Structure of xylans

In view, however, of the most recent work on the structure of xylan,24-26 which has been shown to contain combined L-arabofuranose residues in... 

Sometime after the above review was written the author learned of further unpublished work on xylan through private communications with the research workers concerned. These new findings bring to light important new concepts regarding the structure of xylan and are presented here in brief summary. Happily, part of this work has just been published. 

In the following Sections, a comparison is made of the structures of xylans from different land plants. 

Joseleau, J. P., Comtat, J., and Ruel, K. 1992. Chemical structure of xylans and their intercation in the plant cell walls. In Visser, J., Beldman, G., Kusters-van Someren, M. A., and Voragen, A. G. J. (Eds.), Xylan and xylanases (pp. 1-15). Amsterdam Elsevier Science Publishers. 

J Buchert, A Teleman, V Haijunpaa, M Tenkanen, L Viikari, T Vuorinen. Effect of Cooking and Bleaching on the Structure of Xylan in Conventional Pine Kraft Pulp. Tappi J 7 (n) 125-130,1995. 

A caveat must be made at this point. This model is hypothetical, but it is the simplest that is consistent with the one linkage-one enzyme hypothesis and with the chemical evidence for the structures of xylans. Any further structural elaboration in the xylans must be primarily in their patterns of branching and that implies yet further intricacy in the biosynthesis of these branches. Interpretations other than those assumed here can be placed upon the data obtained by the partial degradation of xylans, but they do not alter one basic conclusion the side-chains of xylans are usually more complex in terms of biosynthesis than are the main core sequences. This suggests that the xylans may be examples of molecules in which encoded information of some sort is held in a three-dimensional array upon a non-informational substructure. In this case the helical core will support a somewhat brush-like array of side chains. Any information contained in the specific side chains probably relates to the construction and organisation of the cell wall, but in what way is unknown. 

Xylan, one of the better known hemicelluloses, is a component of plant cell membranes. This pentosan occurs in association with cellulose. The structure of xylan was shown to be 1,4-polyxylose ... 

Figure 13.3 Structure of xylan from annual plants.

The difference between xylan and cellulose could clearly be seen at the chemical shift of 102 ppm originating from C-1. Comparing the spectra it could be assumed that press lye consisted of pure xylan. It has also been described that the structure of xylan is very sensitive to the close environment having an influence on the spectra (40). No indication for residual lignin could be found in the samples. Also by this method the alkali-insoluble fitictions from 10% NaOH extraction were composed of cellulose I besides cellulose II as indicated by the presence of the signal at 66 ppm. By difference spectra the amount of xylan could be estimated, and the results matched the data from the carbohydrate analysis (Table VII). 

Since the structure of xylan is not well defined, its rate of disappearance was expressed in terms of mass concentration. The units of rx are (mass xylose/vol solution-min). The units of ki are (mass xylose/mass xylan-min), H is the mass concentration of xylan (mass xylan/vol solution), and X is the mass concentration of xylose. The two rate constants ( i, 2) were determined experimentally as a function of acid concentration at 98 °C and were correlated by the following relationship ... 

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