Xylans ester, preparation

Carboxylic acid esters of xylan are prepared under typical conditions used for polysaccharide esterification, i.e., activated carboxylic acid derivatives are allowed to react >vith the polymer both heterogeneously and homogeneously (Figure 1). The heterogeneous esterification of oakwood sawdust and wheat bran... 

Other esters such as the benzoyl,108 stearoyl,101 oleoyl,101 can be prepared by treating xylan with the appropriate chloride in the presence of sodium hydroxide, potassium hydroxide, pyridine or quinoline. These derivatives have not been characterized, perhaps because of their insoluble nature. 

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. 

Furoic acid (furan-2-carboxylic acid, or pyromucic acid) is used as a bactericide, and the furoate esters are used as flavoring agents, as antibiotic and corticosteroid intermediates. It is obtained by the enzymatic or chemical/catalytic aerobial oxidation of furfural (2-furalaldehyde) the latter is the only unsaturated large-volume organic chemical prepared from carbohydrates today. D-Xylose and L-ara-binose, the pentoses contained in the xylan-rich portion of hemicelluloses from agricultural and forestry wastes, under the conditions used for hydrolysis undergo dehydration to furfural. 

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... 

Esters of xylans are notorious for their poor solubility properties. Nitrates of undegraded (4-0-methylglucurono)xylans are insoluble in all solvents tested, and decompose readily at room temperature, especially in the presence of light. Organic esters of xylan are best prepared by first dis-... 

Low amounts of ester LCC structures as well as y-acetylated lignin moieties in the CELs [21] indicate that cellulase preparations should also possess esterase activity [41]. Esterase activity in commercial cellulase preparations is commonly found, since many industrial fungal hosts such as Trichoderma sp. or Aspergillus sp. cosecrete with cellulases a range of esterases, the so-called xylan debranching enzymes , including acetylxylan esterase (AXE) (EC 3.1.1.72), feruloylesterases (EC 3.1.1.73), and p-coumaroylesterases [42]. 

An elegant method based on selective enrichment of specific positions of the lignin side chain with C followed by C NMR studies [79] was applied to the study of LCC isolated from labeled wood [80]. The author claimed the presence in this preparation of LCC linkages of acetal, ether, and ester types at the a-position of the side chain and the absence of LCC bonds at the p- and y-positions of the side chain. However, a comprehensive discussirai revealed that these conclusions were not properly supported [29]. These same problems did not allow reliable NMR characterization of LCC linkages in a model Xylan-DHP substrate [58]. The main conclusion drawn from these studies is that ID C NMR is not a reliable tool to investigate LCC linkages even when using labeled preparations. 

In this preliminary study, extracted willow xylan was not immediately available for use in blending studies. As a result, native, partially acetylated birch xylan and a synthetically acetylated elm xylan were obtained from the sample collection of Professor T. E. Timell, an emeritus faculty member at our institution. Blends of these polymers with cellulose esters and bacterial polyesters were typically prepared by mbcing solutions of the respective polymers dissolved in either dimethylformamide (DMF) or water followed by evaporation in a vacuum oven at 10S°C yielding a thin film. A thermoplastic bacterial co-polyester (poly(hydroxybutyrate)) containing 30% hydroxyvalerate content) was purchased from the Aldrich Chemical Company (cat. no. 28,248-0). The glass transition temperature (Tg) of each polymer and blend was determined using a TA Instruments 2920 differential scanning calorimetry (DSC) instrument. 

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