Environmental Engineering Reference
In-Depth Information
taBle 5.2
General classification of hemicelluloses
Polysaccharide
types
subtypes
composing sugars and linkages
Xylans
Homoxylans (X)
d-Xylopyranosyl (Xylp) linked by β-(1-3) linkages (X3);
β-(1-4) linkages (X4) and/or mixed β-(1-3, 1-4)
linkages (Xm)
Glucuronoxylans (GX or MGX)
α-d-Glucoronic acid (GA) and/or its 4-
O
-methyl
derivative (MeGA) attached at position 2 of the Xylp
monomeric unit
(Arabino)glucuronoxylans (AGX)
Single α-l-arabinofuranosyl residue attached at position
3 of the β-(1-4)-xylopyranan backbone of GX
Arabinoxylans (AX)
α-l-arabinofuranosyl residue attached at position 2, 3, or
both of the same Xylp monomer unit of β-(1-4)-
xylopyranan backbone
(Glucurono)arabinoxylan (GAX)
α-d-Glucoronic acid and α-l-arabinofuranosyl residues
linked to position 2 and 3 of monomeric xylp residues
of β-(1-4)-xylopyranan backbone
Heteroxylans (HX)
β-(1-4)-xylopyranan backbone heavily substituted with
single and oligosaccharide side chains
Mannans
Galactomannan (GaM)
Backbone of d-Mannopyranose (Manp) linked by a
β-(1-4) linkage that is branched variously at position 6
by single α-galacto pyranose residues
Glucomannans (GM)
Backbone contain d-Mannopyranose (Manp) and
d-Glucopyranose (Glcp) linked by β-(1-4) linkage that
is branched variously at position 6 by single α-galacto
pyranose residues
Xyloglucan
Type I (XXXG)
Backbone composed of D-Glucopyranose (Glcp) linked
by β-(1-4) linkage with D-Xylopyranosyl (Xylp)
attached at position 6 of first three Glcp units and fourth
Glcp nonsubstituted
Type II (XXGG)
Backbone composed of d-Glucopyranose (Glcp) linked
by β-(1-4) linkage with d-Xylopyranosyl (Xylp)
attached at position 6 of first two Glcp units and next
two Glcp nonsubstituted
Mixed linked
β-glucans
Unbranched backbone composed of d-Glucopyranose (Glcp) linked by β-(1-3, 1-4) linkages
Source:
Ebringerová, A.,
Macromol Symp
., 232, 1-12, 2006.
a “conformational homology” between these molecules (O'Neill and York 2003). This causes strong
noncovalent association between them (O'Neill and York 2003). This noncovalent interaction also
contributes to the cell wall recalcitrance against various agents of degradation. Treatment with
strong alkali (i.e., 0.1-4 M KOH/NaOH) typically solubilizes hemicelluloses from pectin-removed
cell walls. A broad classification of the hemicellulosic polysaccharides, according to Ebringerová
(2006), is shown in the following table (Table 5.2).
5.2.6.1 xylans
In the plant kingdom, xylans form the most abundant hemicellulosic polysaccharide, given
that they are a prominent component of woody biomass (Ebringerová 2006). Like most of the
hemicellulosic polysaccharides, xylans render integrity to the cell walls by cross-linking with
