Electrospray ionization and matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry of permethylated oligosaccharides

Cracking the Sugar Code by Mass Spectrometry : An Invited Perspective in Honor of Dr. Catherine E. Costello, Recipient of the 2017 ASMS Distinguished Contribution Award

The structural study of glycans and glycoconjugates is essential to assign their roles in homeostasis, health, and disease. Once dominated by nuclear magnetic resonance spectroscopy, mass spectrometric methods have become the preferred toolbox for the determination of glycan structures at high sensitivity. The patterns of such structures in different cellular states now allow us to interpret the sugar codes in health and disease, based on structure-function relationships. Dr. Catherine E. Costello was the 2017 recipient of the American Society for Mass Spectrometry's Distinguished Contribution Award. In this Perspective article, we describe her seminal work in a historical and geographical context and review the impact of her research accomplishments in the field.8 ᅟ Graphical abstract.

Structural Characterization of Carbohydrates by Fourier Transform Tandem Mass Spectrometry

Fourier transform tandem mass spectrometry (MS/MS) provides high mass accuracy, high sensitivity, and analytical versatility and has therefore emerged as an indispensable tool for structural elucidation of biomolecules. Glycosylation is one of the most common posttranslational modifications, occurring in ~50% of proteins. However, due to the structural diversity of carbohydrates, arising from non-template driven biosynthesis, achievement of detailed structural insight is highly challenging. This review briefly discusses carbohydrate sample preparation and ionization methods, and highlights recent developments in alternative high-resolution MS/MS strategies, including infrared multiphoton dissociation (IRMPD), electron capture dissociation (ECD), and electron detachment dissociation (EDD), for carbohydrates with a focus on glycans and proteoglycans from mammalian glycoproteins.

IR and UV photodissociation as analytical tools for characterizing lipid A structures

The utility of 193-nm ultraviolet photodissociation (UVPD) and 10.6-μm infrared multiphoton dissociation (IRMPD) for the characterization of lipid A structures was assessed in an ion trap mass spectrometer. The fragmentation behavior of lipid A species was also evaluated by activated-electron photodetachment (a-EPD), which uses 193-nm photons to create charge reduced radicals that are subsequently dissociated by collisional activation. In contrast to collision-induced dissociation (CID), IRMPD offered the ability to selectively differentiate product ions with varying degrees of phosphorylation because of the increased photoabsorption cross sections and thus dissociation of phosphate-containing species. Both 193-nm UVPD and a-EPD yielded higher abundances and a larger array of product ions arising from C-C cleavages, as well as cross-ring and inter-ring glucosamine cleavages, compared to CID and IRMPD, because of high energy, single-photon absorption, and/or radical-directed dissociation. UVPD at 193 nm also exhibited enhanced cleavage between the amine and carbonyl groups on the 2- and 2'-linked primary acyl chains. Lastly, UVPD of phosphorylethanolamine-modified lipid A species resulted in preferential cleavage of the C-O bond between ethanolamine and phosphate, enabling the selective identification of this modification.

Furanose C-C-linked γ-lactones: a combined ESI FTICR MS and semi-empirical calculations study

Sugars that incorporate the unsaturated carbonyl motif have become important synthetic targets not only as a result of their potential biological properties but also as precursors in the synthesis of many bioactive products. Moreover, little is known about the influence of the γ-lactone moiety in the fragmentation pattern of furanose rings. Therefore, two α,β-unsaturated γ-lactones (butenolides) and two β-hydroxy γ-lactones, C-C linked to a furanose ring were studied using electrospray ionization FTICR mass spectrometry. The behaviour of the protonated and sodiated forms of the compounds under study has been compared considering their structural features. Fragmentation mechanisms were established and ion structures were proposed taking into account the MS(2) and MS(3) experiments, accurate mass measurements and semi-empirical calculations. These inexpensive methods proved to be a valuable resource for proposing protonation sites and for the establishment of fragmentation pathways.

Automated measurement of permethylated serum N-glycans by MALDI-linear ion trap mass spectrometry

The use of N-glycan mass spectrometry for clinical diagnostics requires the development of robust high-throughput profiling methods. Still, structural assignment of glycans requires additional information such as MS(2) fragmentation or exoglycosidase digestions. We present a setting which combines a MALDI ionization source with a linear ion trap analyzer. This instrumentation allows automated measurement of samples thanks to the crystal positioning system, combined with MS(n) sequencing options. 2,5-Dihydroxybenzoic acid, commonly used for the analysis of glycans, failed to produce the required reproducibility due to its non-homogeneous crystallization properties. In contrast, alpha-cyano-4-hydroxycinnamic acid provided a homogeneous crystallization pattern and reproducibility of the measurements. Using serum N-glycans as a test sample, we focused on the automation of data collection by optimizing the instrument settings. Glycan structures were confirmed by MS(2) analysis. Although sample processing still needs optimization, this method provides a reproducible and high-throughput approach for measurement of N-glycans using a MALDI-linear ion trap instrument.

Collision-induced dissociation tandem mass spectrometry for structural elucidation of glycans

The complexity of glycans poses a major challenge for structure elucidation. Tandem mass spectrometry is currently an efficient and powerful technique for the structural characterization of glycans. Collision-induced dissociation (CID) is most commonly used, and involves first isolating the glycan ions of interest, translationally exciting them, and then striking them with inert target gas to fragment the precursor ions. The structural information of the glycan can be obtained from the fragment ions of the tandem MS spectra. In this chapter, sustained off-resonance irradiation-collision-induced dissociation (SORI-CID) implemented with matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI FT ICR MS) is demonstrated to be a useful analysis tool for structural elucidation of mucin-type O-glycans released from mucin glycoproteins. The mechanisms by which the glycans undergo fragmentations in the tandem mass analysis are also discussed.

Infrared multiphoton dissociation mass spectrometry for structural elucidation of oligosaccharides

The structural elucidation of oligosaccharides remains a major challenge. Mass spectrometry provides a rapid and convenient method for structural elucidation on the basis of tandem mass spectrometry. Ions are commonly selected and subjected to collision-induced dissociation (CID) to obtain structural information. However, a disadvantage of CID is the decrease in both the degree and efficiency of dissociation with increasing mass. In this chapter, we illustrate the use of infrared multiphoton dissociation (IRMPD) to obtain structural information for O- and N-linked oligosaccharides. The IRMPD and CID behaviors of oligosaccharides are compared.

Mass spectrometric detection of multiple extended series of neutral highly fucosylated N-acetyllactosamine oligosaccharides in human milk

Complex mixtures of high molecular weight fractions of pooled neutral human milk oligosaccharides (obtained via gel permeation chromatography) have been investigated. The subfractions were each permethylated and analyzed by high-resolution mass spectrometry, using matrix-assisted laser desorption/ionization (MALDI)-Fourier transform ion cyclotron resonance (FTICR) mass spectrometry, in order to investigate their oligosaccharide compositions. The obtained spectra reveal that human milk contains more complex neutral oligosaccharides than have been described previously; the data show that these oligosaccharides can be highly fucosylated, and that their poly-N-acetyllactosamine cores are substituted with up to 10 fucose residues on a an oligosaccharide that has 7-N-acetyllactosamine units. This is the first report of the existence in human milk of this large range of highly fucosylated oligosaccharides which possess novel, potentially immunologically active structures.

Detailed structural elucidation of polyesters and acrylates using Fourier transform mass spectrometry

The detailed structural characterization of complex polymer architectures, like copolymers and polymer mixtures, by mass spectrometry presents a challenge. Even though soft ionization analyses revolutionized the characterization of large molecules and provided a means for determining the polymer's molecular weight distribution, polydispersity, and end groups, full microstructure elucidation and monomer sequencing by soft ionization alone is not possible. The combination of high-resolution Fourier transform mass spectrometry (FTMS) and tandem mass spectrometry (MS(n)) provides a powerful analytical tool for addressing these challenges. This tool was used in our work to separate and identify the products of polymerization between 12-hydroxystearic acid (HSA) and stearic acid (SA), to provide precise information about the exact location of caprolactones on the Tris(2-hydroxyethyl)isocyanurate (THEIC) molecule, and to sequence a glycidyl methacrylate/methyl methacrylate (GMA/MMA) copolymer. The results highlight the value of ultrahigh resolution and tandem mass spectrometry for fine structural characterization and sequencing of polymers.

Identification of isomeric N-glycan structures by mass spectrometry with 157 nm laser-induced photofragmentation

Characterization of structural isomers has become increasingly important and extremely challenging in glycobiology. This communication demonstrates the capability of ion-trap mass spectrometry in conjunction with 157 nm photofragmentation to identify different structural isomers of permethylated N-glycans derived from ovalbumin without chromatographic separation. The results are compared with collision-induced dissociation (CID) experiments. Photodissociation generates extensive cross-ring fragment ions as well as diagnostic glycosidic product ions that are not usually observed in CID MS/MS experiments. The detection of these product ions aids in characterizing indigenous glycan isomers. The ion trap facilitates MS(n) experiments on the diagnostic glycosidic fragments and cross-ring product ions generated through photofragmentation, thus allowing unambiguous assignment of all of the isomeric structures associated with the model glycoprotein used in this study. Photofragmentation is demonstrated to be a powerful technique for the structural characterization of glycans.

Simultaneous fragmentation of multiple ions using IMS drift time dependent collision energies

Ion mobility spectrometry coupled with mass spectrometry (IMS-MS) was utilized to evaluate an ion collision energy ramping technique that simultaneously fragments a variety of species. To evaluate this technique, the fragmentation patterns of a mixture of ions ranging in mass, charge state, and drift time were analyzed to determine their optimal fragmentation conditions. The precursor ions were pulsed into the IMS-MS instrument and separated in the IMS drift cell based on mobility differences. Two differentially pumped short quadrupoles were used to focus the ions exiting the drift cell, and fragmentation was induced by collision induced dissociation (CID) between the conductance limiting orifice behind the second short quadrupole and before the first octopole in the mass spectrometer. To explore the fragmentation spectrum of each precursor ion, the bias voltages for the short quadrupoles and conductance limiting orifices were increased from 0 to 50 V above nonfragmentation voltage settings. An approximately linear correlation was observed between the optimal fragmentation voltage for each ion and its specific drift time, so a linear voltage gradient was employed to supply less collision energy to high mobility ions (e.g., small conformations or higher charge state ions) and more to low mobility ions. Fragmentation efficiencies were found to be similar for different ions when the fragmentation voltage was linearly ramped with drift time, but varied drastically when only a single voltage was used.

Collisionally activated dissociation and electron capture dissociation provide complementary structural information for branched permethylated oligosaccharides

Doubly charged sodiated and permethylated linear malto-oligosaccharides ({Glc}6-{Glc}9), branched N-linked glycans (high-mannose type GlcNAc2Man5-9, and complex asialo- and disialylated-biantennary glycans) were analyzed by tandem mass spectrometry using collisionally-activated dissociation (CAD) and "hot" electron capture dissociation (ECD) available in a custom-built ESI FTICR mass spectrometer. For linear permethylated malto-oligosaccharides, both CAD and "hot" ECD produced glycosidic cleavages (B, Y, C, and Z ions), cross-ring cleavages (A- and X-type), and internal cleavages (B/Y and C/Y type) to provide sequence and linkage information. For the branched N-linked glycans, CAD and "hot" ECD provided complementary structural information. CAD generated abundant B and Y fragment ions by glycosidic cleavages, whereas "hot" ECD produced dominant C and Z ions. A-type cross-ring cleavages were present in CAD spectra. Complementary A- and X-type cross-ring fragmentation pairs were generated by "hot" ECD, and these delineated the branching patterns and linkage positions. For example, 0, 4An and 3, 5An ions defined the linkage position of the major branch as the 6-position of the central core mannose residue. The internal fragments observed in CAD were more numerous and abundant than in "hot" ECD spectra. Since the triply charged (sodiated) molecular ion of the permethylated disialylated-biantennary N-linked glycan has relatively high abundance, it was isolated and fragmented in a "hot" ECD experiment and extensive fragment ions (glycosidic and complementary pairs of cross-ring cleavages) were generated to fully confirm the sequence, branching, and linkage assignments for this glycan.

Infrared multiphoton dissociation of O-linked mucin-type oligosaccharides

Oligosaccharides are known to play important roles in many biological processes. In the study of oligosaccharides, collision-induced dissociation (CID) is the most common dissociation method to elucidate the sequence and connectivity. However, a disadvantage of CID is the decrease in both the degree and efficiency of dissociation with increasing mass. In the present study, we have successfully performed infrared multiphoton dissociation (IRMPD) on 39 O-linked mucin-type oligosaccharide alditols (both neutral and anionic). CID and IRMPD spectra of several oligosaccharides were also compared. They yielded nearly identical fragment ions corresponding to the lowest energy fragmentation pathways. The characteristic fragmentations of structural motifs, which can provide the linkage information, were similarly presented in both CID and IRMPD spectra. Multistage of CID (MS(3) or MS(4)) is commonly needed to completely sequence the oligosaccharides, while IRMPD of the same compounds yielded the fragment ions corresponding to the loss of the first residue to the last residue during a single-stage tandem MS (MS(2)). Finally, it is shown that the fragmentation efficiency of IRMPD increases with the increasing size of oligosaccharides.

Laser-induced photofragmentation of neutral and acidic glycans inside an ion-trap mass spectrometer

Permethylated acidic and neutral N-glycans representing different types of glycan structures, such as linear and branched sialylated structures, high-mannose type and fucosylated complex type, were photodissociated with 157 nm vacuum ultraviolet light in a linear ion trap. Cross-ring fragments corresponding to high-energy fragmentation pathways were observed in abundance for all studied structures. Some product ions appear diagnostic for a linkage of sialic acid residues and the glycan antenna to which these residues are attached. A conclusive assignment of the fucosylation site of the studied glycan structure has been facilitated through measurement of cross-ring fragmentation resulting from photodissociation.

Miniaturized separation techniques in glycomic investigations

High-sensitivity glycomic analyses are becoming of a great interest in modern biomedical and clinical research, as well as in the development of recombinant protein products. The evolution of separation techniques for glycomic analysis at high sensitivity is highlighted in this review. These methodologies include capillary liquid chromatography, capillary electrophoresis (CE) and capillary electrochromatography (CEC). The potential of such methodologies in glycomic analysis is demonstrated for model glycoproteins as well as total glycomes derived from biological samples.

Differentiating structural isomers of sialylated glycans by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight tandem mass spectrometry

Using model acidic glycans, we demonstrate the benefits of permethylation for matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI/TOF-TOF) tandem mass spectrometry. With both the linear and branched structures, extensive cross-ring fragmentation product ions were generated, yielding valuable information on sugar linkages. Elimination of the negative charges commonly associated with sialylated structures through permethylation allowed their structural analysis in the positive ion mode. Extensive A- and X-type ions were observed for the linear structures, and slightly weaker signals for the branched sialylated structures. The diagnostic cross-ring fragments, permitting a distinction between alpha2-3 and alpha2-6 linkages of the sialic acid residues, were seen in abundance. Importantly, the cross-ring fragmentation with the branched structures provides adequate information to assign sialic acid residues, with a specific linkage, to a particular antenna.

New hyphenated methodologies in high-sensitivity glycoprotein analysis

High-sensitivity glycoprotein analyses are of particular interest in modern biomedical and clinical research, as well as in the development of recombinant protein products. The evolution of new hyphenated methodologies in high-sensitivity glycoprotein analysis is highlighted in this thematic review. These methodologies include, in particular, capillary LC/MALDI/TOF/TOF MS in conjunction with online permethylation platform, and silica-based lectin microcolumns interfaced to MS. The potential of these methodologies in glycomic and glycoproteomic analysis is demonstrated for model glycoproteins as well as total glycomes and glycoproteomes derived from biological samples. Additionally, the applications of CE-MS, CEC, and nanoLC with graphitized carbon in the areas of glycomics and glycoproteomics are described.

Atmospheric Pressure MALDI Fourier Transform Mass Spectrometry of Labile Oligosaccharides

An atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) source coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS) under UV laser and solid matrix conditions has been demonstrated to analyze a variety of labile oligosaccharides including O-linked and N-linked complex glycans released from glycoproteins. Spectra were acquired by both AP MALDI and vacuum MALDI and directly compared. The results presented here confirm that AP MALDI can generate significantly less energetic ions than vacuum MALDI and is able to produce the intact molecular ions with little or no fragmentation in both positive and negative ion mode analyses. Under certain conditions, noncovalent complexes of sialylated oligosaccharides were observed. The sensitivity attainable by AP MALDI was found to be comparable to conventional MALDI, and tandem mass spectrometry of oligosaccharides ionized by AP MALDI was shown to allow detailed structural analysis. Analysis of N-glycan mixtures derived from human fibrinogen further demonstrated that AP MALDI-FT ICR MS is ideal for the study of complex glycan samples as it provides high-accuracy, high-resolution mass analysis with no difficulty in distinguishing sample constituents from fragment ions.

Screening and sequencing of complex sialylated and sulfated glycosphingolipid mixtures by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry

A protocol for negative ion nanoelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (-)nanoESI-FTICR MS, investigation of complex biological mixtures consisting of sialylated or sulfated glycosphingolipids (GSL) expressing high heterogeneity in the ceramide portion is described. Different instrumental and solvent conditions were explored and optimized to promote efficient ionization, reduce the in-source fragmentation and consequently enhance the detection of intact molecular species from complex mixtures. Using the novel optimized (-)nanoESI-FTICR MS protocol, a reliable and detailed compositional fingerprint of the polysialylated ganglioside mixture isolated from human brain was obtained. Sustained off-resonance irradiation collision-induced dissociation mass spectrometry (SORI-CID MS2) was introduced for the first time for structural elucidation of polysialylated gangliosides. Under well-defined conditions, an informative fragmentation pattern of the trisialylated ganglioside GT1 was obtained. The compositional mapping of a complex mixture of sulfated glucuronic acid containing neolacto-series GSLs extracted from bovine Cauda equina provided hard evidence upon previously described components and new structures not identified before by any other analytical method. Negative ion nanoESI-FTICR MS at 9.4 T is shown here to represent a valuable method in glycolipidomics, allowing a high resolution and mass accuracy detection of major and minor GSL glycoforms and identification of known and novel biologically relevant structures.

Activation of large ions in FT-ICR mass spectrometry

The advent of soft ionization techniques, notably electrospray and laser desorption ionization methods, has enabled the extension of mass spectrometric methods to large molecules and molecular complexes. This both greatly extends the applications of mass spectrometry and makes the activation and dissociation of complex ions an integral part of these applications. This review emphasizes the most promising methods for activation and dissociation of complex ions and presents this discussion in the context of general knowledge of reaction kinetics and dynamics largely established for small ions. We then introduce the characteristic differences associated with the higher number of internal degrees of freedom and high density of states associated with molecular complexity. This is reflected primarily in the kinetics of unimolecular dissociation of complex ions, particularly their slow decay and the higher energy content required to induce decomposition--the kinetic shift (KS). The longer trapping time of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) significantly reduces the KS, which presents several advantages over other methods for the investigation of dissociation of complex molecules. After discussing general principles of reaction dynamics related to collisional activation of ions, we describe conventional ways to achieve single- and multiple-collision activation in FT-ICR MS. Sustained off-resonance irradiation (SORI)--the simplest and most robust means of introducing the multiple collision activation process--is discussed in greatest detail. Details of implementation of this technique, required control of experimental parameters, limitations, and examples of very successful application of SORI-CID are described. The advantages of high mass resolving power and the ability to carry out several stages of mass selection and activation intrinsic to FT-ICR MS are demonstrated in several examples. Photodissociation of ions from small molecules can be effected using IR or UV/vis lasers and generally requires tuning lasers to specific wavelengths and/or utilizing high flux, multiphoton excitation to match energy levels in the ion. Photodissociation of complex ions is much easier to accomplish from the basic physics perspective. The quasi-continuum of vibrational states at room temperature makes it very easy to pump relatively large amounts of energy into complex ions and infrared multiphoton dissociation (IRMPD) is a powerful technique for characterizing large ions, particularly biologically relevant molecules. Since both SORI-CID and IRMPD are slow activation methods they have many common characteristics. They are also distinctly different because SORI-CID is intrinsically selective (only ions that have a cyclotron frequency close to the frequency of the excitation field are excited), whereas IRMPD is not (all ions that reside on the optical path of the laser are excited). There are advantages and disadvantages to each technique and in many applications they complement each other. In contrast with these slow activation methods, the less widely appreciated activation method of surface induced dissociation (SID) appears to offer unique advantages because excitation in SID occurs on a sub-picosecond time scale, instantaneously relative to the observation time of any mass spectrometer. Internal energy deposition is quite efficient and readily adjusted by altering the kinetic energy of the impacting ion. The shattering transition--instantaneous decomposition of the ion on the surface--observed at high collision energies enables access to dissociation channels that are not accessible using SORI-CID or IRMPD. Finally, we discuss some approaches for tailoring the surface to achieve particular aims in SID.

Fragmentation of oligosaccharide ions with 157 nm vacuum ultraviolet light

The 157 nm photofragmentation of native and derivatized oligosaccharides was studied in a linear ion trap and in a home-built matrix-assisted laser desorption/ionization (MALDI) tandem time-of-flight (TOF/TOF) mass spectrometer, and the results were compared with collision-induced dissociation (CID) experiments. Photodissociation produces product ions corresponding to high-energy fragmentation pathways; for cation-derivatized oligosaccharides, it yields strong cross-ring fragment ions and provides better sequence coverage than low- and high-energy CID experiments. On the other hand, for native oligosaccharides, CID yielded somewhat better sequence coverage than photodissociation. The ion trap enables CID hybrid MS3 experiments on the high-energy fragment ions obtained from photodissociation.

Glycosphingolipid structural analysis and glycosphingolipidomics

Sphingosines, or sphingoids, are a family of naturally occurring long-chain hydrocarbon derivatives sharing a common 1,3-dihydroxy-2-amino-backbone motif. The majority of sphingolipids, as their derivatives are collectively known, can be found in cell membranes in the form of amphiphilic conjugates, each composed of a polar head group attached to an N-acylated sphingoid, or ceramide. Glycosphingolipids (GSLs), which are the glycosides of either ceramide or myo-inositol-(1-O)-phosphoryl-(O-1)-ceramide, are a structurally and functionally diverse sphingolipid subclass; GSLs are ubiquitously distributed among all eukaryotic species and are found in some bacteria. Since GSLs are secondary metabolites, direct and comprehensive analysis (metabolomics) must be considered an essential complement to genomic and proteomic approaches for establishing the structural repertoire within an organism and deducing its possible functional roles. The glycosphingolipidome clearly comprises an important and extensive subset of both the glycome and the lipidome, but the complexities of GSL structure, biosynthesis, and function form the outlines of a considerable analytical problem, especially since their structural diversity confers by extension an enormous variability with respect to physicochemical properties. This chapter covers selected developments and applications of techniques in mass spectrometric (MS) that have contributed to GSL structural analysis and glycosphingolipidomics since 1990. Sections are included on basic characteristics of ionization and fragmentation of permethylated GSLs and of lithium-adducted nonderivatized GSLs under positive-ion electrospray ionization mass spectrometry (ESI-MS) and collision-induced mass spectrometry (CID-MS) conditions; on the analysis of sulfatides, mainly using negative-ion techniques; and on selected applications of ESI-MS and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to emerging GSL structural, functional, and analytical issues. The latter section includes a particular focus on evolving techniques for analysis of gangliosides, GSLs containing sialic acid, as well as on characterizations of GSLs from selected nonmammalian eukaryotes, such as dipterans, nematodes, cestodes, and fungi. Additional sections focus on the issue of whether it is better to leave GSLs intact or remove the ceramide; on development and uses of thin-layer chromatography (TLC) blotting and TLC-MS techniques; and on emerging issues of high-throughput analysis, including the use of flow injection, liquid chromatography mass spectrometry (LC-MS), and capillary electrophoresis mass spectrometry (CE-MS).

Strategy for Profiling and Structure Elucidation of Mucin-Type Oligosaccharides by Mass Spectrometry

A strategy combining accurate mass determination, tandem mass spectrometry, structure homology, and exoglycosidases is described that allows the structural characterization of mucin-type O-linked oligosaccharides. The method is used to profile with quantitation the O-linked oligosaccharide (both neutral and anionic) components of the only diploid Xenopus frog, Xenopus tropicalis. Collision-induced dissociation was used to determine connectivity, to identify previously characterized oligosaccharides, and to determine the presence of structural motifs in unknown oligosaccharides. Exoglycosidase digestion was used to identify the individual residues along with the linkages. The enzymes were also used to cleave larger oligosaccharides to smaller units that are similar to previously elucidated components. By using CID, isomeric structures were compared to determine whether they were identical. In this way, the exoglycosidases were more effectively used, and their use was minimized. A total of 35 oligosaccharides including neutral, sialylated, and sulfated were characterized in this way. The relative abundances of all components were also determined based on HPLC.

Mass spectrometry of oligosaccharides

Glycosylation is a common post-translational modification to cell surface and extracellular matrix (ECM) proteins as well as to lipids. As a result, cells carry a dense coat of carbohydrates on their surfaces that mediates a wide variety of cell-cell and cell-matrix interactions that are crucial to development and function. Because of the historical difficulties with the analysis of complex carbohydrate structures, a detailed understanding of their roles in biology has been slow to develop. Just as mass spectrometry has proven to be the core technology behind proteomics, it stands to play a similar role in the study of functional implications of carbohydrate expression, known as glycomics. This review summarizes the state of knowledge for the mass spectrometric analysis of oligosaccharides with regard to neutral, sialylated, and sulfated compound classes. Mass spectrometric techniques for the ionization and fragmentation of oligosaccharides are discussed so as to give the reader the background to make informed decisions to solve structure-activity relations in glycomics.

Structural characterization of oligosaccharides using MALDI-TOF/TOF tandem mass spectrometry

Extensive cross-ring fragmentation ions, which are very informative of the linkages of the monosaccharide residues constituting these molecules, were readily observed in the MALDI-TOF/TOF/MS/MS spectra of oligosaccharides. These ions, in some cases, were more intense than the commonly observed Y and B ions. The A-type ions observed for the simple oligosaccharides allowed the distinction between alpha(1-4)- and alpha(1-6)-linked isobaric structures. The distinction was based not merely on the differences in the type of ions formed, but also on the ion intensities. For example, both alpha(1-4)- and alpha(1-6)-linked isobaric structures produce ions resulting from the loss of approximately 120 m/z units, but with different intensities, as a result of the fact that they correspond to two different ions (i.e., 0,4A- and 2,4A-ions), requiring different energies to be formed. Abundant A- and X-type ions were also observed for high-mannose N-glycans, allowing the determination of linkages. In addition, the high resolution furnished by MALDI-TOF/TOF allowed determination of certain ions that were commonly overlooked by MALDI-TOF or MALDI-magnetic sector instruments as a result of their lower resolution. Moreover, as a result of the fact that MS/MS spectra for parent ions and all fragment ions are acquired under the same experimental conditions, accurate determination of the molar ratios of isomeric glycans in a mixture analyzed simultaneously by MALDI-TOF/TOF tandem MS becomes possible.

Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. A comparison of fragmentation patterns of linear dextran obtained by in-source decay, post-source decay and collision-induced dissociation and the stability of linear and cyclic glucans studied by in-source decay

In the first part of this study fragmentation patterns from a range of dextran oligomers (containing 4-20 anhydroglucose units) were compared in three different methods of analysis coupled with matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry. Collision-induced-dissociation (CID), prompt in-source decay (ISD) and post-source decay (PSD) all caused cleavage of the glycosidic bonds. Both CID and to a lesser extent ISD caused further cleavage of pyranose rings of the individual sugar residues. There was very little cleavage of pyranose rings detected in the PSD spectrum. Derivatisation of the reducing end-groups of the oligodextrans with 1-phenyl-3-methyl-5-pyrazolone (PMP) restricted cleavage in the MALDI mass spectrometer to the non-reducing end, and further it enabled the saccharides to be separated by HPLC so that a single chain length could be examined as a standard. Maltoheptaose was also used as a standard. In the second part of the study prompt ISD-MALDI mass spectrometry was used to compare the fragmentation of three oligoglucans, dextran, maltodextrin and gamma cyclodextrin, that have different linkages and different secondary structure. The results showed that the degree of fragmentation correlated with the degree of freedom in the saccharide chains in solution determined by NMR. Dextran the most random conformation was fragmented most whereas there was little evidence of any fragments, not even glycosidic bond breakage from cyclodextrin, even when the laser power was increased considerably. The fragmentation pattern of maltodextrin was intermediate. The patterns of fragmentation produced by MALDI mass spectrometry, particularly where standards are available to calibrate the spectrum and the energy of the laser is controlled, can be used to predict the type of linkage present.

Coupling of fully automated chip electrospray to Fourier transform ion cyclotron resonance mass spectrometry for high-performance glycoscreening and sequencing

The NanoMate robot has been coupled to a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer at 9.4 T and implemented for the first time for complex carbohydrate analysis. It was optimized in the negative ion mode to achieve automated sample delivery on the chip along with increased sensitivity, ultra-high resolution and accurate mass determination. A novel bracket has been designed to allow a reliable mounting of the NanoMate to the Apollo electrospray ionization (ESI) source of an APEX II instrument. The notably higher efficiency of ionization for compositional mapping of complex mixtures and feasibility for fragmentation analysis of components by sustained off-resonance irradiation collision-induced tandem mass spectrometry (SORI-CID MS2) has been demonstrated on a glycoconjugate mixture containing O-glycosylated sialylated peptides from urine of a patient suffering from a hereditary N-acetylhexosaminidase deficiency (Schindler's disease), previously analyzed by capillary-based nanoESI-FTICRMS, and of a healthy control person. Due to its potential to generate highly charged ionic species, reduce the in-source fragmentation, increase sensitivity, reproducibility and ionization efficiency, along with the ability to generate a sustained and constant electrospray, this method can be considered as a new platform for advanced glycomics.

Coupling capillary electrochromatography with electrospray Fourier transform mass spectrometry for characterizing complex oligosaccharide pools

To deal with the complexity of the glycan mixtures released from glycoproteins, an efficient form of micro-column separations, capillary electrochromatography, has been combined with high-performance mass spectrometry (Fourier transform ion cyclotron resonance). Contour plots have been generated from the mixtures of O-linked oligosaccharides originated from bovine mucin and bile salt-stimulated lipase, a large glycoprotein enzyme.

Structural characterization of neutral oligosaccharide mixtures through a combination of capillary electrochromatography and ion trap tandem mass spectrometry

A CEC/ESI-MS/MS combined system has been developed for the separation and on-line structural analysis of neutral oligosaccharides. Various types of isomeric oligosaccharides were first successfully separated by CEC using polar monolithic columns, while the on-line tandem mass spectrometry has been explored to differentiate and elucidate the structures of isomeric oligosaccharides. The experimentally obtained tandem spectra usually provide sequence, branching, and linkage information. Oligosaccharide isomers with a different monomeric composition and branching showed different patterns of glycosidic linkage cleavage (B- and Y-ion series), allowing us to deduce their sequence and branching points. Isomers with different linkages were distinguished by identifying cross-ring fragment ions (A-ion series). While (1-->4) linkages yielded dominant (0,2)A ions, (1-->6) linkages showed an extensive and complete cross-ring cleavage series: (0,2)A, (0,3)A, and (0,4)A ions. Although the anomeric configurations and monosaccharide identification are rarely obtained from tandem MS, the relevant mixture components can be completely resolved with high-efficiency CEC columns featuring a polar functionality.

Sialylation analysis of O-glycosylated sialylated peptides from urine of patients suffering from Schindler's disease by Fourier transform ion cyclotron resonance mass spectrometry and sustained off-resonance irradiation collision-induced dissociation

A strategy based on Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) for screening of complex glycoconjugate mixtures containing O-linked glycopeptides and O-glycosylated amino acids with alpha-N-acetylgalactosaminyl residues is presented. To detect and identify O-glycoforms present in urine of patients suffering from hereditary N-acetylhexosaminidase deficiency (known as Schindler's disease), present at 100 times higher concentrations than in urine of healthy controls, new accurate methods for mapping and sequencing were required. In the mass spectrometric analysis particular attention has to be paid to original sialylation patterns, because of the potential lability of the sialic acid moiety during the desorption/ionization process. Negative ion nanoelectrospray ionization (nanoESI) FTICR-MS at 9.4 T is shown here to represent a method of choice for identification of single components in such complex glycomixtures due to high resolution and mass accuracy. By optimization of sustained off-resonance irradiation collision-induced dissociation tandem mass spectrometry (SORI-CID-MS(2)) in the negative ion mode, the type and sequence of the sialylated glycopeptide components were determined from their fragmentation patterns. Additionally, implementation of SORI-CID-MS(3) provides detailed information for sialylation analysis. The potential diagnostic value of this approach is discussed.

Structural analysis of underivatized neutral human milk oligosaccharides in the negative ion mode by nano-electrospray MS(n) (part 1: methodology)

Underivatized neutral oligosaccharides from human milk were analyzed by nano-electrospray ionization (ESI) using a quadrupole ion trap mass spectrometer (QIT-MS) in the negative-ion mode. Under these conditions neutral oligosaccharides are observed as deprotonated molecules [M-H]- with high intensity. CID-experiments of these species with the charge localized at the reducing end lead to C-type fragment ions forming a "new" reducing end. Fragmentations are accompanied by cross-ring cleavages that yield information about linkages of internal monosaccharides. Several isomeric compounds with distinct structural features, such as different glycosidic linkages, fucosylation and branching sites were investigated. The rules governing the fragmentation behavior of this class of oligosaccharides were elucidated and tested for a representative number of certain isomeric glycoforms using the MS/MS and MS(n) capabilities of the QIT. On the basis of the specific fragmentation behavior of deprotonated molecules, the position of fucoses and the linkage type (Gal beta-->3 GlcNAc or Gal beta1-->4 GlcNAc) could be determined and linear and branched could be differentiated. Rules could be established which can be applied in further investigations of these types of oligosaccharides even from heterogenous mixtures.

Gas-phase separations of protein and peptide ion fragments generated by collision-induced dissociation in an ion trap

Ion mobility/time-of-flight mass spectrometry techniques have been used to examine distributions of fragment ions generated by collision-induced dissociation (CID) in a quadrupole ion trap. The mobility-based separation step prior to mass-to-charge (m/z) analysis reduces spectral congestion and provides information that complements m/z-based assignments of peaks. The approach is demonstrated by examining fragmentation patterns of insulin chain B (a 30-residue peptide), and ubiquitin (a protein containing 76 amino acids). Some fragments of ubiquitin show evidence for multiple stable conformations.

A general method for precalculation of parameters for sustained off resonance irradiation/collision-induced dissociation

Sustained off resonance irradiation (SORI) collision-induced dissociation (CID) is a commonly used method of collisionally activating ions for fragmentation in Fourier transform mass spectrometric experiments. To achieve the degree of fragmentation desired, both the irradiation frequency and amplitude must be optimized. This is a time-consuming procedure, particularly when the m/z values of the precursor ions vary over a broad range. We present an approach that simplifies this optimization by precalculating the irradiation frequency of the ions to be fragmented as a constant percentage of the reduced cyclotron frequency. Using this approach, the optimal amplitude was found to be significantly less dependent on the m/z value of the precursor ion, and therefore required little or no adjustment. This method considerably simplified optimization of SORI-CID for analysis of carbohydrates, glycoconjugates, and peptides over the mass range m/z 300-3500, requiring optimization of only a single experimental parameter, the irradiation amplitude, and only for the first MS/MS stage.

Facile sequencing of oligosaccharides by matrix-assisted laser desorption/ionisation on a hybrid quadrupole orthogonal acceleration time-of-flight mass spectrometer

N-Linked oligosaccharide mixtures released from a number of standard glycoproteins were derivatised with 3-acetylamino-6-acetylaminoacridine (AA-Ac) using reductive amination. Analysis of these mixtures using an experimental matrix-assisted laser desorption/ionisation (MALDI) hybrid quadrupole orthogonal acceleration time-of-flight (Q-TOF) mass spectrometer provided detailed information about the mass distribution of the glycan derivatives. Collision-induced dissociation of the singly protonated [M + H](+) ions also gave rise to a number of product ions produced by the sequential cleavage of the glycosidic linkages. As fragmentation of the positively charged species occurred predominantly in one direction, i.e., from the non-reducing end of the glycan to the AA-Ac moiety, a considerable amount of information could be obtained with ease about the sequence in which the sugar residues were attached to one another. This derivatisation procedure and mass spectrometric methodology were applied successfully to neutral and acidic glycans released from proteins separated by gel electrophoresis.

Collision-induced fragmentation of underivatized N-linked carbohydrates ionized by electrospray

The electrospray mass spectra and collision-induced fragmentation of neutral N-linked glycans obtained from glycoproteins were examined with a Q-TOF mass spectrometer. The glycans were ionized most effectively as adducts of alkali metals, with lithium providing the most abundant signal and caesium the least. Singly charged ions generally gave higher ion currents than doubly charged ions. Addition of formic acid could be used to produce [M + H]+ ions, but these ions were always accompanied by abundant cone-voltage fragments. The energy required for collision-induced fragmentation was found to increase in a linear manner as a function of mass with the [M + Na]+ ions requiring about four times as much energy as the [M + H]+ ions for complete fragmentation of the molecular ions. Fragmentation of the [M + H]+ ions gave predominantly B- and Y-type glycosidic fragments whereas the [M + Na]+ and [M + Li]+ ions produced a number of additional fragments including those derived from cross-ring cleavages. Little fragmentation was observed from the [M + K]+ and [M + Rb]+ ions and the only fragment to be observed from the [M + Cs]+ ion was Cs+. The [M + Na]+ and [M + Li]+ ions from all the N-linked glycans gave abundant fragments resulting from loss of the terminal GlcNAc moiety and prominent, though weaker, ions as the result of 0,2A and 2,4A cross-ring cleavages of this residue. Most other ions were the result of successive additional losses of residues from the non-reducing terminus. This pattern was particularly prominent with glycans containing several non-reducing GlcNAc residues where successive losses of 203 u were observed. Many of the ions in the low-mass range were products of several different fragmentation routes but still provided structural information. Possibly of most diagnostic importance was an ion formed by loss of 221 u (GlcNAc molecule) from an ion that had lost the 3-antenna and the chitobiose core. This latter ion, although coincident in mass with some other 'internal' fragments, often provided additional information on the composition of the antennae. Other ions defining antenna composition were weak cross-ring fragments produced from the core branching mannose residue. Glycans containing Gal-GlcNAc residues showed successive losses of this moiety, particularly from the B-type fragments resulting from loss of the reducing-terminal GlcNAc residue. The [M + Na]+ and [M + Li]+ ions from high-mannose and hybrid glycans gave a series of ions of composition (Man)nNa/Li+ where n = 1 to the total number of glycans in the molecule, allowing these sugars to be distinguished from the more highly processed complex glycans. Other ions in the spectra of the high-mannose glycans were diagnostic of chain branching but insufficient information was available to determine their mode of formation.

Structural elucidation of zwitterionic sugar cores from glycosphingolipids by nanoelectrospray ionization-ion-trap mass spectrometry

The use of electrospray ionization (ESI)-ion-trap mass spectrometry (ITMS) for analysis of zwitterionic, glycolipid-derived sugar cores of glycosphingolipids is described. The capability of the method to perform multiple steps of fragmentation (MS(n)) allows structural characterization of these compounds. No derivatization of the released oligosaccharides is necessary when using nano-ESI with sample solution flow rates of about 30 nL/min. Investigations of positive as well as negative ions in fragmentation experiments up to MS(4) permit determination of the sequence of sugar units, their linkage positions, and the exact location of the substituents phosphocholine and phosphoethanolamine. In the case of phosphocholine, chemical cleavage of this substituent was necessary to obtain all the linkage information. Approximately 150-250 ng of sample was needed for each analysis.

Determining anomericity of the glycosidic bond in Zn(II)-diethylenetriamine-disaccharide complexes using MSn in a quadrupole ion trap

Zinc-diethylenetriamine (Zn-dien) N-glycoside complexes of four 1,4 and four 1,6 linked disaccharides are prepared. Each reaction mixture is ionized by electrospray and the resulting species [Zn(dien)(disaccharide)-H]+ is allowed to undergo collision-induced dissociation in a quadrupole ion trap. An MS3 analysis is used to differentiate alpha versus beta anomericity of the glycosidic bond in the disaccharide moiety. In addition, the MS2 and MS3 spectra can be used together to determine the linkage position of this glycosidic bond.

Bioanalytic applications of mass spectrometry

Mass spectrometry has suddenly expanded out of research and assay laboratories into biology, medicine and therapeutics. Electrospray ionization and matrix-assisted laser desorption/ionization yield increased mass-range and sensitivity, leading to novel applications and sparking new analyzer designs, software, and robotics.

Effect of 1-phenyl-3-methyl-5-pyrazolone labeling on the fragmentation behavior of asialo and sialylated N-linked glycans under electrospray ionization conditions

The advantages of labeling free N-linked oligosaccharides with 1-phenyl-3-methyl-5-pyrazolone (PMP), for high performance liquid chromatography (HPLC) and electrospray ionization mass spectrometry (ESI-MS) are discussed. The study focuses on some asialo and sialylated sugars, and compares the HPLC and ESI-MS behaviors of the PMP-labeled substances vs. the native compounds. It is pointed out that native free N-linked carbohydrates have very low affinities for the C18 reversed phases commonly used in HPLC. Native asialo oligosaccharides yield good ESI-MS sensitivity, although they are very susceptible to in-source collision-induced dissociation (CID), and the fragments are produced from any of the branches of the molecules, i.e. do not give specific structural information. Native N-linked standards bearing one sialic acid residue yield a 10-fold loss of ESI-MS sensitivity vs. asialo compounds, and native sugars with two sialic acid moieties were not detectable. The PMP labeling of asialo and sialylated sugars yielded higher affinities for HPLC C18 columns and, even at the early stages of method development, it was possible to separate three PMP-labeled standards to a useful extent. In ESI-MS, PMP-asialo sugars did not yield a significant increase in sensitivity vs. the native species; however, fragmentation produced by in-source CID was more directed as all predominant fragment ions contained the bis-PMP label. This feature is particularly useful when structural determination of an unknown sugar is required. PMP-sialylated sugars gave rise to very clean and informative ESI mass spectra. The monosialo sugar yielded a 100-fold sensitivity improvement vs. its native analog and, in the case of the disialylated compound, a 100% improvement was obtained in the positive mode. Most fragment ions were informative and contained the reducing end on the molecules, thus facilitating spectral interpretation. The combination of PMP derivatization with on-line HPLC/ESI-MS is a promising method for the analysis of asialo and sialylated carbohydrate mixtures.

Structural assignment of permethylated oligosaccharide subunits using sequential tandem mass spectrometry

The sequential tandem mass spectrometry (MSn) capabilities offered by quadrupole ion trap instruments have been explored in a systematic study of permethylated oligosaccharides. Under collision-induced dissociation, protonated molecular species generated in the electrospray ionization mode yield simple and predictable mass spectra. Information on sequence, branching, and, to some extent, interglycosidic linkages can be deduced from fragments resulting from the cleavage of glycosidic bonds. Simple rules for the structural assignment of carbohydrates have been established for the fragmentation of protonated species and subunits thereof and corroborated by 18O-labeling experiments. Moreover, sequential tandem mass spectrometry was demonstrated to allow the straightforward structural characterization of unknown carbohydrate moieties by comparing their CID spectra with those of a set of references. As the collision-induced dissociation patterns are not dependent on the number of prior tandem mass spectrometric steps, structures can be unambiguously assigned by match of the spectra. These findings establish the basis of MSn performed on a quadrupole ion trap instrument for elucidating structures of large carbohydrates, which can be virtually degraded in the mass spectrometer into smaller entities in one or several steps. This powerful technique has been applied, used in conjunction with specific CD3 labeling, to the characterization of series of subunits generated from fucosylated and sialylated oligosaccharides, which are among the most important structures as far as biological activities are concerned.

Two-dimensional mass spectrometry of biomolecules at the subfemtomole level

Multiple dimensions of unique molecular structure information can now be obtained from proteins and DNA using mass spectrometry. Less than 10(-16) mol of the active major histocompatibility complex signaling peptide in a mixture of thousands can be identified. For large proteins (> 40 kDa), the high resolving power (> 10(5) and 10(-17) mol sensitivity of Fourier-transform mass spectrometry provide exact molecular weight values (+/- 1 or 2 Da) for mixture components, indicating error or modifications compared with the predicted DNA sequence. Selecting a specific molecular species, its two-dimensional spectrum indicates the part of the molecule that is modified; a three-dimensional spectrum of that fragment further isolates the modification site.