Glucuronoxylans fractionation

Figure 10, SEC chromatogram and MALDFMS spectra of three sequential fractions (a, b and c shaded) obtained with an arabino-4-O-methyl-glucuronoxylan from spruce wood. Reproduced from Reference 3. Copyright 2001 American Chemical Society.

A total volume of 43 mL pyridine/HAc/H20 (9 1 4) was added to a sample of glucuronoxylan (Ig). After extraction under continuous stirring for Ih at room temperature, the sample was centrifuged at 4000 rpm and the supernatant decanted. The pellet was treated once more with a similar volume of the extractant. The supernatants were combined and chloroform was added (1 1.3). After separation of the chloroform phase, the water phase was washed once more with a similar volume of chloroform. The chloroform phases were combined and all phases were dried by rotavaporation. To remove pyridine from the phases, all fractions were sequentially washed by ethanol-toluene-ethanol, adapted from Lundquist et al. (1990) 28). 

The possibility of an interaction between glucuronoxylan and lignin could give rise to aggregation behaviour in solution. However, one could not rule out the contribution of H-bonding between polysaccharide chains. We therefore perform a fractionation of the heterogeneous glucuronoxylan structure. 

In this experiment the glucuronoxylan was dissolved in 0.5 M NaOH (pHbegin 13.47) and the pH was decreased in steps of 0.5. The pellets that were obtained after fractionation were dried and analysed by capillary zone electrophoresis (CZE) and MALDI-TOF-MS. The results are shown in Figure 2. Those steps where no pellet was obtained are omitted. It can be observed that a decrease in pH corresponds to an increase of the glucuronic acid content. It appears that the unsubstituted xylan chains, the least acidic fractions, are selectively removed in the earlier stages of the pH fractionation. These sparsely substituted xylan chains are expected to be responsible for the intermolecular H-bonding in solution, as has been observed for cellulose chains. Furthermore it... 

Figure 2 pH fractionation of aspen glucuronoxylan. The dotted line represents the glucuronic acid content of the starting material... 

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