Interactions xylans

Interactions with xanthan were investigated for some GAX fractions of wheat bran [109]. Whereas, for lowly substituted GaMs a synergy in viscosity was observed at low total polymer concentrations, yielding a maximum of the relative viscosity at nearly equal proportions of both polysaccharides [124], the xanthan/xylan mixtures at the same experimental conditions showed no synergy. The observed decrease in the relative viscosity values upon addition of the xylan indicates that a certain interaction with xanthan takes place, but that it leads to a contraction in the hydrodynamic volume. The authors suggested that structural and conformational differences between GaM and GAX might be the reason for this observation. 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

Higher conversions of xylan and glucan were seen with increases in both moisture content and inoculum size (Table 4), but no correlation was observed between the conversions and the relative amounts of inoculum and moisture (ratio of inoculum to moisture content not shown). Thus, it is unlikely that these two parameters comprise an interaction effect that is important to the operation of the system. Lower moisture contents gave lower overall amounts of degradation, but seemingly better selectivities for xylan degradation although coefficients of variation for conversions were... 

The list of pyrolysis products of cottonwood shown in Table VII (llj reflects the summation of the pyrolysis products of its three major components. The higher yields of acetone, propenal, methanol, acetic acid, CO, water and char from cottonwood, as compared to those obtained from cellulose and xylan, are likely attributed to lignin pyrolysis. Other results are similar to those obtained from the pyrolysis of cell-wall polysaccharides. This further verifies that there is no significant interaction among the three major components during the thermal degradation of wood. 

At this point, it is worth of noting that typically vaporization enthalpies tend to increase as the vapor pressure decreases. However, the heat of vaporization of possibly aged tar (Table 4) was similar to the fresh tar (Table 3) in the case of tobacco or lower in the case of xylan, even if the vapor pressure lowered significantly (by a factor of ca. 10), This indicates changes in chemical structure (mainly in polar interactions), which are due to loss of volatile species, as well as from condensation type reactions. 

Xylan-based polymers show no convincing evidence for molecular interaction with, or cross-linking of, cellulose under the high dilution fermentation conditions. Whether this is also true in the less dilute cell wall environment remains to be determined. Description of hemicellulose polymers as glucan-binding may therefore not always be accurate. 

Some dyes have a specific interaction with certain polysaccharides e.g. Congo red interacts with (l->4) P-D, (l->3) P-D and (l->3)(l->4) p-D glucans and (l->4) p-D xylans (11, 12). Upon flooding the plate with the dye-solution, undegraded polysaccharide will be stained by the dye whereas degradation zones will remain uncoloured. Congo red was also applied directly in the medium with CMC (13). Another classical example is the blue coloured complex that is formed between intact amylose and Lugol s iodine solution. 

The interaction of agarose with two heavily substituted P1,4-1 inked D -xylans was studied. Using the same gelation and optical rotation criteria, the heavily substituted xylans... 

R 126 M. A. Kabel, H. A. Schols and A.G.J. Voragen, Identification of Structural Features of Various (O-Acetylated) Xylo-Oligosaccharides from Xylan-Rich Agricultural By-Products A Review , p. 108 R 127 P.T. Larsson, Interaction between Cellulose I and Hemicelluloses Studied by Spectral Fitting of CP/MAS C-NMR Spectra , p. 254 Vol. 867, 2004... 

---