Biomedical Engineering Reference
In-Depth Information
two types of arabinogalactan: The common type has terminal galactose residues
linked at
O
-3 or
O
-6, galactose residues linked at
O
-3 or
O
-6, and arabinofuranose
residues linked at
O
-3 or
O
-5. Another type has galactose residues linked by
O
-4
or
O
-3 and
O
-4 bonds and arabinofuranose residues linked terminally or by an
O
-5
bond. Arabinogalactan may also be oligosaccharide constituted by several arabinose
residues. Further, ferulic acid may be linked to some arabinose and galactose
residues. Arabinogalactan in the cell walls may be an independent molecule or as
the side chains on the polysaccharide molecules of pectin [
2
].
2.4.1.6
Chemical Structure of Arabinan
Arabinan entirely consists of arabinose, and
-
L
-arabinofuranosyl residues are
linked with each other at C-5, forming the main chain. Arabinan contains many
branched chains; some side chains of arabinofuranosyl are linked to
O
-2 or
O
-3 or
simultaneously connected to
O
-2 and
O
-3, and some side chains are composed of
arabinose.
'
2.4.2
Chemical Properties of Hemicellulose
Because of the low DP and few crystalline structures, hemicellulose is more
easily degraded in acidic medium than cellulose. But, the category of glycosyl in
hemicellulose varies, including the pyran type, furan type,
'
-glycoside bond-linked
type,
-glycoside bond-linked type,
L
- configuration type,
D
- configuration type,
and so on. The ways of linkage between glycosyls are various, such as 1-2, 1-3, 1-
4 and 1-6 links [
2
]. Most studies showed that hydrolysis of methyl-rabopyranose
is the fastest; the others are arranged in decreasing speed as follows: methyl-
D
-galactopyranoside, methyl-
D
-xylopyranoside, methyl-
D
-mannopyranoside, and
methyl-
D
-galactopyranoside, which is the most stable. The
“
“
-
D
type of glycoside is
easier to hydrolyze than the
-D type. Generally, the hydrolysis rate of the furan type
is faster than that of the pyran type. The hydrolysis rate of glucuronide is 40,000;
perhaps the carboxyl has positive control in the glucoside bond.
Hemicellulose is an inhomogeneous glycan composed of a variety of glycosyls,
so the reducing ends have many kinds of glycosyls and some branch chains.
Similar to cellulose, hemicellulose can have a peeling reaction under mild alkaline
conditions. At high temperature, it would have alkaline hydrolysis. Research
showed that the speed of alkaline hydrolysis of furan glycosides was many times
faster than that of pyran glycosides. Hemicellulose can dissolve in both alkali
solution (5 % Na
2
CO
3
solution) and acid solution (2 % HCl solution). It has a
relative affinity to water, which can make it form a viscous state or become a gelling
agent. In rheologic studies of the viscosity of hemicellulose, this phenomenon can
be well observed. For example, when the concentration of hemicelluloses in water
reaches 0.5 %, the aqueous solution of hemicellulose has a certain consistency
'
