Xylose oligomers

Most biotechnological applications of hemicellulose utilize acid and/or solvent-based hydrolysates, in which the native xylan is cleaved to xylose and xylose oligomers. This saccharification can be achieved through such chemical means, but also by physical and/or enzymatic methods. Forest biomass is often treated with dilute acid or enzymatic hydrolysis as a pretreatment for fermentative application of the hemicellulosic fraction. Dilute acid hydrolysis generally uses low concentrations of mineral acids (H2SO4, HCl, 2-5%) at high temperatures (e.g. 150-170 °C) and pressures (10 atm) (Sun and Cheng, 2002). Concentrated... 

In addition, oligomers of other monosaccharides, such as arabinofuranose [429], galactofuranose [430,431] and xylose [432], were also synthesized efficiently by taking advantage of the Schmidt glycosylation procedure. 

The hydrolysis of acetylated xylo-oligomers from the steaming extract of birchwood using an enzyme mixture containing only xylanase and / -xylosidase was very limited (Table III). Chemical deacetylation showed that the substrate specificities of xylanase and / -xylosidase with respect to substrate DP overlapped, and that /J-xylosidase also hydrolyzed longer oligosaccharides than xylobiose to xylose. The addition of acetyl esterase to the hydrolysis of acetylated xylo-oligomers enhanced xylose production (Table III), but deacetylation was still incomplete (the acetyl content of... 

Table XIII. Methyl Glycosides of Oligomers of Xylose. 213...

C-N.m.r. Data73 in CDC13 of Methyl Glycosides of Peracetates of Oligomers of Xylose ... 

The ratios of arabinose xylose and 4-O-methylglucuronic acid xylose in the reaction solution after acid hydrolysis were about the same as those of holocellulose before xylanase treatment (cf. Experimental). Oligomer or polymer mannan fragments were present in the reaction solution. They were detected after additional acid hydrolysis, and they represented about 12% of the mannan present in the starting material (Table I, Columns 2 and 6). 

One process that may offer benefits of acid catalysis without the drawbacks of H2S04 is the use of carbonic acid. The pH of carbonic acid is determined by the partial pressure of C02 in contact with water, and thus it can be neutralized by releasing the reactor pressure. Carbonic acid is relatively mild and hence does not offer the same hydrolytic capability of H2S04. However, van Walsum (25) has demonstrated that at temperatures on the order of 200°C, carbonic acid does exhibit a catalytic effect on the hydrolysis of xylan. Van Walsum (25) observed an enhanced release of xylose and low-degree-of-polymerization xylan oligomers compared to pretreatment... 

Esterases. Acetyl esterase (EC 3.1.1.6) removes acetyl esters from acetylated xylose and short-chain xylo-oligomers. It s polymeracting counterpart, acetyl xylan esterase (EC 3.1.1.72), has a similar activity, but prefers polymeric xylan.244 In addition to acetate-specific enzyme detection kits, HPLC or GC analysis of acetate release from native extracted xylan and chemically acetylated xylan, colorimetric substrates, such as p-nitrophenol acetate and /3-napthyl acetate, or the fluorometric substrate, 4-methylumbelliferyl acetate are also used to assay acetyl esterases.244,253 The third esterase, ferulic acid esterase (EC 3.1.1.73), hydrolyzes the ester bond between ferulic acid or coumaric acid and the arabinose side chain of arabinoxylan. Assays for this activity are usually carried out using starch-free wheat bran or cellulase-treated gramineous biomass as a substrate and monitoring ferulic or coumaric acid released by HPLC or TLC. When preparing enzyme-treated substrates, care must be taken to employ phenolic-acid-esterase-free cellulases.244 Other substrates include methyl and ethyl esters of the phenolic acids, as well as finely ground plant biomass.240,254,255... 

There are numerous reports of /3-xylosidases that cleave short chain xylo-oligomers to xylose. In these cases, product detection was typically carried out by direct HPLC analysis or hydrolysis of p-nitrophenyl-/3-D-xylopyranoside.284-287... 

Other monomers and oligomers Glycerol Fatty acids Phenolics Ethanol Furfural 5-HMF Glucose Xylose Other carbos Polyols Levulinic acid Levoglucosan... 

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