Accessibility xylan

Esterification is not difficult if xylan is first carefully precipitated from solution and dried to a fluffy, non-horny powder, or if the xylan can be highly swollen so as to make the polysaccharide molecules easily accessible to the esterifying reagents. Solechnik101 finds that swollen undried xylan, when treated with acetic anhydride containing sulfuric... 

Like D-glucose and D-fructose, however, D-xylose can be utilized chemic ly or microbially—to generate a variety of interesting five-ca n c emica s o er than furfural (vide supra) or xylitol, a noncaloric sweetener, both being duectly produced from xylan hydrolysates, that is, without the actual isolation of the sugar. Other readily accessible intermediate products of high preparative utiUty (Scheme 2.14) are the open-chain fixed dithioacetal, the D-xylal, and D-hydroxy-xylal esters, or pyrazol or imidazol A -heterocycles with a hydrophilic trihydroxypropyl side chain. 

The xylanolytic enzyme system of Trichoderma reesei, a well-known producer of cellulolytic enzymes, is versatile and well suited for the total hydrolysis of different xylans. It consists of two major, specific and several non-specific xylanases, at least one / -xylosidase, a-arabinosidase and a-glucuronidase and at least two acetyl esterases. The hydrolysis of polymeric xylans starts by the action of endoxylanases. The side-groupcleaving enzymes have their highest activities towards soluble, short xylo-oligosaccharides, and make the substituted oligosaccharides again accessible for xylanases and / -xylosidase. 

About a quarter of the mannan in the sprucewood holocellulose seems to be accessible to the mannanase. More mannan can be hydrolyzed only when the second hemicellulose becomes at least partly dissolved. The remaining mannan—less than half the initial amount— appears to be removable only together with cellulose degradation. In beechwood holocellulose, most of the main hemicellulose—i.e., the xylan —can be hydrolyzed rather selectively by the sole action of xylanase. This phenomenon could be due to the different molecular size of the two hemicellulases, mannanase about 24 A, xylanase about 18 A (JO), resulting in better diffusing conditions for the xylanase. However, the porosity of the... 

Consistent with the behavior of simple glycosides (Table 2), the homogeneous hydrolysis rate of B-(l-4)-linked polysaccharides, as BeMiller summarized [255, 256], increased in the order cellulose (1) < mannan (2-2.5) < xylan (60-80) < galactan (300). This further demonstrates the significant role of accessibility in acidic degradation reactions. 

Among the pentoses, D-xylose is the cheapest and is readily accessible from wood- or straw-derived xylans. L-Sorbose is the most readily available L-sugar on a large scale due to its technological accessibility from D-sorbitol in the vitamin C industrial production process. 

Bird and Ritter isolated, from wood of white oak, a chlorine holocellu-lose which contained all of the 0-acetyl groups present in the wood. Mitchell and Ritter later extracted a chlorine holocellulose from sugar maple with water and obtained a xylan in a yield of 3.4% of the wood. This polysaccharide contained 9.2% of 0-acetyl groups. A xylan which had been obtained in the same way, from aspen, by Wise and Jones, was, on treatment with periodate, oxidized almost to completion. When the wood itself was similarly treated, most of its xylan escaped oxidation. Although it appears evident that all of the xylan in the wood could not possibly have been accessible to the aqueous reagent, it was concluded that the lack of oxidation was most probably due to the fact that the native xylan was partly 0-acetylated. After treatment of wood from Eucalyptus regnans with methanol at 150°, Stewart and coworkers obtained, on extraction with water, a xylan (in a jdeld of 3.7%) which contained 5-6% of acetate... 

In ARP pretreatment of hybrid poplar, deUgnification is increased with the treatment severity, but xylan removal occurs only to a certain extent. Increase of temperature from 175 to 195 °C in ARP does not affect xylan/lignin removal in hybrid poplar, yet the glucan digestibility of treated solid is increased. This is caused by an increase of cellulase accessibility to cellulose created by breakage of certain bonds and reconfiguration of the components in hemicellulose-lignin matrix. Crystallinity index of hybrid poplar increases after the ARP treatment. It is primarily caused by the removal of amorphous components,... 

Xylanase supplementation in enzymatic hydrolysis is effective for the substrates with high xylan content. Xylanase supplementation not only increases the xylan digestibility of treated biomass but also the glucan digestibility. Xylanase reduces the hindrance caused by resilient hemicellulose layer on the cellulose microfibril, thus, improving cellulase accessibility to the cellulosic part. 

Synthesis of Tri- and Tetra-saccharides. Kovaf and co-workers have produced several D-xylose trisaccharides related to structural components of xylans. Direct D-xylosylation of methyl 2-0-benzyl-j -D-xylopyranoside gave access to the methyl glycoside of 3,4-di-0-( 3-D-xylopyranosyl)-i8-D-xylose, and the 2,4-... 

The enzymatic degradation of (14)-j3-D-xylan single crystals with )3-D-xylanases has been investigated by electron microscopy and electron diffraction.The enzyme attack takes place at the edge of the crystals and progresses towards their centres. This is consistent with an endo-enzyme mechanism, where the enzyme interacts essentially with the accessible D-xylan chains located at the crystal periphery. 

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