Xylan, hydration

Fig. 23.—Conformation of the Xylan Hydrate Chain, Compared with that of Cellulose. (The threefold screw-axis in xylan gives rise to a longer intramolecular hydrogen-bond than in cellulose, which has a twofold axis along the chain. )...

Figure 3, Crystalline conformation of threefold symmetry helix, 3, xylan hydrate chain with J4.85A pitch. (.(Adapted from ref 4)...

Figure 4. Baseplane unit cell projection of white birch xylan hydrate. The unit cell is indexed as hexagonal a-b 9.16 A c (fiber axis) = 14.85 A. The circle represents the position of the water column. (By permission from ref 5)...

The role of the acetate groups in the cell wall of the living tree is not clear. It has been suggested that they serve the function of decreasing the hydration capacity of the xylan, which could, otherwise, conceivably lower the tensile strength of the woody tissue. This appears doubtful, in view of... 

With the exception of cellulose and chitin, plant polysaccharides are usually hydrated. Hydration often occurs in the crystalline regions as well as in the amorphous areas. When water of hydration is found in the crystallites, it may or may not affect the conformation of the polysaccharide backbone and in most cases, it affects the unit-cell dimensions, while in a few cases, the water appears to have no effect on unit-cell dimensions. The structures of six hydrated neutral polysaccharides will be examined with regards to the state of water of hydration in the structure. It wi 11 be seen that water may occur as columns or as sheets in these structures. The structures that will be discussed are (1 4)-3-p-xylan, nigeran, amylose, galactomannan, (1 3)-3-p-gTucan and (1 3)-s-P-xy1 an. The chemical structures of these polysaccharides are shown in Figure 1. 

Although it was not realized at first, the hydrating properties of xylan extend to its crystalline state. The unit-cell is hexagonal (4) with ... 

The base plan unit-cell projection (Figure 2) shows the positions of the water molecules in the unit-cell as deduced from an analysis of the equatorial x-ray diffracted intensities for a fiber diagram recorded at about 50X relative humidity. The water molecules cluster in distinct areas and form a helical column whose symmetry matches that of the xylan chains. In fact, the water of hydration may dictate the symmetry of the xylan chains. The energetically (theoretical) most stable conformation of the xylan chain involves twofold symmetry, whereas in the hydrated crystalline environment, as deduced from x-ray diffraction, the xylan chains possess three-fold symmetry. The water molecules stabilize the three-fold structure by the formation of hydrogen bonds. This structure is an example of columnar hydration which allows a symmetric... 

In conclusion, it is very likely that the hydration of both (1 3)-3-P-glucan and (1 3)-e-D-xylan is in the form of columns between the triple helices which bring about an increase in the unit-cell dimensions of the hydrate. 

The strainTFfects due to dehydration are nowhere better illustrated than in the precautions that must be taken to dry and season wood before its use. This problem relates to water removal from capillaries but certainly the dehydration of the hemicelluloses which are in a paracrystal1ine order at the surface of the microfibrils ( ) must play a role. Xylan (4) in hardwoods and galactoglucomannans (IJ ) in softwoods have the characteristics of columnar and sheet hydration, respectively. 

A xylan has been found in Chlorophyta, Rhodophyta, and Phaeophyta (brown algae). The skeletal polysaccharide of die green algae Caulerpa, Halimeda, and Udotea is a (1 3)-j8-D-xylopyranan, coiled helically to form microfibrils. In contrast to the crystalline lattice of cellulose, which could not be penetrated by water, the helical arrangement in the )8-D-(l — 3)-linked xylan allows in the lattice considerable hydration and hydrogen bonding, and these contribute to... 

Fig. 6.15. Tentative molecular conformation and repeat unit of the xylan molecule in the hydrate unit cell. (Marchessault and Liang, 1962.)...

In the past SO years wood chemists have learned much about the composition and physical properties of hardwood xylans. Partially acetylated glucuronoxylans are model native hardwood xylans. They exhibit thermoplasticity, film forming properties, crystallization potential and are oriented in the secondary cell wall. Their crystal structure has been determined and the hydration of this crystalline polysaccharide has been defined. The structural regularity of these abundant polysaccharides can be interpreted using the principle of optical superposition. 

The pronounced susceptibility of xylan to hydration is nature s method of plasticizing and the random acetyl groups assist further by preventing crystallization. Polarized infrared spectroscopy provided evidence that the xylan chains are oriented along the tree axis and one can conclude that this is equivalent to axial orientation in the fiber. (6)... 

The thermoplasticity phenomenon was also displayed by the arabino-glucurono-xylan films from white pine, but the latter were far more extensible than the native 0-acetyl counterparts from white birch. For example, while the white birch 0-acetyl xylan could extend no more than 50%, the white birch samples readily allowed up to 200% stretch. There is little doubt that the heterogeneity of the xylans hinders crystallization but the well-established fact that hydrated and dry crystalline states exist should help to understand many of the unusual features of these materials. 

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. 

---