Xylan periodate oxidation

D-xylopyranose units. This structural concept is substantiated by estimation of the formic acid obtained when the xylan is oxidized by periodate ions. On hydrolysis of the fully oxidized xylan there is obtained a small amount of D-xylose which presumably occupied the branch points in the polysaccharide and consequently was protected from periodate oxidation by possessing no adjacent free hydroxyl groups. 

Other workers140 have also examined the products derived by hydrolysis of periodate-oxidized xylan. Both wheat straw and com cob xylan, after oxidation and hydrolysis, yield small amounts of L-arabinose and D-xylose. These sugars are obtained even after the xylans have been subjected to extended periods of oxidation. It is concluded that the D-xylose constituted branch points in the xylan. Likewise, the L-arabi-nose molecules must not have been terminal units in a xylan chain but must have been either interior units in the xylan molecule or have constituted an araban-like polysaccharide which is in combination or admixture with the xylan polysaccharide. 

D-xylose on hydrolysis with dilute nitric acid. Percival and Chanda7 isolated a xylan from the same plant. They found that the methylated xylan produced on hydrolysis 2-methyl-D-xylose, 2,3-dimethyl-D-xylose, 2,4-dimethyl-D-xylose and 2,3,4-trimethyl-D-xylose. From this and from the results of periodate oxidation, Percival and Chanda considered that the polysaccharide contains 1 — 3 and 1 —> 4 linkages with a non-reducing endgroup (yielding the 2,3,4-trimethyl-D-xylose) for every 20-21 D-xylose units. They considered that this xylan was not a mixture of 1 —> 3 and 1 —> 4 linked polysaccharides because careful fractionation of its diacetate and dimethyl ether failed to establish any polymer heterogeneity. Barry, Dillon, Hawkins and O Colla74 confirmed the conclusion of Percival and Chanda. 

Painter, T. and Larsen, B., Transient hemiacetal formed during the periodate oxidation of xylan, Acta Chem. Scand., 24, 2366-2378, 1970. 

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... 

Complex side-reactions, including overoxidation, are likely to occur in the oxidations, and information on the mechanism of periodate oxidation and the periodate-oxidation products of starch, cellulose, xylan, and other polysaccharides have been discussed. Thus, the limit of periodate oxidation is not always that expected, and the formation of interresidue hemiacetal structures during the oxidation can cause an early halt. - It is not until these structures have been reduced that oxidation can proceed once more. Other oxidants, such as dichromate-sulfuric acid, dichromate-oxalic acid, and sodium hypochlorite (which cause more extensive reactions), have been investigated with respect to cellulose and periodate-oxidized cellulose. Further oxidation of 2,3-dialdehydocel-lulose with halogen peroxy acids gives 2,3-dicarboxycellulose. ... 

T. 1. Painter and B. Larsen, Formation of hemiacetals between neighboring hexuronic acid residues during the periodate oxidation of alginate, Acta Chem. Scand, 24 (1970) 813-833 T. 1. Painter and B. Larsen, Transient hemiacetal structures formed during the periodate oxidation of xylan, Acta Chem. Scand, 24 (1970) 2366-2378. 

The sodium periodate oxidation of xylan isolated from Palmaria decipiens yields 2,3-dialdehyde xylan (26). The dialdehyde moieties can be converted into a Schiff base type compound by the reaction with p-chloroaniline (Figure. 4). 

On treatment with periodatd ion, straw and beechwood xylan oxidize rapidly.104 As with other polysaccharides, the oxidation comes to a more definite end point in solutions which are buffered to pH 4-5. While somewhat more than the theoretical amount of periodate ion is consumed, the reaction apparently proceeds uniformly with oxidative cleavage of the 2,3 carbon bond to produce the structure indicated in Figure 4. In the course of the reaction the xylan passes into solution. The optical rotation of the oxidized product is surprisingly high (co. 100°). 

These results may be explained on the basis that xylan possesses a linear chain of 1,4-linked anhydroxylose units. However, it should be pointed out that on hydrolysis of the oxidized xylan, some D-xylose is found in addition to the expected glyceraldehyde and glyoxal.I04(b) While it is possible that the oxidation reaction is not complete, another explanation is that a branched xylan chain may be present. If branching occurs on a pentose chain unit, only one free hydroxyl would remain on the unit hence, it would not be oxidized or degraded during the course of the periodate reaction. 

Woods contain a number of acyl (mainly acetyl) groups, and it is probable that these are associated with the xylan components of the hemicellulose fraction. Evidence for the original esterification of aspen-wood xylan comes from the observation that the isolated xylan (obtained by alkaline extraction) is readily cleaved by periodate, whereas only a small proportion of the D-xylose residues in the wood itself are oxidized under similar conditions.66 Hemicelluloses still containing acyl groups may be extracted from wood holocelluloses by means of dimethyl sulfoxide, and further quantities... 

Moreover, 2,3-dialdehyde xylan may be obtained in the well-known glycol cleavage oxidation of vicinal diol units with sodium periodate, which can be further oxidized to give 2,3-dicarboxyl xylan. Figure 3 summarizes the structures of the possible oxidized repeating units. 

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