High-field NMR as a technique for the determination of polysaccharide structures

Gluco-oligosaccharides as potential prebiotics: Synthesis, purification, structural characterization, and evaluation of prebiotic effect

Prebiotics have long been used to modulate the gut microbiota and improve host health. Most established prebiotics are nondigestible carbohydrates, especially short-chain oligosaccharides. Recently, gluco-oligosaccharides (GlcOS) with 2-10 glucose residues and one or more O-glycosidic linkage(s) have been found to exert prebiotic potentials (not fully established prebiotics) because of their selective fermentation by beneficial gut bacteria. However, the prebiotic effects (non-digestibility, selective fermentability, and potential health effects) of GlcOS are highly variable due to their complex structure originating from different synthesis processes. The relationship between GlcOS structure and their potential prebiotic effects has not been fully understood. To date, a comprehensive summary of the knowledge of GlcOS is still missing. Therefore, this review provides an overview of GlcOS as potential prebiotics, covering their synthesis, purification, structural characterization, and prebiotic effect evaluation. First, GlcOS with different structures are introduced. Then, the enzymatic and chemical processes for GlcOS synthesis are critically reviewed, including reaction mechanisms, substrates, catalysts, the structures of resultant GlcOS, and the synthetic performance (yield and selectivity). Industrial separation techniques for GlcOS purification and structural characterization methods are discussed in detail. Finally, in vitro and in vivo studies to evaluate the non-digestibility, selective fermentability, and associated health effects of different GlcOS are extensively reviewed with a special focus on the GlcOS structure-function relationship.

Structural analysis of glucans

Glucans are most widespread polysaccharides in the nature. There is a large diversity in their molecular weight and configuration depending on the original source. According to the anomeric structure of glucose units it is possible to distinguish linear and branched α-, β- as well as mixed α,β-glucans with various glycoside bond positions and molecular masses. Isolation of glucans from raw sources needs removal of ballast compounds including proteins, lipids, polyphenols and other polysaccharides. Purity control of glucan fractions is necessary to evaluate the isolation and purification steps; more rigorous structural analyses of purified polysaccharides are required to clarify their structure. A set of spectroscopic, chemical and separation methods are used for this purpose. Among them, NMR spectroscopy is known as a powerful tool in structural analysis of glucans both in solution and in solid state. Along with chemolytic methods [methylation analysis (MA), periodate oxidation, partial chemical or enzymatic hydrolysis, etc.], correlation NMR experiments are able to determine the exact structure of tested polysaccharides. Vibration spectroscopic methods (FTIR, Raman) are sensitive to anomeric structure of glucans and can be used for purity control as well. Molecular weight distribution, homogeneity and branching of glucans can be estimated by size-exclusion chromatography (SEC), laser light scattering (LLS) and viscometry.

A gel-state 2D-NMR method for plant cell wall profiling and analysis: a model study with the amorphous cellulose and xylan from ball-milled cotton linters

Amorphous cellulose and xylan structures were analyzed using high-resolution 2D-NMR, and the NMR data were obtained in a DMSO-d₆/pyridine-d₅ (4 : 1) solvent system.

Aerobic sludge granulation: a tale of two polysaccharides?

Aerobic sludge granules are suspended biofilms with the potential to reduce the cost and footprint of secondary wastewater treatment. Attempts to answer how and why they form leads to a consideration of the role of their extracellular polymeric substances (EPS) in determining their physical and microbiological properties. The exopolysaccharide components of this matrix, in particular, have received attention as putative structural, gel-forming agents. Two quite different exopolysaccharides have been proposed as the gel-forming constituents, with their gel properties clearly different from those of activated sludge EPS. This review aims to address the question of whether more than one gel-forming exopolysaccharide exist in granules. Based on the available structural data, it seems likely that they are different gel-forming polymers and their differences are not artifacts of the analytical methods used. Nonetheless, both proposed structural gel polymers are extracted and purified based on procedures selecting for anionic polar polysaccharides soluble at high pH, and both contain hexuronic acids. Granulation does not result from EPS synthesis by any single microbial population, nor from production of a single exopolysaccharide. Future studies using solvents suitable for recalcitrant polysaccharides are likely to reveal important structural roles for other polysaccharides. It is hoped that this article will serve as a guide for subsequent studies into understanding the roles of exopolysaccharides in aerobic granular sludge.

Heparan sulfate separation, sequencing, and isomeric differentiation: ion mobility spectrometry reveals specific iduronic and glucuronic acid-containing hexasaccharides

We describe the resolution of heparan sulfate (HS) isomers by chromatographic methods and their subsequent differentiation by mass spectrometry (MS), ion mobility, and (1)H nuclear magnetic resonance (NMR) analysis. The two purified hexasaccharide isomers produced nearly identical MS spectra, quantitative disaccharide profiles, and partial enzymatic digestions. However, both tandem spectrometry (MS(2)) and ion mobility spectrometry (IMS) indicated structural differences existed. All data suggested the distinction between the two hexasaccharides resided in their uronic acid stereochemistries. Glucuronic (GlcA) and iduronic acids (IdoA) were subsequently defined by (1)H NMR analysis completing the structural analysis and verifying the unique structures initially indicated by MS(2) and IMS. Our results suggest that IMS may be a powerful tool in the rapid differentiation of GlcA and IdoA containing isomers in the absence of prior structural knowledge.

Structure of a major glycolipid from Thermus oshimai NTU-063

The structure of a major glycolipid isolated from the thermophilic bacteria Thermus oshimai NTU-063 was elucidated. The sugar and fatty acid compositions were determined by GC-MS and HPLC analysis on their methanolysis and methylation derivatives, respectively. After removal of both O- and N-acyl groups by alkaline treatment, the glycolipid was converted to a fully acetylated tetraglycosyl glycerol derivative, the structure of which was then determined by NMR spectroscopy (TOCSY, HSQC, HMBC). Thus, the complete structure of the major glycolipid from T. oshimai NTU-063 was established as beta-Glcp-(1-->6)-beta-Glcp-(1-->6)-beta-GlcpNAcyl-(1-->2)-alpha-Glcp-(1-->1)-glycerol diester. The N-acyl groups on the 2-amino-2-deoxy-glucopyranose residue are C15:0 and C17:0 fatty acids, whereas the fatty acids of glycerol diester are more heterogeneous including both straight and branched fatty acids from C15:0 to C18:0.

Liquid Chromatography/Mass Spectrometry Sequencing Approach for Highly Sulfated Heparin-derived Oligosaccharides

Liquid chromatography/mass spectrometry (LC/MS) is applied to the analysis of complex mixtures of oligosaccharides obtained through the controlled, heparinase-catalyzed depolymerization of heparin. Reversed-phase ion-pairing chromatography, utilizing a volatile mobile phase, results in the high resolution separation of highly sulfated, heparin-derived oligosaccharides. Simultaneous detection by UV absorbance and electrospray ionization-mass spectrometry (ESI-MS) provides important structural information on the oligosaccharide components of this mixture. Highly sensitive and easily interpretable spectra were obtained through post-column addition of tributylamine in acetonitrile. High resolution mass spectrometry afforded elemental composition of many known and previously unknown heparin-derived oligosaccharides. UV in combination with MS detection led to the identification of oligosaccharides arising from the original non-reducing end (NRE) of the heparin chain. The structural identification of these oligosaccharides provided sequence from a reading frame that begins at the non-reducing terminus of the heparin chain. Interestingly, 16 NRE oligosaccharides are observed, having both an even and an odd number of saccharide residues, most of which are not predicted based on biosynthesis or known pathways of heparin catabolism. Quantification of these NRE oligosaccharides afforded a number-averaged molecular weight consistent with that expected for the pharmaceutical heparin used in this analysis. Molecular ions could be assigned for oligosaccharides as large as a tetradecasaccharide, having a mass of 4625 Da and a net charge of -32. Furthermore, MS detection was demonstrated for oligosaccharides with up to 30 saccharide units having a mass of >10000 Da and a net charge of -60.

The fine structure of Caenorhabditis elegans N-glycans

We report the fine structure of a nearly contiguous series of N-glycans from the soil nematode Caenorhabditis elegans. Five major classes are revealed including high mannose, mammalian-type complex, hybrid, fuco-pausimannosidic (five mannose residues or fewer substituted with fucose), and phosphocholine oligosaccharides. The high mannose, complex, and hybrid N-glycan series show a high degree of conservation with the mammalian biosynthetic pathways. The fuco-pausimannosidic glycans contain a novel terminal fucose substitution of mannose. The phosphocholine oligosaccharides are high mannose type and are multiply substituted with phosphocholine. Although phosphocholine oligosaccharides are known immunomodulators in human nematode and trematode infections, C. elegans is unique as a non-parasitic nematode containing phosphocholine N-glycans. Therefore, studies in C. elegans should aid in the elucidation of the biosynthetic pathway(s) of this class of biomedically relevant compounds. Results presented here show that C. elegans has a functional orthologue for nearly every known enzyme found to be deficient in congenital disorders of glycosylation types I and II. This nematode is well characterized genetically and developmentally. Therefore, elucidation of its N-glycome, as shown in this report, may place it among the useful systems used to investigate human disorders of glycoconjugate synthesis such as the congenital disorders of glycosylation syndromes.

Development and validation of an NMR-based identity assay for bacterial polysaccharides

A method utilizing NMR spectroscopy has been developed to confirm the identity of bacterial polysaccharides used to formulate a polyvalent pneumococcal polysaccharide vaccine. The method is based on 600 MHz proton NMR spectra of individual serotype-specific polysaccharides. A portion of the anomeric region of each spectrum (5.89 to 4.64 ppm) is compared to spectra generated for designated reference samples for each polysaccharide of interest. The selected region offers a spectral window that is unique to a given polysaccharide and is sensitive to any structural alteration of the repeating units. The similarity of any two spectral profiles is evaluated using a correlation coefficient (rho), where rho >/= 0.95 between a sample and reference profile indicates a positive identification of the sample polysaccharide. This method has been shown to be extremely selective in its ability to discriminate between serotype-specific polysaccharides, some of which differ by no more than a single glycosidic linkage. Furthermore, the method is rapid and does not require extensive sample manipulations or pretreatments. The method was validated as a qualitative identity assay and will be incorporated into routine quality control testing of polysaccharide powders to be used in preparation of the polyvalent pneumococcal vaccine PNEUMOVAX 23. The specificity and reproducibility of the NMR-based identity assay is superior to the currently used colorimetric assays and can be readily adapted for use with other bacterial polysaccharide preparations as well.

Structure/function studies of anticoagulant sulphated polysaccharides using NMR

Sulphated polysaccharides have many biological functions, which depend on binding of highly specific carbohydrate structures to proteins. NMR spectroscopy is a technique capable of detailed structural elucidation of these polysaccharides, and can be used in applications ranging from routine analysis to research into covalent and conformational aspects of polysaccharide structure. This technique can be used to characterise sequence variations in heparin samples. The NMR-determined solution conformation of heparin has been used to predict binding sites on the surface of heparin-binding proteins. Sulphation patterns for dermatan sulphates of marine invertebrates have been determined. Their anticoagulant effects depend on an exact pattern of sulphate substitution. A small alteration in dermatan sulphate structure, from 4-O-sulphated to 6-O-sulphated galactosamine, leads to almost complete loss of anticoagulant activity in spite of an overall high level of sulphation. A fucosylated chondroitin sulphate isolated from sea cucumber has anticoagulant and antithrombotic activity depending on its sulphated fucose branches. The anticoagulant activity of algal fucans has been compared with that of regular, linear sulphated fucans from marine echinoderms; again high activity appears to correlate with the presence of sulphated fucose branches.