Ionization and fragmentation of N-linked glycans as silver adducts by electrospray mass spectrometry

Metal adduction in mass spectrometric analyses of carbohydrates and glycoconjugates

Glycans, carbohydrates, and glycoconjugates are involved in many crucial biological processes, such as disease development, immune responses, and cell-cell recognition. Glycans and carbohydrates are known for the large number of isomeric features associated with their structures, making analysis challenging compared with other biomolecules. Mass spectrometry has become the primary method of structural characterization for carbohydrates, glycans, and glycoconjugates. Metal adduction is especially important for the mass spectrometric analysis of carbohydrates and glycans. Metal-ion adduction to carbohydrates and glycoconjugates affects ion formation and the three-dimensional, gas-phase structures. Herein, we discuss how metal-ion adduction impacts ionization, ion mobility, ion activation and dissociation, and hydrogen/deuterium exchange for carbohydrates and glycoconjugates. We also compare the use of different metals for these various techniques and highlight the value in using metals as charge carriers for these analyses. Finally, we provide recommendations for selecting a metal for analysis of carbohydrate adducts and describe areas for continued research.

Exploring Gas-Phase MS Methodologies for Structural Elucidation of Branched N-Glycan Isomers

Structural isomers of N-glycans that are identical in mass and atomic composition provide a great challenge to conventional mass spectrometry (MS). This study employs additional dimensions of structural elucidation including ion mobility (IM) spectroscopy coupled to hydrogen/deuterium exchange (HDX) and electron capture dissociation (ECD) to characterize three main A2 N-glycans and their conformers. A series of IM-MS experiments were able to separate the low abundance N-glycans and their linkage-based isomers (α1-3 and α1-6 for A2G1). HDX-IM-MS data indicated the presence of multiple gas-phase structures for each N-glycan including the isomers of A2G1. Identification of A2G1 isomers by their collision cross section was complicated due to the preferential collapse of sugars in the gas phase, but it was possible by further ECD fragmentation. The cyclic IM-ECD approach was capable of assigning and identifying each isomer to its IM peak. Two unique cross-ring fragments were identified for each isomer: m/z = 624.21 for α1-6 and m/z = 462.16 for α1-3. Based on these key fragments, the first IM peak, indicating a more compact conformation, was assigned to α1-3 and the second IM peak, a more extended conformer, was assigned to α1-6.

Mass Spectrometry-Based Techniques to Elucidate the Sugar Code

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

Polysaccharides from lichen Xanthoria parietina: 1,4/1,6-α-d-glucans and a highly branched galactomannan with macrophage stimulating activity via Dectin-2 activation

Hot-water soluble polysaccharides H-1-3 and H-2-1 were isolated from the thalli of the lichen Xanthoria parietina (L.) Th. Fr. and purified by ion exchange and gel permeation chromatography. Structure elucidation was mainly based on 2D-NMR and nano-ESI-Q-TOF MS/MS experiments. H-1-3 (13.7 kDa) was shown to be linear α-glucan with α-d-Glcp-(1 → [→[4)-α-d-Glcp-(1]₂ → [6)-α-d-Glcp-(1]₃ → 4)]_n core backbone. The (1,4)- and (1,6)-α-d-Glcp linkages were in a 2:3 M ratio. H-2-1 (525 kDa) was characterized as a complex branched β-galacto-α-mannan with →[6)-α-d-Manp-(1 → [2,6)-α-d-Manp-(1]₂ → [2)-α-d-Manp-(1]₂→]_n core units and main side chains of (1,3)-β-d-Galf linked at O-6 to →2)-α-d-Manp-(1→, together with minor terminal units of 1,4/1,6-α-D -Glcp units attached to the core chain at O-6 position and α-L-Rhap linked to Galf side chain at O-2 position (Manp: Galf: Glcp: Rhap linkage ratio = 9:3:2:1). H-2-1 exerted strong immunoactivity in vitro and activated murine RAW macrophages 264.7 towards significantly increased phagocytosis, TNF-α and IL-1β secretion. These effects are due to an interaction of the galactomannan with the transmembrane pattern-recognition protein Dectin-2 of the macrophages.

Laser Desorption/Ionization Mass Spectrometric Imaging of Endogenous Lipids from Rat Brain Tissue Implanted with Silver Nanoparticles

Mass spectrometry imaging (MSI) of tissue implanted with silver nanoparticulate (AgNP) matrix generates reproducible imaging of lipids in rodent models of disease and injury. Gas-phase production and acceleration of size-selected 8 nm AgNP is followed by controlled ion beam rastering and soft landing implantation of 500 eV AgNP into tissue. Focused 337 nm laser desorption produces high quality images for most lipid classes in rat brain tissue (in positive mode: galactoceramides, diacylglycerols, ceramides, phosphatidylcholines, cholesteryl ester, and cholesterol, and in negative ion mode: phosphatidylethanolamides, sulfatides, phosphatidylinositol, and sphingomyelins). Image reproducibility in serial sections of brain tissue is achieved within <10% tolerance by selecting argentated instead of alkali cationized ions. The imaging of brain tissues spotted with pure standards was used to demonstrate that Ag cationized ceramide and diacylglycerol ions are from intact, endogenous species. In contrast, almost all Ag cationized fatty acid ions are a result of fragmentations of numerous lipid types having the fatty acid as a subunit. Almost no argentated intact fatty acid ions come from the pure fatty acid standard on tissue. Graphical Abstract ᅟ.

Liquid chromatography electrospray ionization Fourier transform ion cyclotron resonance mass spectrometric characterization of N-linked glycans and glycopeptides

We combine liquid chromatography, electrospray ionization, and Fourier transform ion cyclotron resonance mass spectrometry (LC ESI FT-ICR MS) to determine the sugar composition, linkage pattern, and attachment sites of N-linked glycans. N-linked glycans were enzymatically released from glycoproteins with peptide N-glycosidase F, followed by purification with graphitized carbon cartridge solid-phase extraction and separation over a TSK-Gel Amide80 column under hydrophilic interaction chromatography (HILIC) conditions. Unique glycopeptide compositions were determined from experimentally measured masses for different combinations of glycans and glycopeptides. The method was validated by identifying four peptides glycosylated so as to yield 12 glycopeptides unique in glycan composition for the standard glycoprotein, bovine alpha-2-HS-glycoprotein. We then assigned a total of 137 unique glycopeptide compositions from 18 glycoproteins from fetal bovine serum, and the glycan structures for most of the assigned glycopeptides were heterogeneous. Highly accurate FT-ICR mass measurement is essential for reliable identification.

Using a nanoelectrospray-differential mobility spectrometer-mass spectrometer system for the analysis of oligosaccharides with solvent selected control over ESI aggregate ion formation

Differential mobility spectrometry (DMS), also commonly referred to as high field asymmetric waveform ion mobility spectrometry (FAIMS) is a rapidly advancing technology for gas-phase ion separation. The interfacing of DMS with mass spectrometry (MS) offers potential advantages over the use of mass spectrometry alone. Such advantages include improvements to mass spectral signal/noise, orthogonal/complementary ion separation to mass spectrometry, enhanced ion and complexation structural analysis, and the potential for rapid analyte quantitation. In this report, we demonstrate the successful use of our nanoESI-DMS-MS system, with a methanol drift gas modifier, for the separation of oligosaccharides. The tendency for ESI to form oligosaccharide aggregate ions and the negative impact this has on nanoESI-DMS-MS oligosaccharide analysis is described. In addition, we demonstrate the importance of sample solvent selection for controlling nanoESI oligosaccharide aggregate ion formation and its effect on glycan ionization and DMS separation. The successful use of a tetrachloroethane/methanol solvent solution to reduce ESI oligosaccharide aggregate ion formation while efficiently forming a dominant MH(+) molecular ion is presented. By reducing aggregate ion formation in favor of a dominant MH(+) ion, DMS selectivity and specificity is improved. In addition to DMS, we would expect the reduction in aggregate ion complexity to be beneficial to the analysis of oligosaccharides for other post-ESI separation techniques such as mass spectrometry and ion mobility. The solvent selected control over MH(+) molecular ion formation, offered by the use of the tetrachloroethane/methanol solvent, also holds promise for enhancing MS/MS structural characterization analysis of glycans.

Evaluation of different quantitative approaches for the determination of noneasily ionizable molecules by different atmospheric pressure interfaces used in liquid chromatography tandem mass spectrometry: abamectin as case of study

The liquid chromatography tandem mass spectroscopy residue determination of compounds without any acidic or basic centers such as abamectin has been investigated. Several approaches regarding the interface used and adduct formation have been compared. The low acidity of the hydroxyl groups only made deprotonation feasible using the atmospheric pressure chemical ionization (APCI) interface. To obtain sufficient sensitivity for residue analysis, the Ion Sabre APCI interface was necessary. However, the sensitivity attained was lower than for monitoring adducts in positive ion mode. Using electrospray ionization, different adducts with Na+, NH4+, and Li+ were tested and compared. The best results were obtained for the ammoniated adduct in electrospray ionization (ESI) because of its high sensitivity and the presence of several product ions with similar abundance. The highest sensitivity was reached using an in-source fragment as precursor ion, leading to a limit of detection (LOD) of 2 microg/L with low relative standard deviation. The relatively high abundance of other transitions allowed abamectin confirmation at concentrations close to the LOD (6 microg/L). Alkali ions were found to be a suitable alternative to determine and confirm abamectin at residue levels. The [M + Na]+ also presented various product ions with similar abundance, which allowed confirmation at LOD levels. However, this LOD was found to be almost four times higher than with [M + NH4]+ because of the poor sensitivity of the transitions obtained. Although the use of Li+ facilitated the fragmentation of the adduct [M + Li]+, with similar sensitivity to [M + NH4]+, this fragmentation preferentially generated only one product ion, which did not allow confirmation at concentration levels lower than 15 microg/L. The use of APCI for monitoring adducts was also feasible, but with less sensitivity. The sensitivity increased with the Ion Sabre APCI, although it was still five times lower than with ESI. Other adduct formers such as Co2+ and Ni2+ also were tested with unsatisfactory results.