New NHR Techniques for Structure Determination and Resonance Assignments of Complex Carbohydrates

A review of the neurotoxic effect of palmyrah flour

The present review covers the history of the neurotoxic effect of palmyrah (Borassus flabellifer L). The chemical nature of the active synergists is isomers of a spirostane tetraglycoside containing three rhamnosyl residues and one glucosamine where the position of the NH(2) appears to be the difference in the saponins. As neurotoxicity has not been reported in humans consuming palmyrah flour, it is hypothesized that this may be due to one or more of the following: a species effect; the mode of processing flour and cooking palmyrah flour recipes containing these water-soluble and dry-heat decomposable saponin primary amines; frequency of consumption of palmyrah flour-based products; and the nutritive value of other dietary components. It is hypothesized that the Hepatotoxic syndrome as reported previously is due to a collective effect of a number of biologically active compounds, most of which are water-soluble saponins, like neurotoxins.

Synthesis of N- and S-bis-protected lactosyl isothiobiurets

Several N,S-bis(hepta-O-acetyllactosyl)-1-aryl-2-isothiobiuret compounds have been synthesised for the first time by the reaction of S-(hepta-O-acetyl-beta-lactosyl)-1-arylisothiocarbamides and hepta-O-acetyl-beta-lactosyl isocyanate. The identities of these newly synthesised compounds were established on the basis of elemental analysis and IR, (1)H NMR, (13)C NMR and mass spectral studies.

A Facile Method for the Synthesis of Hepta-O-Acetyl-β-D-lactosyl substituted Benzoates

Hepta-O-acetyl-β-D-lactosyl substituted benzoates were synthesised by the condensation of acetobromolactose with substituted benzoic acids in DMF at room temperature in the presence of Et3N.

Conformational analysis of an alpha-galactosyl trisaccharide epitope involved in hyperacute rejection upon xenotransplantation

alpha-Galactosyl epitopes are carbohydrate structures bearing an alpha-Gal-(1-->3)-Gal terminus (alpha-Gal epitopes). The interaction of these epitopes on the surface of animal cells with anti alpha-Gal antibodies in human serum is believed to be the main cause in antibody-mediated hyperacute rejection in xenotransplantation. In this paper, conformational analysis of an N-linked alpha-D-Galp-(1-->3)-beta-D-Galp-(1-->4)-beta-D-Glcp trisaccharide epitope was conducted in terms of each monosaccharide residue conformation, primary hydroxymethyl group configuration, and interglycosidic conformations. Selective 2D J-delta INEPT experiments have been carried out at three different temperatures to evaluate three-bond, long-range 13C-1H coupling constants for the crucial alpha-(1-->3) linkage. The NMR experimental data were complemented by theoretical calculations. The flexibility and dynamics of the trisaccharide have been studied by Metropolis Monte Carlo simulations. Ensemble-averaged three-bond, long-range 13C-1H coupling constants and nuclear Overhauser effects were in good agreement with the experimental data. The alpha-(1-->3) glycosidic linkage has shown a restricted flexibility as indicated by NMR spectroscopy and molecular modeling.

Synthesis of 1,2,3-tri-O-beta-lactosyl-D-threitol and 1-O-benzyl-2,3,4-tri-O-beta-lactosyl-D-threitol

The coupling of 2,3,6,2',3',4',6-hepta-O-acetyl-alpha-lactosyl bromide with 1,4-di-O-benzyl-D-threitol using mercury(II) cyanide as a promoter, with subsequent deprotection of one or both of the benzyl groups, further glycosylation, and deacetylation afforded the title compounds. This class of compound is useful in the assessment of binding properties of D-galactopyranose to human and rabbit hepatocytes.

Complete NMR structural elucidation of the capsular polysaccharide adjuvant from Klebsiella I-714

The capsular polysaccharide (CPS) produced by the non-pathogenic Klebsiella strain I-714, selected for its immunomodulating activity, has been purified and almost completely detoxified in a previous study [Adam, O., Vercellone, A., Paul, F., Monsan, P. F. & Puzo, G. (1995) Anal. Biochem. 225, 321-327]. The present report concerns the structural elucidation of this CPS by several one-dimensional and two-dimensional 1H-NMR and 13C-NMR experiments performed on the native molecule. It was found to be a high molecular-mass branched polymer constituted by a hexasaccharide repeating unit of following structure: 4)-alpha-L-Rhap-(1-->3)-beta-D-Galp-(1-->2)-alpha-L-Rhap-(1- ->4)-beta-D- GlcpA-(1-->3)-¿alpha-L-Rhap-(1-->2)-¿-alpha-D-Galp-(1. The presence of two glycosidic substitutions on the Galp residue resulted in a strong overlapping of its proton signals, preventing direct assignment of both proton and carbon resonances. However, assignment could be achieved using the two-bond and three-bond 1H-13C heteronuclear coupling observed in the 1H-13C heteronuclear multiple-bond correlation (HMBC) spectrum.

NMR spectroscopy in the structural elucidation of oligosaccharides and glycosides

The potential of one- and two-dimensional NMR techniques for the identification of individual sugar residues, their anomeric configuration, interglycosidic linkages, sequencing and the site of any appended group, in establishing the structures of naturally occurring oligosaccharides and glycosides is presented.

Synthesis and characterization of 6-O-beta-lactosyl-alpha,beta-lactoses, 1-O-(6-O-beta-lactosyl-beta-lactosyl)-(R,S)-glycerols, and 4,6-di-O-beta-D-galactopyranosyl-alpha,beta-D-glucoses

1,2,3,2',3',4',6'-Hepta-O-acetyl-beta-lactose (4) was coupled with 2,3,6,2',3',4',6'-hepta-O-acetyl-alpha-lactosyl bromide (7) in the presence of Hg(CN)2 to afford 1,2,3,2',3',4',6'-hepta-O-acetyl-6-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-b eta- lactosyl)-beta-lactose (11) which, upon O-deacetylation, gave 6-O-beta-lactosyl-alpha,beta-lactoses (64% from 4). In contrast, the reaction of 7 with benzyl 2,3,2',3',4',6'-hexa-O-acetyl-beta-lactoside in the presence of Hg(CN)2 produced 3,6,2',3',4',6'-hexa-O-acetyl-1,2-O- (2,3,2',3',4',6'-hexa-O-acetyl-1-O-benzyl-beta-lactos-6-yl orthoacetyl)-alpha-lactose (63%) and 3,6,2',3',4',6'-hexa-O-acetyl-1,2-O-(1- cyanoethylidene)-alpha-lactose (27%). The glycosidation of 4 using 2,3,4,6-tetra-O-acetyl-alpha-D-galactopyranosyl bromide in the presence of Hg(CN)2 afforded, after deprotection, 4,6-di-O-beta-D-galactopyranosyl-alpha,beta-D-glucoses (66%). The reaction of 11 with 1,2-di-O-benzyl-(R,S)-glycerols and trimethylsilyl trifluoromethanesulfonate yielded, after deprotection, 1-O-(6-O-beta-lactosyl-beta-lactosyl)-(R,S)-glycerols (18%). Under the same coupling conditions 11 reacted with 2-O-benzylglycerol to form 3-O-acetyl-2-O-benzyl-1-O-[2',3',4',6'-hexa-O-acetyl-6-O-(2,3,6,2',3',4' ,6'- hepta-O-acetyl-beta-lactosyl)-beta-lactosyl]-(R,S)-glycerols (16%).

Synthesis and characterization of 2-O-beta-lactosylglycerol, 1,2-di-O-beta-lactosyl-(R,S)-glycerols, and 1,2,3,-tri-O-beta-lactosylglycerol

The reaction of 2,3,6,2',3',4',6'-hepta-O-acetyl-alpha-lactosyl bromide (5) and 1,3-di-O-benzylglycerol in the presence of mercury(II) cyanide in benzene-nitromethane afforded 1,3-di-O-benzyl-2-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)glyc erol (70%), which was converted into 2-O-beta-lactosylglycerol. 1,2-Di-O-beta-lactosyl-(R,S)-glycerols were obtained by way of the coupling of 5 to either 1-O-benzyl-(R,S)-glycerol or 1-O-benzyl-2-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)-(R,S)-gl ycerols. The most efficient route to 1,2, 3-tri-O-beta-lactosylglycerol (17) involved treatment of 2-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)glycerol with 3 mol. equiv. of 5 followed by removal of the blocking groups, to give 17 (47%).

Synthesis and characterization of 6-O-beta-lactosyl-alpha,beta-D-mannopyranoses and 2,6-di-O-beta-lactosyl-alpha,beta-D-mannopyranoses

The reaction of 2,3,6,2',3',4',6'-hepta-O-acetyl-alpha-lactosyl bromide (4) and benzyl 3,4-di-O-benzyl-alpha-D-mannopyranoside (3) in the presence of mercury(II) cyanide in benzene-nitromethane produced benzyl 3,4-di-O-benzyl-2,6-bis-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lact osy l)-alph a D-mannopyranoside (5) and benzyl 3,4-di-O-benzyl-6-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)-alp ha-D- mannopyranoside (6), as part of a complex mixture. Column chromatography, followed by acetylation of the fraction containing 5 and 6, gave a sample of 5 and benzyl 2-O-acetyl-3,4-di-O-benzyl-6-O (2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)-alpha-D-mannopyranoside (7) in approximately 35% and 17% yields (based on 4), respectively. Deprotection of 5 and 7 afforded the target compounds, namely 2,6-di-O-beta-lactosyl-alpha,beta-D-mannopyranoses and 6-O-beta-lactosyl-alpha,beta-D-mannopyranoses, respectively. If the coupling of 4 with 3 were performed in the presence of silver trifluoromethanesulfonate and 2,4,6-trimethylpyridine, only a mixture of 3,6,2',3',4',6'-hexa-O-acetyl- alpha-lactose-1,2-[( 3,6,2',3',4',6'-hexa-O-acetyl-alpha-lactose 1,2-(benzyl 3,4-di-O-benzyl-alpha-D-mannopyranosid-6-yl orthoacetyl)-2-yl]orthoacetate) and 3,6,2',3',4',6'-hexa-O-acetyl-alpha-lactose 1,2-(benzyl 3,4-di-O-benzyl-alpha-D-mannopyranosid-6-yl orthoacetate) was obtained. The orthoacetates were characterized by n.m.r. spectroscopy. The two target materials are useful in the assessment of the binding properties of galactose-terminated ligands to the asialoglycoprotein receptor of normal rabbit and human hepatocytes.

O-specific polysaccharide of Hafnia alvei lipopolysaccharide isolated from strain 1211. Structural study using chemical methods, gas-liquid chromatography/mass spectrometry and NMR spectroscopy

The Hafnia alvei strain 1211 O-specific polysaccharide is composed of 3-amino-N-(D-3'-hydroxybutyryl)-3,6-dideoxy-D-galactose, N-acetyl-D-galactosamine, N-acetyl-D-glucosamine and D-glucose (1:1:2:2). On the basis of sugar and methylation analyses, Smith degradation, and one- and two-dimensional 1H- and 13C-NMR spectroscopy, the polysaccharide was shown to be an O-acetylated polymer of the repeating hexasaccharide unit, ----2D(4-OAc)Fucp3NAcyl beta 1----6DGlcpNAc alpha 1---- (DGlcp beta 1----3)4DGalpNAc alpha 1----3DGlcpNAc beta 1----2DGlcp beta 1----, where DFucp3NAcyl = 3-amino-N-(D-3'-hydroxybutyryl)-3,6-dideoxy-D- galactopyranose. The O-specific polysaccharide showed some microheterogeneity due to incomplete substitution by terminal glucose.

Elucidation of the structure of the Pasteurella haemolytica serotype T10 lipopolysaccharide O-antigen by n.m.r. spectroscopy

The aqueous-phase lipopolysaccharide isolated from Pasteurella haemolytica serotype T10 cells by the phenol-water extraction method was found to be S-type lipopolysaccharide which possessed O-antigenic polysaccharide chains composed only of D-galactose residues. Structural analysis of the O-polysaccharide, using a combination of 1D and 2D 1H- and 13C-n.m.r. methods, led to the identification of the disaccharide repeating-unit as [----3)-alpha-D-Galp-(1----3)-beta-D-Galf-(1----]n. The serological cross-reactivity between P. haemolytica serotypes T4 and T10 can now be related to the structural similarity of the antigenic LPS O-polysaccharides.

A two-dimensional 1H-n.m.r. (500 MHz) and 13C-n.m.r. (125 MHz) study of N-linked glycopeptides derived from calf fetuin

The oligosaccharide part of an N-linked triantennary glycopeptide from calf fetuin with fourteen carbohydrate residues and its smaller derivatives obtained by successive enzymic cleavage of the terminal residues were investigated using 2D 1H-n.m.r. (500 MHz) and 13C-n.m.r. (125 MHz) spectroscopy. Assignments have been made of the resonances of almost all the protons of the constituent carbohydrate residues in these glycopeptides. A comparison of the 1H chemical shifts and coupling constants, as determined from the cross-peaks, has shown the dependence of these parameters on the interactions of spatially related neighbouring carbohydrates. Small conformational changes take place upon elongation of the oligosaccharide side-chains.

Determination of the structure of the Escherichia coli K100 capsular polysaccharide, cross-reactive with the capsule from type b Haemophilus influenzae

The structure of the Escherichia coli K100 capsular polysaccharide, cross-reactive with that from type b Haemophilus influenzae, was determined by using a combination of chemical and spectroscopic techniques. The structure of the K100 repeating unit was found to be----3)-beta-D-Ribf-(1----2)-D-ribitol-5-(PO4----. The K100 polysaccharide is thus identical in composition to, but different in linkage from, the H. influenzae type b capsular polysaccharide, which has beta-D-Ribf-(1----1)-D-ribitol linkages.

Structural studies of the Escherichia coli K93 and K53 capsular polysaccharides

The structures of the Escherichia coli K93 and K53 capsular polysaccharides have been investigated by chemical and spectroscopic methods. The repeating unit of both polymers was found to be----3)-beta-D-Galf-(1----f)-beta-D-GlcAp-(1. The O-5 and O-6 atoms of D-galactose are acetylated in the repeating unit of the K93 polymer, but only O-2 is acetylated in the K53 polymer. The K93 polysaccharide is cross-reactive with the Neisseria meningitidis Group A capsular polysaccharide (of known structure). The K53 polysaccharide, although structurally similar to that from K93 organisms, does not cross-react with the Group A polymer.

Chemically modified capsular polysaccharides as vaccines

Capsular polysaccharides have assumed an important role as vaccines against disease caused by bacteria in humans. The concept of using pure definable polysaccharides devoid of their accompanying complex bacterial mass is technically elegant and is obviously capable of extension into other areas of immunoprophylaxis. However, problems have been identified which will need to be solved in order that the concept may be more widely adopted. Focusing on the meningococcal polysaccharides, possible solutions to two of these important problems, namely, the poor immunogenicity of the A and C polysaccharides in infants, and the poor immunogenicity of the B polysaccharide in all humans, are proposed. These solutions involve the use of a new generation of artificial synthetic antigens for modulating the immune response. For instance, conjugation of the A and C polysaccharides to tetanus toxoid converted them to T-cell dependent antigens in mice, thus making these conjugates potential infant vaccine candidates. Although a similar conjugation of the B polysaccharide failed to substantially enhance its immunogenicity in mice, this could be achieved by further chemical manipulation of the basic structure of the B polysaccharide. N-propionylation of the B polysaccharide, followed by its conjugation to tetanus toxoid, yielded an antigen, which when injected in mice, induced in them high titers of cross-reactive B polysaccharide-specific IgG antibodies. The chemical modification of polysaccharides requires an understanding of the interrelation between their structures and immunospecificities, and the structural elucidation of polysaccharides and the resultant monitoring of their structural modifications, can be conveniently accomplished using a wide range of NMR spectroscopic techniques. The capsular polysaccharides of many of the bacteria which cause meningitis in humans contain sialic acid and have extensive structural homology with human tissue. As a result of this homology the immunospecificities of these polysaccharides are complex, being based on unconventional conformational determinants.

Assignments of the 1H- and 13C-n.m.r. spectra of tobramycin at low and high pH

The 1H-n.m.r. spectra (200 MHz) of protonated (pD 3.9) and non-protonated (pD 11.9) tobramycin have been analysed completely by 2D methods. The 3JH,H values are consistent with an essentially undistorted 4C1 conformation for each of the three moieties and are practically independent of the state of protonation. The resonances in the 13C-n.m.r. spectrum (50 MHz) have been reassigned at both pD values on the basis of 2D1H-13C chemical-shift correlation and 1D selective INEPT measurements.

New n.m.r.-spectroscopic approaches for structural studies of polysaccharides: application to the Haemophilus influenzae type a capsular polysaccharide

The extension of several modern nuclear magnetic resonance (n.m.r.) spectroscopic techniques to polysaccharides is discussed and illustrated, using the native Haemophilus influenzae type a capsular polysaccharide. These techniques provide for the unambiguous assignment of all n.m.r. resonances (1H, 13C, and 31P) via high-sensitivity homonuclear and 1H-detected heteronuclear correlations, and they are capable of locating the intersaccharide linkages (both O-linked and phosphoric diester-linked) and appended groups (e.g. O-acetyl groups). To illustrate the power and sensitivity of these methods, a 10-mg sample of the H. Influenzae type a polysaccharide (repeat unit mol. wt. = 376) was studied. The combined acquisition time for the two-dimensional 1H-13C correlation data (one-bond and multiple-bond), the 1H-31P correlation data, and the 1H-1H (homonuclear Hartmann-Hahn) data was approximately 18 h.

The application of 1H/13C inversely correlated NMR spectroscopy to the determination of acylation and glycosylation sites in the O-specific polysaccharide from Hafnia alvei 1187

An inversely correlated 1H/13C NMR spectrum defined the amino sugars acylated by acetyl or 3-hydroxybutyryl groups and revealed partial sequences and glycosylation sites in a tetrasaccharide repeating unit of the title polysaccharide, (----2DGlc alpha 1----3DGlcNAcyl alpha 1----4DGalNAc alpha 1----3DGalNAc beta 1----)n, where Acyl = 3-hydroxybutyryl.

One- and two-dimensional 90.5-MHz 13C-NMR spectroscopy of the N-linked triantennary oligosaccharide units of calf fetuin

Complete assignments of all anomeric resonances in the proton and carbon spectra of the N-linked oligosaccharide units of fetuin were made using one- and two-dimensional NMR spectroscopy. We are able to confirm the presence of microheterogeneity in the N-acetylneuraminic acid linkages to the galactose residues and the presence of a unique triantennary structure which carries a side chain: NeuAc alpha(1-3)Gal beta(1-3)GlcNac beta(1-4)-. Anomeric carbon chemical shifts changes resulting from long-range conformational effects were observed.

Synthesis of the tetrasaccharide repeating-unit of the lipopolysaccharide isolated from Pseudomonas maltophilia

The title tetrasaccharide having the structure 3-O-Me-beta-L-Xylp-(1----4)- alpha-L-Rhap-(1----4)-alpha-L-Rhap-(1----2)-L-Rhap was obtained by reaction of the alpha-acetobromo derivative of 4-O-(3-O-methyl-beta-L-xylopyranosyl)-L-rhamnopyranose and benzyl 3,4-di-O-benzyl-2-O-(2,3-O-isopropylidene-alpha-L-rhamnopyranosyl)- alpha-L-rhamnopyranoside, followed by removal of the protecting groups. The synthesised compounds were characterised on the basis of n.m.r. data.

Two-dimensional nuclear magnetic resonance spectroscopy

Great spectral simplification can be obtained by spreading the conventional one-dimensional nuclear magnetic resonance (NMR) spectrum in two independent frequency dimensions. This so-called two-dimensional NMR spectroscopy removes spectral overlap, facilitates spectral assignment, and provides a wealth of additional information. For example, conformational information related to interproton distances is available from resonance intensities in certain types of two-dimensional experiments. Another method generates 1H NMR spectra of a preselected fragment of the molecule, suppressing resonances from other regions and greatly simplifying spectral appearance. Two-dimensional NMR spectroscopy can also be applied to the study of 13C and 15N, not only providing valuable connectivity information but also improving sensitivity of 13C and 15N detection by up to two orders of magnitude.