A new technique distinguishing alpha2-3 sialyl linkage from alpha2-6 linkage in sialyllactoses and sialyl-N-acetyllactosamines by post-source decay fragmentation method of MALDI-TOF mass spectrometry

Nitro Indole Derivatives as Novel Dual-Polarity Matrices for MALDI Mass Spectrometry and Imaging with Broad Applications

A new matrix framework is presented in this study for the improved ionization efficiency of complex mixtures by matrix-assisted laser desorption ionization (MALDI) mass spectrometry/imaging. Five nitro indole (NI) derivatives [3-methyl-4-nitro-1H-indole (3,4-MNI), 3-methyl-6-nitro-1H-indole (3,6-MNI), 2,3-dimethyl-4-nitro-1H-indole (2,3,4-DMNI), 2,3-dimethyl-6-nitro-1H-indole (2,3,6-DMNI), and 4-nitro-1H-indole (4-NI)] were synthesized and shown to produce both positive and negative ions with a broad class of analytes as MALDI matrices. NI matrices were compared to several common matrices, such as 2,5-dihydroxybenzoic acid (DHB), alpha-cyano-4-hydroxylcinnamic acid (CHCA), sinapinic acid (SA), 1,5-diaminonaphthelene (1,5-DAN), and 9-aminoacridine (9-AA), for the analysis of lipid, peptide, protein, glycan, and perfluorooctanesulfonic acid (PFOS) compounds. 3,4-MNI demonstrated the best performance among the NI matrices. This matrix resulted in reduced ion suppression and better detection sensitivity for complex mixtures, for example, egg lipids/milk proteins/PFOS in tap water, while 2,3,6-DMNI was the best matrix for blueberry tissue imaging. Several important aspects of this work are reported: (1) dual-polarity ion production with NI matrices and complex mixtures; (2) quantitative analysis of PFOS with a LOQ of 0.5 ppb in tap water and 0.05 ppb in MQ water (without solid phase extraction enrichment), with accuracy and precision within 5%; (3) MALDI imaging with 2,3,6-DMNI as a matrix for plant metabolite/lipid identification with ionization enhancement in the negative ion mode m/z 600-900 region; and (4) development of a thin film deposition under/above tissue method for MALDI imaging with a vacuum sublimation matrix on a high-vacuum MALDI instrument.

Electrospray ionization in-source decay of N-glycans and the effects on N-glycan structural identification

RATIONAL

Electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are soft ionization techniques commonly used in mass spectrometry. Although in-source and post-source decays of MALDI have been investigated extensively, the analogous decays of ESI have received little attention. Previous studies regarding the analogous decays of ESI focus on the dissociation of multiply charged proteins and peptides. The decay of carbohydrates in ESI has not been investigated yet, and it may have interference in carbohydrate structural determination.

METHODS

Commercial apparatus, including a high-performance liquid chromatography (HPLC), an ESI source, and a linear ion trap mass spectrometer, were used to investigate the fragmentation of several N-glycans during the ESI process.

RESULTS

About 0.2%-3% of neutral N-glycans and more than 50% of N-glycans consisting of a sialic acid are dissociated into small N-glycans by ESI in-source decay in typical ESI operating conditions. The efficiencies of most dissociation channels increase as the temperature of ion transfer capillary increases, indicating that part of the energy deposited into the precursor ions for cracking is from the heated capillary. The cracking patterns of ESI in-source decay are slightly different from those of gaseous phase collision-induced dissociation.

CONCLUSIONS

Large N-glycans are dissociated into small N-glycans in ESI in-source decay that may result in the interference of the structural identification of small N-glycans. Separation of large N-glycans from small N-glycans, for example, using HPLC, prior to ESI ionization is necessary to eliminate the interference. This is particularly important when N-glycans consist of sialic acid or large N-glycans have much higher concentration than that of small N-glycans in ESI solution.

Cross-Ring Fragmentation Patterns in the Tandem Mass Spectra of Underivatized Sialylated Oligosaccharides and Their Special Suitability for Spectrum Library Searching

Reference spectral library searching, while widely used to identify compounds in other areas of mass spectrometry, is not commonly used in glycomics. Building on a study by Cotter and coworkers on analysis of sialylated oligosaccharides using atmospheric pressure-matrix-assisted laser-induced tandem mass spectrometry (MS/MS), we show that library search methods enable the automated differentiation of such sialylated oligosaccharide isomers using MS/MS derived from electrospray collision-induced dissociation in ion trap and beam-type fragmentation mass spectrometers. We compare MS/MS spectra of five sets of native sialylated oligosaccharide isomers and show a spectral library search method that can distinguish between these isomers using the precursor ion [M+2X-H]⁺, where X=Li, Na, or K. Sialic acid linkage (α2,3 vs. α2,6) is known to have a dramatic effect on the fragmentation of the sialylated compounds. We found that ^2,4A3 cross-ring fragment at the terminal monosaccharide in sialyllactoses, sialyllactosamines, and sialyl pentasaccharides is highly abundant in the MS/MS spectra of [M+2X-H]⁺ species of α2,6-NeuAc glycans, while (^2,4A3-H2O) fragment is highly abundant in α2,3-NeuAc moiety. The ^2,4A3-H2O peak is specific to NeuAc-α2,3-Gal-β1,4-Y (Y=GlcNAc or Glc). To our knowledge, this observation was not reported previously. Theoretical calculations reveal major conformational differences between α2,6-NeuAc and α2,3-NeuAc structures that provide reasonable explanations for the observed fragmentation patterns. Other singly-charged ions ([M+X]⁺) do not show similar cross-ring cleavages. Implemented in a searchable library, these spectral differences provide a facile method to distinguish sialyl isomers without derivatization. We also found good spectral matching across instruments. MS/MS spectra and tools are available at http://chemdata.nist.gov/glycan/spectra . Graphical Abstract.

Exploration of the Sialic Acid World

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Norharmane Matrix Enhances Detection of Endotoxin by MALDI-MS for Simultaneous Profiling of Pathogen, Host, and Vector Systems

The discovery of novel pathogenic mechanisms engaged during bacterial infections requires the evolution of advanced techniques. Here, we evaluate the dual polarity matrix norharmane (NRM) to improve detection of bacterial lipid A (endotoxin), from host and vector tissues infected withFrancisella novicida (Fn). We evaluated NRM for improved detection and characterization of a wide range of lipids in both positive and negative polarities, including lipid A and phospholipids across a range of matrix-assisted laser desorption-ionization-coupled applications. NRM matrix improved the limit of detection (LOD) for monophosphoryl lipid A (MPLA) down to picogram level representing a 10-fold improvement of LOD versus 2,5-dihydroxybenzoic acid and 100-fold improvement of LOD versus 9-aminoacridine (9-AA). Improved LOD for lipid A subsequently facilitated detection of theFn lipid A major ion (m/z 1665) from extracts of infected mouse spleen and the temperature-modifiedFn lipid A atm/z 1637 from infectedDermacentor variabilis ticks. Finally, we simultaneously mapped bacterial phospholipid signatures within anFn-infected spleen along with an exclusively host-derived inositol-based phospholipid (m/z 933) demonstrating coprofiling of the host-pathogen interaction. Expanded use of NRM matrix in other infection models and endotoxin-targeting imaging experiments will improve our understanding of the lipid interactions at the host-pathogen interface.

Technologies and strategies for glycoproteomics and glycomics and their application to clinical biomarker research

Several approaches and technologies are currently available to study the glycosylated proteome (glycoproteomics) or the entire repertoire of glycans in a biological system (glycomics). The biological importance of glycosylation has driven the development of novel, sensitive separation and detection methods. New and improved methodologies, such as high throughput array systems and liquid chromatography-mass spectrometry for glycan profiling and sequencing, are emerging and are being applied in clinical research. A major thrust of glycoproteomics and glycomic clinical research is the application of these analytical tools to cancer research and is aimed at the discovery of glycan-based biomarkers for diagnosis of early stage human cancers, monitoring disease progression, measuring response to therapy, and detecting recurrence. The identification of cancer biomarkers requires a multidisciplinary approach and therefore this review discusses the strategies, technologies and methods currently used for N-glycoprotein/glycanbiomarker research.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update covering the period 2001-2002

This review is the second update of the original review on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates that was published in 1999. It covers fundamental aspects of the technique as applied to carbohydrates, fragmentation of carbohydrates, studies of specific carbohydrate types such as those from plant cell walls and those attached to proteins and lipids, studies of glycosyl-transferases and glycosidases, and studies where MALDI has been used to monitor products of chemical synthesis. Use of the technique shows a steady annual increase at the expense of older techniques such as FAB. There is an increasing emphasis on its use for examination of biological systems rather than on studies of fundamental aspects and method development and this is reflected by much of the work on applications appearing in tabular form.

Structural assignment of disialylated biantennary N-glycan isomers derivatized with 2-aminopyridine using negative-ion multistage tandem mass spectral matching

To investigate the possibility of structural assignment based on negative-ion multistage tandem mass (MS(n)) spectral matching, four isomers of disialylated biantennary N-glycans (alpha2-6 and/or alpha2-3 linked sialic acid on alpha1-6 and alpha1-3 antennae) derivatized with 2-aminopyridine (PA) were analyzed by employing high-performance liquid chromatography/electrospray ionization linear ion trap time-of-flight mass spectrometry (HPLC/ESI-LIT-TOFMS), which uses helium gas for ion trapping and collision-induced dissociation (CID). It is shown that the MS(2) spectra derived from each precursor ion [M-2H](2-) are reproducible and useful for distinguishing the four isomers. Thus, they can be assigned by negative-ion MS(2) spectral matching based on correlation coefficients. In addition, MS(3) spectra derived from D-type fragment ions clearly differentiate the alpha2-3- or alpha2-6-linked sialic acid on the alpha1-6 antenna due to their characteristic spectral patterns. The C(4)-type fragment ions, which are produced from both the alpha1-6 and alpha1-3 antennae, show the characteristic MS(3) spectra reflecting alpha2-3- or alpha2-6- linkage type or a mixture of both types. Thus, the differentiation and assignment of these disialylated biantennary N-glycan isomers can also be supported with the MS(3) spectra of C(4)- and D-type ions.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update covering the period 1999-2000

This review describes the use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates and continues coverage of the field from the previous review published in 1999 (D. J. Harvey, Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates, 1999, Mass Spectrom Rev, 18:349-451) for the period 1999-2000. As MALDI mass spectrometry is acquiring the status of a mature technique in this field, there has been a greater emphasis on applications rather than to method development as opposed to the previous review. The present review covers applications to plant-derived carbohydrates, N- and O-linked glycans from glycoproteins, glycated proteins, mucins, glycosaminoglycans, bacterial glycolipids, glycosphingolipids, glycoglycerolipids and related compounds, and glycosides. Applications of MALDI mass spectrometry to the study of enzymes acting on carbohydrates (glycosyltransferases and glycosidases) and to the synthesis of carbohydrates, are also covered.

The influence of sialylation on glycan negative ion dissociation and energetics

For the analysis of native glycans using tandem mass spectrometry (MS), it is desirable to choose conditions whereby abundances of cross-ring cleavages indicative of branch positions are maximized. Recently, negative ion tandem mass spectrometry has been shown to produce significantly higher abundances of such ions in glycans compared to the positive ion mode. Much of this prior work has concerned fragmentation patterns in asialo glycans. The present work compares the abundances of critical cross-ring cleavage ions using negative mode tandem mass spectrometry for milk oligosaccharides and N-linked glycans. For comparison, product ion formation was studied for deprotonated and nitrated ions formed from asialo glycans and deprotonated ions from sialylated glycans. Breakdown profiles demonstrate clearly that more energy was required to fragment sialylated compounds to the same extent as either their asialo or nitrate adducted counterparts. The extraction of a proton from a ring hydroxyl group during the ionization process may be viewed, qualitatively, as imparting significantly more energy to the ion than would that from a molecule bearing an acidic group, so that acidic glycans are more stable in the gas phase, as the negative charge resides on the carboxyl group. These results have strong practical implications because a major portion of glycans released from mammalian proteins will be sialylated.

Systematic identification of N-acetylheparosan oligosaccharides by tandem mass spectrometric fragmentation

Recently, a useful procedure for the preparation of both even- and odd-numbered series of N-acetylheparosan (NAH) oligosaccharides was established. The present report describes findings when these NAH oligosaccharides were subjected to comparative mass spectrometry (MS)/MS fragmentation analysis by matrix-assisted laser desorption/ionization (MALDI)-LIFT-time-of-flight (TOF)/TOF-MS/MS, and electrospray ionization (ESI) collision-induced dissociation (CID) MS/MS. The resultant fragment ions were systematically assigned to elucidate fragmentation characteristics. In the MALDI-LIFT-MS/MS experiments, all the NAH oligosaccharides underwent unique glycosidic cleavages that included B-Y ion cleavages (nomenclature system of Domon and Costello, Glycoconjugate J. 1988; 5: 397) at the C-1 side, and C-Z ion cleavages at the C-4 side, with respect to glucuronic acid (GlcA). In addition, (0,2)A and/or (0,2)X cross-ring cleavages were observed for relatively small oligosaccharides. The former observation clearly reflects the occurrence of a GlcA-N-acetylglucosamine (GlcNAc) alternating structure of NAH, while the latter feature implies the occurrence of the -beta-1-4-glucuronide linkage. Extensive glycosidic cleavages were also observed in the ESI-CID-MS/MS fragmentation, though cleavage specificity was less evident than in the case of MALDI-LIFT-TOF/TOF-MS/MS. The information obtained in this study should be valuable for understanding both biosynthetic and degradation processes of NAH and its derivatives including heparin and heparan sulfate, as well as artificially modified NAH oligosaccharides.

Structural assignment of isomeric 2-aminopyridine-derivatized monosialylated biantennary N-linked oligosaccharides using negative-ion multistage tandem mass spectral matching

To investigate the possibility of structural assignment based on negative-ion tandem multistage (MSn) mass spectral matching, four isomers of 2-aminopyridine (PA)-derivatized monosialylated oligosaccharides (i.e., complex-type N-glycans with an alpha2-3- or alpha2-6-linked sialic acid on alpha1-6 or alpha1-3 antennae) were analyzed using high-performance liquid chromatography/electrospray ion trap time-of-flight mass spectrometry (HPLC/ESI-IT-TOFMS). The negative ion [M-2H]2- is observed predominantly in the MS1 spectra without the loss of a sialic acid. The MS2 spectra derived from it are sufficiently reproducible that MS2 spectral matching based on correlation coefficients can be applied to the assignment of these isomers. The isomers containing a sialic acid on alpha1-6 or alpha1-3 antennae can be distinguished by MS2 spectral matching, but the alpha2-3 and alpha2-6 linkage types of sialic acid cannot be distinguished by their MS2 spectra. However, MS3 spectra derived from fragment ions containing a sialic acid (i.e., C4- and D-type ions) clearly differentiate the alpha2-3 and alpha2-6 linkage types of sialic acid in their MS3 spectral patterns. This difference might be rationalized in terms of a proton transfer from the reducing-end mannose to the negatively charged sialic acid. These two moieties are very close in the structural conformations of the precursor C4-type fragment ions of alpha2-6 linkage type, as predicted by molecular mechanics calculations. Thus, negative-ion MSn (n = 2, 3) spectral matching was demonstrated to be useful for the structural assignment of these four monosialylated PA N-glycan isomers.

Analysis of sialyl oligosaccharides by high-performance liquid chromatography-electrospray ionisation-mass spectrometry with differentiation of α2-3 and α2-6 sialyl linkages

A method for analysing sialyl oligosaccharides from bovine colostrum using high-performance liquid chromatography-electrospray ionisation-mass spectrometry (HPLC-ESI-MS) is described. Under positive ionisation mode, mass spectra of alpha2-3 and alpha2-6 linkages were different, and the former produced a prominent B2 (or B3 in disialyl lactose) mass fragment. This fragment was absent from mass spectra with alpha2-6 linkages. Two sialyl oligosaccharides, which have not been reported previously, were tentatively identified. One comprises a N-acetyl neuraminic acid (Neu5Ac), two hexoses (Hex), and one N-acetyl hexosamine (HexNAc) residue ((Neu5Ac)1 (Hex)2 (HexNAc)1), and the other comprises one Neu5Ac and one Hex residue ((Neu5Ac)1(Hex)1).

Discrimination of neolacto-series gangliosides with alpha2-3- and alpha2-6-linked N-acetylneuraminic acid by nanoelectrospray ionization low-energy collision-induced dissociation tandem quadrupole TOF MS

A combined strategy of thin-layer chromatography immunostaining and negative ion nanoelectrospray low-energy CID mass spectrometry was established for the differentiation of isomeric alpha2-3 and alpha2-6 sialylated neolacto-series monosialogangliosides from human granulocytes. The gangliosides investigated differed in the ceramide moiety by substitution with C16:0 or C24:1 fatty acid and in their oligosaccharide chains due to nLc4 and nLc6 core structures. With respect to the type of sialylation, the homogeneity of the HPLC-purified ganglioside fractions was verified by use of specific anti-Neu5Acalpha2-3Galbeta1-4GlcNAc-R and anti-Neu5Acalpha2-6Galbeta1-4GlcNAc-R antibodies. A clear-cut series of fragment ions for both types of isomeric gangliosides, carrying alpha2-3- and alpha2-6-linked neuraminic acid, respectively, was obtained by low-energy CID. Additionally, a characteristic ring cleavage was detected exclusively in all species with Neu5Acalpha2-6Galbeta1-4GlcNAc terminus, regardless of ceramide fatty acid and oligosaccharide chain lengths. The diagnostic (0,2)X(4/6) ions, generated by ring cleavage of an alpha2-6-linked neuraminic acid are accompanied by a simultaneous decrease of the corresponding Y(4)/Y(6) ions. These results suggest the unequivocal discrimination of individual alpha2-3- and alpha2-6-sialylated neolacto-series monosialogangliosides by distinct fragmentation patterns in low-energy CID tandem MS.

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.

Analysis of glycoproteins and the oligosaccharides thereof by high-performance capillary electrophoresis-significance in regulatory studies on biopharmaceutical products

This review describes the recent development in the analysis of glycoproteins using capillary electrophoresis with various separation techniques, and focuses especially on the analysis of recombinant glycoprotein pharmaceuticals. We include the analysis of glycoprotein multiforms (ie glycoform) as well as glycan analysis. The relationship between glycoprotein multiforms and oligosaccharide distributions in a glycoprotein sample is also discussed. Further, recent development in capillary electrophoresis-mass spectrometry is described.

Capillary electrophoresis of sialic acid-containing glycoprotein. Effect of the heterogeneity of carbohydrate chains on glycoform separation using an alpha1-acid glycoprotein as a model

alpha1-Acid glycoprotein (AGP) showed multiple peaks on separation using capillary electrophoresis in a chemically modified capillary with dimethylpolysiloxane at slightly acidic conditions. We analyzed glycoforms of AGP species after separation by ion-exchange chromatography, Con A affinity chromatography, and Cu(II)-chelating affinity chromatography. The AGP species thus obtained were digested with N-glycosidase F, and the released carbohydrate chains were analyzed by high-performance liquid chromatography after labeling with 3-aminobenzoic acid. The results afforded basic information on the contribution of carbohydrate chains to the separation mechanism of glycoforms of AGP by capillary electrophoresis. In addition, we describe an easy method for AGP analysis in serum samples using the electrokinetic injection.

Stabilization and linkage analysis of metal-ligated sialic acid containing oligosaccharides

The dissociation of metal-ligated sialyllactose and sialyl-N-acetyllactosamine was investigated. Metal-ligand derivatization of the carbohydrate samples with the diethylenetriamine ligand and one of four transition metals [Co(II), Ni(II), Cu(II), Zn(II)] suppressed sialic acid loss in the collision-induced dissociation process. Suppression of sialic acid loss allows sialic acid linkage information to be gained through tandem mass spectrometry. Sialic acid stabilization is postulated to occur due to the doubly charged metal ion which allows for deprotonation of the sialic acid moiety. Furthermore, a connection between the metal center and the amount of sialic acid loss was found. These results were rationalized using the Irving-Williams series and a competition between different sites of deprotonation. Analysis of the product ion spectra showed a clear differentiation of sialic acid linkage. Linkage determination is proposed to be effective due to the available conformations allowed by the different linkages. A more flexible linkage will allow more coordination of the sialic acid residue with the metal center, whereas a less flexible linkage will make this interaction unlikely.

Structural characterization of fucose-containing oligosaccharides by high-performance liquid chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Eight pyridylamino (PA) derivatives of fucose-containing oligosaccharides, which occur as free oligosaccharides in human milk and also are derived from glycosphingolipids, have been analyzed by high-performance liquid chromatography (HPLC) on normal-phase and reversed-phase columns, and by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Six out of eight PA-oligosaccharides were clearly separated by both normal- and reversed-phase HPLC at a column temperature of 40 degrees C, but two PA-oligosaccharides, lacto-N-fucopentaose II [Gal beta1-3(Fuc alpha1-4)GlcNAc beta1-3Gal beta1-4GIcPA] and lacto-N-fucopentaose III [Gal beta1-4(Fuc alpha1-3)GlcNAc beta1-3Gal beta1-4GIcPA], were not separated. The two unresolved PA-oligosaccharides were finally separated by reversed-phase HPLC at a column temperature of 11 degrees C. MALDI-TOF mass spectra of PA-oligosaccharides demonstrated pseudo-molecular ions as the predominant signals, therefore information about the molecular mass of each PA-oligosaccharide was easily obtained. Post-source decay (PSD) MALDI-TOF mass spectra of PA-oligosaccharides gave information about the carbohydrate sequences and carbohydrate species of each PA-oligosaccharide by detecting the ions responsible for the cleavage of the glycosidic bonds. The detection limits of the PA-oligosaccharides by HPLC, MALDI-TOF mass spectrometry, and PSD MALDI-TOF mass spectrometry were 20 fmol, 20 fmol, and 2 pmol, respectively. These results suggest that a system including HPLC and MALDI-TOF mass spectrometry or HPLC and PSD MALDI-TOF mass spectrometry is quite useful for the structural characterization of sub-pmol or pmol levels of fucose-containing oligosaccharides, and that these methods could be used for the analysis of various types of oligosaccharides derived from glycoproteins and glycosphingolipids.

Distinguishing of linkage isomers of lactotetra oligosaccharides by using the relative ion intensity analysis of post-source decay fragment ions in curved-field reflectron matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The native oligosaccharides of lacto-N-neotetraose (Gal beta1-4GlcNAc beta1-3Gal beta1-4Glc; LNnT) and lacto-N-tetraose (Gal beta1-3GlcNAc beta1-3Gal beta1-4Glc; LNT) were analyzed by using curved-field reflectron matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). Since a curved-field reflectron TOFMS enables a simultaneous focusing of a wide mass range of metastable fragment ions, the relative ion intensities in the post-source decay (PSD) mass spectra can be discussed. The PSD mass spectra of LNnT and LNT were distinguishable in their relative ion intensities. In the case of LNT, beta-elimination could occur in the N-acetyl glucosamine (GlcNAc) at the C-3 position, which was bonded by galactose (Gal); however, it did not occur in LNnT. The 3-O elimination caused a difference in the relative ion intensities in the PSD mass spectra of LNnT and LNT. The beta1-3 glycosyl linkage cleaved more easily than the beta1-4 glycosyl linkage in the MALDI-PSD fragmentation. An analysis of the relative ion intensities in the MALDI-PSD mass spectra of oligosaccharides was very useful for distinguishing the linkage isomers and for characterizing the types of glycosyl linkages.

Negative ion mass spectrometry of sialylated carbohydrates: discrimination of N-acetylneuraminic acid linkages by MALDI-TOF and ESI-TOF mass spectrometry

Negative ion MALDI and electrospray fragmentation spectra were recorded from 12 sialylated carbohydrates ranging from trisaccharides to biantennary N-linked glycans. D-Arabinosazone was found to be the most satisfactory MALDI matrix for these compounds. Fragmentation mechanisms were investigated with the aid of several synthesized analogues of the sugars labeled with 13C and 2H. The substitution position of the sialic acid (alpha2-->3 or alpha2-->6) was found to have a dramatic effect on the overall fragmentation pattern of these compounds, and several features of the spectra were identified that allowed the substitution pattern to be determined. In particular, the appearance of an ion at m/z 306 appeared to be diagnostic of the presence of an alpha2-->6-linked sialic acid. Selection and further fragmentation of the in-source (conevoltage) fragment ion corresponding to the trisaccharide Neu5Acalpha2-->3(or 6)Galbeta1-->4GlcNAc from larger, N-linked glycans, ionized by electrospray, gave fragmentation patterns identical to those of the reference trisaccharides, thus providing a method for confirming the sialic acid linkage.