Arabinoxylan acetate

Figure 3. HNMR spectrum (DMSO-d, 80 °C) of an arabinoxylan acetate. Reproduced with permission from reference 19. Copyright 2002 Elsevier...

Figures 7 and 8 illustrate the effect of type of substituent and DS on the Tg of these arabinoxylan esters. In the series of parent arabinoxylan, AXA, AXP, and AXB, the respective Tg s are 198, 138, 97, and 61 °C. In the case of arabinoxylan acetates, as the DS decreased, the observed Tg increased. That is, the Tg of these arabinoxylan esters decreases as the length of the side chain and the total DS increases. The decrease in Tg with increasing DS is likely the result of a decrease in polymer-polymer interactions arising from hydrogen bonding. The decrease in Tg as the length of the side chain increases is likely due to both a decrease in polymer-polymer interactions and to an increase in free volume.

Feruloyl esterase activity was first detected in culture filtrates of Strepto-myces olivochromogenes (49), and has thereafter also been reported for some hemicellulolytic fungi (Table III). A partially purified feruloyl esterase from S. commune liberated hardly any ferulic acid without the presence of xylanase (65). Very recently a feruloyl esterase was purified from Aspergillus oryzae (Tenkanen, M. Schuseil, J. Puls, J. Poutanen, K., /. Biotechnol, in press). The enzyme is an acidic monomeric protein having an isoelectric point of 3.6 and a molecular weight of 30 kDa. It has wide substrate specificity, liberating ferulic, p-coumaric, and acetic acids from steam-extracted wheat straw arabinoxylan. 

Esterases. Acetyl esterase (EC 3.1.1.6) removes acetyl esters from acetylated xylose and short-chain xylo-oligomers. It s polymeracting counterpart, acetyl xylan esterase (EC 3.1.1.72), has a similar activity, but prefers polymeric xylan.244 In addition to acetate-specific enzyme detection kits, HPLC or GC analysis of acetate release from native extracted xylan and chemically acetylated xylan, colorimetric substrates, such as p-nitrophenol acetate and /3-napthyl acetate, or the fluorometric substrate, 4-methylumbelliferyl acetate are also used to assay acetyl esterases.244,253 The third esterase, ferulic acid esterase (EC 3.1.1.73), hydrolyzes the ester bond between ferulic acid or coumaric acid and the arabinose side chain of arabinoxylan. Assays for this activity are usually carried out using starch-free wheat bran or cellulase-treated gramineous biomass as a substrate and monitoring ferulic or coumaric acid released by HPLC or TLC. When preparing enzyme-treated substrates, care must be taken to employ phenolic-acid-esterase-free cellulases.244 Other substrates include methyl and ethyl esters of the phenolic acids, as well as finely ground plant biomass.240,254,255... 

The arabinoxylan was precipitated and stored wet in EtOH. The EtOH was exchanged for acetic acid prior to esterification. 

When TFAA was used as a promoter for the esterification of com fiber arabinoxylan instead of catalytic amounts of MSA, acetic acid activated arabinoxylan was successfully acetylated. The AXA was essentially identical to that obtained from MSA catalyzed esterification (cf. entries 6 and 9). With TFAA, when the reaction time was extended from 1 h to 3.5 h, the molecular weight was observed to drop from 486,000 to 295,000 (entry 10). Entry 11 illustrates TFAA promoted acetylation of a com fiber arabinoxylan having a high salt content. The high salt content of the arabinoxylan is due to the method of isolation (10). Entry 11 demonstrates that it is not necessary to highly purify the arabinoxylan prior to esterification as the salts can be removed from the product during precipitation of the arabinoxylan ester. 

As noted earlier, the hemicellulose A component of com fiber arabinoxylan has very limited solubility in water at a pH less than ca. 10. Entry 12 illustrates that hemicellulose is also quite difficult to esterify. The hemicellulose was allowed to swell in pH 7 water before exchanging the water for acetic acid. Using the same reaction conditions used for esterification of the hemicellulose B component (cf. entries 9 and 12), essentially no esterification was observed. This difficulty in esterifying the hemicellulose A component extends to esterification of mixtures of hemicellulose A and B (entries 13 and 14). Treatment of a mixture of hemicellulose A and B with acetic anhydride in the presence of TFAA resulted in a decreased product yield and an AXA with a much lower molecular weight. Clearly, it is advantageous to separate hemicellulose A and B prior to esterification of com fiber arabinoxylan. 

Figure 5. TGA spectra for unmodified corn fiber arabinoxylan (AX) and for the acetate (AXA), propionate (AXP), and butyrate (AXB) esters of the arabinoxylan. To remove the effects of residual moisture, the TGA spectra for AX and AXA were normalized to 100 wt% at 140 °C Reproduced with permission from reference 19, Copyright 2002 Elsevier Science Ltd.

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