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.

RADICAL-INDUCED FRAGMENTATION OF PHOSPHOLIPID CATIONS USING METASTABLE ATOM-ACTIVATED DISSOCIATION MASS SPECTROMETRY (MAD-MS)

The fragmentation pattern of several protonated 1+ phosphatidylcholines (PCs) were studied using low energy collision induced dissociation (CID) and helium metastable atom-activated dissociation (He-MAD). He-MAD of the protonated compounds produced a dominant phosphocholine head group at m/z 184 as well as typical sn-1 and sn-2 glycerol fragments such as [M+H-R_x-1CHC=O]⁺ and [M+H-R_x-1CO₂H]⁺. Within the aliphatic chain, He-MAD showed fragments consistent with high-energy collision induced dissociation (HE-CID) and products/pathways consistent with Penning ionization of the 1+ precursor ions to their respective radical dications. These Penning ionization products included both singly and doubly charged radical fragments, and the fragment ions are related to the number and position of double bonds in the acyl chains. Fragments created through HE-CID-like fragmentation followed classic charge remote fragmentation pathways including ladder-like fragmentation along the acyl chain, except for additional or missing peaks due to predictable rearrangement reactions. He-MAD therefore shows utility in being able to effectively fragment singly charged lipids into a variety of useful product ions using both radical and high-energy processes in the confines of a 3D ion trap.

Sphingolipid analysis by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS)

Sphingolipid (SPL) metabolism (Fig. 1) serves a key role in the complex mechanisms regulating cellular stress responses to environment. Several SPL metabolites, especially ceramide (Cer), sphingosine (Sph) and sphingosinel-phosphate (S1P) act as key bioactive molecules governing cell growth and programmed cell death (Fig. 2). Perturbations in sphingolipids of one type may enhance or interfere with the action of another. To monitor changes in SPL composition therefore, reliable analytical methods are necessary. Here we present the liquid chromatography tandem mass spectrometry (LC-MS/MS) approach for simultaneous qualitative and quantitative monitoring of SPL components (classes and molecular species) in biological material as an effective tool to study sphingolipid signaling events. The LC-MS/MS methodology is the only available technique that provides high specificity and sensitivity, along with a wealth of structural identification information.

Simultaneous quantitative analysis of bioactive sphingolipids by high-performance liquid chromatography-tandem mass spectrometry

There has been a recent explosion in research concerning novel bioactive sphingolipids (SPLs) such as ceramide (Cer), sphingosine (Sph) and sphingosine 1-phosphate (Sph-1P) that necessitates development of accurate and user-friendly methodology for analyzing and quantitating the endogenous levels of these molecules. ESI/MS/MS methodology provides a universal tool used for detecting and monitoring changes in SPL levels and composition from biological materials. Simultaneous ESI/MS/MS analysis of sphingoid bases (SBs), sphingoid base 1-phosphates (SB-1Ps), Cers and sphingomyelins (SMs) is performed on a Thermo Finnigan TSQ 7000 triple quadrupole mass spectrometer operating in a multiple reaction monitoring (MRM) positive ionization mode. Biological materials (cells, tissues or physiological fluids) are fortified with internal standards (ISs), extracted into a one-phase neutral organic solvent system, and analyzed by a Surveyor/TSQ 7000 LC/MS system. Qualitative analysis of SPLs is performed by a Parent Ion scan of a common fragment ion characteristic for a particular class of SPLs. Quantitative analysis is based on calibration curves generated by spiking an artificial matrix with known amounts of target synthetic standards and an equal amount of IS. The calibration curves are constructed by plotting the peak area ratios of analyte to the respective IS against concentration using a linear regression model. This robust analytical procedure can determine the composition of endogenous sphingolipids (ESPLs) in varied biological materials and achieve a detection limit at 1 pmol or lower level. This and related methodology are already defining unexpected specialization and specificity in the metabolism and function of distinct subspecies of individual bioactive SPLs.

Identification of phospholipid structures in human blood by direct-injection quadrupole-linear ion-trap mass spectrometry

Direct-injection electrospray ionization mass spectrometry in combination with information-dependent data acquisition (IDA), using a triple-quadrupole/linear ion trap combination, allows high-throughput qualitative analysis of complex phospholipid species from child whole blood. In the IDA experiments, scans to detect specific head groups (precursor ion or neutral loss scans) were used as survey scans to detect phospholipid classes. An enhanced resolution scan was then used to confirm the mass assignments, and the enhanced product ion scan was implemented as a dependent scan to determine the composition of each phospholipid class. These survey and dependent scans were performed sequentially and repeated for the entire duration of analysis, thus providing the maximum information from a single injection. In this way, 50 different phospholipids belonging to the phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, phosphatidylcholine and sphingomyelin classes were identified in child whole blood.

Analysis of sphingosine 1-phosphate, ceramides, and other bioactive sphingolipids by high-performance liquid chromatography-tandem mass spectrometry

The lipid backbones of sphingolipids and their metabolites are highly bioactive compounds that affect diverse cellular functions. The metabolites that have been most extensively studied with respect to their effects on cell behavior are ceramides, sphingosine (and other sphingoid bases), and sphingosine 1-phosphate. Additionally, there is interest in other naturally occurring species such as lysosphingolipids (sphingosine, phosphorylcholine, and psychosines) and N-methyl (di- and tri-methyl)-sphingosines. In many cases, studies of cell signaling mediated by these compounds have focused on a single category (such as ceramides or sphingosine 1-phosphate) because of the technical difficulty of more comprehensive analyses. One obstacle in such studies is that most of these compounds are metabolically interconvertable, so it is difficult to assign a conclusive relationship. In this article, we describe the analytical methods for extraction, identification, and quantitation of sphingolipids using state-of-the-art tandem mass spectrometry (MS/MS). Precursor ion scans are used to distinguish various species of sphingolipids in crude extracts by their unique molecular decomposition products. Specific headgroup, sphingoid base, and fatty acid chain combinations can be readily determined. Quantitation is achieved by multiple reaction monitoring (MRM) in conjunction with high-pressure liquid chromatography (HPLC). Compared to precursor ion scans alone, MRM experiments yield greater sensitivity and lower limits of detection by monitoring a specific precursor and product ion pair. This sensitivity facilitates detection of dozens of individual molecular species under optimal ion formation and decomposition conditions for each species, eliminating any ambiguity that may arise from differences in the kinetics of dissociation.

Applications and mechanisms of charge-remote fragmentation

Studies on the applications, energetics, and mechanisms of charge-remote fragmentations are reviewed, with emphasis given to those articles published after 1992. Independent of the charge status, charge-remote fragmentations are analogous to gas-phase thermolysis. Under collisional activation and with a fixed charge, ions containing long-chain or poly-ring structures undergo charge-remote fragmentations, generating productions that are structurally informative. Interpretation of the production spectra enables one to elucidate molecular structures. Although charge-remote fragmentations have been successfully used in the structural determination of fatty acids, phospholipids, glycolipids, triacylglycerols, steroids, peptides, ceramides, and other systems, the energetics and mechanisms of these reactions are still debated because none of the existing mechanisms can explain all the experimental data.

Structural determination of sphingomyelin by tandem mass spectrometry with electrospray ionization

Alkaline metal adduct ions of sphingomyelin were formed by electrospray ionization in positive ion mode. Under low energy collisionally activated dissociation (CAD), the product ion spectra yield abundant fragment ions representative of both long chain base and fatty acid which permit unequivocal determination of the structure. Tandem spectra obtained by constant neutral loss scanning permit identification of sphingomyelin class and specific long chain base subclass in the mixture. The fragmentation pathways under CAD were proposed, and were further confirmed by source CAD tandem mass spectrometry. The total analysis of sphingomyelin mixtures from bovine brain, bovine erythrocytes, and chicken egg yolk is also presented.

Structure determination of soybean and wheat glucosylceramides by tandem mass spectrometry

Glucosylceramide (GluCer) is a major sphingolipid of plant tissue and, thus, abundant in nature and in dietary food sources. The lipid backbones of mammalian GluCer (sphingosine, d18:1(delta4), and ceramide) induce cell death (apoptosis) and inhibit colon carcinogenesis, it is critical to know the structures of GluCer present in plants as a first step toward understanding this potential link between diet and cancer. This study characterized the molecular species of GluCer from soybean and wheat by low-resolution, high-resolution and tandem mass spectrometry. Soybean GluCer was comprised primarily (>95%) of ceramide with 4,8-sphingadiene (d18:2(delta4,delta8)) and alpha-hydroxypalmitic acid (h16:0); the remainder had the same backbone with h18:0, h20:0, h22:0 and h24:0 fatty acids. Wheat GluCer had three major ceramide, d18:2(delta4,delta8) with h16:0, d18:1(delta8) with h16:0 and d18: 2(delta4,delta8) with h20:0, and smaller amounts of other homologs. These backbones differ from those of mammalian sphingolipids, which often have a delta4-double bond (but rarely a delta8-double bond), and have alpha-hydroxy fatty acids in only some cases. Previously unexplained fragmentations that were diagnostic for the type of sphingoid base backbone (i.e. by homolytic cleavage of the doubly allylic C-6-C-7 bond to yield a stable distonic allylic radical cation and an allylic radical neutral) were also identified. Hence this method should be useful in the identification of double bonds in sphingolipids, and structure-function relationships between sphingolipids and colon carcinogenesis.

Quantitative analysis of biological membrane lipids at the low picomole level by nano-electrospray ionization tandem mass spectrometry

Nano-electrospray tandem mass spectrometry allows qualitative and quantitative analysis of complex membrane lipid mixtures at the subpicomole level. We have exploited this technique to selectively detect individual classes of phospholipids from unprocessed total cellular lipid extracts by either precursor ion or neutral loss scanning. This way phosphatidylcholine, sphingomyelin, phosphatidylinositol and -phosphates, phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerol, phosphatidic acid, and their plasmalogen analogues can be detected. The optimized ionization and fragmentation conditions described together with the principle of internal standardization by nonnatural analogues allow the rapid and quantitative determination of membrane lipid compositions down to sample amounts of 1000 cells.

Application of tandem mass spectrometry for the analysis of long-chain carboxylic acids

The application of MS-MS for the analysis of long-chain carboxylic acids and their esters has proved enormously successful but expensive. It is discussed mainly on basis of results obtained with different instruments with lesser attention to principles of the method, which have been adequately reviewed elsewhere. The use of electrospray ionization (ESI) has greatly increased the sensitivity of the method and has permitted assay of total lipid extracts. The combination of HPLC with electrospray and single quadrupole mass spectrometry, LC-ESI-CID-MS, rivals the triple quadrupole MS-MS application in many instances at considerably lower cost. However, LC-ESI-MS-MS remains the most desirable system at the present time for lipid ester analyses.