Xylan esters

FAE was specific for esterified xylan-oligomers, but did not show selectivity towards a specific ester. This enzyme could release ferulic acid as well as acetyl groups from esterified arabinoxylans in the presence of an endoxylanase. 

Sulfuric acid esters of xylan can be prepared118 by the method of Karrer. 

Other esters such as the benzoyl,108 stearoyl,101 oleoyl,101 can be prepared by treating xylan with the appropriate chloride in the presence of sodium hydroxide, potassium hydroxide, pyridine or quinoline. These derivatives have not been characterized, perhaps because of their insoluble nature. 

Figure 3.8(a) Possible ester linkage between an a-OH of lignin and a carboxyl group of a 4-0-methyl-/3-d-glucuronic acid residue on the xylan backbone. 

Like D-glucose and D-fructose, however, D-xylose can be utilized chemic ly or microbially—to generate a variety of interesting five-ca n c emica s o er than furfural (vide supra) or xylitol, a noncaloric sweetener, both being duectly produced from xylan hydrolysates, that is, without the actual isolation of the sugar. Other readily accessible intermediate products of high preparative utiUty (Scheme 2.14) are the open-chain fixed dithioacetal, the D-xylal, and D-hydroxy-xylal esters, or pyrazol or imidazol A -heterocycles with a hydrophilic trihydroxypropyl side chain. 

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). 

Furoic acid (furan-2-carboxylic acid, or pyromucic acid) is used as a bactericide, and the furoate esters are used as flavoring agents, as antibiotic and corticosteroid intermediates. It is obtained by the enzymatic or chemical/catalytic aerobial oxidation of furfural (2-furalaldehyde) the latter is the only unsaturated large-volume organic chemical prepared from carbohydrates today. D-Xylose and L-ara-binose, the pentoses contained in the xylan-rich portion of hemicelluloses from agricultural and forestry wastes, under the conditions used for hydrolysis undergo dehydration to furfural. 

Figure 2.8 Summary of behavior of phenolic acid esters and free forms in the GIT and the liver. 5-O-CQA, 5-O-caffeoylquinic acid 5-O-FAAF, 5-O-feruloyl-L-arabinofura-nose C, conjugate (sulfate, glucuronide or methyl) CA, caffeic acid FA, ferulic acid FA-FA, dimer of ferulic acid FA-oligosac., ferulic acid esterified to an oligosaccharide FA-Xy, ferulic acid esterified to xylan R, alkyl group.

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