The 13C-n.m.r. spectra of peracetylated cello-oligosaccharides

The reaction of 1,4-anhydro-2-deoxy-5,6-O-isopropylidene-d-arabino-hex-1-enitol (1) with m-chloroperbenzoic acid in ethanol gives 2,3-unsaturated ethyl glycosides together with saturated ethyl glycosides formed by trans-ring opening of 1,2-epoxide intermediates. Similar results are obtained on peroxidation of 1,4-anhydro-2-deoxy-3-O-(2,3:5,6-di-O-isopropylidene-α-d-mannofuranosyl)-5,6-O-isopropylidene-d-arabino-hex-1-enitol (2). Products resulting from osmylation of 1 and 2 and cleavage of the osmate esters are also described. 2-Deoxy derivatives are prepared from 1 and 2 by methoxymercuration-demercuration and also by reduction of 2-bromo-2-deoxy derivatives obtained by ethoxybromination.     

1H- and 13C-n.m.r.-spectral study of synthetic methyl d-manno-oligosaccharides

¹H- and ¹³C-n.m.r. spectra for 16 synthetic methyl manno-oligosaccharides were recorded, and the signals for the anomeric protons and anomeric carbon atoms in branched manno-pentaosides and -hexaosides were assigned, based on the data for methyl manno-biosides and -triosides. These n.m.r. data identified the branching pattern of high-mannose types of glycans of glycopeptides with those of unambiguously synthesized manno-oligosaccharides, and confirmed the structures proposed for such glycans.     

13C-n.m.r. analysis of some sulphate derivatives of chitosan.

Positions of substitution with sulphate in three water-soluble sulphated derivatives of chitosan were analysed by 13C n.m.r. The structures of N-acetylchitosan 3,6-O-disulphate, sodium chitosan N-, 6-O-disulphate, and sodium chitosan 6-O-monosulphate were confirmed.     

High resolution 13C-n.m.r. spectroscopy of 'mixed linkage' xylans.

Three xylan fractions, obtained by stepwise precipitation with ethanol, were analysed by 75-MHz 13C-n.m.r. spectroscopy. Diad frequencies, determined from the C-2 resonances, show that the (1----3)-linkages are interspersed throughout the chain rather than grouped contiguously. This type of distribution is in agreement with a random coil conformation and with the constancy of the optical rotation in solvents of different ionic strength and chaotropic power. These diad frequencies were compared with the theoretical values calculated for a random distribution from the ratio of (1----4)-:(1----3)-linkages in the 1H-n.m.r. spectra, and from the methylation analysis for one of the fractions.     

C-n.m.r. spectral study of C reductively methylated glycopeptides derived from glycophorin A

¹³C-n.m.r. spectral data for ¹³C reductively methylated intact homozygous and heterozygous glycophorins A were compared with the ¹³C-n.m.r. spectral data for the ¹³C reductively methylated homozygous and heterozygous N-terminal glycopeptides derived from the trypsin digest of glycophorin A. The results indicate that pronounced aggregation of this glycoprotein in solution does not affect the structural differences that we have previously observed for glycophorins A^M and A^N at and/or near the N-terminal amino acid. Moreover, the data suggest that two structural states exist for glycophorin A^M.     

13C n.m.r. study of L-alanine peptides

The di-, tri-, and tetrapeptides of L-alanine have been studied in aqueous solution by 13C n.m.r. spectroscopy at 25 and 50 MHz. By using selectively 13C enriched analogs containing either 90% 13C methyl or carbonyl carbons and measurements as a function of pH, assignment of the chemical shifts of the peptides has been made. T1 and NOE measurements of the peptides in their cationic, anionic, and zwitterionic states have been recorded as a function of concentration. The results show considerable segmental motion along the backbone carbons of the peptides, with only small changes occurring in the dynamic motions of the peptides as their charge states are altered. The lack of concentration dependence of the chemical shift and T1 values, as well as the similarity of T1 values for individual peptides in the three charge states, indicate that the peptides do not self-associate in aqueous solution.     

The application of 13C-n.m.r. spectroscopy to products derived from sucrose

Glycogen has been isolated from the livers of rats which had been fasted and then intubated with d-glucose. The structure of the glycogen, as determined by iodine staining and enzymic methods, was shown to be very similar to that from control animals. There were slight differences in the iodine-staining properties, but not as marked as that previously reported in the literature.     

13C-n.m.r.-spectral study of galactosyltransferase specificity with regard to the carbohydrate side-chain of native ovalbumin

As a prelude to studies using bovine N-acetylglucosaminide-β-(1→4)-galactosyltransferase to label membrane-surface glycoproteins with isotopically enriched d-galactose, the structural specificity of the enzymic reaction with water-soluble, hen ovalbumin has been examined. The enzyme-catalyzed transfer of d-galactose from UDP-d-galactose requires a (nonreducing) terminal 2-acetamido-2-deoxy-d-glucosyl group and exhibits selectivity towards saccharide chains containing d-mannose. This study considers the structural specificity of the enzyme with regard to the anomeric linkage between 2-acetamido-2-deoxy-d-glucose and d-mannose in the carbohydrate chains of hen ovalbumin. Uniformly ¹³C-enriched d-galactose was enzymically attached to the ovalbumin carbohydrate chain (which exhibits microheterogeneity in its structure), the protein was hydrolyzed, and separate glycopeptide fractions were chromatographically isolated. The ¹³C-n.m.r. spectra (60.5 MHz) of the fractions revealed two peaks for the anomeric carbon atom of d-galactose. The two peaks, at 104.20 and 104.39 p.p.m., were ascribed to d-galactosyl groups attached to 2-acetamido-2-deoxy-d-glucose respectively linked β-(1→4) and β-(1→2), to d-mannose in the glycopeptide chains. Quantifying of the spectral data revealed no specificity of d-galactosyltransferase towards the linkage from the terminal 2-acetamido-2-deoxy-d-glucosyl group to the penultimate d-mannosyl residue.     

Room temperature, low-field 13C-n.m.r. spectra of degraded kappa/iota carrageenans

Using samples previously degraded by autohydrolysis, it is possible to obtain valuable low-field 13C-n.m.r. spectra of samples of kappa/iota carrageenans at room temperature. Spectroscopy of samples extracted at different stages of autohydrolysis helps to determine the mechanism of the reaction and the presence of low proportion kinks and other irregularities in the molecules.     

13C-substituted pentos-2-uloses: synthesis and analysis by 1H- and 13C-n.m.r. spectroscopy

D-erythro-Pentos-2-ulose and D-threo-pentos-2-ulose and their 1-13C- and 2-13C-substituted derivatives have been prepared by oxidizing the corresponding natural and 13C-substituted D-aldopentoses (D-arabinose, D-xylose) with cupric acetate, and purifying the products by chromatography on a cation-exchange resin in the calcium or barium form. The equilibrium compositions of the pentos-2-uloses in 2H2O were determined by 13C-n.m.r. spectroscopy (75 MHz) at 25 degrees and 80 degrees. Among the eighteen possible monomeric acyclic, cyclic, and bicyclic forms, the anomeric pairs of the unhydrated aldopyranoses, aldopyranose endocyclic hydrates, aldofuranose endocyclic hydrates, and ketofuranose exocyclic hydrates were identified on the basis of 13C chemical shifts and 13C-1H and 13C-13C spin-coupling constants. 1H-N.m.r. (300, 500, and 620 MHz) and 13C-n.m.r. (75 MHz) spectroscopic data in one and two dimensions (DQF-COSY, homonuclear 2D-J) were used to evaluate the conformational properties of the cyclic structures. The unhydrated pyranoses are highly conformationally homogeneous; the erythro and threo isomers prefer 1C4 and 4C1 conformations, respectively. D-threo-Pentos-2-ulopyranose hydrate prefers the 4C1 conformation whereas the erythro isomers exists in both the 4C1 and 1C4 conformations. The furanoid forms favor structures having quasi-axial anomeric hydroxyl groups and quasi-equatorial exocyclic hydroxymethyl or dihydroxymethyl groups.     

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 arrangement of isoprene units in pig liver dolichol by 13C-n.m.r. spectroscopy.

The arrangement of isoprene units in pig liver dolichol-18, -19 and -20 was determined by 1H- and 13C-n.m.r. spectroscopies. The alignment of trans and cis isoprene units was found to be in the order: dimethylallyl unit, two trans units, a sequence of 14-16 cis units, and a saturated isoprene unit terminated with a hydroxyl group, which verified the presumed chemical structure of dolichol. The absence of geometric isomers was confirmed. A slight amount of impurity was detected in each reversed-phase h.p.l.c. fraction of dolichol obtained by a conventional method. Detailed assignments of the 13C-n.m.r. spectrum were given for these dolichols by using model compounds and INEPT (insensitive nuclei enhanced by polarization transfer) measurement. The chemical structure of synthetic dolichol-19, which was prepared by the addition of a saturated isoprene unit to the polyprenol-18 isolated from Ginkgo biloba, was confirmed to be identical with that of pig liver dolichol-19.     

Complete 1H- and 13C-n.m.r. assignments for two sulphated oligosaccharide alditols of hen ovomucin

The complete 1H- and 13C-n.m.r. assignments for beta-D-Galp-(1----4)-beta-D-GlcpNAc-6-SO3H-(1----6)-[beta-D-Galp-(1----3 )]- D-GalNAcol and alpha-NeuAcp-(2----3)-beta-D-Galp-(1----3)-[beta-D-Galp-(1----4)-b eta-D- GlcpNAc-6-SO3H-(1----6)]-D-GalNAcol were made by a combination of 2-D correlation experiments (Relayed-Cosy; and 13C,1H Correlation-shift n.m.r. spectroscopy), and 1-D n.m.r. spectroscopy. The results illustrate the ability of these methods to locate sulphate and NeuAc groups in anionic mucinous glycoproteins.