Search

A 4-O-methylglucuronoxylan was converted into a hexenuronoxylan at high temperature and alkalinity similar to the conditions used during kraft pulping. The hexenuronoxylan was hydrolysed with enzymes, and acidic xylooligosaccharides were separated from the hydrolysate by anion-exchange and size-exclusion chromatography. The primary structure of the two main hexenuronic acid-substituted xylooligosaccharides (a tetramer and a pentamer) was determined by two-dimensional 1H and 13C NMR spectroscopy. The 4-deoxy-hexenutronic acid is not stable under the acid hydrolysis step of conventional carbohydrate analysis. Here, we have identified the acidic degradation products of 4-deoxy-hexenuronic acid by NMR spectroscopy. Two degradation pathways were observed, both resulting in a furan derivative.

A new acidic sidegroup in xylans, from both kraft pulp and pulping liquor, was identified by NMR spectroscopy. Unmodified oligosaccharides from kraft pulp xylan were obtained by enzymatic hydrolysis with xylanase (Trichoderma reesei). The acidic oligosaccharides were separated from the neutral forms on an anion exchange resin. The new acidic sidegroup was identified as 4-deoxy-β-l-threo-hex-4-enopyranosyluronic acid (hexenuronic acid) by 1H and 13C NMR spectroscopy. Hexenuronic acid is a β-elimination product of 4-O-methylglucuronic acid and is formed during kraft pulping. HMBC and NOESY experiments showed that hexenuronic acid is attached β-(1→2) to xylose. The NOESY data further indicated that hexenuronic acid protrudes from the main xylan chain. The pKa values for hexenuronic acid (3.03) and 4-O-methylglucuronic acid (3.14) attached (1→2) to xylose were determined from pH-dependent chemical shifts.

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 facile synthetic scheme for the preparation of methyl 4-deoxy-beta-L-threo-hex-4-enopyranosiduronic acid utilizing the commercially available methyl alpha-D-galactopyranoside as starting material has been developed. The synthesis sequence comprises six high yielding reaction steps: TEMPO oxidation, acetylation, methanolysis of the lactone, acetylation, beta-elimination, and final removal of the protecting groups. Only one column chromatographic purification is needed throughout the whole sequence. The overall yield is 60%. The final product has been characterized by NMR, Raman, UVRR, FTIR, and HRMS.

Deprotonation of d -mannitol was studied in aqueous basic solutions by means of potentiometry and 13 C NMR spectroscopy. Two-step dissociation in the pH range from 12 to 13.8 was shown, and successive dissociation constants K a1 and K a2 were determined. In a solution with ionic strength I = 1.0 M (NaOH + NaNO 3 ) p K a1 = 13.1 ± 0.1 and p K a2 = 13.8 ± 0.2. With increasing ionic strength from 0.75 to 3.0 M, both p K a1 and p K a2 values decrease. Deprotonation-induced chemical shifts in pH-variable 13 C NMR spectra show that the OH-groups next to internal carbon atoms C-3 and C-4 dissociate to a greater extent compared to OH-groups next to external carbon atoms C-1 and C-6.

The formation of four Cu(II)-xylitol complexes was observed in aqueous alkaline solutions (11.0or =pHor =14.0, I=1.0, 20 degrees C) by means of direct current polarography and VIS spectrophotometry. Mononuclear hydroxy complexes, CuXyl(OH)- (log beta=17.7 +/- 0.5), CuXyl(OH)2(2-) (log beta=20.2 +/- 0.3) and CuXyl2(OH)2(4-) (log beta=22.4 +/- 0.3), are formed at high ligand-to-metal ratios (L:Mor =10), whereas dinuclear complex Cu2Xyl (log beta=29.2 +/- 0.3) is the predominant species at low ligand-to-metal ratio (L:M=0.5). Diffusion coefficients and molar absorptivities of the complex species were determined. pH variable 13C NMR suggested that pKa values of xylitol are rather similar and equal to 13.8 +/- 0.2, 13.9 +/- 0.1 and 13.9 +/- 0.2 for OH-groups adjacent to (C-1,C-5), (C-3) and (C-2,C-4) carbon atoms, respectively.

The pentuloses d -erythro-2-pentulose (1) and d -threo-2-pentulose (2) and their 1-13C- and 2-13C-substituted derivatives were prepared by hydrogenating the corresponding isotopically normal and 13C-substituted d -pentos-2-uloses with a Pd-carbon catalyst. The threo isomer and its labeled derivatives were alternatively prepared from isotopically normal and 13C-substituted d -xyloses with immobilized d -xylose ( d -glucose) isomerase (E.C. 5.3.1.5). The equilibrium compositions of 1 and 2 (furanose anomers and acyclic keto forms) in 2H2O were determined from 13C-n.m.r. spectra (75 MHz) of the 2-13C-labeled derivatives. The conformational properties of the cyclic and acyclic forms in 2H2O were assessed with the use of 1H1H, 13C1H, and 13C13C spin-coupling constants obtained from 1H-n.m.r. (620 MHz) and 13C-n.m.r. (75 MHz) spectra. Compared with the structurally related aldotetrofuranoses the 2-pentulofuranoses more strongly prefer conformations in which the anomeric hydroxyl group is oriented quasi-axially. The strongly dipolarized carbonyl group in the acyclic keto forms of 1 and 2 appears to stabilize chain conformations having O-1 and O-3 eclipsed with the carbonyl oxygen.

The kinetics of the alkaline isomerization of D -glucose and D -fructose in 0.1 M ethanol-water solutions of sodium hydroxide were studied. To calculate the hydroxylion-independent rate-constants, the ionization equilibria of the respective sugar were determined separately by potentiometric titration in the same solvent system at 21°. The sugars ( D -glucose and D -fructose) in the reaction mixture were analyzed by g.l.c. as the trimethylsilylated oximes. By the micro-scale technique developed, reliable data could be obtained for reactions shorter than 10 seconds. The isomerization of D -glucose to D -fructose, and vice versa , accelerated markedly as a function of the alcohol concentration, and in 70% ethanol, their rates were 2.4 and 1.7 times the corresponding values in water solution. The activation energies of these isomerization reactions were found to be 122 kJ.mol −1 ( D -glucose) and 112 kJ.mol −1 ( D -fructose), and they were independent of the ethanol concentration.

The kinetics of the alkaline degradation of maltose in solution in 0.1 m sodium hydroxide in ethanol-water of up to 53% (w/w) concentration of ethanol were studied over a temperature range of 30–60°. The isomerization of maltose to maltulose and its further degradation proceeded at approximately the same rate, whereas the isomerization of maltulose to maltose was significantly slower. All these reactions were markedly accelerated with increasing concentration of ethanol in the solvent. At 53% (w/w) concentration of ethanol, the isomerization of maltose and the degradation of maltulose took place 2.0 and 2.4 times as fast as the corresponding reactions in water. The activation energies of these reactions were 115 (isomerization) and 110 (degradation) kJ.mol −1 , and they were independent of the concentration of the alcohol.