Xylan, spectra

Fig. 4. FT-IR spectrum of xylan powder extracted from corn cobs.

Fig. 5. Solid-state i C nuclear magnetic resonance spectrum of corn cob xylan.

Several Py-MS studies were done on homoglycans. A Py-FI MS study on a (1- 4)-p-xylan [2] showed several characteristic ions such as m/z = 132 (associated with the monomer unit C5H8O4), the ion m/z = 96 (associated with furfural, C5H4O2), and the ion m/z = 114 (probably associated with 3-hydroxy-2-penteno-1,5-lactone, CsHsOs). The study showed that retro-aldolization between C-1 and C-2 atoms does not take place. Smaller ions seen in the spectrum were probably generated from further pyrolysis of the initially formed fragments. 

Chitinase from Bacillus sp. was added to a solution of Xyl (l— 4)GlcNAc oxazoline 13 dissolved into a phosphate buffer. The consumption rate of 13 was accelerated by the enzyme addition and chitin-xylan hybrid polysaccharide 14 was produced. During the polymerization, the reaction proceeded homogenously throughout the reaction. The yield was 76% and its average molecular weight value of 14 was M = 1500 with MJMa = 1.76, determined by GPC. Furthermore, MALDI-TOF mass spectrum of 14 indicated the peaks at every m/z 335, which corresponds to the molecular mass of the repeating disaccharide unit. 

The xylan obtained was studied by NMR spectroscopy. One major and two minor signals were observed in the anomeric proton region between 4.4 and 5.5 ppm in the H NMR spectrum (Fig. 3). The chemical shifts and three-bond coupling constants in the H NMR spectrum indicate that the major structural element in the xylan was a P-(l->4)-D-xylopyranosyl residue and that a 4-0-methylglucuronic acid side-group and a disubstituted xylopyranosyl residue were minor structural elements. The spectrum resembles the spectrum for alkali-extracted 4-0-methylglucuronoxylan from aspen described earlier. It can thus be concluded that the isolated... 

After deacetylation by alkaline hydrolysis, the MALDI-MS spectrum of the aspen xylan (Figure 5, lower spectrum) revealed that this xylan contained acidic xylooligosaccharides with one or two 4-0-MeGlcA substituents (DP 10-28). Thus, the molar mass parameters (i.e., average molar masses) and the degree of polymerization and the degree of acetyl substitution of this xylan could be conveniently determined by this MALDI-MS analysis procedure. 

The positive-ion MALDI mass spectrum obtained for the oligo- and polysaccharides obtained from a hardwood (birch) xylan after mild acid hydrolysis is shown in Figure 7 (upper spectrum). This spectrum consists of several partially over-lapping series of peaks, which were identified on the basis of their exact molar masses as corresponding to neutral xylosaccharides with DP up to 30 and acidic xylosaccharides with one or two 4-0-MeGlcA substituents. 

Figure 6. MALDI mass spectrum (negative-ion mode) of the oligosaccharides obtained from a spruce xylan after mild acid hydrolysis. Reproduced from Reference 6. Copyright 2001 American Chemical Society.

In this study, MALDI-TOF-MS was employed to determine die structure and amount of side chains as die presence of uronic acid is known to have effects on the molecular properdes of xylan (25). The MALDI-TOF-MS spectrum of an acid-insoluble hemicellulose fraction isolated from the press lye, which was methylated (17,18) prior to measurement, is shown in Figure 2. 

PL-Beta was assumed to consist of pure xylan and the spectrum was subtracted form all other spectra for carbohydrate estimation... 

Isolated cellulose I and xylan extracted finm bleached birch pulp was mixed in order to study the process opposite to the removal of hemicelluloses fiom bleached kraft pulp by acid hydrolysis (16). The spectrum of hydrated tylan is shown in Figure 6. 

A comparison between the results obtained when fitting a spectrum recorded on isolated cellulose I with results obtained when fitting a spectrum recorded on the mixture of isolated cellulose I and itylan is shown in Figure 8. The mixing was done by heating a mbcture of colloidal sols of isolated cellulose I and isolated xylan for 48 h at 90 C (excess of water present at all times). The mixture was then centrifuged and placed in a NMR rotor and spectra were recorded. 

Figure 6. The CP/MAS C-NMR spectrum recorded on xylan isolated from bleached birch krqft pulp. The xylan sample is a colloidal sol (Reproduced with permission from reference 16, Copyright 1999).

Reference experiments showed that no detectable spectral changes occurred in the spectra of cellulose or in the spectrum of xylan when they were heated (48 h 90 C in water) separately (16). 

Since the signal at 82.0 ppm (X) is not present in neither the spectrum recorded on isolated cellulose I nor in the xylan Sfl ectrum it can unambiguously be assigned as a signal resulting from the interaction between cellulose I and xylan. 

The neutral xylan fraction from Palmaria decipiens was treated with aqueous bromine solution (25). This xylan contains l->4- as well as l->3-linked sugar units. Carbonyl groups are formed which was revealed by means of FTIR (vco at 1741 cm ) and NMR spectroscopy. The peaks found in the C-NMR spectrum were assigned (Table 5). The presence of C=0 was also revealed... 

Yellowing of paper under UV irradiation is a result of photo-oxidation of hemicellulose, which includes xylan and glueomanan [773, 981, 1859]. A seven-line ESR spectrum has been recorded during UV irradiation of xylan [968]. The photolytic chain scission of hemicellulose may occur either at the end of the molecule or in the middle of the chain. Hemicellulose can yield monosaccharides after photoirradiation, followed by transformation into coloured materials. 

N.N.M.E. van Peij, J. Brinkmann, M. Vrsanska, J. Visser and L.Hi de Graaff, 5-Xylosidase activity, encoded by xlnD, is essential for complete hydrolysis of xylan by Aspergillus niger but not for induction of the xylanolytic enzyme spectrum. Eur. J. Biochem., 1997, 245, 164-173. 

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