Biomedical Engineering Reference
In-Depth Information
2.4.1
Chemical Structure of Hemicellulose
The content and structure of hemicellulose in various plants are different. The
research on the chemical structure is mainly about the composition of the main chain
and branched chains of glucans in hemicellulose. The main chain may consist of one
or more types of glycosyls, and the connections between glycosyl are also different.
Raw materials from different producing areas and different parts have different
glycan compositions. Therefore, to illustrate the chemical structure, glycans must
be classified first. It is generally believed that hemicellulose is the glucan in the
matrix of the cell, and the main components are xylan, xyloglucan, glucomannan,
manna, galactomannan, callose, and so on [
27
].
2.4.1.1
Chemical Structure of Xylan Hemicelluloses
Almost all plants contain xylan.
D
-Xylosyls are linked with each other to form
homopolymer linear molecules as the main chain. Xylan hemicellulose is the glucan
with a backbone of 1,4-
“
-
D
-xylopyranose and with branch chains of 4-oxymethyl-
glucuronic acid.
The hemicellulose of hardwoods and gramineous forbs is mainly composed
of this kind of polysaccharide. The hemicellulose of Gramineae also contains
L
-
arabinofuranose linking to the main chain as branch chains. The number of branch
chains depends on different kinds of plants. The typical molecular structure of
hemicellulose of Gramineae is chiefly composed of
“
-
D
-xylopyranosyl, which is
linked by
-1,4-glucosidic bonds. Branch chains consist of
L
-arabinofuranosyl
and
D
-glucuronopyranosyl, respectively, on C3 and C2 of the main chain; there
are also branch chains composed of xylosyl and acetyl (xylosyl acetate). The
DP of hemicellulose in Gramineae is less than 100. Xylan hemicellluloses in
timber are composed of linear xylans linked by
“
-1,4-glucosidic bonds, with some
different short-branch chains linked to the main chain, similar to the Gramineae.
However, average polymerization is higher than 100. In addition, hemicellulose
from softwoods and hardwoods also has the distinction. Hemicellulose of hardwood
is chiefly acidic xylans that have been partly acetylized; for example, the content of
this hemicellulose in birch is about 35 %, while this content in
Euonymus bungenus
isonly13%[
28
]. Xylan hemicellulose in softwoods is 4-O-methyl-glucuronic acid
arabinose-xylan with almost no acetyl, while O-acety-
L
-4-O-methyl-glucuronic
acid xylan is the most important hemicellulose in hardwoods [
2
].
“
2.4.1.2
Chemical Structure of Mannan Hemicellulose
Softwoods contain the highest content of mannan hemicellulose; some hardwoods
also have mannan hemicellulose, but grass has little. Mannose and glucose are
linked by
“
(1
!
4) bonds to form inhomogeneous polymer as the main chain.
