Light beige powder
> 300 ° C
Insoluble in water
Xylans are herbal heteropolysaccharide and belong to the hemicelluloses, their main repeating unit ( monomer ) is the D- xylose. In nature, xylans are the second most abundant polysaccharide. Because of different modification options xylans are not a clearly defined component.
Xylans as polysaccharides of plant
The xylans were given this name in 1890 by EW Allen and Bernhard Tollens, because they assumed, xylose was the only hydrolysis product of these natural products. They are next to cellulose as the most important polysaccharide in the plant world. Xylans are the most important representatives of the hemicelluloses or polyoses.
The functions and structural properties of xylans can be most clearly represented in comparison with the cellulose. In contrast to cellulose based is no only one, only different degrees polymerized monomer. Rather, they differ in the proportion and in their chemical structure, depending on the plant families from each other. According to their monosaccharide, uronic acid and acetyl - blocks, a distinction is roughly in arabino - 4-O- methylglucuronoxylan from softwoods and in more varied form of monocots and O -acetyl- 4-O- methylglucuronoxylan from hardwoods, and some other, less common structures and subsequent modifications, eg by digestion method. Monosaccharides can function as D-xylose, D-mannose, D- glucose, D- arabinose, D-galactose, D -glucuronic acid and D- galacturonic acid.
Another difference from cellulose is the much shorter chain length of the xylans, which is normally a maximum of 200 blocks in lignocelluloses.
The side groups of xylan does not allow the same degree as in the case of the cellulose build -crystalline structures, so that in vivo only xylan probably amorphous occurs, although the formation of mixed crystals with the cellulose is contemplated. The tasks of the xylan hemicellulose as the Association of scaffold materials differ from the cellulose. The hemicelluloses not the strength properties are important, rather they act a mediator between cellulose and lignin. Their function is therefore described as that of a natural softener and support material. For the substance, or mixture of substances (primarily xylans ), which can be extracted with dilute liquor from wood, the term wood gum was formerly used.
The representation of the distribution of the hemicelluloses and thus also of the xylan in the composite cell wall a number of different models have been developed, which have in common that the hemicelluloses, cellulose fibrils to form a connecting layer, which allows the embedding of the cellulose in the Ligninmatrix. Indication of this offering, the absence of hemicelluloses in lignin natural cellulose such as cotton. A further reason for the presence of hemicelluloses would be to look into their hygroscopic property, which could contribute to an equilibrium moisture content of the cell wall.
The totality of the different chemical and physical structural characteristics of xylans that are in addition to their origin also influenced by isolation methods and technical processes that make a rather complex system. The ultimate properties and, where appropriate, the suitability for the intended use, either in the pulp or in isolated form, occur only within the interplay of all these points.
Occurrence of xylans
As already mentioned, the xylans are the most widespread in nature hemicelluloses, they are found in all land plants and in some algae. In the coniferous and deciduous trees they each represent about 10-15 % and 10-35 % of the wood substance. However, the span of this information already shows that there are significant differences in the levels between different species, in different tissues and in different layers of the cell wall. According to the task in the plant, the Xylangehalte focus eg on the different cell wall layers. In tracheids of late wood can be found more xylan than in those of early wood.
Since xylans are also at high levels included (up to 40 %) in the biomass of monocots and thus also in agricultural waste products, they are at least theoretically, in enormous quantities as a raw material available.
Average degree of polymerization (DP)
The degree of polymerization (Pn) of Holzxylanen be found average values for arabino - 4-O- methylglucuronoxylan of greater than 120, and O -acetyl- 4-O- methylglucuronoxylan of 200 However, there is a large fluctuation width, which depends on the examined species. The Nadelholzxylanen the literature values range 73-185 and the Laubholzxylanen from 86 to 218 degrees of polymerization in monocots, considerably higher as can be, for water-soluble xylan from rye bran exist DP values of 418 for the corresponding water-insoluble fraction even of 674 However, the values obtained are strongly influenced by the isolation and measurement method used and thus subject to large fluctuations.
The main and side chains
Linking the main chain consisting always only β -D - xylopyranose units ( Xyl ), either in the β - (1 → 3) - or β - (1 → 4) glycosidic form possible, wherein the β - ( 1 → 3 )-form was found only in algae and seaweeds. Normally, it is assumed in deciduous and Nadelholzxylanen of unbranched chains, but could per molecule also one or two branches occur.
From the backbone in addition to the possible branches occur in varying degrees side groups of sugars and sugar derivatives on. While esparto xylan only side chains of other xylose units, so real branches has, one finds the most complex xylans esp. in specialized tissues (eg seed coats ) of monocots. The branches usually only consist of monomeric units, but may also be dimeric or oligomeric. As sugar units come of it before L - arabinofuranose (Ara ), D - xylopyranose and in rare cases hexoses. Rhamnose is found in small amounts as a component of the end group in the xylan. Another possible side chains are the uronic acids D-glucuronic acid ( GluA ), and 4-O- methyl-D- glucuronic acid ( Me GluA ), and O-acetyl groups (Ac).
As already mentioned, the O -acetyl- 4-O- methylglucuronoxylan is typical of the hardwoods and the arabino - 4-O- methylglucuronoxylan typical of the conifers. It arabinoxylans but have also been isolated from hardwoods such as laurel plants or cinnamon bark.
4 -O- methylglucuronic acid side chains are linked glycosidically to the 2-carbon atom of the Xyloserings α - (1 → 2) arabinofuranose components of deciduous and Monokotyledonenxylane α - ( 1 → 3) - glycoside at the carbon atom 3 of xylose. In Laubholzxylanen ( the temperate zone) also contribute 50 % to 60 % of the xylose O -acetyl groups, but the content can vary a total of about 3 % to about 12 % of the mass. In Birkenxylan the ratio of unsubstituted, C2 -, C3 - and di -substituted building blocks 44: 24: 22: a high degree of acetylation of xylan 10 is favorable for the water solubility. In the wood degree of acetylation decreases with age, as va formed by an acidic medium of the acetyl side groups of acetic acid in heartwood.
Very important for the properties of xylans, in particular, the solubility, in addition to the frequency of side groups is also the composition of pendant groups and their distribution along the chain, there can be prevented by the "disturbances", which represent the substituents intermolecular interaction bearings.
For the most frequently found Laubholzxylan average ratio of xylose to 4-O- methyl -glucuronic acid is 10: 1, the variation width is 6: 1 to 11: 1, whereby fractions at ratios of 3: 1 were found. The distribution of the side chains within the xylan of a plant is therefore not constant.
For softwoods, the 4-O- methylglucuronic shares higher. There, the ratio of xylan is to 4-O- methyl -glucuronic acid usually about 5-6: 1, values 3-4: 1 occur. The arabinose groups meet on xylan in ratios of average ( Xyl: Ara ) 6-10: 1 in, and thus also show a large variability. As an average ratio of the three components in softwood can 8: 1.6: 1 ( Xyl: Me - GluA: Ara ) are given, the values between 10: 3: 1 and 2.5: 0.8: 1 fluctuate. As has been shown in various coniferous and deciduous trees, the distribution of the 4-O- methylglucuronic residues along the chain in the two wood species groups is different. While in Nadelholzxylan on the xylan backbone of time there is a more or less uniform distribution of the side groups on each 7 to 8 Xyloseeinheit, areas alternate in Laubholzxylan with intensive substitution ( approximately at every other Xyloseeinheit ) and those without side groups irregularly from. Due to the many in hemicelluloses available hydroxyl groups and the amorphous structure, they exhibit a strong hygroscopicity and are therefore largely responsible through their embedding in the cell wall association for the swelling and shrinkage of lignocellulosic materials.
In nature, xylans are degraded by so-called xylanases. These enzymes catalyze the hydrolysis of the main and side chains. Xylanases have been found in marine algae, protozoa, crustaceans, insects, snails and seeds of land plants. Mainly takes place the removal of microorganisms such as filamentous fungi that are mainly responsible for the biological degradation. The proportion xylanases secreted is higher than that of other microorganisms, such as yeasts or bacteria.
However, because of the inhomogeneous structure of xylans of degradation does not take place by a xylanase, but by an enzyme complex. Here are endoxylanases ( EC 18.104.22.168 ) and β -D - xylosidases ( EC 22.214.171.124 ) are particularly important. In order to cleave the modified side residues also ferulic acid esterases, coumaric acid esterases, acetyl esterases (EC 126.96.36.199 ), α - glucuronidases and arabinases be used ( EC 188.8.131.52 ).