D-Xylan

P -D-xylan with 3-hnked a-L-arabiaose branches (1 4)-P -D-xylan with 2-Unked 4-0-methyl-a-D-glucuronic acid and arabiaose branches... 

Fig. 9. — Antiparallel packing arrangement of the 3-fold helices of (1— 4)-(3-D-xylan (7). (a) Stereo view of two unit cells roughly normal to the helix axis and along the short diagonal of the ab-plane. The two helices, distinguished by filled and open bonds, are connected via water (crossed circles) bridges. Cellulose type 3-0H-0-5 hydrogen bonds stabilize each helix, (b) A view of the unit cell projected along the r-axis highlights that the closeness of the water molecules to the helix axis enables them to link adjacent helices.

Most of the glucuronoxylans have single 4-0-methyl-o -D-glucopyranosyl uronic acid residues (MeGlcA) attached always at position 2 of the main chain Xylp units (Fig. 2). This structural type is usually named as 4-0-methyl-D-glucurono-D-xylan (MGX). However, the glucuronic acid side chain may be present in both the 4-0-methylated and non-methylated forms (GlcA). 

Fig. 3 Primary structure of (L-arabino)-4-0-methyl-D-glucurono-D-xylan (AGX)...

Fig. 5 Primary structure of water-soluble L-arabino-D-xylan (AX)...

Naturally, very long T,c values are expected for solids as viewed from the expected correlation time at the low temperature side of the T c minimum (0.1-0.2 s) as shown in Figure 1. Indeed, their values turns out to be the order of 10-30 s for carbon sites in the absence of internal fluctuations as in polysaccharides such as (1 — 3)-p-D-glucan and (1 —> 3)-p-D-xylan,46 8 fibrous proteins such as collagen49 and silk fibroin,50 free and metal-complexed ionophores,51 or in some instances up to 1000 s as in crystalline polyethylene.52... 

Endo-xylanases (1,4-p-D-xylan xylanohydrolase, EC 3.2.1.8), which hydrolyze mainly interior p-(l- 4)-D-xylosyl bonds in the plant hemicellulose xylan, have been isolated from many fungi and bacteria and occasionally from plants and invertebrates. Because xylan is one of the most common of all natural materials, and its removal or conversion is often desirable, the endo-xylanase family has high commercial as well as scientific interest. 

This enzyme [EC 3.2.1.8], also referred to as endo-1,4-)3-xyIanase and l,4-/3-D-xylan xylanohydrolase, catalyzes the endohydrolysis of l,4-/3-D-xylosidic linkages in xylans. 

This enzyme, officially known as l,4-/3-D-xylan synthase [EC 2.4.2.24], catalyzes the reaction of UDP-xylose with (l,4)-)3-D-xylan to yield UDP and (l,4)-/3-D-xylan +i. UDP-D-glucose is also a substrate. 

The triple-helical model previously proposed55 for (1— 3)-/3-D-xylan was refined in the hexagonal unit-cell, with a = b = 1.54 nm, c = 0.612 nm, and y = 120°, by using least-squares methods. A procedure that obviated the use of Bessel functions was described. The R factor was 41%. 

In describing and interpreting some of the more important properties of plant galactomannans, comparisons will be made with structurally similar polysaccharides, including the closely related glucomannans and galactoglucomannans, and those based on (1 — 4)-/3-D-xylan main-chains (for example, the arabinoxylans) and (1 — 4)-/3-D-glucan main-chains [for example, the amyloids and sodium 0-(carboxymethyl)cellulose]. 

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