Xylans, acetates solubility

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. 

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. 

The xylan of hardwoods (O-acetyl-4-O-methylglucuronoxylan) consists of at least 70 p-xylopyranose residues (average degree of polymerization between 150 and 200) linked by P-l,4-glycosidic bonds (Fig. 11.4-4) 139. Every tenth xylose residue carries a 4-O-methylglucuronic acid attached to the C-2 of xylose 131. In addition, hardwood xylans are highly acetylated e. g. birchwood xylan contains more than 1 mol of acetic acid per 2 mols of xylose 140. Acetylation occurs usually at the C-3 rather than the C-2 position of xylose. Acetylation at both positions has also been reported 141, 142. The presence of these acetyl groups is responsible for the partial solubility of xylan in water 133. The alkali extraction of xylan leads to the deacetylation of this substrate 140. ... 

In early investigations on hardwood xylans, aqueous potassium hydroxide (instead of sodium hydroxide) was often used, presumably because of the high solubility of potassium acetate in ethanol. The later investigations of Hamilton and Quimby showed that this accidental choice was actually the best that could have been made. These workers found that lithium and sodium hydroxide are much more efficient than potassium hydroxide in removing the glucomannans present in softwoods and hardwoods. The latter contain ten times more xylan than glucomannan, and, if aqueous potassium hydroxide is used, all of the glucomannan remains in the wood. It should also be noted in this connection that 16% and 24% aqueous potassium hydroxide, concentrations which have often been used, allow a much more efficient removal of xylan from hardwoods than the sodium hydroxide solutions used by Schuerch and his coworkers, and that yields of 90% can be attained. 

In several cases, separation of acidic xylans from neutral glucomannans has been achieved by way of their acetates, by a method originally suggested by Perlin. As mentioned earlier (see Part I, p. 295), the acetates of acidic xylans are seldom readily soluble in chloroform. On the other hand, acetylated hexosans dissolve readily. When a chloroform solution of a glucuronoxylan acetate and a glucomannan acetate is shaken with water, an emulsion is formed which contains the pentosan acetate while the hexosan acetate remains in the chloroform. This method has been successfully applied by other workers, but it is somewhat time-consuming. 

In the case of water-soluble O-acetylated xylans and glucomannans, an ammonium acetate solution with a pH of 7 was employed as eluent in connection with the SEC separation step (7). However, for dkali extracted xylans and glucomannans, a more alkaline (pH 13) sodium hydroxide/acetate eluent was required (3). In all cases pretreatment of the SEC fractions by passage through a cation-exchange resin prior to MALDI-MS analysis and/or the use of MALDI probes coated with a Nafion film (2), were necessary in order to minimize the disturbance by buffer ions during the MALDI analysis step. 

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