Xylan acetate

Note. The name ending in -an refers to the unsubstituted polysaccharide. Thus xylan occurs in nature in unacetylated and partially acetylated forms. Xylan designates the unacetylated material, and xylan acetate an acetylated derivative. 

Although most polysaccharides may be acylated without degradation, acyl derivatives of xylans29 are often unsatisfactory for molecular-weight determinations owing to their poor solubility characteristics. Mixed esters of xylans143 are often superior in this respect, but there are obvious difficulties in the differential analysis of acyl groups. Hemicellulose nitrates have been used for measurements of molecular size,168 but xylan nitrates,29 like xylan acetates, are reported to have poor solubility characteristics. 

Derivatives of hemicellulose components have properties similar to the ceUulosic equivalents but modified by the effects of thek lower molecular weight, more extensive branching, labile constituents, and more heterogeneous nature. Acetates, ethers, carboxymethylxylan (184), and xylan—poly(sodium acrylate) (185) have been prepared. 

Xylan-based micro- and nanoparticles have been produced by simple coacervation (Garcia et al., 2001). In the study, sodium hydroxide and chloride acid or acetic acid were used as solvent and non-solvent, respectively. Also, xylan and surfactant concentrations and the molar ratio between sodium hydroxide and chloride acid were observed as parameters for the formation of micro- and nanoparticles by the simple coacervation technique (Garcia et al., 2001). Different xylan concentrations allowed the formation of micro- and nanoparticles. More precisely, microparticles were found for higher concentrations of xylan while nanopartides were produced for lower concentrations of the polymer solution. When the molar ratio between sodium hydroxide and chloride acid was greater than 1 1, the partides settled more rapidly at pH=7.0. Regarding the surfactant variations, an optimal concentration was found however, at higher ones a supernatant layer was observed after 30 days (Garda et al., 2001). 

Natural xylan may occur partially esterified with acetic acid. This possibility arises from the observation that most holocellulose preparations contain a small amount of acetic acid. So far, however, it has not been established which polysaccharides are esterified. Treatment of the holocellulose with the alkaline solutions necessary for xylan extraction brings about complete deacetylation. 

The film-forming property of xylan derivatives also leads to the inference of an average D. P. greater than about 50. This deduction arises from the general observation that pliable polysaccharide acetate films are only formed from molecules with a D. P. of 50 or more. 

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... 

High quality xylan diacetate is insoluble in most reagents although the acetates of degraded xylan become progressively more soluble as the molecular weight decreases. Solubility in pyridine first appears, while with further decrease in molecular size, solubility in chloroform occurs. Because of their insolubility, the acetates of undegraded xylan cause filtration difficulties when present in commercial cellulose acetates. 

Xylans are the major hemicelluloses of many plant materials, where they often contribute to the rigidity of plant cell walls. Most xylans are heteropolysaccharides with a homopolymeric backbone chain of 1,4-linked j8-D-xylo-pyranose units. The degree and t pe of substitution of the backbone is dependent on the plant origin of a xylan. In addition to xylose, xylans may contain L-arabinose, D-glucuronic acid or its 4-O-methyl ether, and acetic, p-coumaric, and ferulic acids. 

The late discovery of acetyl xylan and feruloyl esterases has been partly due to the lack of suitable substrates. Xylans are often isolated by alkaline extraction, in which ester groups are saponified. Treatment of plant materials under mildly acidic conditions, as in steaming or aqueous-phase thermomechanical treatment, leaves most of the ester groups intact. These methods, however, partly hydrolyze xylan to shorter fragments (63,69). Polymeric acetylated xylan can be isolated from delignified materials by dimethyl sulfoxide extraction (70). The choice of substrate is especially important in studies of esterases for deacetylation of xylans. The use of small chromophoric substrates (p-nitrophenyl acetate, a-naphthyl acetate, and methylumbelliferyl acetate) analogously to the assays of disaccharidases may lead to the monitoring of esterases unable to deacetylate xylan (33, 63, 64). 

Blake and Richards26 carefully examined all of the steps involved in the analysis of complex polysaccharides, and preferred to use the alditol acetates in studies on the classification,428 fractionation,427 and molecular aggregation428 of hemicelluloses and speargrass xylan.420,430... 

Before discussing the various derivatives that have been used, it should be remembered that oligosaccharides are often obtained by chromatography on paper or on cellulose columns, and this may cause them to become contaminated by xylan this may be eliminated by extraction of the crude fractions with hot ethanol.474 For similar reasons, glucose that is really extraneous may be detected in fractions separated by electrophoresis on cellulose acetate.475 Gas-liquid chromatography was used to show the presence in pituitary glyco-... 

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