Structural Analysis of Unsaturated Glycosphingolipids Using Shotgun Ozone-Induced Dissociation Mass Spectrometry

In-Depth Characterization of Sphingoid Bases via Radical-Directed Dissociation Tandem Mass Spectrometry

Sphingoid base (SPH) is a basic structural unit of all classes of sphingolipids. A sphingoid base typically consists of an aliphatic chain that may be desaturated between C4 and C5, an amine group at C2, and a variable number of OH groups located at C1, C3, and C4. Variations in the chain length and the occurrence of chemical modifications, such as methyl branching, desaturation, and hydroxylation, lead to a large structural diversity and distinct functional properties of sphingoid bases. However, conventional tandem mass spectrometry (MS/MS) via collision-induced dissociation (CID) faces challenges in characterizing these modifications. Herein, we developed an MS/MS method based on CID-triggered radical-directed dissociation (RDD) for in-depth characterization of sphingoid bases. The method involves derivatizing the sphingoid amine with 3-(2,2,6,6-tetramethylpiperidin-1-yloxymethyl)-picolinic acid 2,5-dioxopyrrolidin-1-yl ester (TPN), followed by MS2 CID to unleash the pyridine methyl radical moiety for subsequent RDD. This MS/MS method was integrated on a reversed-phase liquid chromatography-mass spectrometry workflow and further applied for in-depth profiling of total sphingoid bases in bovine heart and Caenorhabditis elegans. Notably, we identified and relatively quantified a series of unusual sphingoid bases, including SPH id17:2 (4,13) and SPH it19:0 in C. elegans, revealing that the metabolic pathways of sphingolipids are more diverse than previously known.

Chemical derivatization strategy for mass spectrometry-based lipidomics

Lipids, serving as the structural components of cellular membranes, energy storage, and signaling molecules, play the essential and multiple roles in biological functions of mammals. Mass spectrometry (MS) is widely accepted as the first choice for lipid analysis, offering good performance in sensitivity, accuracy, and structural characterization. However, the untargeted qualitative profiling and absolute quantitation of lipids are still challenged by great structural diversity and high structural similarity. In recent decade, chemical derivatization mainly targeting carboxyl group and carbon-carbon double bond of lipids have been developed for lipidomic analysis with diverse advantages: (i) offering more characteristic structural information; (ii) improving the analytical performance, including chromatographic separation and MS sensitivity; (iii) providing one-to-one chemical isotope labeling internal standards based on the isotope derivatization regent in quantitative analysis. Moreover, the chemical derivatization strategy has shown great potential in combination with ion mobility mass spectrometry and ambient mass spectrometry. Herein, we summarized the current states and advances in chemical derivatization-assisted MS techniques for lipidomic analysis, and their strengths and challenges are also given. In summary, the chemical derivatization-based lipidomic approach has become a promising and reliable technique for the analysis of lipidome in complex biological samples.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2017-2018

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2018. Also included are papers that describe methods appropriate to glycan and glycoprotein analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, new methods, matrices, derivatization, MALDI imaging, fragmentation and the use of arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Most of the applications are presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. The reported work shows increasing use of combined new techniques such as ion mobility and highlights the impact that MALDI imaging is having across a range of diciplines. MALDI is still an ideal technique for carbohydrate analysis and advancements in the technique and the range of applications continue steady progress.

Computational mass spectrometry accelerates C = C position-resolved untargeted lipidomics using oxygen attachment dissociation

Mass spectrometry-based untargeted lipidomics has revealed the lipidome atlas of living organisms at the molecular species level. Despite the double bond (C = C) position being a crucial factor in biological system, the C = C defined structures have not yet been characterized comprehensively. Here, we present an approach for C = C position-resolved untargeted lipidomics using a combination of oxygen attachment dissociation and computational mass spectrometry to increase the annotation rate. We validated the accuracy of our platform as per the authentic standards of 85 lipids and the biogenic standards of 52 molecules containing polyunsaturated fatty acids (PUFAs) from the cultured cells fed with various fatty acid-enriched media. By analyzing human and mice-derived samples, we characterized 648 unique lipids with the C = C position-resolved level encompassing 24 lipid subclasses defined by LIPIDMAPS. Our platform also illuminated the unique profiles of tissue-specific lipids containing n-3 and/or n-6 very long-chain PUFAs (carbon [Formula: see text] 28 and double bonds [Formula: see text] 4) in the eye, testis, and brain of the mouse.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

Mass Spectrometry-Based Techniques to Elucidate the Sugar Code

Cells encode information in the sequence of biopolymers, such as nucleic acids, proteins, and glycans. Although glycans are essential to all living organisms, surprisingly little is known about the "sugar code" and the biological roles of these molecules. The reason glycobiology lags behind its counterparts dealing with nucleic acids and proteins lies in the complexity of carbohydrate structures, which renders their analysis extremely challenging. Building blocks that may differ only in the configuration of a single stereocenter, combined with the vast possibilities to connect monosaccharide units, lead to an immense variety of isomers, which poses a formidable challenge to conventional mass spectrometry. In recent years, however, a combination of innovative ion activation methods, commercialization of ion mobility-mass spectrometry, progress in gas-phase ion spectroscopy, and advances in computational chemistry have led to a revolution in mass spectrometry-based glycan analysis. The present review focuses on the above techniques that expanded the traditional glycomics toolkit and provided spectacular insight into the structure of these fascinating biomolecules. To emphasize the specific challenges associated with them, major classes of mammalian glycans are discussed in separate sections. By doing so, we aim to put the spotlight on the most important element of glycobiology: the glycans themselves.

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

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

In-Depth Structural Characterization and Quantification of Cerebrosides and Glycosphingosines with Gas-Phase Ion Chemistry

Cerebrosides (n-HexCer) and glycosphingosines (n-HexSph) constitute two sphingolipid subclasses. Both are comprised of a monosaccharide headgroup (glucose or galactose in mammalian cells) linked via either an α- or β-glycosidic linkage to the sphingoid backbone (n = α or β, depending upon the nature of the linkage to the anomeric carbon of the sugar). Cerebrosides have an additional amide-bonded fatty acyl chain linked to the sphingoid backbone. While differentiating the multiple isomers (i.e. glucose vs galactose, α- vs β-linkage) is difficult, it is crucial for understanding their specific biological roles in health and disease states. Shotgun tandem mass spectrometry has been a powerful tool in both lipidomics and glycomics analysis but is often limited in its ability to distinguish isomeric species. This work describes a new strategy combining shotgun tandem mass spectrometry with gas-phase ion chemistry to achieve both differentiation and quantification of isomeric cerebrosides and glycosphingosines. Briefly, deprotonated cerebrosides, [n-HexCer-H]^-, or glycosphingosines, [n-HexSph-H]^-, are reacted with terpyridine (Terpy) magnesium complex dications, [Mg(Terpy)₂]²⁺, in the gas phase to produce a charge-inverted complex cation, [n-HexCer-H+MgTerpy]⁺ or [n-HexSph-H+MgTerpy]⁺. The collision-induced dissociation (CID) of the charge-inverted complex cations leads to significant spectral differences between the two groups of isomers, α-GalCer, β-GlcCer, and β-GalCer for cerebrosides and α-GlcSph, α-GalSph, β-GlcSph, and β-GalSph for glycosphingosines, which allows for isomer distinction. Moreover, we describe a quantification strategy with the normalized percent area extracted from selected diagnostic ions that quantify either three isomeric cerebroside or four isomeric glycosphingosine mixtures. The analytical performance was also evaluated in terms of accuracy, repeatability, and interday precision. Furthermore, CID of the product ions resulting from 443 Da loss from the charge-inverted complex cations ([n-HexCer-H+MgTerpy]⁺) has been performed and demonstrated for localization of the double-bond position on the amide-bonded monounsaturated fatty acyl chain in the cerebroside structure. The proposed strategy was successfully applied to the analysis of total cerebroside extracts from the porcine brain, providing in-depth structural information on cerebrosides from a biological mixture.

Recent advances in the mass spectrometric analysis of glycosphingolipidome - A review

Aberrant expression of glycosphingolipids has been implicated in a myriad of diseases, but our understanding of the strucural diversity, spatial distribution, and biological function of this class of biomolecules remains limited. These challenges partly stem from a lack of sensitive tools that can detect, identify, and quantify glycosphingolipids at the molecular level. Mass spectrometry has emerged as a powerful tool poised to address most of these challenges. Here, we review the recent developments in analytical glycosphingolipidomics with an emphasis on sample preparation, mass spectrometry and tandem mass spectrometry-based structural characterization, label-free and labeling-based quantification. We also discuss the nomenclature of glycosphingolipids, and emerging technologies like ion mobility spectrometry in differentiation of glycosphingolipid isomers. The intrinsic advantages and shortcomings of each method are carefully critiqued in line with an individual's research goals. Finally, future perspectives on analytical sphingolipidomics are stated, including a need for novel and more sensive methods in isomer separation, low abundance species detection, and profiling the spatial distribution of glycosphingolipid molecular species in cells and tissues using imaging mass spectrometry.

Ozone-Induced Cleavage of Endocyclic C═C Double Bonds within Steroid Epimers Produces Unique Gas-Phase Conformations

Herein we demonstrate the first application of ozone-induced cleavage of endocyclic C═C double bonds for improved steroid isomer separation using ion mobility-mass spectrometry. Steroids represent a challenging biomolecular class for ion mobility (IM) separations due to their structural rigidity and subtle stereochemical differences. In this work, we compare the effects of ozonolysis on the relative mobilities of a model stereoisomer pair, testosterone and epitestosterone. A solution-phase ozonolysis approach is used due to its simplicity, relatively low cost, and potential for rapid, online analysis. Despite the presence of solvent-based addition products, we observe that these steroids undergo an ozone-based cleavage resulting in unique, stable gas-phase conformations. The resulting resolution between testosterone and epitestosterone, with collision cross section values of 176.6 and 193.3 Ų, respectively, demonstrates a significant improvement in comparison with previous IM-based approaches. The significantly smaller conformation observed for epitestosterone is stabilized by a three-point interaction between the oxygen-containing functional groups and a sodium ion; this same conformation cannot be sterically achieved by testosterone. Identification of this specific structural difference is strengthened by experimental results showing the disappearance of this conformation following in-source water loss, which eliminates the potential for that three-point interaction. Computational modeling of the lowest energy gas-phase structures for these ozone products corroborates the experimental results. In conclusion, this approach provides tremendous potential as a rapid IM separation method for steroid isomers and other endocyclic C═C double bond containing molecules.

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

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

Orthogonal Method for Double-Bond Placement via Ozone-Induced Dissociation Mass Spectrometry (OzID-MS)

Most often, the structures of secondary metabolites are solved using a suite of NMR techniques. However, there are times when it can be challenging to position double bonds, particularly those that are fully substituted or when there are multiple double bonds in similar chemical environments. Ozone-induced dissociation mass spectrometry (OzID-MS) serves as an orthogonal structure elucidation tool, using predictable fragmentation patterns that are generated after ozonolysis across a carbon-carbon double bond. This technique is finding growing use in the lipidomics community, suggestive of its potential value for secondary metabolites. This methodology was evaluated by confirming the double-bond positions in five fungal secondary metabolites, specifically, ent-sartorypyrone E (1), sartorypyrone A (2), sorbicillin (3), trichodermic acid A (4), and AA03390 (5). This demonstrated its potential with a variety of chemotypes, ranging from polyketides to terpenoids and including those in both conjugated and nonconjugated polyenes. In addition, the potential of using this methodology in the context of a mixture was piloted by studying Aspergillus fischeri, first examining a traditional extract and then sampling a live fungal culture in situ. While the intensity of signals varied from pure compound to extract to in situ, the utility of the technique was preserved.

Mapping Unsaturation in Human Plasma Lipids by Data-Independent Ozone-Induced Dissociation

Over 1500 different lipids have been reported in human plasma at the sum composition level. Yet the number of unique lipids present is surely higher, once isomeric contributions from double bond location(s) and fatty acyl regiochemistry are considered. In order to resolve this ambiguity, herein, we describe the incorporation of ozone-induced dissociation (OzID) into data-independent shotgun lipidomics workflows on a high-resolution hybrid quadrupole-Orbitrap platform. In this configuration, [M + Na]⁺ ions generated by electrospray ionization of a plasma lipid extract were transmitted through the quadrupole in 1 Da segments. Reaction of mass-selected lipid ions with ozone in the octopole collision cell yielded diagnostic ions for each double bond position. The increased ozone concentration in this region significantly improved ozonolysis efficiency compared with prior implementations on linear ion-trap devices. This advancement translates into increased lipidome coverage and improvements in duty cycle for data-independent MS/MS analysis using shotgun workflows. Grouping all precursor ions with a common OzID neutral loss enables straightforward classification of the lipidome by unsaturation position (with respect to the methyl terminus). Two-dimensional maps obtained from this analysis provide a powerful visualization of structurally related lipids and lipid isomer families within plasma. Global profiling of lipid unsaturation in plasma extracts reveals that most unsaturated lipids are present as isomeric mixtures. These new insights provide a unique picture of underlying metabolism that could in the future provide novel indicators of health and disease.

Fragmentation Behavior and Gas-Phase Structures of Cationized Glycosphingolipids in Ozone-Induced Dissociation Mass Spectrometry

The role of cationization in the fragmentation behavior of glycoconjugates is amply documented in collisional activation techniques but remains less explored in ozone-induced dissociation mass spectrometry (OzID-MS). OzID-MS has been used to elucidate the location of carbon-carbon double bonds in unsaturated lipids. Previously, we demonstrated the structural analysis of unsaturated glycosphingolipids using OzID-MS by mass-selecting the [M+Na]+ adduct for fragmentation. In this work, we aimed to examine the effect of different adducts, namely [M+Na]+, [M+Li]+, and [M+H]+ on the OzID-MS fragmentation behavior of a representative unsaturated glycosphingolipid, LacCer d18:1/18:1(9Z). Our data show that [M+H]+ primarily undergoes dehydration followed by collision-induced dissociation-like loss of the headgroup, while [M+Li]+ and [M+Na]+ dissociate at the double bonds albeit with slightly different intensities of the resulting fragments. Using molecular mechanics and theoretical calculations at the semiempirical level, we report for the first time the gas-phase structure of cationized glycosphingolipids, which helps rationalize the observed bond cleavage. Our findings highlight that the type of adducts can influence gas-phase ion structure of glycosphingolipids and subsequently affect their fragmentation in OzID-MS. This study contributes to the growing body of knowledge on OzID-MS and gas-phase structures of ionized lipids and the findings have the potential to be extended to other more complex glycoconjugates.

Differential Isotope Labeling by Permethylation and Reversed-Phase Liquid Chromatography-Mass Spectrometry for Relative Quantification of Intact Neutral Glycolipids in Mammalian Cells

Probing the role of glycolipids in health and disease warrants development of practical strategies to determine these molecules at the intact structural level, namely to simultaneously characterize and quantify the glycan and lipid moieties without breaking the linkage between them. Herein we present such an approach utilizing differential isotope labeling and reversed phase liquid chromatography-tandem mass spectrometry (RPLC-MS/MS) for structural characterization and relative quantification of intact neutral glycolipids. In this approach, each individual sample and a pooled aliquot of each sample were permethylated using ¹²CH₃I and ¹³CH₃I, respectively, with the latter one serving as internal reference standard. The individual ¹²C-permethylated samples were spiked with equal amounts of the ¹³C-permethylated pooled sample and analyzed by RPLC-MS/MS. Permethylation not only increased the ionization efficiency of glycolipids but also facilitated structural characterization of both moieties. The ratio of the peak areas between the ¹²C- and ¹³C-labeled glycolipids served as surrogate measure of their relative concentrations. The coefficient of variation of the method was <6% measured across four representative glycolipids in five different ratios and triplicate experiments, after correction of natural isotopic distribution. When analyzing the low abundant glycolipids in total lipid extract, permethylation can dramatically reduce the analytical background by depleting most of the highly abundant ester-linked lipids. Application to conduritol B epoxide-, a β-glucocerebrosidase inhibitor, treated RAW 264.7 cells demonstrated the practical utility of this method in profiling the temporal accumulation of different glycolipids. Overall, this methodology offers a practical LC-MS based identification and quantification strategy to advance intact glycolipids analysis in mammalian cells.

New Frontiers in Lipidomics Analyses using Structurally Selective Ion Mobility-Mass Spectrometry

The growth of lipidomics and the high isomeric complexity of the lipidome has revealed a need for analytical techniques capable of structurally characterizing lipids with a high degree of specificity. Lipids are morphologically diverse molecules that can exist as any one of a large number of isomeric species, and as such are often indistinguishable by mass spectrometry without a complementary separation method. Recent developments in the field of lipidomics aim to address these challenges by utilizing a combination of multiple analytical techniques which are selective to lipid primary structure. This review summarizes two emerging strategies for lipidomic analysis, namely, ion mobility-mass spectrometry and ion fragmentation via ozonolysis.

Categorizing and qualifying nutritional lipidomic data: defining brutto, medio, genio, and infinio lipid species within macrolipidomics and microlipidomics

PURPOSE OF REVIEW

Lipidomic profiling of biological samples is increasing in nutritional research applications. 'Lipidomic analyses' however can be quite variable in specific methods and the type of information about the specific lipids that is revealed. The lack of defined and simple terminology to describe aspects of lipidomics presents a challenge in the use of lipidomics across interdisciplinary research groups.

RECENT FINDINGS

We propose the use of macrolipidomics and microlipidomics to define lipidomic strategies based on analytical outcomes. Macrolipidomics involves the global characterization of the most abundant lipids in a system, whereas microlipidomics examines low abundant lipids with potent bioactivity that typically require specialized analyses. We also propose that in addition to the term 'brutto', the terms 'medio, genio, and infinio' be used to indicate when information about the lipid molecule increases from isobars/isomers to regio-isomers with carbon-carbon double bond information.

SUMMARY

The use of these terms will help establish a common language around the field of lipidomics and improve communication and uptake in the field of clinical nutrition. Macrolipidomic and microlipidomic terms quickly convey the general purpose of the approach. Brutto, medio, genio, and infino quickly convey the nature of the lipid identification.

An On-Tissue Paternò-Büchi Reaction for Localization of Carbon-Carbon Double Bonds in Phospholipids and Glycolipids by Matrix-Assisted Laser-Desorption-Ionization Mass-Spectrometry Imaging

Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) visualizes the distribution of phospho- and glycolipids in tissue sections. However, C=C double-bond (db) positional isomers generally cannot be distinguished. Now an on-tissue Paternò-Büchi (PB) derivatization procedure that exploits benzaldehyde as a MALDI-MSI-compatible reagent is introduced. Laser-induced postionization (MALDI-2) was used to boost the yields of protonated PB products. Collision-induced dissociation of these species generated characteristic ion pairs, indicative of C=C position, for numerous singly and polyunsaturated phospholipids and glycosphingolipids in mouse brain tissue. Several db-positional isomers of phosphatidylcholine and phosphatidylserine species were expressed with highly differential levels in the white and gray matter areas of cerebellum. Our PB-MALDI-MS/MS procedure could help to better understand the physiological role of these db-positional isomers.

Differential-Mobility Spectrometry of 1-Deoxysphingosine Isomers: New Insights into the Gas Phase Structures of Ionized Lipids

Separation and structural identification of lipids remain a major challenge for contemporary lipidomics. Regioisomeric lipids differing only in position(s) of unsaturation are often not differentiated by conventional liquid chromatography-mass spectrometry approaches leading to the incomplete, or sometimes incorrect, assignation of molecular structure. Here we describe an investigation of the gas phase separations by differential-mobility spectrometry (DMS) of a series of synthetic analogues of the recently described 1-deoxysphingosine. The dependence of the DMS behavior on the position of the carbon-carbon double bond within the ionized lipid is systematically explored and compared to trends from complementary investigations, including collision cross-sections measured by drift tube ion mobility, reaction efficiency with ozone, and molecular dynamics simulations. Consistent trends across these modes of interrogation point to the importance of direct, through-space interactions between the charge site and the carbon-carbon double bond. Differences in the geometry and energetics of this intramolecular interaction underpin DMS separations and influence reactivity trends between regioisomers. Importantly, the disruption and reformation of these intramolecular solvation interactions during DMS are proposed to be the causative factor in the observed separations of ionized lipids which are shown to have otherwise identical collision cross-sections. These findings provide key insights into the strengths and limitations of current ion-mobility technologies for lipid isomer separations and can thus guide a more systematic approach to improved analytical separations in lipidomics.

Isobaric Labeling of Intact Gangliosides toward Multiplexed LC-MS/MS-Based Quantitative Analysis

Gangliosides are sialic acid-containing glycosphingolipids recognized to play essential role in biological processes. Both the glycan and lipid structures influence their biological function and thus necessitate their determination as intact molecular species. To our knowledge, no multiplexed method for intact gangliosides currently exists. In this paper, we aimed to demonstrate an approach for isobaric labeling of intact gangliosides. Specifically, we carried out the rapid, chemoselective oxidation of sialic acid side chain in common ganglioside core structures using NaIO₄ followed by ligation with a carbonyl-reactive isobaric tandem mass tag (TMT) reagent and subsequent RPLC-MS/MS analysis. Attachment of the isobaric label was observed to improve the ionization efficiency of complex gangliosides using electrospray ionization. Fragmentation of the resulting [M + 2H]²⁺ ions of TMT-labeled gangliosides provided information-rich spectra containing fragments from the glycan, lipid, and TMT reporter ions. This facile approach enabled simultaneous quantification of up to six samples as well as identification of glycan and lipid compositions in a single injection. As a proof-of-concept, using porcine brain total ganglioside extracts pooled at known ratios, we obtained overall sample-to-sample precision of <12% RSD and mean error of <10%. This showcased the great promise and feasibility of this strategy for high-throughput analysis of intact gangliosides in biological extracts.

Online Ozonolysis Combined with Ion Mobility-Mass Spectrometry Provides a New Platform for Lipid Isomer Analyses

One of the most significant challenges in contemporary lipidomics lies in the separation and identification of lipid isomers that differ only in site(s) of unsaturation or geometric configuration of the carbon-carbon double bonds. While analytical separation techniques including ion mobility spectrometry (IMS) and liquid chromatography (LC) can separate isomeric lipids under appropriate conditions, conventional tandem mass spectrometry cannot provide unequivocal identification. To address this challenge, we have implemented ozone-induced dissociation (OzID) in-line with LC, IMS, and high resolution mass spectrometry. Modification of an IMS-capable quadrupole time-of-flight mass spectrometer was undertaken to allow the introduction of ozone into the high-pressure trapping ion funnel region preceding the IMS cell. This enabled the novel LC-OzID-IMS-MS configuration where ozonolysis of ionized lipids occurred rapidly (10 ms) without prior mass-selection. LC-elution time alignment combined with accurate mass and arrival time extraction of ozonolysis products facilitated correlation of precursor and product ions without mass-selection (and associated reductions in duty cycle). Unsaturated lipids across 11 classes were examined using this workflow in both positive and negative ion modalities, and in all cases, the positions of carbon-carbon double bonds were unequivocally assigned based on predictable OzID transitions. Under these conditions, geometric isomers exhibited different IMS arrival time distributions and distinct OzID product ion ratios providing a means for discrimination of cis/trans double bonds in complex lipids. The combination of OzID with multidimensional separations shows significant promise for facile profiling of unsaturation patterns within complex lipidomes including human plasma.

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.