Multistage Mass Spectrometry of Phospholipids using Collision-Induced Dissociation (CID) and Metastable Atom-Activated Dissociation (MAD)

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.

Novel Structure-Driven Predict-to-Hit Strategy for PC Double Bond Positional Isomer Identification Based on Negative LC-MRM Analysis

As the predominant phospholipids in mammalian cells, phosphatidylcholines (PCs) have been demonstrated to play a crucial role in a multitude of vital biological processes. Research has highlighted the significance of the diversity in PC isomers as instigators of both physiological and pathological responses, particularly those with variations in the position of double bonds within their fatty chains. Profiling these PC isomers is paramount to advancing our understanding of their biological functions. Despite the availability of analytical methods utilizing high-resolution secondary mass spectrometry (MS2) fragmentation, a novel approach was imperative to facilitate large-scale profiling of PC isomers while ensuring accessibility, facility, and reliability. In this study, an innovative strategy centered around structure-driven predict-to-hit profiling of the double bond positional isomers for PCs was meticulously developed, employing negative reversed-phase liquid chromatography-multiple reaction monitoring (RPLC-MRM). This novel methodology heightened the sensitivity. The analysis of rat lung tissue samples resulted in the identification of 130 distinct PC isomers. This approach transcended the confines of available PC isomer standards, thereby broadening the horizons of PC-related biofunction investigations. By optimizing the quantitation reliability, the scale of sample analysis was judiciously managed. This work pioneers a novel paradigm for the exploration of PC isomers, distinct from the conventional methods reliant on high-resolution mass spectrometry (HRMS). It equips researchers with potent tools to further explore the biofunctional aspects of lipids.

Ion-molecule reactions of mass-selected ions

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

A Comprehensive Insight and Mechanistic Understanding of the Lipidomic Alterations Associated With DCM

Dilated cardiomyopathy (DCM) is one of the major causes of heart failure characterized by the enlargement of the left ventricular cavity and contractile dysfunction of the myocardium. Lipids are the major sources of energy for the myocardium. Impairment of lipid homeostasis has a potential role in the pathogenesis of DCM. In this review, we have summarized the role of different lipids in the progression of DCM that can be considered as potential biomarkers. Further, we have also explained the mechanistic pathways followed by the lipid molecules in disease progression along with the cardioprotective role of certain lipids. As the global epidemiological status of DCM is alarming, it is high time to define some disease-specific biomarkers with greater prognostic value. We are proposing an adaptation of a system lipidomics-based approach to profile DCM patients in order to achieve a better diagnosis and prognosis of the disease.

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.

Structural Elucidation and Relative Quantification of Sodium- and Potassium-Cationized Phosphatidylcholine Regioisomers Directly from Tissue Using Electron Induced Dissociation

Comprehensive structural characterization of phosphatidylcholines (PCs) is essential to understanding their biological functions and roles in metabolism. Electron induced dissociation (EID) of protonated PCs directly generated from biological tissues has previously been shown to provide in-depth structural information on the lipid headgroup, regiosiomerism of fatty acyl tails and double bond positions. Although phosphatidylcholine ions formed via alkali metal cationization (i.e., [M + Na]+ and [M + K]+) are commonly generated during matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry experiments, the gas-phase ion chemistry behavior of EID on sodium- and potassium-cationized phosphatidylcholine ion types has not been studied for ions generated directly from tissue. Herein, we demonstrate EID on [M + Na]+ and [M + K]+ ion types in a MALDI imaging mass spectrometry workflow for lipid structural characterization. Briefly, near-complete structural information can be obtained upon EID of sodium- and potassium-cationized PCs, including diagnostic fragmentation of the lipid headgroup as well as identification of fatty acyl chain positions and double bond position. EID of cationized lipids generates sn-specific glycerol backbone cleavages as well as a favorable combined loss of sn-2 fatty acid with choline over sn-1, allowing for facile differentiation and relative quantification of PC regioisomers. Moreover, relative quantification of sn-positional isomers from biological tissue reveals that the relative percentages of sodium- and potassium-cationized sn-positional isomers varies significantly in different regions of rat brain tissue.

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.

Structural Characterization of Natural and Synthetic Macrocycles Using Charge-Transfer Dissociation Mass Spectrometry

Research in natural products (NPs) has gained interest as drug developers turn to nature to combat problems with drug resistance, drug delivery, and emerging diseases. Whereas NPs offer a tantalizing source of new pharmacologically active compounds, their structural complexity presents a challenge for analytical characterization and organic synthesis. Of particular concern is the characterization of cyclic-, polycyclic-, or macrocyclic compounds. One example of endogenous compounds as inspiration for NP development are cobalamins, like vitamin B₁₂. An example of exogenous NPs is the class of macrolides that includes erythromycin. Both classes of macrocycles feature analogues with a range of modifications on their macrocyclic cores, but because of their cyclic nature, they are generally resistant to fragmentation by collision-induced dissociation (CID). In the present work, charge-transfer dissociation (CTD) was employed, with or without supplemental collisional activation, to produce radical-driven, high-energy fragmentation products of different macrocyclic precursors. With the assistance of collisional activation of CTnoD products, CTD frequently cleaved two covalent bonds within the macrocycle cores to reveal rich, informative spectra that helped identify sites of modification and resolve structural analogues. In a third example of macrocycle fragmentation, CTD enabled an impurity in a biological sample to be characterized as a cyclic polymer of nylon-6,6. In each example, CTD spectra are starkly different from CID and are highly reminiscent of other high-energy fragmentation techniques like extreme ultraviolet dissociative photoionization (XUV-DPI) and electron ionization-induced dissociation (EID). The results indicate that CTD-MS is a useful tool for the characterization of natural and synthetic macrocycles.

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

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

Structural Characterization of Sulfated Glycosaminoglycans Using Charge-Transfer Dissociation

Glycosaminoglycans (GAGs) participate in a broad range of physiological processes, and their structures are of interest to researchers in structural biology and medicine. Although they are abundant in tissues and extracellular matrices, their structural heterogeneity makes them challenging analytes. Mass spectrometry, and more specifically, tandem mass spectrometry, is particularly well suited for their analysis. Many tandem mass spectrometry techniques have been examined for their suitability toward the structural characterization of GAGs. Threshold activation methods such as collision-induced dissociation (CID) produce mainly glycosidic cleavages and do not yield a broad range of structurally informative cross-ring fragments. Considerable research efforts have been directed at finding other means of dissociating gas-phase GAG ions to produce more comprehensive structural information. Here, we compare the structural information on GAGs obtained by charge-transfer dissociation (CTD) and electron detachment dissociation (EDD). EDD has previously been applied to GAGs and is known to produce both glycosidic and cross-ring cleavages in similar abundance. CTD has not previously been used to analyze GAGs but has been shown to produce abundant cross-ring cleavages and no sulfate loss when applied to another class of sulfated carbohydrates like algal polysaccharides. In contrast to EDD, which is restricted to FTICR mass spectrometers, CTD can be implemented on other platforms, such as ion trap mass spectrometers (ITMS). Here, we show the capability of CTD-ITMS to produce structurally significant details of the sites of modification in both heparan sulfate (HS) and chondroitin sulfate (CS) standards ranging in length from degree of polymerization (dp) 4 to dp6. EDD and CTD both yield more structural information than CID and yield similar fractional abundances to one another for glycosidic fragments, cross-ring fragments, and neutral losses.

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.

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

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

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

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

In-depth structural characterization of phospholipids by pairing solution photochemical reaction with charge inversion ion/ion chemistry

Shotgun lipid analysis based on electrospray ionization-tandem mass spectrometry (ESI-MS/MS) is increasingly used in lipidomic studies. One challenge for the shotgun approach is the discrimination of lipid isomers and isobars. Gas-phase charge inversion via ion/ion reactions has been used as an effective method to identify multiple isomeric/isobaric components in a single MS peak by exploiting the distinctive functionality of different lipid classes. In doing so, fatty acyl chain information can be obtained without recourse to condensed-phase separations or derivatization. This method alone, however, cannot provide carbon-carbon double bond (C=C) location information from fatty acyl chains. Herein, we provide an enhanced method pairing photochemical derivatization of C=C via the Paternò-Büchi reaction with charge inversion ion/ion tandem mass spectrometry. This method was able to provide gas-phase separation of phosphatidylcholines and phosphatidylethanolamines, the fatty acyl compositions, and the C=C location within each fatty acyl chain. We have successfully applied this method to bovine liver lipid extracts and identified 40 molecular species of glycerophospholipids with detailed structural information including head group, fatty acyl composition, and C=C location. Graphical Abstract ᅟ.

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

The ability to control the charge state and ionization efficiency of lipids and hydrocarbons by means of in-source Paternò-Büchi functionalization in nano-electrospray ionization mass spectrometry experiments is investigated. Ultraviolet light irradiation of acetylpyridine filled nano-electrospray emitter tips, containing unsaturated analytes, generates protonated lipid and hydrocarbon ions. Comparison of reaction yields and fragment ion abundances of functionalized unsaturated fatty acids indicate that acetylpyridine Paternò-Büchi functionalization allows to readily detect fatty acids and determine double bond positions, but fragmentation efficiency and reactivity depend on double bond position and varies between different acetylpyridine isomers. Results for methyl oleate and olefins suggest that fragment ion abundances of unsaturated compounds depend on interactions between acetylpyridine and nearby functional groups. Paternò-Büchi functionalization with acetylpyridine was used to detect and assign double bond positions of mono- and polyunsaturated fatty acid, cholesterol ester, triglyceride, and hydrocarbon standards with ion abundances that are up to 631 times higher than abundances of the same compounds prior Paternò-Büchi reaction. To demonstrate the scope and analytical robustness of the newly developed method, free fatty acids in mouse brain as well as male Schistosoma mansoni extracts and hydrocarbons in an olefin mixture are investigated. For this complex set of analytes, charging and charge switching using acetylpyridine Paternò-Büchi functionalization enable double bond position assignment and relative quantification in positive ion mode. Graphical Abstract.

Desorption Electrospray Ionization Coupled with Ultraviolet Photodissociation for Characterization of Phospholipid Isomers in Tissue Sections

Desorption electrospray ionization (DESI) mass spectrometry imaging has become a powerful strategy for analysis of tissue sections, enabling differentiation of normal and diseased tissue based on changes in the lipid profiles. The most common DESI workflow involves collection of MS1 spectra as the DESI spray is rastered over a tissue section. Relying on MS1 spectra inherently limits the ability to differentiate isobaric and isomeric species or evaluate variations in the relative abundances of key isomeric lipids, such as double-bond positional isomers which may distinguish normal and diseased tissues. Here, 193 nm ultraviolet photodissociation (UVPD), a technique capable of differentiating double-bond positional isomers, is coupled with DESI to map differences in the double-bond isomer composition in tissue sections in a fast, high throughput manner compatible with imaging applications.

Selective phosphatidylcholine double bond fragmentation and localisation using Paternò-Büchi reactions and ultraviolet photodissociation

The effect of double bond functionalisation for selective double bond localisation by ultraviolet photodissociation of phosphatidylcholines is investigated. Paternò-Büchi reactions in nanoESI emitter tips enable attachment of acetophenone to double bonds of unsaturated phosphatidylcholines after 100 s of 254 nm light irradiation with about 50-80% reaction yield. Functionalized phosphatidylcholines dissociate upon 266 nm irradiation yielding double bond selective fragment ions in contrast to results for ultraviolet photodissociation of unmodified lipids. Ultraviolet photodissociation of Paternò-Büchi modified lipids results in a selectivity increase of up to 2.2 towards double bond localisation compared collision-induced dissociation experiments. Double bond localisation is also possible with ultraviolet photodissociation when alkali metal ion attachment to Paternò-Büchi modified phosphatidylcholines occurs in contrast to classic collision-induced dissociation experiments. The developed methodology is used to differentiate lipid double bond isomers and applied to phosphatidylcholines from egg yolk to identify 15 phosphatidylcholines. Results from this study demonstrate that locally depositing energy in close vicinity to cleavable bonds via ultraviolet photodissociation can result in increased dissociation selectivity. This method can help to disentangle contributions from different structural elements in complex tandem mass spectra of lipids and aid to the structural characterization of phospholipids in a "top-down" approach.

Top-Down Charge Transfer Dissociation (CTD) of Gas-Phase Insulin: Evidence of a One-Step, Two-Electron Oxidation Mechanism

Top-down analyses of protonated insulin cations of charge states of 4+, 5+, or 6+ were performed by exposing the isolated precursor ions to a beam of helium cations with kinetic energy of more than 6 keV, in a technique termed charge transfer dissociation (CTD). The ~100 ms charge transfer reaction resulted in approximately 20% conversion efficiency to other intact charge exchange products (CTnoD), and a range of low abundance fragment ions. To increase backbone and sulfide cleavages, and to provide better structural information than straightforward MS2 CTD, the CTnoD oxidized products were isolated and subjected to collisional activation at the MS3 level. The MS3 CTD/CID reaction effectively broke the disulfide linkages, separated the two chains, and yielded more structurally informative fragment ions within the inter-chain cyclic region. CTD also provided doubly oxidized intact product ions at the MS2 level, and resonance ejection of the singly oxidized product ion revealed that the doubly oxidized product originates directly from the isolated precursor ion and not from consecutive CTD reactions of a singly oxidized intermediate. MS4 experiments were employed to help identify potential radical cations and diradical cations, but the results were negative or inconclusive. Nonetheless, the two-electron oxidation process is a demonstration of the very large potential energy (>20 eV) available through CTD, and is a notable capability for a 3D ion trap platform. Graphical Abstract ᅟ.

Recent advances in lipid separations and structural elucidation using mass spectrometry combined with ion mobility spectrometry, ion-molecule reactions and fragmentation approaches

Lipids are a vital class of molecules that play important and varied roles in biological processes, however, fully understanding these roles is extremely difficult due to the immense number and diversity of possible lipid species. While recent advances in chromatography and high resolution mass spectrometry have greatly progressed knowledge about distinct lipid species and functions, effectively separating many lipids still remains problematic. Isomeric lipids have made lipid characterization especially difficult and occur due to subclasses having the same chemical composition, or species having multiple acyl chain connectivities (sn-1, sn-2, or sn-3), double bond positions and orientations (cis or trans), and functional group stereochemistries (R versus S). To aid in isomer characterization, ion mobility spectrometry separations, ion-molecule reactions and fragmentation techniques have increasingly been added to lipid analysis workflows. In this manuscript, we review the current state of these approaches and their capabilities for improving the identification of lipid species.

Charge Transfer Dissociation (CTD) Mass Spectrometry of Peptide Cations: Study of Charge State Effects and Side-Chain Losses

1+, 2+, and 3+ precursors of substance P and bradykinin were subjected to helium cation irradiation in a 3D ion trap mass spectrometer. Charge exchange with the helium cations produces a variety of fragment ions, the number and type of which are dependent on the charge state of the precursor ions. For 1+ peptide precursors, fragmentation is generally restricted to C-CO backbone bonds (a and x ions), whereas for 2+ and 3+ peptide precursors, all three backbone bonds (C-CO, C-N, and N-Cα) are cleaved. The type of backbone bond cleavage is indicative of possible dissociation channels involved in CTD process, including high-energy, kinetic-based, and ETD-like pathways. In addition to backbone cleavages, amino acid side-chain cleavages are observed in CTD, which are consistent with other high-energy and radical-mediated techniques. The unique dissociation pattern and supplementary information available from side-chain cleavages make CTD a potentially useful activation method for the structural study of gas-phase biomolecules. Graphical Abstract ᅟ.

Uncovering Structural Diversity of Unsaturated Fatty Acyls in Cholesteryl Esters via Photochemical Reaction and Tandem Mass Spectrometry

Mass spectrometry analysis of cholesteryl esters (CEs) faces several challenges, with one of them being the determination of the carbon-carbon double bond (C=C) locations within unsaturated fatty acyl chains. Paternὸ-Büchi (PB) reaction, a photochemical reaction based on the addition of acetone to C=C, is capable of C=C location determination when coupled with tandem mass spectrometry (MS/MS). In this study, the PB reaction conditions were tailored for CEs and subsequent nanoelectrospray ionization (nanoESI). A solvent system containing acetone/methanol/dichloromethane/water (40/30/20/10, volume ratios) and 100 μM LiOH was determined to be optimal, resulting in reasonable PB reaction yield (~30%) and good ionization efficiency (forming lithium adduct of CEs). Collision-induced dissociation (CID) of the PB reaction products produced characteristic fragment ions of CE together with those modified by the PB reactions, such as lithiated fatty acyl ([FA + Li]+) and its PB product ([FA - PB + Li]+). MS3 CID of [FA - PB + Li]+ led to abundant C=C diagnostic ion formation, which was used for C=C location determination and isomer quantitation. A PB-MS3 CID approach was developed and applied for CE analysis from human plasma. A series of unsaturated CEs was identified with specific C=C locations within fatty acyl chains. Absolute quantitation for each CE species was achieved including coexisting C=C location isomers, such as Δ9 and Δ11 isomers of CE 18:1 and ω-6 and ω-3 isomers of CE 18:3. These results show that PB-MS/MS is useful in uncovering structural diversity of CEs due to unsaturation in fatty acyls, which is often undetected from current lipid analysis approach. Graphical Abstract ᅟ.

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.

Structural Characterization of Phosphatidylcholines Using 193 nm Ultraviolet Photodissociation Mass Spectrometry

Advances in mass spectrometry have made it a preferred tool for structural characterization of glycerophospholipids. Collisional activation methods commonly implemented on commercial instruments do not provide fragmentation patterns that allow elucidation of certain structural features, including acyl chain positions on the glycerol backbone and double bond positions within acyl chains. In the present work, 193 nm ultraviolet photodissociation (UVPD) implemented on an Orbitrap mass spectrometer is used to localize double bond positions within phosphatidylcholine (PC) acyl chains. Cleavage of the carbon-carbon bonds adjacent to the double bond provides a diagnostic mass difference of 24 Da and enables differentiation of double-bond positional isomers. The UVPD method was extended to the characterization of PCs in a bovine liver extract via a shotgun strategy. Positive mode higher energy collisional dissociation (HCD) and UVPD, and negative mode HCD were undertaken in a complementary manner to identify species as PCs and to localize double bonds, respectively.

Stability of Odd- Versus Even-Electron Gas-Phase (Quasi)Molecular Ions Derived from Pyridine-Substituted N-Heterotriangulenes

Electrospray ionisation of N-heterotriangulenes (i.e., dimethylmethylene-bridged triphenylamines) with up to three pyridyl groups at their periphery, produces the true radical cation ([M]^{+ .} ) and the protonated molecule ([M+H]⁺ ) simultaneously. These ions are studied as model systems to illustrate the stability alternation of odd- versus even-electron ions in energy-dependent collision-induced dissociation (CID) experiments. All ions show the same fragmentation pattern, the consecutive loss of three methyl radicals (^. CH₃ ) from the dimethylmethylene bridges of the central triangulene core. [M]^{+ .} ions dissociate at much lower collision energies than their [M+H]⁺ counterparts. The radical cation forms a singlet fragment with an extended aromatic system that is energetically favoured. Ab initio and density functional theory calculations support this interpretation and allow the assignment of the electronic structure of the fragment ions. Consecutive collision-induced dissociations provide a better match with theory when studied with an ion trap, rather than a linear quadrupole. This is attributed to the resonant nature of the excitation of intermediate ions.