Rapid molecular mass and structural determination of plant cell wall-derived oligosaccharides using off-line high-performance anion-exchange chromatography/mass spectrometry

Screening of Underivatized Oligosaccharides Extracted from the Stems of Triticum aestivum Using Porous Graphitized Carbon Liquid Chromatography−Mass Spectrometry

Highly polar oligosaccharide analytes are notoriously difficult to separate by HPLC without prior derivatization or the use of highly alkaline eluent systems. Using a porous graphitic carbon (PGC) HPLC column, we have studied a pool of endogenous underivatized water-soluble oligosaccharides that were extracted from the stems of a range of wheat cultivars. The aqueous/organic eluents that are used with this stationary phase are ideal electrospray solvents and hence facilitate the on-line coupling of the analysis to mass spectrometry. Our on-line PGC-LC-MS method has allowed the separation of native oligosaccharides, dp 2-20, in under 30 min. The method is robust and suitable for the separation of other complex oligosaccharide mixtures. We propose that isomers of fructan structures are separated and that the branching in these structures can affect their elution order. Further, our findings on the size and type of oligosaccharides extracted from wheat stems have been compared to grain yield data. Cultivars known to be high in stem carbohydrate content have been shown to contain larger oligosaccharide structures than cultivars classified as low in stem carbohydrate content. Interestingly, the largest oligosaccharides were present in the stems of wheat plants harvested 14 days after flowering, which correlates directly with the time that grain filling occurs.

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.

Structures and structure-activity relationships of three mitogenic and complement fixing pectic arabinogalactans from the malian antiulcer plants Cochlospermum tinctorium A. Rich and Vernonia kotschyana Sch. Bip. ex Walp

Structures of three pectic arabinogalactans, one from Vernonia kotschyana (Vk2a) and two from Cochlospermum tinctorium (Ct50A1 and Ct50A2), and their complement fixation and induction of B cell proliferation in vitro were compared. The polysaccharide Vk2a expressed potent biological activity in both assays compared with Ct50A1 and Ct50A2. Vk2a possessed a very high molecular weight (1150 +/- 20 kDa) compared with Ct50A1 and Ct50A2 which both showed a polydisperse nature with the highest molecular weight polymers in each fraction estimated at approximately 105 kDa (Ct1a) and 640 +/- 100 kDa (Ct2a), respectively. The HMW polymers showed complement fixation in the same range as the native fractions. The arabinogalactan II content was low in Vk2a (2%) compared with that in Ct50A1 (23%) and Ct50A2 (12%). The high molecular weight polymers were subjected to digestion with a beta-d-(1, 3)-galactanase-rich fraction from Driselase, oligomers were isolated by HPAEC, and their finer structures were determined by MALDI- and ES-qoToF-MS, linkage, and monosaccharide composition analyses. Vk2a consists of both a galacturonan core and a rhamnogalacturonan core rich in neutral side chains. The backbones of both Ct-polysaccharides consist mainly of RG-I regions with numerous neutral side chains dominated by galactosyl residues, whereas the homogalacturonan regions seem to be small. Differences in the chain lengths of the 6-linked galacto-oligosaccharides attached to the 3-linked galactan core could not be related to the differences in the potencies of the biological activities observed.

Differentiation of isomeric oligosaccharide structures by ESI tandem MS and GC–MS

A mixture of arabinoxylan oligosaccharides from wheat seedling was permethylated and analyzed by electrospray ion trap MS and GC-MS. The presence of isomeric structures differing in degree of branching and position of the branched residue along the xylose backbone was demonstrated for oligosaccharides with four and five monosaccharide residues. No isomeric structures were found for oligosaccharides with three monosaccharide residues. Linkage analysis by GC-MS reveals that xylose residues were substituted with single arabinoxyl residues at C-3.

Structural characterization by both positive and negative electrospray ion trap mass spectrometry of oligogalacturonates purified by high-performance anion-exchange chromatography

The off-line coupling of high-performance anion-exchange chromatography to electrospray ion trap mass spectrometry (ESI-IT-MS) is described. Two sets of isocratic conditions were optimised for the semi-preparative purification of oligogalacturonates of degree of polymerisation from 4 to 6 by monitoring eluates with either pulsed amperometric detection or evaporative light scattering detection in the presence of an online Dionex Carbohydrate Membrane Desalter (CMD). In these conditions, purified oligogalacturonate solutions were suitable, without further desalting steps, for infusion ESI-IT-MS experiments. This paper provides some interesting features of positive and negative ESI-IT-multiple MS (MSn) of these acidic oligosaccharides. The spectra acquired in both ion modes show characteristic fragments resulting from glycosidic bond and cross-ring cleavages. Under negative ionization conditions, the fragmentation of the singly-charged [M-H]- ions, as well as the Ci-, and Zi-, fragment ions through sequential MSn experiments, was always dominated by product ions from C- and Z-type glycosidic cleavages. All spectra also displayed 0.2 A-type cross-ring cleavage ions which carry linkage information. Collision-induced dissociation (CID) spectra of sodium-cationized species obtained under positive ionization conditions were more complex. Successive MSn experiments also led to the 0.2 A-type cross-ring cleavage ions observed together with B- and Y-type ions. The presence of the 0.2 A ion series was related to Mr 60 (C2H4O2) losses. Combined with the absence of the Mr 30 (CH2O) and the Mr 90 (C3H6O3) ions, these ions were indicative of 1-4 type glycosidic linkage.

Location of O-acetyl substituents in xylo-oligosaccharides obtained from hydrothermally treated Eucalyptus wood

A combination of techniques was used to localise the O-acetyl substituents in xylo-oligosaccharides, which are present in hydrolysates of hydrothermally treated Eucalyptus wood. Reversed-phase (RP)-high performance liquid chromatography (HPLC) coupled on-line to both a mass spectrometer and an evaporating light scattering (ELS) detector provided data about the order of elution of the various O-acetylated oligomers. The retention of the oligomers on the column depended on the number and position of the O-acetyl substituents within the xylo-oligosaccharides. One dimensional (1D)- and two dimensional (2D)-(1)H NMR spectroscopy was used to study the structural features of several xylotetramers separated by RP-HPLC, each having one O-acetyl substituent. O-Acetyl migration was proven to have occurred in these xylo-oligosaccharides. Mainly O-acetyl migration within the same xylosyl residue was observed. RP-HPLC-NMR was performed in order to study the structural features of the acetylated oligomers 'on-line' avoiding O-acetyl migration. Finally, the precise location of the 2-O- or 3-O-acetyl substituent in 6 xylotetramers and 4 xylotrimers separated by RP-HPLC was determined.

Structural characterisation of underivatised olive pulp xylo-oligosaccharides by mass spectrometry using matrix-assisted laser desorption/ionisation and electrospray ionisation

Purified olive pulp glucuronoxylans, with a Xyl/GlcA ratio of 7:1, were subjected to mild acid hydrolysis and the mixture of oligosaccharides obtained was fractionated by size exclusion chromatography. One elution fraction representative of low molecular weight oligosaccharides was analysed by mass spectrometry using matrix-assisted laser desorption/ionisation (MALDI) and electrospray ionisation (ESI) as ionisation methods, in the positive mode. Both types of spectra showed cationised molecules [M + Na](+) of xylo-oligosaccharides in a range below m/z 1,000. The xylo-oligosaccharide structures identified were series of neutral oligosaccharides of xylose (Xyl(n), n = 3-7), of acidic oligosaccharides substituted by one glucuronic acid (Xyl(n)GlcA, n = 3-5) and by two glucuronic acid residues (Xyl(n)GlcA(2), n = 2 and 3), and also of acidic oligosaccharides substituted with one 4-O-methylglucuronic acid residue (Xyl(n)meGlcA, n = 2-4). The proposed structures were confirmed by tandem mass (MS/MS) spectra obtained using collision induced dissociation of the molecular ions. Fragmentation of cationised adducts of neutral Xyl(n) yielded C- and A-type fragments, while ammonium adducts mainly yielded B-type fragments. The fragmentation of the sodium adducts of acidic oligosaccharides (Xyl(n)meGlcA, Xyl(n)GlcA) resulted in the loss of the substituting residue (GlcA or meGlcA) as the predominant fragment, while the corresponding ammonium adducts yielded B-type fragments.

The occurrence of internal (1 --> 5)-linked arabinofuranose and arabinopyranose residues in arabinogalactan side chains from soybean pectic substances

CDTA-extractable soybean pectic substances were subjected to enzymatic digestion with arabinogalactan degrading enzymes yielding a resistant polymeric pectic backbone and arabino-, galacto-, and arabinogalacto-oligomers. The complex digest was fractionated using size-exclusion chromatography. Monosaccharide composition analysis, HPAEC fractionation and MALDI-TOF MS analysis of the resulting fractions showed that each contained a mixture of oligosaccharides of essentially the same degree of polymerisation, composed of only arabinose and galactose. MALDI-TOF MS analysis was used for molecular mass screening of oligosaccharides in underivatised HPAEC fractions. The monosaccharide sequence and the branching pattern of oligosaccharides (degree of polymerisation from 4 to 8) were determined using linkage analysis and ES-CID tandem MS analysis of the per-O-methylated oligosaccharides in each of the HPAEC fractions. These analyses indicated the presence of common linear (1 --> 4)-linked galacto-oligosaccharides, and both linear and branched arabino-oligosaccharides. In addition, the results unambiguously showed the presence of oligosaccharides containing (1 --> 4)-linked galactose residues bearing an arabinopyranose residue as the non-reducing terminal residue, and a mixture of linear oligosaccharides constructed of (1 --> 4)-linked galactose residues interspersed with an internal (1 --> 5)-linked arabinofuranose residue. The consequences of these two new structural features of pectic arabinogalactan side chains are discussed.

Size exclusion chromatography/mass spectrometry applied to the analysis of polysaccharides

A novel method for analysing polysaccharide materials is described which employs size-exclusion chromatography (SEC) followed by detection by on-line electrospray ionisation mass spectrometry (ESI-MS) and off-line matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS). It is demonstrated through SEC/ESI ion trap mass spectrometry that the formation of multiply charged oligomer ions, which bind up to five sodium cations, allows the rapid analysis of polysaccharide ions with molecular weights in excess of 9 kDa. MALDI spectra generated from fractionation of the effluent collected from the same SEC separation are shown to be in good agreement with the ESI spectra with respect to molecular weight distributions and types of ions generated. ESI and MALDI mass spectra of samples obtained from sequential graded ethanol precipitation and SEC fractionation of acid and enzymatically digested arabinoxylan polysaccharides show important structural differences between polysaccharide fragments. In addition, a comparison is made between the mass spectra of native and permethylated SEC-separated fragments of acid and enzymatically treated arabinogalactan. Linkage information of the permethylated arabinogalactan oligomers can be rapidly established through the use of on-line SEC/ESI-MS( n) experiments.

Offline coupling of low-pressure anion-exchange chromatography with MALDI-MS to determine the elution order of human milk oligosaccharides

Pooled human milk oligosaccharides were separated into neutral and several acidic oligosaccharide fractions by preparative anion-exchange chromatography (AEC) using AG 1-X2. The oligosaccharides were eluted stepwise using deionized water and three different concentrations of ammonium acetate buffer, pH 6.8. The elution order of the compounds was determined directly by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analysis of the AEC effluent without any cleanup or concentration steps. Up to a concentration of 500 mM ammonium acetate, the masses of acidic oligosaccharides could be detected by screening the fractions in an automated mode. The combination of the improved chromatographic procedure, the applied MALDI matrices, and operating parameters is suitable for the detection of neutral oligosaccharides as well as acidic oligosaccharides. The method provides high sensitivity and mass accuracy, including for the high-molecular-weight monosialylated oligosaccharides up to 2751.5 Da. The applied ionic strength of the anion-exchange eluents enables a rapid and an unambiguous composition assignment by MALDI-MS for neutral, monosialylated, and disialylated oligosaccharides from human milk. The acidic fractions have to be desalted by electrodialysis and were finally analyzed by HPAEC-PAD to get a high-resolution "fingerprint" of structures present in each fraction. From these analyses, it can be concluded that the isomeric variety of monosialylated oligosaccharides occurring in human milk is higher than estimated before.

Structural analysis of (methyl-esterified) oligogalacturonides using post-source decay matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

The use of post-source decay matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the structural analysis of ((partly) methyl-esterified) oligogalacturonides (oligoGalA) is described. The fragmentation behavior of purified (un)saturated oligoGalA (degree of polymerization 3-6), methyl-esterified and methyl-glycosydated oligoGalA was studied. General fragmentation patterns are described and used for the elucidation of the positions of methyl esters on partly methyl-esterified oligoGalA. This technique now permits the determination of the position of methyl esters or other substituents on pectic fragments, helping in understanding the mode of action of pectinolytic enzymes.

Structural analysis of calcium spirulan (Ca-SP)-derived oligosaccharides using electrospray ionization mass spectrometry

Detailed structural analyses of calcium spirulan (Ca-SP)-derived oligosaccharides were performed by ESI-MS and collision-induced dissociation tandem mass spectrometry. This study indicates that Ca-SP is composed of two types of disaccharide repeating units, O-rhamnosyl-acofriose and O-hexuronosyl-rhamnose (aldobiuronic acid).