Liquid secondary ion mass spectrometry/Fourier transform mass spectrometry of oligosaccharide anions

Mass Spectrometry Approach for Differentiation of Positional Isomers of Saccharides: Toward Direct Analysis of Rare Sugars

Rare sugars have gained popularity in recent years due to their use in antiaging treatments, their ability to sweeten with few calories, and their ability to heal infections. Rare sugars are found in small quantities in nature, and they exist typically as isomeric forms of traditional sugars, rendering some challenges in their isolation, synthesis, and characterization. In this work, we present the first direct mass spectrometric approach for differentiating structural isomers of sucrose that differ only by their glycosidic linkages. The method employed a noncontact nanoelectrospray (nESI) platform capable of analyzing minuscule volumes (5 μL) of saccharides via the formation of halide adducts ([M+X]-; X = Cl and Br). Tandem mass spectrometry analysis of the five structural isomers of sucrose afforded diagnostic fragment ions that can be used to distinguish each isomer. Detailed mechanisms showcasing the distinct fragmentation pattern for each isomer are discussed. The method was applied to characterize and confirm the presence of all five selected rare sugars in raw honey complex samples. Aside from the five natural α isomers of sucrose, the method was also suitable for differentiating some β isomers of the same glycosidic linkages, provided the monomeric sugar units are different. The halide adduct formation via the noncontact nESI source was also proven to be effective for oligosaccharides such as raffinose, β-cyclodextrin, and maltoheptaose. The results from this study encourage the future development of methods that function with simple operation to enable straightforward characterization of small quantities of rare sugars.

Gas-Phase Fragmentation of Cyclic Oligosaccharides in Tandem Mass Spectrometry

Modern mass spectrometry, including electrospray and MALDI, is applied for analysis and structure elucidation of carbohydrates. Cyclic oligosaccharides isolated from different sources (bacteria and plants) have been known for decades and some of them (cyclodextrins and their derivatives) are widely used in drug design, as food additives, in the construction of nanomaterials, etc. The peculiarities of the first- and second-order mass spectra of cyclic oligosaccharides (natural, synthetic and their derivatives and modifications: cyclodextrins, cycloglucans, cyclofructans, cyclooligoglucosamines, etc.) are discussed in this minireview.

Direct and Regioselective Di-α-fucosylation on the Secondary Rim of β-Cyclodextrin

A straightforward glycosylation method is described to regio- and stereoselectively introduce two α-l-fucose moieties directly to the secondary rim of β-cyclodextrin. Using NMR and MS fragmentation studies, the nonasaccharide structure was determined, which was also visualized using molecular dynamics simulations. The reported glycosylation method proved to be robust on gram-scale, and may be generally applied to directly glycosylate β-cyclodextrins to make well-defined multivalent glycoclusters.

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.

Anionic fructose-related conformational and positional isomers assigned through PES experiments and DFT calculations

Fructose⁻ exists as an open chain structure with substrate dependent specific conformational isomers. (Fructose-H₂O)⁻ evidences two types of positional isomers.

Negative electrospray ionization mass spectrometry: a method for sequencing and determining linkage position in oligosaccharides from branched hemicelluloses

Xyloglucans of apple, tomato, bilberry and tamarind were hydrolyzed by commercial endo β-1-4-D-endoglucanase. The xylo-gluco-oligosaccharides (XylGos) released were separated on CarboPac PA 200 column in less than 15 min, and, after purification, they were structurally characterized by negative electrospray ionization mass spectrometry using a quadrupole time-of-flight (ESI-Q-TOF), a hybrid linear ion trap (LTQ)/Orbitrap and a hybrid quadrupole Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers. In order to corroborate the fragmentation routes observed on XylGos, some commercial galacto-manno-oligosaccharides (GalMOs) and glucurono-xylo-oligosaccharides were also studied. The fragmentation pathways of the ionized GalMos were similar to those of XylGos ones. The product ion spectra were mainly characterized by prominent double cleavage (D) ions corresponding to the entire inner side chains. The directed fragmentation from the reducing end to the other end was observed for the main glycosylated backbone but also for the side-chains, allowing their complete sequencing. Relevant cross-ring cleavage ions from (0,2)X(j)-type revealed to be diagnostic of the 1-2-linked- glycosyl units from XylGos together with the 1-2-linked glucuronic acid unit from glucuronoxylans. Resonant activation in the LTQ Orbitrap allowed not only determining the type of all linkages but also the O-acetyl group location on fucosylated side-chains. Moreover, the fragmentation of the different side chains using the MS(n) capabilities of the LTQ/Orbitrap analyzer also allowed differentiating terminal arabinosyl and xylosyl substituents inside S and U side-chains of XylGos, respectively. The CID spectra obtained were very informative for distinction of isomeric structures differing only in their substitution pattern. These features together makes the fragmentation in negative ionization mode a relevant and powerful technique useful to highlight the subtle structural changes generally observed during the development of plant organs such as during fruit ripening and for the screening of cell wall mutants with altered hemicellulose structure.

Linkage determination of linear oligosaccharides by MS(n) (n > 2) collision-induced dissociation of Z₁ ions in the negative ion mode

Obtaining unambiguous linkage information between sugars in oligosaccharides is an important step in their detailed structural analysis. An approach is described that provides greater confidence in linkage determination for linear oligosaccharides based on multiple-stage tandem mass spectrometry (MS(n), n >2) and collision-induced dissociation (CID) of Z1 ions in the negative ion mode. Under low energy CID conditions, disaccharides (18)O-labeled on the reducing carbonyl group gave rise to Z1 product ions (m/z 163) derived from the reducing sugar, which could be mass-discriminated from other possible structural isomers having m/z 161. MS(3) CID of these m/z 163 ions showed distinct fragmentation fingerprints corresponding to the linkage types and largely unaffected by sugar unit identities or their anomeric configurations. This unique property allowed standard CID spectra of Z1 ions to be generated from a small set of disaccharide samples that were representative of many other possible isomeric structures. With the use of MS(n) CID (n = 3 - 5), model linear oligosaccharides were dissociated into overlapping disaccharide structures, which were subsequently fragmented to form their corresponding Z1 ions. CID data of these Z1 ions were collected and compared with the standard database of Z1 ion CID using spectra similarity scores for linkage determination. As the proof-of-principle tests demonstrated, we achieved correct determination of individual linkage types along with their locations within two trisaccharides and a pentasaccharide.

Electron detachment dissociation of underivatized chloride-adducted oligosaccharides

Chloride anion attachment has previously been shown to aid determination of saccharide anomeric configuration and generation of linkage information in negative ion post-source decay MALDI tandem mass spectrometry. Here, we employ electron detachment dissociation (EDD) and collision activated dissociation (CAD) for the structural characterization of underivatized oligosaccharides bearing a chloride ion adduct. Both neutral and sialylated oligosaccharides are examined, including maltoheptaose, an asialo biantennary glycan (NA2), disialylacto-N-tetraose (DSLNT), and two LS tetrasaccharides (LSTa and LSTb). Gas-phase chloride-adducted species are generated by negative ion mode electrospray ionization. EDD and CAD spectra of chloride-adducted oligosaccharides are compared to the corresponding spectra for doubly deprotonated species not containing a chloride anion to assess the role of chloride adduction in the stimulation of alternative fragmentation pathways and altered charge locations allowing detection of additional product ions. In all cases, EDD of singly chloridated and singly deprotonated species resulted in an increase in observed cross-ring cleavages, which are essential to providing saccharide linkage information. Glycosidic cleavages also increased in EDD of chloride-adducted oligosaccharides to reveal complementary structural information compared to traditional (non-chloride-assisted) EDD and CAD. Results indicate that chloride adduction is of interest in alternative anion activation methods such as EDD for oligosaccharide structural characterization.

Characterizing oligosaccharides using injected-ion mobility/mass spectrometry

Injected-ion mobility/mass spectrometry techniques have been used to measure the reduced ion mobilities for negatively charged raffinose, melezitose and α-, β-, and γ-cyclodextrins formed by electrospray ionization. At low injection energies, the mass spectra are dominated by negatively charged (deprotonated) parent ions. At high injection energies, the mass spectra recorded for the cyclodextrins and raffinose display peaks that result from cross-ring cleavage of individual sugar units. Melezitose dissociates by cleavage of the glycosidic bonds. The ion mobility distributions can be used to distinguish between different isomeric forms of parent and fragment ions having the same mass-to-charge ratios.

Structural identification of novel oligosaccharides produced by Lactobacillus bulgaricus and Lactobacillus plantarum

β-Galactosidases (β-Gal) of lactic acid bacteria produce oligosaccharides from lactose when suitable acceptor carbohydrates are present. This study aimed to elucidate the structure of oligosaccharides formed by galactosylation of N-acetylglucosamine (GlcNAc) and fucose. Crude cellular extract of Lactobacillus bulgaricus and LacLM of Lactobacillus plantarum were used as sources of β-Gal activity. Disaccharides obtained by galactosylation of GlcNAc were identified as Gal-β-(1→4)-GlcNAc or Gal-β-(1→6)-GlcNAc by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and comparison with external standards. Trisaccharides were identified as Gal-β-(1→6)-Gal-β-(1→[4 or 6])-GlcNAc by LC-MS, analysis of the MS/MS spectra of selected in-source fragment ions, and their relative retention times. LC-MS analysis revealed the presence of five galactosylated fucosides, but their linkage type could not be identified, partly due to the lack of reference compounds. β-Gal of lactic acid bacteria may serve as suitable tools for the chemoenzymatic synthesis of therapeutic oligosaccharides.

Differentiation of the stereochemistry and anomeric configuration for 1-3 linked disaccharides via tandem mass spectrometry and 18O-labeling

Collision-induced dissociation (CID) of deprotonated hexose-containing disaccharides (m/z 341) with 1-2, 1-4, and 1-6 linkages yields product ions at m/z 221, which have been identified as glycosyl-glycolaldehyde anions. From disaccharides with these linkages, CID of m/z 221 ions produces distinct fragmentation patterns that enable the stereochemistries and anomeric configurations of the non-reducing sugar units to be determined. However, only trace quantities of m/z 221 ions can be generated for 1-3 linkages in Paul or linear ion traps, preventing further CID analysis. Here we demonstrate that high intensities of m/z 221 ions can be built up in the linear ion trap (Q3) from beam-type CID of a series of 1-3 linked disaccharides conducted on a triple quadrupole/linear ion trap mass spectrometer. (18)O-labeling at the carbonyl position of the reducing sugar allowed mass-discrimination of the "sidedness" of dissociation events to either side of the glycosidic linkage. Under relatively low energy beam-type CID and ion trap CID, an m/z 223 product ion containing (18)O predominated. It was a structural isomer that fragmented quite differently than the glycosyl-glycolaldehydes and did not provide structural information about the non-reducing sugar. Under higher collision energy beam-type CID conditions, the formation of m/z 221 ions, which have the glycosyl-glycolaldehyde structures, were favored. Characteristic fragmentation patterns were observed for each m/z 221 ion from higher energy beam-type CID of 1-3 linked disaccharides and the stereochemistry of the non-reducing sugar, together with the anomeric configuration, were successfully identified both with and without (18)O-labeling of the reducing sugar carbonyl group.

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.

MALDI linear-field reflectron TOF post-source decay analysis of underivatized oligosaccharides: determination of glycosidic linkages and anomeric configurations using anion attachment

Six different anionic species (fluoride, chloride, bromide, iodide, nitrate, and acetate) are tested for their abilities to form anionic adducts with neutral oligosaccharides that are detectable by MALDI-TOF mass spectrometry. Fluoride and acetate cannot form anionic adducts with the oligosaccharides in significant yields. However, bromide, iodide, and nitrate anionic adducts consistently appear in higher abundances relative to [M - H](-), just like the highly stable chloride adducts. Post-source decay (PSD) decompositions of Br(-), I(-), and NO(3)(-) adducts of oligosaccharides provide no structural information, i.e., they yield the respective anions as the main product ions. However, determination of linkage types is achieved by analysis of structurally-informative diagnostic peaks offered by negative ion PSD spectra of chloride adducts of oligosaccharides, whereas the relative peak intensities of pairs of diagnostic fragment ions allow differentiation of anomeric configurations of glycosidic bonds. Thus, simultaneous identification of the linkage types and anomeric configurations of glycosidic bonds is achieved. Our data indicate that negative ion PSD fragmentation patterns of chloride adducts of oligosaccharides are mainly determined by the linkage types. Correlation may exist between the linkage positions and fragmentation mechanisms and/or steric requirements for both cross-ring and glycosidic bond fragmentations. PSD of the chloride adducts of saccharides containing a terminal Glcalpha1-2Fru linkage also yields chlorine-containing fragment ions which appear to be specifically diagnostic for a fructose linked at the 2-position on the reducing end. This also allows differentiation from saccharides with a 1-1 linked pyranose on the same position.

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.

Electron detachment dissociation of neutral and sialylated oligosaccharides

Electron detachment dissociation (EDD) has recently been shown by Amster and coworkers to constitute a valuable analytical approach for structural characterization of glycosaminoglycans. Here, we extend the application of EDD to neutral and sialylated oligosaccharides. Both branched and linear structures are examined, to determine whether branching has an effect on EDD fragmentation behavior. EDD spectra are compared to collisional activated dissociation (CAD) and infrared multiphoton dissociation (IRMPD) spectra of the doubly and singly deprotonated species. Our results demonstrate that EDD of both neutral and sialylated oligosaccharides provides structural information that is complementary to that obtained from both CAD and IRMPD. In all cases, EDD resulted in additional cross-ring cleavages. In most cases, cross-ring fragmentation obtained by EDD is more extensive than that obtained from IRMPD or CAD. Our results also indicate that branching does not affect EDD fragmentation, contrary to what has been observed for electron capture dissociation (ECD).

Characterization of the glycosidic linkage of underivatized disaccharides by interaction with Pb(2+) ions

Electrospray ionization in combination with tandem mass spectrometry and lead cationization is used to characterize the linkage position of underivatized disaccharides. Lead(II) ions react mainly with disaccharides by proton abstraction to generate [Pb(disaccharide)(m)-H](+) ions (m = 1-2). At low cone voltages, an intense series of doubly charged ions of general formula [Pb(disaccharide)(n)](2+) are also observed. Our study shows that MS/MS experiments have to be performed to differentiate Pb(2+)-coordinated disaccharides. Upon collision, [Pb(disaccharide)-H](+) species mainly dissociate according to glycosidic bond cleavage and cross-ring cleavages, leading to the elimination of C(n)H(2n)O(n) neutrals (n = 2-4). The various fragmentation processes allow the position of the glycosidic bond to be unambiguously located. Distinction between glc-glc and glc-fru disaccharides also appears straightforward. Furthermore, for homodimers of D-glucose our data demonstrate that the anomericity of the glycosidic bond can be characterized for the 1 --> n linkages (n = 2, 4, 6). Consequently, Pb(2+) cationization combined with tandem mass spectrometry appears particularly useful to identify underivatized disaccharides.

The Stereochemical Dependence of Unimolecular Dissociation of Monosaccharide-Glycolaldehyde Anions in the Gas Phase: A Basis for Assignment of the Stereochemistry and Anomeric Configuration of Monosaccharides in Oligosaccharides by Mass Spectrometry via a Key Discriminatory Product Ion of Disaccharide Fragmentation, m/z 221

Mass spectrometry of hexose-containing disaccharides often yields product ions of m/z 221 in the negative ion mode. Using a Paul trap, isolation and collision-induced dissociation of the m/z 221 anions yielded mass spectra that easily differentiated their stereochemistry and anomeric configuration, for all 16 stereochemical variants. The ions were shown to be glycopyranosyl-glycolaldehydes through chemical synthesis of their standards. The stereochemistry dramatically affected fragmentation which was dependent on four relative stereochemical arrangements: (1) the relationship between the hydroxyl group at position 2 and the anomeric configuration, (2) a cis relationship of the anomeric position and positions 2 and 3 (1,2,3-cis), (3) a 1,2 trans-2,3 cis relationship, and (4) the relationship between the hydroxyl group at position 4 and the anomeric configuration. After labeling the reducing carbonyl oxygen of a series of disaccharides with 18O to mass-discriminate between their monosaccharide components, it was demonstrated that m/z 221 anions are comprised of an intact nonreducing sugar glycosidically linked to a 2-carbon aglycon derived from the reducing sugar, irrespective of the linkage position between monosaccharides. This enabled the location of the intact sugar to be assigned to the nonreducing side of a glycosidic linkage. Detailed studies of experimental factors necessary for reproducibility demonstrated that the unique mass spectrum for each m/z 221 anion could be obtained from month-to-month through the use of an internal energy-input calibrant ion that ensured reproducible energy deposition into the ions. The counterparts to these ions for the 2-acetamido-2-deoxyhexoses were m/z 262 anions, and the anomeric configuration and stereochemistry of these anions could also be reproducibly discriminated for N-acetylglucosamine and N-acetylgalactosamine. The fragmentation patterns of m/z 221 anions provide a firm reproducible basis for assignment of sugar stereochemistries in the gas phase.

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.

Differentiation of the anomeric configuration and ring form of glucosyl-glycolaldehyde anions in the gas phase by mass spectrometry: isomeric discrimination between m/z 221 anions derived from disaccharides and chemical synthesis of m/z 221 standards

Mass spectrometry of disaccharides in the negative-ion mode frequently generates product anions of m/z 221. With glucose-containing disaccharides, dissociation of isolated m/z 221 product ions in a Paul trap yielded mass spectra that easily differentiated between both anomeric configurations and ring forms of the ions. These ions were shown to be glucosyl-glycolaldehydes through chemical synthesis of their standards. By labeling the reducing carbonyl oxygen of disaccharides with 18O to mass discriminate between monosaccharides, it was established that the m/z 221 ions are comprised solely of an intact nonreducing sugar with a two-carbon aglycon derived from the reducing sugar, regardless of the disaccharide linkage position. This enabled the anomeric configuration and ring form of the ion to be assigned and the location of the ion to the nonreducing side of a glycosidic linkage to be ascertained. Detailed studies of experimental factors necessary for reproducibility in a Paul trap demonstrated that the unique dissociation patterns that discriminate between the isomeric m/z 221 ions could be obtained from month-to-month in conjunction with an internal energy-input calibrant ion that ensures reproducible energy deposition into isolated m/z 221 ions. In addition, MS/MS fragmentation patterns of disaccharide m/z 341 anions in a Paul trap enabled linkage positions to be assigned, as has been previously reported with other types of mass spectrometers.

Structural characterization of underivatized arabino-xylo-oligosaccharides by negative-ion electrospray mass spectrometry

Various arabino-xylo-oligosaccharides with known substitution patterns were assessed by negative ESI-Q-TOFMS and ESI-ITMS. The CID spectra of linear xylo-oligosaccharides and of nine isomeric mono- and disubstituted arabino-xylo-oligosaccharides established that structures differing in their substitution pattern can be differentiated by this approach. The negative-ion fragmentation spectra of the deprotonated quasi-molecular ions are mainly characterized by glycosidic cleavage ions from the C-series, which provide sequence informations, and by cross-ring cleavage (0,2)A(i) ions, which provide partial linkage information. When the collision energy increased, the cross-ring cleavage (0,2)A(i) ions underwent consecutive loss of water to produce (0,2)A(i)-18 fragment ions and glycosidic cleavage ions of the B-series are also produced besides the C(i) ions. Contrary to linear xylo-oligosaccharides, C(i) ions, which originate from C-3 monosubstituted xylosyl residues never produce the related cross-ring cleavage (0,2)A(i) ions. Disubstitution at O-2 and O-3 of xylosyl residues appears to enhance the production of the (0,2)A(i) ions compared to monosubstitution. For the differentiation of the mono- and disubstitution patterns of the penultimate xylosyl residue, the relative abundance of the glycosidic cleavage ions at m/z 263 and 299 found on Q-TOF CID spectra plays a relevant role and appears to be more informative than MS(n) spectra obtained on a ion trap instrument.

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.

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.

Mass spectrometric decompositions of cationized β-cyclodextrin

The mass spectrometric decompositions of beta-cyclodextrin (beta-CD) complexed with a number of common divalent metal cations (Mg, Ca, Cd, Cu, Co and Pb), obtained under electrospray ionization conditions, are reported. The main fragmentation pathways of [beta-CD+Cat](2+) ions studied (Cat stands for divalent cation) consist of consecutive losses of sugar units. The rupture of C-C bond in sugar units, which occurs via hydrogen atom transfer from the fragment ion formed to the eliminated species, was also observed. Isotope labelling consisting of the exchange of all hydroxyl hydrogens for deuteriums, has been applied in order to understand better the formation of fragment ions. It was found that C-H hydrogen transfer proceeds only during fragmentation across C-C bonds.

Application of Fourier transform ion cyclotron resonance mass spectrometry to oligosaccharides

The application of Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) to the structural elucidation of oligosaccharides is described. This review covers the analyses of oligosaccharides in the context of the unique features of FTICR MS and the improvements in instrumentation that make it possible to study this class of compounds. It consists of work performed initially to understand the fundamental aspects of oligosaccharide ionization and unimolecular fragmentation. More recent investigation includes the application of the technique to samples of direct biological origin. Chemical and enzymatic degradation methods in conjunction with mass spectrometry (MS) and the use front-end methods with FTICR MS are also discussed. The current applications including the characterization of bacterial lipooligosaccharides and phosporylated carbohydrates are described.

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.

Rapid and sensitive differentiation of anomers, linkage, and position isomers of disaccharides using High-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS)

A challenging aspect of structural elucidation of carbohydrates is gaining unambiguous information for anomers, linkage, and position isomers. Such isomers with identical mass can't be easily distinguished in mass spectrometry and a separation step is required prior to mass spectrometry identification. In our laboratory, gas-phase separation and differentiation of anomers, linkage, and position isomers of disaccharides was achieved using High-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS). The FAIMS method responds to changes in ion mobility at high field rather than absolute values of ion mobility, and was shown to provide efficient separation and identification of disaccharide isomers at high sensitivity. Separation of analyzed disaccharide isomers can be accomplished at low nM level in a matter of seconds without sample purification or fractionation. Capability for examining a large population of ionic species of disaccharides by this method allowed for correlating structural details of disaccharide isomers with their separation properties in FAIMS. Results for disaccharide isomers indicate that this method could be applied to a larger group of carbohydrates.

Ranking of gas-phase acidities and chloride affinities of monosaccharides and linkage specificity in collision-induced decompositions of negative ion electrospray-generated chloride adducts of oligosaccharides

Negative ion electrospray-tandem mass spectrometry has been employed to study chloride adducts of saccharide molecules. Decompositions of [M + Cl]- obtained under identical low-energy collision conditions allow the approximate ranking of chloride affinities and gas-phase acidities of a series of isomeric monosaccharides. The ketohexoses are found to be more acidic than the aldohexoses. Chloride adduct decompositions are examined for a glucopyranosyl fructose and a glucopyranosyl glucose series. For each disaccharide series, the linkage position is shown to markedly influence the favored pathways of [M + Cl]- decompositions, initiated either by loss of neutral HCl to form [M - H]- and possibly leading to further (consecutive) decompositions, or by loss of M to form Cl-. Upon formation of [M - H]-, both cross-ring cleavages and glycosidic bond decompositions were observed in varying degrees for the two series of disaccharides. Remarkably, for three non-reducing polysaccharides that each contain a terminal sucrose group at the "downstream" end, chlorine-containing product ions arising from cleavage of the Glcalpha-2Fru linkage have been observed. Apart from Cl-, chlorine-containing product ions are not observed for any of the other disaccharides investigated, and they appear to be specifically diagnostic of a terminal Glcalpha-2Fru linkage. Their appearance is rationalized based upon a substantially reduced tendency for HCl loss from these non-reducing polysaccharides.

Ionization and collision-induced fragmentation of N-linked and related carbohydrates using divalent cations

Maltoheptaose and several N-linked glycans were ionized by electrospray as adducts with the divalent cations Mg2+, Ca2+, Mn2+, Co2+ and Cu2+. [M + metal]2+ ions were the major species in all cases with calcium giving the highest sensitivity. In addition, copper gave [M + Cu]+ ions. Other cations gave singly charged ions only by elimination of a protonated monosaccharide. Fragmentation of the [M + metal]2+ ions produced both singly and doubly charged ions with the relative abundance of doubly charged ions decreasing in the order Ca > Mg > Mn > Co > Cu. Singly charged ions were formed by elimination of a protonated monosaccharide residue followed, either by successive monosaccharide residue losses, or by a 2,4A cross-ring cleavage of the reducing-terminal monosaccharide. Formation of doubly charged fragments from [M + metal]2+ ions involved successive monosaccharide-residue losses either with or without O,2A or 2,4A cross-ring cleavages of the reducing-terminal monosaccharide. Abundant diagnostic doubly charged ions formed by loss of the 3-antenna from the O,2A cross-ring product were specific to [M + Ca]2+ ions. Fragmentation of [M + Cu]+ ions was similar to that of the corresponding [M + H]+ ions in that most cross-ring fragments were absent.

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.

Theoretical study of deprotonated glucopyranosyl disaccharide fragmentation

Molecular orbital calculations were used to investigate the fragmentation of deprotonated glucopyranosyl disaccharides. Based on data from collisional activation and isotopic labeling experiments, fragmentation mechanisms are proposed, with calculated transition states being used to study the energetics of fragmentation. The calculations suggest that deprotonation at the C(2) hydroxyl of the non-reducing ring, following ring opening, may be important for disaccharide fragmentation. It is also shown that the stereochemistry at the 2-position of the non-reducing ring may have a significant effect on disaccharide fragmentation, particularly with regard to determination of the anomeric configuration.

A dual vacuum chamber fourier transform mass spectrometer with rapidly interchangeable LSIMS, MALDI, and ESI sources: initial results with LSIMS and MALDI

A design is presented involving two separate vacuum chambers to provide nearly simultaneous capabilities of liquid secondary ion mass spectrometry (LSIMS), matrix-assisted laser desorption/ionization (MALDI), and electrospray ionization (ESI) in an external source Fourier transform mass spectrometer. The instrument consists of two vacuum chambers, one with five stages of differential pumping for a combined LSIMS/MALDI source. The chamber dedicated to ESI was formerly a three-stage chamber with LSIMS and electron ionization. Two additional stages were added with the ESI source. LSIMS and MALDI have similar vacuum requirements and were moved to a newly built chamber with two stages of pumping. We present our first results obtained on the new vacuum chamber. Data presented for the MALDI source show that, with only two stages of pumping, and with shorter radio frequency-only quadrupole rods for ion injection, spectra comparable to those obtained on the formerly three-stage instrument can be obtained. Characterization of the MALDI source and data on linear, cyclic, and branched oligosaccharides are given. Finally, the design of the secon chamber is proposed as a low-cost prototype for an external source FTMS instrument.

"Colored" noise waveforms and quadrupole excitation for the dynamic range expansion of Fourier transform ion cyclotron resonance mass spectrometry

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry offers unparalleled analytical performance in most regards but has a dynamic range of typically no better than 10(2)-10(3). This limitation reportedly arises from two opposing constraints, involving the maximum number of ions that can be effectively trapped (10(6)-10(7)) and the minimum number of ions required to produce a detectable signal (10(2)-10(3)). A potential solution to this dynamic range limitation is presented, based on the application of selected-ion accumulation using quadrupole excitation. We show that lower concentration species can be effectively accumulated in the FTICR trapped ion cell, while the more abundant species are continually removed by the application of quadrupolar excitation in the form of band-limited or "colored" noise waveforms. The result is that "room" is made in the cell for lower abundance species, even during extended accumulation periods. This approach was demonstrated with mixtures of the bovine proteins, insulin, ubiquitin, and cytochrome c. For normal accumulation, the dynamic range was approximately 100. The application of selected-ion accumulation in the form of colored noise allowed the extension by 2 orders of magnitude and the detection of species of 1 x 10(-8) M concentration from a solution also containing another component at 9 x 10(-5) M. With this method, a putative new low abundance variant of bovine insulin was observed, and selected-ion accumulation and subsequent collisionally activated dissociation were used for its identification. Dipolar magnetron excitation was also explored to enhance selected-ion accumulation and was found to reduce the amount of buffer gas required for complete removal of the undesired species by a factor of 5. Further possible improvements are discussed, as are the complications due to the required balance between magnetron and cyclotron damping rates.

Mass spectrometric analyses of beta-ketolactone oligomers, macrocyclic or catenane structures?

Mass spectrometry is used to develop an analytical method for a new class of organic oligomeric material, beta-ketolactones. Three different series with varying oligomeric sizes are examined. The oligomers may form at least two different structures, a macrocyclic and a catenane ring. Fast atom bombardment coupled with Fourier transform mass spectrometry provides a rapid and convenient method that provides both molecular weight information and abundant fragment ions that are structurally relevant. All the compounds are found to work well with the mass spectrometric analysis. Electrospray ionization coupled with triple-quadrupole mass spectrometry was also evaluated and found to yield results that are similar to FAB. On the basis of the FAB and the low-energy collisionally activated dissociation spectra, we conclude that these compounds are macrocyclic rings and not catenanes.

Negative ion fast atom bombardment and collision-induced dissociation mass spectrometry of the 2-, 3-, 4- and 6-deoxy derivatives from methyl beta-D-galactopyranoside and related compounds

The fast atom bombardment collision-induced dissociation mass spectra of the [M-H]- ions of the 2-, 3-, 4- and 6-deoxy derivatives from methyl beta-D-galactopyranoside and some related compounds have been recorded. The fragmentation reactions of these quasimolecular ions and of OD-labelled analogues have been examined and related to the molecular structure. In some cases distinct and common mechanisms can be derived, but it is also clear from these experiments that not only the site of deprotonation of the molecules, but also the anticipated charge localization in the fragment ions strongly direct the fragmentation reactions.