultra-high-performance liquid chromatography/electrospray ionization tandem mass spectrometry analysis of phospholipid regioisomers

RATIONALE

Phospholipids are important components of cell membranes that are linked to several beneficial health effects such as increasing plasma HDL cholesterol levels, improving cognitive abilities and inhibiting growth of colon cancer. The role of phospholipid (PL) regioisomers in all these health effects is, however, largely not studied due to lack of analytical methods.

METHODS

Electrospray ionization mass spectrometry in negative mode produces structurally informative fragment ions resulting from differential dissociation of fatty acids (FAs) from the sn-1 and sn-2 positions, primarily high-abundance [RCOO]^- ions. The fragment ion ratios obtained with different ratios of regiopure phospholipid reference compounds were used to construct calibration curves, which allow determination of regioisomeric ratios of an unknown sample. The method was developed using both direct infusion mass spectrometry (MS) and ultra-high-performance liquid chromatography and hydrophilic interaction liquid chromatography mass spectrometry (UHPLC-HILIC-MS).

RESULTS

The produced calibration curves have high coefficients of determination (R²  >0.98) and the fragment ion ratios in replicate analyses were very consistent. A test mixture containing 60/40% ratios of all available regioisomer pairs was analyzed to test and validate the functionality of the calibration curves. The results were accurate and reproducible. However, regioisomeric quantification of certain chromatographically overlapping compounds is restricted by the relatively wide window in precursor ion selection of the MS instrument used.

CONCLUSIONS

This method establishes a framework for analysis of phospholipid regioisomers. Specific regioisomers can be quantified using the existing data, and method development will continue with improving chromatographic separation and exploring the fragmentation patterns and efficiencies of different PL classes and FA combinations, ultimately to refine this method for routine analysis of natural fats and oils.

HILIC-ESI-MS analysis of phosphatidic acid methyl esters artificially generated during lipid extraction from microgreen crops

The uncontrolled activation of endogenous enzymes may introduce both qualitative and quantitative artefacts when lipids are extracted from vegetal matrices. In the present study, a method based on hydrophilic interaction liquid chromatography coupled either to high-resolution/accuracy Fourier-transform mass spectrometry (HILIC-ESI-FTMS) or to linear ion trap multiple stage mass spectrometry (HILIC-ESI-MSn , with n = 2 and 3) with electrospray ionization was developed to unveil one of those artefacts. Specifically, the artificial generation of methyl esters of phosphatidic acids (MPA), catalysed by endogenous phospholipase D (PLD) during lipid extraction from five oleaginous microgreen crops (chia, soy, flax, sunflower and rapeseed), was studied. Phosphatidylcholines (PC) and phosphatidylglycerols (PG) were found to be the most relevant precursors of MPA among glycerophospholipids (GPLs), being involved in a transphosphatidylation process catalysed by PLD and having methanol as a coreactant. The combination of MS2 and MS3 measurements enabled the unambiguous recognition of MPA from their fragmentation pathways, leading to distinguish them from isobaric PA including a further CH2 group on their side chains. PLD was also found to catalyse the hydrolysis of PC and PG to phosphatidic acids (PAs). The described transformations were confirmed by the remarkable decrease of MPA abundance observed when isopropanol, known to inhibit PLD, was tentatively adopted instead of water during the homogenization of microgreens. The unequivocal identification of MPA might be exploited to assess if GPL alterations are actually triggered by endogenous PLD during lipid extractions from specific vegetal tissues.

Analytical Methodologies for Lipidomics in Hemp Plant

The chemical composition of Cannabis sativa L. has been extensively studied for tens of years, but little is known about its lipidome. This chapter describes an analytical workflow for polar lipid determination in hemp. After extraction, lipids are enriched and isolated by graphitized carbon black sorbent, and the isolated lipid is analyzed by liquid chromatography (LC) coupled with high resolution mass spectrometry, leading to identification of many lipid species. We have developed a semi-automated platform using commercially available Lipostar software for lipid identification. Our approach affords the identification of 189 polar lipids in hemp extract, including sulfolipids and phospholipids. The number of the identified lipid species is by far the highest ever reported for Cannabis sativa.

Characterization of the Uncommon Lipid Families in Corynebacterium glutamicum by Mass Spectrometry

This book chapter provides readers the step-by-step instruction for cell growth, lipid isolation, and lipid analysis to obtain the lipidome of Corynebacterium glutamicum (C. glutamicum) in the genus Corynebacterium, a biotechnologically important bacterium. We separate the lipid families by preparative HPLC with an analytical C-8 column, followed by linear ion-trap multiple stage mass spectrometry (LIT MSⁿ) with high-resolution mass measurement to define the structures of cytidine diphosphate diacylglycerol (CDP-DAG), glucuronosyl diacylglycerol (GlcA-DAG), α-D-mannopyranosyl-(1 → 4)-α-D-glucuronyl diacylglycerol (Man-GlcA-DAG), 1-mycolyl-2-acyl-phosphatidylglycerol (MA-PG), and acyl trehalose monomycolate (acyl-TMM) whose structures have been previously mis-assigned or not defined by mass spectrometric means. We also define the structures of mycolic acid, phosphatidylglycerol, phosphatidylinositol, cardiolipin, trehalose dimycolate lipids in the cell wall. The similarity of the lipidome to that in the Mycobacterium genera is consistent with the notion that Corynebacterium and Mycobacterium are gram-positive bacteria belonging to the suborder Corynebacterineae.

Lipidomics of the Edible Brown Alga Wakame (Undaria pinnatifida) by Liquid Chromatography Coupled to Electrospray Ionization and Tandem Mass Spectrometry

The lipidome of a brown seaweed commonly known as wakame (Undaria pinnatifida), which is grown and consumed around the world, including Western countries, as a healthy nutraceutical food or supplement, was here extensively examined. The study was focused on the characterization of phospholipids (PL) and glycolipids (GL) by liquid chromatography (LC), either hydrophilic interaction LC (HILIC) or reversed-phase LC (RPLC), coupled to electrospray ionization (ESI) and mass spectrometry (MS), operated both in high and in low-resolution mode. Through the acquisition of single (MS) and tandem (MS/MS) mass spectra more than 200 PL and GL of U. pinnatifida extracts were characterized in terms of lipid class, fatty acyl (FA) chain composition (length and number of unsaturations), and regiochemistry, namely 16 SQDG, 6 SQMG, 12 DGDG, 5 DGMG, 29 PG, 8 LPG, 19 PI, 14 PA, 19 PE, 8 PE, 38 PC, and 27 LPC. The FA (C16:0) was the most abundant saturated acyl chain, whereas the monounsaturated C18:1 and the polyunsaturated C18:2 and C20:4 chains were the prevailing ones. Odd-numbered acyl chains, iJ., C15:0, C17:0, C19:0, and C19:1, were also recognized. While SQDG exhibited the longest and most unsaturated acyl chains, C18:1, C18:2, and C18:3, in the sn-1 position of glycerol, they were preferentially located in the sn-2 position in the case of PL. The developed analytical approach might pave the way to extend lipidomic investigations also for other edible marine algae, thus emphasizing their potential role as a source of bioactive lipids.

Structural identification of sour compounds in wine and tea by ambient ionization mass spectrometry according to characteristic product ion and neutral loss

Sourness is an important food taste for human. A rapid, accurate method was used to generalize the structure similarity and diversity of sour compounds. Based on the product ion and neutral loss of sour compounds, ambient ionization techniques combined with quadrupole-Orbitrap mass spectrometry (AI-Q-Orbitrap) was employed. According to the behavior of sour compounds in the process of high collision dissociation (HCD) of MS/MS, three fragmentation pathway schemes were proposed: (1) charge-driven fragmentation and CO₂ loss, (2) six-membered ring rearrangement and C_α-C_β cleavage, and (3) elimination rearrangement and H₂O, CO₂ and CO loss in succession. Besides, structure information about characteristic product ions and characteristic neutral losses was summarized. Finally, multi-class sour compounds including monoacids, diacids, polyacids and phenolic acids in wine and tea were identified and compared. Therefore, sour compounds and their structure information can be determined by AI-MS based on characteristic product ion and neutral loss.

Identification of unusual phospholipids from bovine heart mitochondria by HPLC-MS/MS

Phospholipids, including ether phospholipids, are composed of numerous isomeric and isobaric species that have the same backbone and acyl chains. This structural resemblance results in similar fragmentation patterns by collision-induced dissociation of phospholipids regardless of class, yielding complicated MS/MS spectra when isobaric species are analyzed together. Furthermore, the presence of isobaric species can lead to misassignment of species when made solely based on their molecular weights. In this study, we used normal-phase HPLC for ESI-MS/MS analysis of phospholipids from bovine heart mitochondria. Class separation by HPLC eliminates chances for misidentification of isobaric species from different classes of phospholipids. Chromatography yields simple MS/MS spectra without interference from isobaric species, allowing clear identification of peaks corresponding to fragmented ions containing monoacylglycerol backbone derived from losing one acyl chain. Using these fragmented ions, we characterized individual and isomeric species in each class of mitochondrial phospholipids, including unusual species, such as PS, containing an ether linkage and species containing odd-numbered acyl chains in cardiolipin, PS, PI, and PG. We also characterized monolysocardiolipin and dilysocardiolipin, the least abundant but nevertheless important mitochondrial phospholipids. The results clearly show the power of HPLC-MS/MS for identification and characterization of phospholipids, including minor species.

Coupled Mass-Spectrometry-Based Lipidomics Machine Learning Approach for Early Detection of Clear Cell Renal Cell Carcinoma

A discovery-based lipid profiling study of serum samples from a cohort that included patients with clear cell renal cell carcinoma (ccRCC) stages I, II, III, and IV (n = 112) and controls (n = 52) was performed using ultraperformance liquid chromatography coupled to quadrupole-time-of-flight mass spectrometry and machine learning techniques. Multivariate models based on support vector machines and the LASSO variable selection method yielded two discriminant lipid panels for ccRCC detection and early diagnosis. A 16-lipid panel allowed discriminating ccRCC patients from controls with 95.7% accuracy in a training set under cross-validation and 77.1% accuracy in an independent test set. A second model trained to discriminate early (I and II) from late (III and IV) stage ccRCC yielded a panel of 26 compounds that classified stage I patients from an independent test set with 82.1% accuracy. Thirteen species, including cholic acid, undecylenic acid, lauric acid, LPC(16:0/0:0), and PC(18:2/18:2), identified with level 1 exhibited significantly lower levels in samples from ccRCC patients compared to controls. Moreover, 3α-hydroxy-5α-androstan-17-one 3-sulfate, cis-5-dodecenoic acid, arachidonic acid, cis-13-docosenoic acid, PI(16:0/18:1), PC(16:0/18:2), and PC(O-16:0/20:4) contributed to discriminate early from late ccRCC stage patients. The results are auspicious for early ccRCC diagnosis after validation of the panels in larger and different cohorts.

Identification and quantification of phospholipids in strawberry seeds and pulp (Fragaria × ananassa cv San Andreas) by liquid chromatography with electrospray ionization and tandem mass spectrometry

An extensive characterization and quantification of intact phospholipids (PLs) in strawberry (Fragaria × ananassa cv San Andreas) seed and pulp was carried out by hydrophilic interaction liquid chromatography (HILIC) and electrospray ionization (ESI) coupled to either Fourier-transform (FT) orbital-trap or linear ion-trap tandem mass spectrometry (LIT-MS/MS). More than 150 intact polar lipids including phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), phosphatidylglycerols (PGs), phosphatidic acids (PAs), phosphatidylinositols (PIs), lysophosphatidylcholines (LPCs), and lysophosphatidylethanolamines (LPEs) were identified in negative ESI mode. PC 18:2/18:2 and 18:2/18:3 were found to be the major components of strawberry lipid extracts at concentrations of 230 ± 36 and 189 ± 32 μg/g, respectively, in seeds and at concentrations of 330 ± 50 and 140 ± 22 μg/g, respectively, in pulp. The lipidic extracts of both strawberry seeds and pulp exhibited the dominance of LPC 16:0/0:0 at a content of 132 ± 19 and 114 ± 16 μg/g, respectively, and LPC 0:0/18:2 at 236 ± 20 and 150 ± 20 μg/g, respectively. The other most abundant species of strawberry seeds and pulp were PE 18:2/18:2, 40 ± 9 and 190 ± 40 μg/g, followed by PI 16:0/18:2, 51 ± 15 and 24 ± 8 μg/g, respectively, while PG, PA, and LPE show comparable abundance below 10 μg/g. The most recurrent fatty acyl substituents of PLs were C18:3 (α-linolenic acid), C18:2 (linoleic acid), C18:1 (oleic acid), C18:0 (stearic acid), C16:0 (palmitic acid), and relatively high contents of a shorter chain such as C14:0 (myristic acid).

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.

Regiochemical Assignment of N-Acylphosphatidylethanolamines (NAPE) by Liquid Chromatography/Electrospray Ionization with Multistage Mass Spectrometry and Its Application to Extracts of Lupin Seeds

1,2-Diacyl-sn-glycero-3-phospho-N-acyl-ethanolamines (NAPE) are low abundance phospholipids but important constituents of intracellular membranes of plant tissues, responsible for generating bioactive N-acylethanolamine (NAE), which participates in several physiological processes such as regulation of seed germination and protection against pathogenic attacks. From an analytical point of view, the critical aspect of these bioactive lipids lies in the determination of fatty acyl chains located in sn-1/sn-2 position on the glycerol backbone (O-linked), along with the amide-bound (N-linked) fatty acyl chain. Here, the identity and occurrence of NAPE in lipid extracts of lupin seeds (Lupinus luteus L.) was assessed by electrospray ionization in negative ion mode upon reversed-phase liquid chromatography (RPLC-ESI) coupled to mass spectrometry (MS) either at high- (i.e., Orbitrap FTMS) or low- (linear ion trap, LIT) resolution/accuracy. Collisional induced dissociation (CID)-tandem MS and MS³ acquisitions of chemically prepared NAPE allowed to unequivocally recognize the N-linked fatty acyl chain and to establish the diagnostic product ions that were successfully applied to identify NAPE in lipid extracts of yellow lupin seeds. The most abundant NAPE species were those containing N-acyl groups C18:1, C18:2; a minor prevalence was found for C16:0, C18:0, and C18:3, and almost the same acyl chains O-linked on the glycerol backbone in several sn-1/sn-2 combinations were observed. The positional isomers of NAPE species were identified as deprotonated molecules ([M-H]^-) at m/z 978.7541 (three isomers 52:3), m/z 980.7694 (two isomers 52:2), m/z 1002.7535 (four isomers 54:5), m/z 1004.7686 (two isomers 54:4), m/z 1006.7837 (two isomers 54:3), and m/z 1008.8026 (single isomer 54:2). The total amount of NAPE in lupin seeds ranged in the interval of 2.00 ± 0.13 mg/g dw, in agreement with other edible legumes. We anticipate our approach to be a robust assessment method potentially applicable to biological extracts containing NAPE species and can provide comprehensive profiles and contents.

Arsenosugar Phospholipids (As-PL) in Edible Marine Algae: An Interplay between Liquid Chromatography with Electrospray Ionization Multistage Mass Spectrometry and Phospholipases A1 and A2 for Regiochemical Assignment

The chemical identity of arsenosugar phospholipids (As-PL) as mono- (i.e., lyso, L-As-PL) and diacyl-arsenosugar PL in four edible and common marine alga samples, such as nori (Porphyra spp.), wakame (Undaria pinnatifida), dulse (Palmaria palmata), and kombu (Saccharina japonica), was successfully investigated. Adopting negative polarity electrospray ionization (ESI), not common for As-PL, conjugated with hydrophilic interaction liquid chromatography (HILIC) and mass spectrometry (MS), performed either at low resolution using a linear ion trap (LIT) with sequential MSn (n = 2, 3) or at high resolution using a high-resolution/high-accuracy Fourier-transform MS (FTMS), based on an orbital trap instrument, more than 20 As-PL and 2 L-As-PL species were identified. The absence of As-PL standard compounds encouraged us to generate an in-house-built database of As-PL/L-As-PL for a rapid and simple classification. Despite their compositional diversity, tandem MS of deprotonated As-PL and L-As-PL ([M - H]-) demonstrated the occurrence of a highly diagnostic product ion at m/z 389.0 ([AsC10H19O9P]-). The fatty acid composition and distribution of As-PL were easily assigned on the basis of the ratio intensity between sn-1 and sn-2 product ions. Indeed, the preferential formation of [R1C3H5O4P]- ions over [R2C3H5O4P]- ions, both containing the glycerol backbone, enabled the regiochemical assignment of As-PL. These outcomes were confirmed by MSn (n = 2, 3) analyses and using sn-1- and sn-2-regioselective hydrolase enzymes (i.e., phospholipases A1 and A2). The predominant As-PL's in samples of nori (red alga), wakame, and kombu (both brown algae) were identified as containing palmitic acyl chains (i.e., As-PL958 (As-PL 16:0/16:0) with ca. 66 ± 3, 82 ± 4, and 58 ± 3% as relative abundances, respectively), while the main species in dulse (red alga) samples was As-PL982 (As-PL 18:1/16:1) at ca. 38 ± 3%.

Charge migration fragmentation in the negative ion mode of cyclopentenone and cyclopentanone intermediates in the biosynthesis of jasmonates

RATIONALE

Jasmonates are formed from 12-oxo-10,15(Z)-phytodienoic acid (12-OPDA) in plants and also from 12-oxo-10-phytoenoic acid (12-OPEA) in fungi. Collision-induced dissociation (CID) of [M-H]^- generates characteristic product anions at m/z 165 [C₁₁ H₁₇ O]^- . Our goal was to investigate the structure and mode of formation of this anion by CID of 12-OPDA, 12-OPEA, and 12-oxophytonoic acid (12-OPA).

METHODS

We investigated the CID of the [M-H]^- , [M-H-CO₂ ]^- , and [M-H-H₂ O]^- anions using electrospray ionization and MS/MS analysis of 12-OPDA, 12-OPEA, and 12-OPA, and compared the results with the data obtained with the corresponding compounds labeled with ² H at C-6 and C-7 and with structural and side chain analogs.

RESULTS

CID of [6,6,7,7-² H₄ ]12-OPEA and [6,6-² H₂ ]12-OPDA ([M-H]^- and [M-H-CO₂ ]^- ) showed that one or two ² H atoms were transferred to anions at m/z 165 as judged by the signal intensities of m/z 165 + 1 or 165 + 2, respectively. CID of [6,6-² H₂ ]- and [6,6,7,7-² H₄ ]-12-OPA ([M-H]^- and [M-H-CO₂ ]^- ) yielded the loss of H₂ from the cyclopentanone and displayed the transfer of one ² H atom in analogy to 12-OPEA. In contrast, CID of [6,6,7,7-² H₄ ]12-OPEA and [6,6,7,7-² H₄ ]12-OPA [M-H-H₂ O]^- demonstrated the transfer of two ² H atoms (m/z 165 + 2). All spectra obtained by CID of [6,6,7,7-² H₄ ]12-OPDA and [6,6,7,7-² H₄ ]12-oxo-9(13),15(Z)-phytodienoic acid showed that one or two additional ² H atoms could be transferred to this anion at m/z 167 of [6,6-² H₂ ]12-OPDA due to isotope scrambling.

CONCLUSIONS

CID of 12-OPDA and 12-OPEA generates cyclopentanone enolate anions at m/z 165 by charge-driven hydride transfer as a common mechanism and by bond cleavage between C-7 and C-8 of the carboxyl side chains with either gain or loss of a hydrogen atom.

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

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

Comprehensive Evaluation of a Quantitative Shotgun Lipidomics Platform for Mammalian Sample Analysis on a High-Resolution Mass Spectrometer

Shotgun lipidomics is a powerful tool that enables simultaneous and fast quantification of diverse lipid classes through mass spectrometry based analyses of directly infused crude lipid extracts. We present here a shotgun lipidomics platform established to quantify 38 lipid classes belonging to four lipid categories present in mammalian samples and show the fine-tuning and comprehensive evaluation of its experimental parameters and performance. We first determined for all the targeted lipid classes the collision energy levels optimal for the recording of their lipid class- and species-specific fragment ions and fine-tuned the energy levels applied in the platform. We then performed a series of titrations to define the boundaries of linear signal response for the targeted lipid classes, and demonstrated that the dynamic quantification range spanned more than 3 orders of magnitude and reached sub picomole levels for 35 lipid classes. The platform identified 273, 261, and 287 lipid species in brain, plasma, and cultured fibroblast samples, respectively, at the respective optimal working sample amounts. The platform properly quantified the majority of these identified lipid species, while lipid species measured to be below the limit of quantification were efficiently removed from the data sets by the use of statistical analyses of data reproducibility or a cutoff threshold. Finally, we demonstrated that a series of parameters of cell culture conditions influence lipidomics outcomes, including confluency, medium supplements, and use of transfection reagents. The present study provides a guideline for setting up and using a simple and efficient platform for quantitatively exploring the mammalian lipidome.

Analysis of Phospholipids, Lysophospholipids, and Their Linked Fatty Acyl Chains in Yellow Lupin Seeds (Lupinus luteus L.) by Liquid Chromatography and Tandem Mass Spectrometry

Hydrophilic interaction liquid chromatography (HILIC) and electrospray ionization (ESI) coupled to either Fourier-transform (FT) orbital-trap or linear ion-trap tandem mass spectrometry (LIT-MS/MS) was used to characterize the phospholipidome of yellow lupin (Lupinus luteus) seeds. Phosphatidylcholines (PC) were the most abundant species (41 ± 6%), which were followed by lyso-forms LPC (30 ± 11%), phosphatidylethanolamines (PE, 13 ± 4%), phosphatidylglycerols (PG, 5.1 ± 1.7%), phosphatidic acids (PA, 4.9 ± 1.8%), phosphatidylinositols (PI, 4.7 ± 1.1%), and LPE (1.2 ± 0.5%). The occurrence of both isomeric forms of several LPC and LPE was inferred by a well-defined fragmentation pattern observed in negative ion mode. An unprecedented characterization of more than 200 polar lipids including 52 PC, 42 PE, 42 PA, 35 PG, 16 LPC, 13 LPE, and 10 PI, is reported. The most abundant fatty acids (FA) as esterified acyl chains in PL were 18:1 (oleic), 18:2 (linoleic), 16:0 (palmitic), and 18:3 (linolenic) with relatively high contents of long fatty acyl chains such as 22:0 (behenic), 24:0 (lignoceric), 20:1 (gondoic), and 22:1 (erucic). Their occurrence was confirmed by reversed-phase (RP) LC-ESI-FTMS analysis of a chemically hydrolyzed sample extract in acid conditions at 100 °C for 45 min.

Toward Complete Structure Elucidation of Glycerophospholipids in the Gas Phase through Charge Inversion Ion/Ion Chemistry

Shotgun lipidomics has recently gained popularity for lipid analysis. Conventionally, shotgun analysis of glycerophospholipids via direct electrospray ionization tandem mass spectrometry (ESI-MS/MS) provides glycerophospholipid (GPL) class (i.e., headgroup composition) and fatty acyl composition. Reliant on low-energy collision-induced dissociation (CID), traditional ESI-MS/MS fails to define fatty acyl regiochemistry along the glycerol backbone or carbon-carbon double bond position(s) in unsaturated fatty acyl substituents. Therefore, isomeric GPLs are often unresolved, representing a significant challenge for shotgun-MS approaches. We developed a top-down shotgun-MS method utilizing gas-phase ion/ion charge inversion chemistry that provides near-complete GPL structural identification. First, in negative ion mode, CID of mass-selected GPL anions generates fatty acyl carboxylate anions via fragmentation of ester bonds linking the fatty acyl substituents at the sn-1 and sn-2 positions of the glycerol backbone. Product anions, including fatty acyl carboxylate ions, were then derivatized in the mass spectrometer via an ion/ion charge inversion reaction with tris-phenanthroline magnesium dications. Subsequent CID of charge-inverted fatty acyl complex cations yielded isomer-specific product ion spectra that permit (i) unambiguous assignment of carbon-carbon double bond position(s) and (ii) relative quantitation of isomeric fatty acyl substituents. The outlined strategy was applied to the analysis of targeted GPLs extracted from human plasma, including several proposed plasma biomarkers. A single experiment thus facilitates assignment of the GPL headgroup, fatty acyl composition, carbon-carbon double bond position(s) in unsaturated fatty acyl chains, and, in some cases, fatty acyl sn-position and relative abundances for isomeric fatty acyl substituents. Ultimately, this MSn platform paired with ion/ion chemistry permitted identification of major, and some minor, isomeric contributors that are unresolved using conventional ESI-MS/MS.

Detection and Structural Characterization of Ether Glycerophosphoethanolamine from Cortical Lysosomes Following Traumatic Brain Injury Using UPLC-HDMSE

The use of ultra performance liquid chromatography coupled to data independent tandem mass spectrometry with traveling wave ion mobility for detection and structural identification of ether-linked glycerophosphoethanolamine is described. The experimental design generates 4D data (chromatographic retention time, precursor accurate mass, drift time with associated calculated collisional cross-section, and time-aligned accurate mass diagnostic product ions) for each ionization mode. Confident structure identification depends on satisfying 4D data confirmation in both positive and negative ion mode. Using this methodology, a number of ether-linked glycerophosphoethanolamine lipids are structurally elucidated from mouse brain lysosomes. It is further determined that several ether-linked glycerophosphoethanolamine structures are differentially abundant between lysosomes isolated from mouse cortex following traumatic brain injury as compared to that of sham animals. The combined effort of aligning multi-dimensional mass spectrometry data with a well-defined traumatic brain injury model lays the foundation for gaining mechanistic insight in the role lysosomal membrane damage plays in neuronal cell death following brain injury.

Structural characterization of phosphoethanolamine-modified lipid A from probiotic Escherichia coli strain Nissle 1917

Gut microbiota, a complex microbial community inhabiting human or animal intestines recently regarded as an endocrine organ, has a significant impact on human health. Probiotics can modulate gut microbiota and the gut environment by releasing a range of bioactive compounds. Escherichia coli (E. coli) strain Nissle 1917 (EcN), a Gram-negative bacterial strain, has been used to treat gastrointestinal (GI) disorders (i.e., inflammatory bowel disease, diarrhea, ulcerative colitis, and so on). However, endotoxicity of lipopolysaccharide (LPS), a major component of the cell wall of Gram-negative bacteria in the gut, is known to have a strong influence on gut inflammation and maintenance of gut homeostasis. Therefore, characterizing the chemical structure of lipid A which determines the toxicity of LPS is needed to understand nonpathogenic colonization and commensalism properties of EcN in the gut more precisely. In the present study, MALDI multiple-stage mass spectrometry analysis of lipid A extracted from EcN demonstrates that hexaacylated lipid A (m/z 1919.19) contains a glucosamine disaccharide backbone, a myristate, a laurate, four 3-hydroxylmyristates, two phosphates, and phosphoethanolamine (PEA). PEA modification of lipid A is known to contribute to cationic antimicrobial peptide (CAMP) resistance of Gram-negative bacteria. To confirm the role of PEA in CAMP resistance of EcN, minimum inhibitory concentrations (MICs) of polymyxin B and colistin were determined using a wild-type strain and a mutant strain with deletion of eptA gene encoding PEA transferase. Our results confirmed that MICs of polymyxin B and colistin for the wild-type were twice as high as those for the mutant. These results indicate that EcN can more efficiently colonize the intestine through PEA-mediated tolerance despite the presence of CAMPs in human gut such as human defensins. Thus, EcN can be used to help treat and prevent many GI disorders.

Targeting Modified Lipids during Routine Lipidomics Analysis using HILIC and C30 Reverse Phase Liquid Chromatography coupled to Mass Spectrometry

Lipids are important biomolecules in all biological systems and serve numerous essential cellular functions. The global analysis of complex lipids is very challenging due to the extreme diversity in lipid structures. Variation in linkages and positions of fatty acyl chain(s) on the lipid backbone, functional group modification, occurrence of the molecular species as isomers or isobars are among some of the greatest challenges to resolve in lipidomics. In this work, we describe a routine analytical approach combining two liquid chromatography platforms: hydrophilic interaction (HILIC) and C30 reversed-phase chromatography (C30RP) coupled to high resolution mass spectrometry (HRMS) as complementary high throughput platforms to analyze complex lipid mixtures. Vascular plants (kale leaves and corn roots), rat brain and soil microbes were used as proxies to evaluate the efficiency of the enhanced approach to resolve traditional, as well as, modified lipids during routine lipidomics analysis. We report for the first time, the observation of a modified class of acylphosphatidylglycerol (acylPG) in corn roots by HILIC, and further resolution of the isomers using C30RP chromatography. We also used this approach to demonstrate the presence of high levels of N-monomethyl phosphatidylethanolamine (MMPE) in soil microbes, as well as to determine the regioisomers of lysophospholipids in kale leaves. Additionally, neutral lipids were demonstrated using C30RP chromatography in positive ion mode to resolve triacylglycerol isomers in rat brain. The work presented here demonstrates how the enhanced approach can more routinely permit novel biomarker discovery, or lipid metabolism in a wide range of biological samples.

A method for the quantitative determination of glycerophospholipid regioisomers by UPLC-ESI-MS/MS

Diacyl glycerophospholipids (GPs) belong to the most abundant lipid species in living organisms and consist of a glycerol backbone with fatty acyl groups in sn-1 and sn-2 and a polar head group in the sn-3 position. Regioisomeric mixed diacyl GPs have the same fatty acyl composition but differ in their allocation to sn-1 or sn-2 of the glycerol unit. In-depth analysis of regioisomeric mixed diacyl GP species composed of fatty acyl moieties that are similar in length and degree of saturation typically requires either chemical derivatization or sophisticated analytical instrumentation, since these types of regioisomers are not well resolved under standard ultra-performance liquid chromatography (UPLC) conditions. Here, we introduce a simple and fast method for diacyl GP regioisomer analysis employing UPLC tandem mass spectrometry (MS/MS). This GP regioisomer analysis is based both on minor chromatographic retention time shifts and on major differences in relative abundances of the two fatty acyl anion fragments observed in MS/MS. To monitor these differences with optimal precision, MS/MS spectra are recorded continuously over the UPLC elution profile of the lipid species of interest. Quantification of relative abundances of the regioisomers was performed by algorithms that we have developed for this purpose. The method was applied to commercially available mixed diacyl GP standards and to total lipid extracts of Escherichia coli (E. coli) and bovine liver. To validate our results, we determined regioisomeric ratios of phosphatidylcholine (PC) standards using phospholipase A₂-specific release of fatty acids from the sn-2 position of the glycerol backbone. Our results show that most analyzed mixed diacyl GPs of biological origin exhibit significantly higher regioisomeric purity than synthetic lipid standards. In summary, this method can be implemented in routine LC-MS/MS-based lipidomics workflows without the necessity for additional chemical additives, derivatizations, or instrumentation.

Mass spectrometry-based shotgun lipidomics - a critical review from the technical point of view

Over the past decade, mass spectrometry (MS)-based "shotgun lipidomics" has emerged as a powerful tool for quantitative and qualitative analysis of the complex lipids in the biological system. The aim of this critical review is to give the interested reader a concise overview of the current state of the technology, focused on lipidomic analysis by mass spectrometry. The pros and cons, and pitfalls associated with each available "shotgun lipidomics" method are discussed; and the new strategies for improving the current methods are described. A list of important papers and reviews that are sufficient rather than comprehensive, covering all the aspects of lipidomics including the workflow, methodology, and fundamentals is also compiled for readers to follow. Graphical abstract ᅟ.

Phospholipids as cancer biomarkers: Mass spectrometry-based analysis

Lipids, particularly phospholipids (PLs), are key components of cellular membrane. PLs play important and diverse roles in cells such as chemical-energy storage, cellular signaling, cell membranes, and cell-cell interactions in tissues. All these cellular processes are pertinent to cells that undergo transformation, cancer progression, and metastasis. Thus, there is a strong possibility that some classes of PLs are expected to present in cancer cells and tissues in cellular physiology. The mass spectrometric soft-ionization techniques, electrospray ionization (ESI), and matrix-assisted laser desorption/ionization (MALDI) are well-established in the proteomics field, have been used for lipidomic analysis in cancer research. This review focused on the applications of mass spectrometry (MS) mainly on ESI-MS and MALDI-MS in the structural characterization, molecular composition and key roles of various PLs present in cancer cells, tissues, blood, and urine, and on their importance for cancer-related problems as well as challenges for development of novel PL-based biomarkers. The profiling of PLs helps to rationalize their functions in biological systems, and will also provide diagnostic information to elucidate mechanisms behind the control of cancer, diabetes, and neurodegenerative diseases. The investigation of cellular PLs with MS methods suggests new insights on various cancer diseases and clinical applications in the drug discovery and development of biomarkers for various PL-related different cancer diseases. PL profiling in tissues, cells and body fluids also reflect the general condition of the whole organism and can indicate the existence of cancer and other diseases. PL profiling with MS opens new prospects to assess alterations of PLs in cancer, screening specific biomarkers and provide a basis for the development of novel therapeutic strategies. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 37:107-138, 2018.

Natural Organochlorines as Precursors of 3-Monochloropropanediol Esters in Vegetable Oils

During high-temperature refining of vegetable oils, 3-monochloropropanediol (3-MCPD) esters, possible carcinogens, are formed from acylglycerol in the presence of a chlorine source. To investigate organochlorine compounds in vegetable oils as possible precursors for 3-MCPD esters, we tested crude palm, soybean, rapeseed, sunflower, corn, coconut, and olive oils for the presence of organochlorine compounds. Having found them in all vegetable oils tested, we focused subsequent study on oil palm products. Analysis of the chlorine isotope mass pattern exhibited in high-resolution mass spectrometry enabled organochlorine compound identification in crude palm oils as constituents of wax esters, fatty acid, diacylglycerols, and sphingolipids, which are produced endogenously in oil palm mesocarp throughout ripening. Analysis of thermal decomposition and changes during refining suggested that these naturally present organochlorine compounds in palm oils and perhaps in other vegetable oils are precursors of 3-MCPD esters. Enrichment and dose-response showed a linear relationship to 3-MCPD ester formation and indicated that the sphingolipid-based organochlorine compounds are the most active precursors of 3-MCPD esters.

Mechanisms governing the fragmentation of glycerophospholipids containing choline and ethanolamine polar head groups

Glycerophospholipids are the major amphiphilic molecules found in the plasma membrane bilayer of all vertebrate cells. Involved in many biological processes, their huge structural diversity and large concentration scale make their thorough characterization extremely difficult in complex biological matrices. Mass spectrometry techniques are now recognized as being among the most powerful methods for the sensitive and comprehensive characterization of lipids. Depending on the experimental conditions used during electrospray ionization mass spectrometry experiments, glycerophospholipids can be detected as different molecular species (e.g. protonated, sodiated species) when analyzed either in positive or negative ionization modes or by direct introduction or hyphenated mass spectrometry-based methods. The observed ionized forms are characteristic of the corresponding phospholipid structures, and their formation is highly influenced by the polar head group. Although the fragmentation behavior of each phospholipid class has already been widely studied under low collision energy, there are no established rules based on charge-induced dissociation mechanisms for explaining the generation of fragment ions. In the present paper, we emphasize the crucial roles played by ion-dipole complexes and salt bridges within charge-induced dissociation processes. Under these conditions, we were able to readily explain almost all the fragment ions obtained under low-energy collision-induced dissociation for particular glycerophospholipids and lysoglycerophospholipids species including glycerophosphatidylcholines and glycerophosphatidylethanolamines. Thus, in addition to providing a basis for a better comprehension of phospholipid fragmentation processes, our work also highlighted some potentially new relevant diagnostic ions to signal the presence of particular lipid species.

Unambiguous regiochemical assignment of sulfoquinovosyl mono- and diacylglycerols in parsley and spinach leaves by liquid chromatography/electrospray ionization sequential mass spectrometry assisted by regioselective enzymatic hydrolysis

RATIONALE

Sulfoquinovosylmonoglycerides (SQMG) and sulfoquinovosyldiglycerides (SQDG) in the lipid extracts of parsley (Petroselinum crispum) and spinach (Spinacia oleracea) leaves were investigated. The aim of this work was to assess and establish the chemical characterization of fatty acyl chains in sulfolipids (SQMG and SQDG) and their regiochemistry.

METHODS

A key component of this approach is a combination of hydrolysis reactions catalyzed by Lecitase® Ultra, which is a sn₁ -regioselective hydrolase enzyme, and reversed-phase liquid chromatography with electrospray ionization and sequential mass spectrometry (RPLC/ESI-MS) by collision-induced dissociation (CID)-MSⁿ (n = 2, 3).

RESULTS

The occurrence of SQMG bearing 16:0 or 18:3 acyl chains was established for the first time. A regiochemistry-dependent fragmentation pattern of SQMG was attained whereby the sulfoquinovosyl anion ([C₆ H₁₁ O₈ S]^- at m/z 243.0) provides a diagnostic product ion. Regioselective enzymatic treatment also provided a posteriori confirmation of a widely accepted fragmentation rule for SQDG. The sulfoquinovosyl anion was found to play a role also in the fragmentation pattern of SQDG, whose regiochemical assignment could be ultimately confirmed by MS³ experiments.

CONCLUSIONS

The predominant sulfolipid in leaf extracts of raw parsley (Petroselinum crispum) and spinach (Spinacia oleracea) was identified as SQDG 18:3/16:0, along with SQMG 18:3/0:0 and SQMG 16:0/0:0. The present CID-MS-based method can be considered a successful approach to validate the regiochemical characterization of sulfolipids paving the way for their unambiguous characterization.

Charge transfer dissociation of phosphocholines: gas-phase ion/ion reactions between helium cations and phospholipid cations

Phospholipid cations formed by electrospray ionization were subjected to excitation and fragmentation by a beam of 6 keV helium cations in a process termed charge transfer dissociation (CTD). The resulting fragmentation pattern in CTD is different from that of conventional collision-induced dissociation, but analogous to that of metastable atom-activated dissociation and electron-induced dissociation. Like collision-induced dissociation, CTD yields product ions indicative of acyl chain lengths and degrees of unsaturation in the fatty acyl moieties but also provides additional structural diagnostic information, such as double bond position. Although CTD has not been tested on a larger lipid sample pool, the extent of structural information obtained demonstrates that CTD is a useful tool for lipid structure characterization, and a potentially useful tool in future lipidomics workflows. CTD is relatively unique in that it can produce a relatively strong series of 2+ product ions with enhanced abundance at the double bond position. The generally low signal-to-noise ratios and spectral complexity of CTD make it less appealing than OzID or other radical-induced methods for the lipids studies here, but improvements in CTD efficiency could make CTD more appealing in the future. Copyright © 2017 John Wiley & Sons, Ltd.

Lipidomics by ultrahigh performance liquid chromatography-high resolution mass spectrometry and its application to complex biological samples

An improved approach for selective and sensitive identification and quantitation of lipid molecular species using reversed phase chromatography coupled to high resolution mass spectrometry was developed. The method is applicable to a wide variety of biological matrices using a simple liquid-liquid extraction procedure. Together, this approach combines multiple selectivity criteria: Reversed phase chromatography separates lipids according to their acyl chain length and degree of unsaturation and is capable of resolving positional isomers of lysophospholipids, as well as structural isomers of diacyl phospholipids and glycerolipids. Orbitrap mass spectrometry delivers the elemental composition of both positive and negative ions with high mass accuracy. Finally, automatically generated tandem mass spectra provide structural insight into numerous glycerolipids, phospholipids, and sphingolipids within a single run. Calibration showed linearity ranges of more than four orders of magnitude, good values for accuracy and precision at biologically relevant concentration levels, and limits of quantitation of a few femtomoles on column. Hundreds of lipid molecular species were detected and quantified in three different biological matrices, which cover well the wide variety and complexity of various model organisms in lipidomic research. Together with a software package, this method is a prime choice for global lipidomic analysis of even the most complex biological samples.

Tandem Mass Spectrometry in Combination with Product Ion Mobility for the Identification of Phospholipids

Concerted tandem and traveling wave ion mobility mass spectrometry (CTS analysis) is a unique method that results in a four-dimensional data set including nominal precursor ion mass, product ion mobility, accurate mass of product ion, and ion abundance. This nontargeted lipidomics CTS approach was applied in both positive- and negative-ion mode to phospholipids present in human serum, and the data set was used to evaluate the value of product ion mobility in identifying lipids in a complex mixture. It was determined that the combination of diagnostic product ions and unique collisional cross-section values of product ions is a powerful tool in the structural identification of lipids in a complex biological sample.

Desorption electrospray ionization mass spectrometry imaging reveals chemical defense of Burkholderia seminalis against cacao pathogens

DESI-MSI of metabolites identified inB. seminalis.

Determination of ester position in isomeric (O-acyl)-hydroxy fatty acids by ion trap mass spectrometry

RATIONALE

(O-acyl)-hydroxy fatty acids (OAHFAs) are a recently discovered class of endogenous lipids, generating significant interest for their correlation with enhanced glucose tolerance. Structural variants that differ in the position of the ester linkage have been described, including the ω-OAHFA sub-class, that plays a key role in stabilizing the human tear film. Developing analytical tools for rapid and unambiguous structural elucidation of OAHFAs is essential to understanding their diverse physiological functions.

METHODS

Commercially available and synthesized OAHFA standards were dissolved in chloroform and subsequently diluted into methanol with 1.5 mM ammonium acetate. Negative ion collision-induced dissociation (CID) MSⁿ spectra were acquired using chip-based nano-electrospray ionization (Advion TriVersa NanoMate) coupled to an Orbitrap Elite mass spectrometer (Thermo Fisher Scientific).

RESULTS

Major product ions observed during CID of [OAHFA - H]^- ions readily identify the constituent fatty acid and hydroxy fatty acid; however, isomers are not easily distinguished. Interrogation of the hydroxy fatty acid and dehydrated hydroxy fatty acid product ions by MSⁿ and ion-molecule reactions yielded diagnostic ions that readily pinpoint hydroxylation position and, thus, the OAHFA ester location. Conversely, these ions are characteristically absent in the MS³ spectra of ω-OAHFAs. Unimolecular dissociation mechanisms are proposed, which are shown to be consistent with prior isotopic labelling experiments.

CONCLUSIONS

A mechanistic rationale is provided to explain the unimolecular dissociation of [OAHFA - H]^- ions in an ion trap mass spectrometer, thus enabling near-complete de novo structural elucidation of OAHFAs in shotgun lipidomics workflows, even if synthetic standards are unavailable for comparison. Copyright © 2016 John Wiley & Sons, Ltd.

Enhanced Lipidome Coverage in Shotgun Analyses by using Gas-Phase Fractionation

A high resolving power shotgun lipidomics strategy using gas-phase fractionation and data-dependent acquisition (DDA) was applied toward comprehensive characterization of lipids in a hen ovarian tissue in an untargeted fashion. Using this approach, a total of 822 unique lipids across a diverse range of lipid categories and classes were identified based on their MS/MS fragmentation patterns. Classes of glycerophospholipids and glycerolipids, such as glycerophosphocholines (PC), glycerophosphoethanolamines (PE), and triglycerides (TG), are often the most abundant peaks observed in shotgun lipidomics analyses. These ions suppress the signal from low abundance ions and hinder the chances of characterizing low abundant lipids when DDA is used. These issues were circumvented by utilizing gas-phase fractionation, where DDA was performed on narrow m/z ranges instead of a broad m/z range. Employing gas-phase fractionation resulted in an increase in sensitivity by more than an order of magnitude in both positive- and negative-ion modes. Furthermore, the enhanced sensitivity increased the number of lipids identified by a factor of ≈4, and facilitated identification of low abundant lipids from classes such as cardiolipins that are often difficult to observe in untargeted shotgun analyses and require sample-specific preparation steps prior to analysis. This method serves as a resource for comprehensive profiling of lipids from many different categories and classes in an untargeted manner, as well as for targeted and quantitative analyses of individual lipids. Furthermore, this comprehensive analysis of the lipidome can serve as a species- and tissue-specific database for confident identification of other MS-based datasets, such as mass spectrometry imaging. Graphical Abstract ᅟ.

Fragmentation reactions using electrospray ionization mass spectrometry: an important tool for the structural elucidation and characterization of synthetic and natural products

Over the last decade, the number of studies reporting the use of electrospray ionization mass spectrometry (ESI-MS) in combination with collision cells (or other activation methods) to promote fragmentation of synthetic and natural products for structural elucidation purposes has considerably increased. However, the lack of a systematic compilation of the gas-phase fragmentation reactions subjected to ESI-MS/MS conditions still represents a challenge and has led to many misunderstood results in the literature. This review article exploits the most common fragmentation reactions for ions generated by ESI in positive and negative modes using collision cells in an effort to stimulate the use of this technique by non-specialists, undergraduate students and researchers in related areas.

Gas-Phase Chemical Separation of Phosphatidylcholine and Phosphatidylethanolamine Cations via Charge Inversion Ion/Ion Chemistry

The [M + H](+) cations formed upon electrospray ionization of the glycerophospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE) show distinct reactivities upon gas-phase reactions with doubly deprotonated 1,4-phenylenedipropionic acid (PDPA). PC cations undergo charge inversion via adduct formation with subsequent methyl cation and proton transfer to the acid to yield [PC - CH3](-) anions. These demethylated PC anions fragment upon ion trap collision-induced dissociation (CID) to yield products that reveal fatty acid chain lengths and degrees of unsaturation. PE cations, on the other hand, undergo charge inversion via double proton transfer to the two carboxylate moieties in doubly deprotonated PDPA to yield [PE - H](-) anions. These anions also fragment upon ion trap CID to yield product ions indicative of chain lengths and degrees of unsaturation in the fatty acyl moieties. Advantage is taken of this distinct reactivity to separate isomeric and isobaric PC and PE cations present in mass spectra of lipid mixtures. A cation precursor ion population containing a mixture of PE and PC cations is mass-selected and subjected to ion/ion charge inversion reactions that result in separation of PC and PE anions into different mass-to-charge ratios. Mass selection and subsequent ion trap CID of the lipid anions allows for the characterization of the isomeric lipids within each subclass. The charge inversion approach described here is demonstrated to provide increased signal-to-noise ratios for detection of PCs and PEs relative to the standard negative ionization approach as well as improved mixture analysis performance.