Xylans, acetates isolation

The late discovery of acetyl xylan and feruloyl esterases has been partly due to the lack of suitable substrates. Xylans are often isolated by alkaline extraction, in which ester groups are saponified. Treatment of plant materials under mildly acidic conditions, as in steaming or aqueous-phase thermomechanical treatment, leaves most of the ester groups intact. These methods, however, partly hydrolyze xylan to shorter fragments (63,69). Polymeric acetylated xylan can be isolated from delignified materials by dimethyl sulfoxide extraction (70). The choice of substrate is especially important in studies of esterases for deacetylation of xylans. The use of small chromophoric substrates (p-nitrophenyl acetate, a-naphthyl acetate, and methylumbelliferyl acetate) analogously to the assays of disaccharidases may lead to the monitoring of esterases unable to deacetylate xylan (33, 63, 64). 

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... 

The distribution of the 0-acetyl groups along the xylan chains has not been studied. The figures quoted above are very close to what would be expected if the acetate groups were distributed according to the laws of chance. An 0-acetylated xylan has also been isolated from the same birch wood by way of a peroxyacetic acid holocellulose. ... 

An elegant method based on selective enrichment of specific positions of the lignin side chain with C followed by C NMR studies [79] was applied to the study of LCC isolated from labeled wood [80]. The author claimed the presence in this preparation of LCC linkages of acetal, ether, and ester types at the a-position of the side chain and the absence of LCC bonds at the p- and y-positions of the side chain. However, a comprehensive discussirai revealed that these conclusions were not properly supported [29]. These same problems did not allow reliable NMR characterization of LCC linkages in a model Xylan-DHP substrate [58]. The main conclusion drawn from these studies is that ID C NMR is not a reliable tool to investigate LCC linkages even when using labeled preparations. 

Recently, Bi et al. (2009) isolated En. asburiae JDR-1, a strain capable of fermenting both xylose and methylglucuronoxylose, which is released from hemicellulose upon acid treatment. The strain produced predominantly ethanol and acetate from hydrolysates of sweet gum xylan. To enhance ethanol production, the PET operon was introduced into En. asburiae JDR-1. Introduction of the PET operon led to homoethanol fermentation with acetate as a by-product. In addition, deletion of the native pfl gene enhanced ethanol production from xylan hydrolysates with 99% of theoretical yield and a rate of 0.11 g ethanol/L-h, which was 1.57 times the yield and 1.48 times the rate obtained with the ethanologenic strain E. coli KOI 1. 

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