13C-n.m.r. study of C hordein.

Insoluble xylan was prepared from ground birch (Betula pubescens) pulp by alkali extraction and precipitation with ethanol. The only sugar detected after acid hydrolysis of the preparation was xylose. The insoluble xylan was used as substrate in a nephelometric assay to determine the xylanase (EC 3.2.1.8, 1,4-beta-D-xylan xylanohydrolase and EC 3.2.1.37, 1,4-beta-D-xylan xylohydrolase) activities of Aspergillus and Trichoderma enzymes. The nephelometric method is reliable in evaluating xylanase hydrolysis of insoluble xylan.     

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.     

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.     

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 computer-assisted structural analysis of regular polysaccharides on the basis of 13C-n.m.r. data

A computerised approach to the structural analysis of unbranched regular polysaccharides is described, which is based on an evaluation of the 13C-n.m.r. spectra for all possible primary structures within the additive scheme starting from the chemical shifts of the 13C resonances of the constituent monosaccharides and the average values of the glycosylation effects. The analysis reveals a structure (or structures), the evaluated spectrum of which resembles most closely that observed. The approach has been verified by using a series of bacterial polysaccharides of known structure and, in combination with methylation analysis data, for the determination of the presently unknown structures of the O-specific polysaccharides from Salmonella arizonae O59 and O63, and Proteus hauseri O19.     

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.     

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. study of citrate metabolism in rabbit renal proximal-tubule cells.

U.v.-visible-absorption and e.p.r. spectroscopy were used to study the type 2 and type 3 copper centres in the mercury derivative of laccase. After treatment with peroxide the mercury derivative of laccase exhibits a fully developed absorption band at 330 nm (delta epsilon = 2900 +/- 100 M-1.cm-1, which is characteristic of type 3 copper in the oxidized state. In addition, there is a weak ligand-field absorption at 740 nm (epsilon = 380 +/- 30 M-1.cm-1), which can be assigned to the type 3 pair. Because the e.p.r. spectrum of the type 2 copper is well resolved in the case of the mercury derivative of laccase, for the first time we have been able to observe spectroscopic evidence for a pH-dependent structural transition that has been invoked to explain the kinetics of enzyme reduction [Andréasson & Reinhammar (1979) Biochim. Biophys. Acta 568, 145-156]. According to the e.p.r. data the pKa lies in the range 6-7, and comparisons with a model compound show that the spectral changes can plausibly be interpreted in terms of the deprotonation of a water molecule in the co-ordination sphere of the type 2 copper.     

Structure determination of Cryptococcus neoformans serotype A-variant glucuronoxylomannan by 13C-n.m.r. spectroscopy

A series of polysaccharides was derived by physical and chemical methods from an antigenic, O-acetyl-containing, glucuronoxylomannan (GXM), isolated from the growth medium of Cryptococcus neoformans (CDC B2550) serotype A-variant having composition ratios of Man:Xyl:GlcA:OAc = 10:4:3:6. 13C-N.m.r. spectra of derivatives provided new structural evidence for GXM. Treatment of GXM with Li in ethylenediamine gave a xylomannan (XM, with Man:Xyl = 5:2). Smith degradation of XM gave a mannan (M). Ultrasonic treatment of GXM gave GXM-sonicated (GXMS). Treatment of GXM with 3-(3-dimethylaminopropyl)-1-ethylcarbodiimide.HCl and then with NaBH4 gave reduced GXMS (RGXMS), or with aq. trifluoroacetic acid gave partially acid-hydrolyzed GXMS. Periodate oxidation of GXM and NaBH4 reduction of the product gave a polyalcohol-mannan (PM). Treatment of GXMS, RGXMS, and PM with NH4OH at pH 11 gave the respective O-deacetylated analogs. Comparison among the 13C-n.m.r. spectra of GXM, the various derivatives, and reference monosaccharides allowed the following conclusions: M is (1----3)-alpha-D-mannopyranan; XM consists of the M backbone with 91% of the Xyl on nonadjacent Man residues as 2-O-beta-D-Xylp substituents and with 9% as 4-O-D-Xylp substituents on other Man residues. GXM consists of the XM structure, but with non-D-xylosylated Man residues substituted with 2-O-beta-D-GlcpA substituents and with 6-O-acetyl groups distributed approximately equally on Man residues that have other substituents and those that have none. The molecular mechanics program MM2 was used to estimate the relative energies of anomeric orientations of the typical glycosidic linkage in M. The results suggest that 6'-OH----O-2 H-bonding is significant in the minimal-energy orientation of M, with phi = -36 degrees and psi = 51 degrees, and that two other glycosidic orientations may be important in the 2-O- or 6-O-substituted derivatives of M.     

A 13C-n.m.r. study of intermediates in the L-type pentose phosphate cycle

The structures in aqueous solution of all major contributing forms of D-altro-heptulose 1,7-diphosphate, and D-glycero-D-altro and D-glycero-D-ido-octulose 1,8-diphosphates have been established by 13C-n.m.r. spectroscopy. Assignments to individual carbon atoms were made with the aid of isotopically enriched analogues and by comparison with related sugars.     

Scope and limitations of the simple 13C-n.m.r. method of structural analysis of carbohydrates: glucodisaccharides

The scope and limitations of the SIMPLE n.m.r. method (secondary isotope multiplet n.m.r. spectroscopy of partially labelled entities) has been investigated for a series of glucodisaccharies. ¹³C-SIMPLE n.m.r. measurements have been made on solutions of (1→1)- (α,α-trehalose), (1→2)- (sophorose and kojibiose), (1→3)- (laminaribiose), and (1→6)-linked (gentiobiose and isomaltose) glucodisaccharides in (CD₃)₂SO and the results combined with those previously published for (1→4)-linked analogues (maltose and cellobiose). Each linkage (and substitution) type gives rise to a unique pattern of ¹³C isotopomers which, in principle, may be used for complete assignment of the spectra and structural analysis of the molecule. The glucodisaccharides are difficult to analyse, compared with other disaccharides, because the presence of two glucose moieties leads to degeneracies of a few isotopic multiplets which cannot be differentiated by the magnitudes of the isotope effects. Assignments in aqueous solutions were obtained by using the DIS (differential isotope shift) n.m.r. method in conjunction with the results from SIMPLE n.m.r. In practice, nearly all of the signals can be assigned unequivocally and the remaining signals are choices between two possible assignments.