Semiquantitative analysis of isomeric oligosaccharides by negative-ion mode UV-MALDI TOF postsource decay mass spectrometry and their fragmentation mechanism study at N-acetyl hexosamine moiety

Fragmentation mechanisms of protonated cyclodextrins in tandem mass spectrometry

Tandem mass spectrometry has found widespread application as a powerful tool for the characterization of linear and branched oligosaccharides. Though the technique has been applied to the analysis of cyclic oligosaccharides as well, the underlying fragmentation mechanisms have hardly been investigated. This study focuses on the mechanistic aspects of the gas-phase dissociation of protonated β-cyclodextrins. Elucidation of the dissociation mechanisms is supported by tandem mass spectrometric experiments and by experiments on di- and trimethylated cyclodextrin derivatives. The fragmentation pathway comprises the linearization of the macrocyclic structure as the initial step of the decomposition, followed by the elimination of glucose subunits and the subsequent release of water and formaldehyde moieties from the glucose monomer and dimer fragment ions. Linearization of the macrocycle occurs due to proton-driven scission of the glycosidic bond adjacent to carbon atom C1 in conjunction with the formation of a new hydroxy group. The resulting ring-opened structure further decomposes in charge-independent processes forming either zwitterionic fragments, a 1,4-anhydroglucose moiety, or a new macrocyclic structure, that is lost as a neutral, and an oxonium ion. Since the hydroxy group formed at the ring-opening site can be regarded as the non-reducing end of the linearized structure, the fragment ion nomenclature commonly used for linear and branched oligosaccharides, which relies on the designation of a reducing and a non-reducing end, can also be applied to the description of fragment ions derived from cyclic structures.

Development and validation of a bioanalytical method of analyzing 3'- and 6'-sialyllactose using liquid chromatography-tandem mass spectrometry in minipig plasma and its application in a pharmacokinetic study

Sialyllactose (SL) is an acidic oligosaccharide, consisting of a combination of sialic acid and lactose. It is found in human milk. It has immune-protective effects against pathogens in newborns and helps with the development of the immune system and intestinal microorganisms. We developed and validated a method by which 3'-SL and 6'-SL levels were simultaneously analyzed via liquid chromatography-tandem mass spectrometry (LC-MS/MS), and evaluated the pharmacokinetics of the materials after systemic delivery to minipigs. To improve chromatographic selectivity, several types of columns (C18, amide, and HILIC phase) were used to separate the peaks of 3'-SL and 6'-SL. Ultimately the HILIC phase column was selected, as it had a good peak shape and quick resolution. The mobile phase comprised ammonium acetate buffer and acetonitrile with gradient elution. MS was performed in the negative ion and multiple reaction monitoring modes. Plasma samples were prepared using the protein precipitation method with methanol. A surrogate matrix was used for quantification because SLs are endogenous plasma compounds. The method developed was validated according to U.S. Food and Drug Administration guidance. A pharmacokinetic study was performed with intravenous administration of 3'-SL and 6'-SL in minipigs (Sus scrofa/Yucatan). The concentrations of 3'-SL and 6'-SL were readily measurable in the plasma samples, which suggests that the method adequately determined systemic exposure in minipigs.

Development and Validation of a Bioanalytical Method for 3′- and 6′-Sialyllactose in Minipig Liver and Kidney Using Liquid Chromatography-Tandem Mass Spectrometry and Its Application to Analysis of Tissue Distribution

Breast milk contains human milk oligosaccharides (HMOs), including sialyllactose (SL). SL is composed of sialic acid and lactose, and is divided into 3′-SL and 6′-SL according to the binding position. SL has immunoprotective effects against bacteria and viruses, and acts as a probiotic in the gastrointestinal tract. In this study, we developed a bioanalytical method for simultaneous analysis of 3′-SL and 6′-SL in liver and kidney tissues of Yucatan minipigs using liquid chromatography–tandem mass spectrometry (LC-MS/MS) under conditions optimized in our previous study. LC-MS/MS was performed using a hydrophilic interaction liquid chromatography (HILIC) column (50 mm × 2.1 mm, 3 μm) with a mobile phase consisting of 10 mM ammonium acetate in water (pH 4.5) and acetonitrile with gradient elution at a flow rate of 0.3 mL/min. A surrogate matrix method using water was applied for analysis of endogenous SL. The developed method was validated with regard to selectivity, linearity, precision, accuracy, the matrix effect, recovery, parallelism, dilution integrity, carryover, and stability according to the US Food and Drug Administration guidelines. We performed a tissue distribution study of minipigs, and analyzed liver and kidney tissues using the developed method to determine the tissue distribution of 3′-SL and 6′-SL. The tissue concentrations of 3′-SL and 6′-SL were readily measurable, suggesting that the method would be useful for evaluating the tissue distributions of these compounds in minipigs.

Linkage and sequence analysis of neutral oligosaccharides by negative-ion MALDI tandem mass spectrometry with laser-induced dissociation

Mass spectrometry (MS) has become the primary method for high-sensitivity structural determination of oligosaccharides. Fragmentation in the negative-ion MS can provide a wealth of structural information and these can be used for sequence determination. However, although negative-ion MS of neutral oligosaccharide using the deprotonated molecule [M-H]^- as the precursor has been very successful for electrospray ionization (ESI), it has only limited success for matrix-assisted laser desorption/ionization (MALDI). In the present study, the features of negative-ion MALDI primary spectra were investigated in detail and the product-ion spectra using [M-H]^- and [M+Cl]^- as the precursors were carefully compared. The formation of [M-H]^- was the main difficulty for MALDI while [M+Cl]^- was proved to be useful as alternative precursor anion for MALDI-MS/MS to produce similar fragmentation for sequencing of neutral oligosaccharides. N-(1-naphthyl)ethylenediamine dihydrochloride was then used as both the matrix and the Cl^- dopant to evaluate the extent of structural information that can be obtained by negative-ion fragmentation from [M+Cl]^- using laser-induced dissociation (LID)-MS/MS for linkage assignment of gluco-oligosaccharides and for typing of blood-group ABO(H) and Lewis antigens on either type 1 or type 2 backbone-chains.

Analysis of matrix-assisted laser desorption/ionization quadrupole time-of-flight collision-induced dissociation spectra of simple precursor ions and isobaric oligosaccharide ion mixtures based on product ion intensities and pattern recognition

RATIONALE

Qualitative analysis of glycomic tandem mass spectrometry (MS/MS) data based on m/z values of product ions alone is widely used, and often sufficient for analysis of single analytes. However, most complex glycomic mixtures contain multiple isobaric oligosaccharides, in which case this approach is often limited. Here we show how ion intensity information can be used in order to enhance MS/MS data analysis, and extract both qualitative and semiquantitative information from complex glycomic MS/MS datasets.

METHODS

A matrix-assisted laser desorption/ionization quadrupole time-of-flight (MALDI QTOF) mass spectrometer was used in this study. We compared the intensities of product ions within a single product ion series, determined by their length, across the whole glycomic MS/MS dataset. In order to detect discernable patterns, the intensity data was normalized to the intensity of each product ion within the series. In most cases, normalized data yielded easily discernable patterns, relevant either for analysis of specific glycomic structure types, or mechanistic MS studies.

RESULTS

We used our approach on a glycomic sample consisting of human milk oligosaccharides. The approach yielded useful results for both qualitative and semiquantitative analyses. All normalizations performed were not equally rich in information and the information content of generated tables was not possible to predict. These analyses were shown to be independent of instrument manufacturer.

CONCLUSIONS

Our approach enabled more detailed qualitative analysis of MS/MS spectra of precursor ions containing isobaric oligosaccharide structures. While limited semiquantitative information could be extracted from the raw data as well, the accuracy of this method should be significantly enhanced when standard calibration mixtures can be prepared. Copyright © 2017 John Wiley & Sons, Ltd.

Ultra high performance liquid chromatography-tandem mass spectrometry method for the determination of soluble milk glycans in rat serum

The main objective of the present work was to develop and validate a multicompound method to measure soluble milk glycans (SMGs) in biological fluids such as serum. An ultra high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method for the identification and quantification of the following SMGs and their precursors 2'-fucosyllactose, 3'-sialyllactose, 6'-sialyllactose, lacto-N-neotetraose, N-acetylneuraminic acid, fucose, lactose and glucose in rat serum samples was set up. These analytes were separated in an Acquity UPLC BEH Amide column using acetonitrile-water gradient with ammonia as additive, in a 10 min run, and were detected and quantified using a triple quadrupole (QqQ) mass spectrometer. The mass spectrometric conditions in negative electrospray ionization mode (ESI-) were individually optimized for each analyte to obtain maximum sensitivity in the Selected Reaction Monitoring (SRM) mode. Selection of two specific fragmentation reactions for each compound allowed simultaneous quantification and identification in one run, ensuring a high specificity of the method. The limits of detection (LODs) ranged from 5 to 70 ng mL(-1) and the limits of quantification (LOQs) from 20 to 200 ng mL(-1). The inter- and intra-day variability was lower than 15% and the recoveries ranged from 85% to 115%. A biological application of the method was also described, specifically the time-course of SMGs in rat serum after an oral challenge.

MALDI-MS/MS with traveling wave ion mobility for the structural analysis of N-linked glycans

The preference for singly charged ion formation by MALDI makes it a better choice than electrospray ionization for profiling mixtures of N-glycans. For structural analysis, fragmentation of negative ions often yields more informative spectra than fragmentation of positive ones but such ions are more difficult to produce from neutral glycans under MALDI conditions. This work investigates conditions for the formation of both positive and negative ions by MALDI from N-linked glycans released from glycoproteins and their subsequent MS/MS and ion mobility behaviour. 2,4,6-Trihydroxyacetophenone (THAP) doped with ammonium nitrate was found to give optimal ion yields in negative ion mode. Ammonium chloride or phosphate also yielded prominent adducts but anionic carbohydrates such as sulfated N-glycans tended to ionize preferentially. Carbohydrates adducted with all three adducts (phosphate, chloride, and nitrate) produced good negative ion CID spectra but those adducted with iodide and sulfate did not yield fragment ions although they gave stronger signals. Fragmentation paralleled that seen following electrospray ionization providing superior spectra than could be obtained by PSD on MALDI-TOF instruments or with ion traps. In addition, ion mobility drift times of the adducted glycans and the ability of this technique to separate isomers also mirrored those obtained following ESI sample introduction. Ion mobility also allowed profiles to be obtained from samples whose MALDI spectra showed no evidence of such ions allowing the technique to be used in conditions where sample amounts were limiting. The method was applied to N-glycans released from the recombinant human immunodeficiency virus glycoprotein, gp120.

Identification and relative-quantification of glycans by matrix-assisted laser desorption/ionization in-source decay with hydrogen abstraction

The use of specific matrixes allows enhancing the scope of in-source decay (ISD) applications in matrix-assisted laser desorption/ionization (MALDI) thanks to the specificity of analyte-matrix chemistry. The use of an oxidizing matrix, 5-nitrosalicylic acid (5-NSA), for MALDI-ISD of glycans is shown to promote fragmentation pathways involving radical precursors. Both glycosidic and cross-ring cleavages are promoted by hydrogen abstraction from hydroxyl group of glycans by 5-NSA molecules. Cross-ring cleavage ions are potentially useful in linkage analysis, one of the most critical steps of glycan characterization. Moreover, we show here that isobaric glycans could be distinguished by structure specific ISD ions and that the molar ratio of glycan isomers in the mixture can be estimated from their fragment ions abundance. The use of 5-NSA also opens the possibility to perform pseudo-MS(3) analysis of glycans. Therefore, MALDI-ISD with 5-NSA is a useful method for identification of glycans and semiquantitative analysis of mixture of glycan isomers.

Calorimetric and theoretical study of the interaction between some saccharides and sodium halide in water

Dilution enthalpies and mixing enthalpies of sodium halide and some saccharides (glucose, galactose, xylose, arabinose, fructose, and sucrose) in aqueous solution were determined by calorimetric measurements at 298.15 K. The values were used to determine enthalpic pair interaction parameters. Combined with Gibbs energy pair parameters, entropic pair interaction parameters were also obtained. Theoretical calculations at the B3LYP/6-311++G(d,p) level were carried out to provide the information of structures and thermodynamic functions. The information reveals the thermodynamic essence of the interactions between sodium halide and saccharides in aqueous solutions. The experimental results and theoretical calculations show that the sign of enthalpic pair interaction parameter 2υh(ES) is determined by the direct interaction between saccharides and ions, whereas the difference in value of 2υh(ES) for different saccharides or electrolytes depends on the partial dehydration of saccharides or anions in aqueous solution. The difference in value of entropic pair interaction parameters depends partly on the different dominant interactions in the process of partial dehydration of saccharides or ions. An enthalpy-entropy compensation relationship was observed for the sodium bromide-aldopyranose-water systems. Remarkably, it can be conjectured that the hydration entropy of glucose is lower than for other monosaccharides. Perhaps it is one of the reasons why glucose plays an important role in living organisms rather than other monosaccharides.

Production and fragmentation of negative ions from neutral N-linked carbohydrates ionized by matrix-assisted laser desorption/ionization

Although negative ion fragmentation mass spectra of neutral N-linked carbohydrates (those attached to Asn in glycoproteins) provide much more structural information than spectra recorded in positive ion mode, neutral carbohydrates are reluctant to form negative ions by matrix-assisted laser desorption/ionization (MALDI) unless ionized from specific matrices such as nor-harmane or adducted with anions such as chloride. This paper reports the results of experiments to optimize negative ion formation from adducts of N-linked glycans with respect to ion abundance and fragment ion production. The best results were obtained with 2,4,6-trihydroxyacetophenone (THAP) as the matrix with added ammonium nitrate as the salt providing the anion. This approach is demonstrated to be applicable for a wide range of N-linked glycan structures. Phosphate adducts, analogous to those that are usually encountered in electrospray spectra from N-glycans released by protein N-glycosidase F, were produced by addition of ammonium phosphate to the matrix but in relatively low yield allowing competitive ionization of endogenous anionic compounds leading to complex spectra. Fragmentation of the nitrate adducts, which were formed in higher yield, generally paralleled that seen by collision-induced dissociation following ionization by electrospray, with the first stage of the dissociation being the elimination of the nitrate with a proton from one of the hydroxyl groups of the sugar. The spectra of the resulting [M-H](-) species displayed very specific fragment ions, mainly cross-ring and C-type glycosidic cleavage products, that revealed more structural (linkage and branching) information of the compounds than the mainly glycosidic cleavage products that dominated the positive ion spectra.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

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.

Methods for the quantitation of human milk oligosaccharides in bacterial fermentation by mass spectrometry

Oligosaccharides are the third most abundant component in human milk. In the past decades, it became apparent that they would be able to protect against pathogens and participate in the development of the gut microflora for infants. However, their role in infants' nutrition and development remains poorly understood. To better understand this function, it is extremely important to have a quantitative tool for profiling oligosaccharides. In this article, we show the development of a method to quantitatively differentiate the relative amounts of oligosaccharides fermented by different intestinal bacteria. To determine the oligosaccharide consumption, bacteria were grown in a medium using human milk oligosaccharides (HMOs) as the only carbon source purified from breast milk and further analyzed by matrix-assisted laser desorption/ionization-Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FTICR MS). A method using an internal deuterium-labeled standard was developed and compared with an external standard method, with the internal standard method giving better precision and unambiguous measurements than the external standard method and providing to be a novel and robust tool for following bacterial fermentation of milk oligosaccharides.