Xylan containing only xylose residues has been prepared from esparto grass, the combined glucose and arabinose extracted with xylan was removed by fractional precipitation. The sugar obtained on hydrolysis was estimated by several methods - particularly by the standardised filter paper chromatography, and the polysaccharide was found to be 98-99% pure. It was completely methylated with caustic soda and dimethyl sulphate without any degradation, and the components of the hydrolysate of the methylated xylan was effectively separated in a cellulose column with petrol-n-butanol (7:3) mixture. The end group was characterised as 2:3:4 trimethyl D-xylopyranose and amounted to one mol. in every 35 + 3 residues. No arabofuranose derivative could be detected. 2-Methyl xylose was also obtained in almost the same proportion. The different methylated sugars obtained on hydrolysis were also estimated by paper partition chromatography by stoichiometric oxidation of the extracted sugars in an improved alkaline-phosphate buffer instead of carbonate-bicarbonate buffer, and was found to be of the same order. Viscosity measurements of the acetylated and the methylated xylan , and the osmotic pressure measurement of the latter showed a degree of polymerisation of 70-80. The same number of residues in the molecule was also indicated by periodate oxidation of the xylan and the estimation of the reducing group by colorimetric determination reduction of 3:5 dinitrosalicylic acid. On the basis of these results it is suggested that an esparto xyla molecule contains about 75 + 5 xylopyranose units joined by 1:4 B-linkages with a single branching point formed by a 1:3 union at Some point in the chain. A similar examination of xylan from pear cell-wall material was found to be structurally different from that of esparto grass. The amount of end group separated in the column on hydrolysis of the methylated polysaccharide corresponded to one non-reducing end 60 residues, and it was also identified as 2:3:4 trimethyl D-xylopyranose. The cellulose column was so efficient as to quantitatively separate a mixture of 2:3:4 trimethyl xylose and 2:3 dimethyl xylose on a semi-micro scale. The estimation of the end group by paper chromatography and the determination of the chain length by periodate oxidation showed a similar degree of polymerisation, but the molecular weight calculated from the specific viscosity of methylate xylan showed value of about half that obtained for the corresponding derivative of xylan from esparto grass. These result are in favour of a straight chain formulation of 55-60 residues as the interpretation of viscosity measurement is open to doubt; however, a branched chain formulation cannot be entirely dismissed without an accurate determination of the molecular weight by osmotic pressure measurements.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:602610 |
| Date | January 1949 |
| Creators | Chanda, S. K. |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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