Arabinoxylans are naturally found in the bran of grasses (Graminiae).
consist of α-L-arabinofuranose residues attached as branch-points to β-(14)-linked D-xylopyranose polymeric backbone
chains.a These may be 2- or 3-substituted or 2- and 3- di-substituted.
In wheat flour, the distribution of the type of substitution
is not random (contiguous similarly substituted residues being
preferred due to the enzymatic mechanism) but the distribution
of substituted (irrespective of of the substitution type)
residues along the chain appears random .
The arabinose residues may also be linked to other groups
attached such as glucuronic acid residues, ferulic acid
crosslinks  and acetyl groups .
The most stable conformations
(top) and β-(14)-linked
D-xylopyranose residues. The furanose can, however,
take up a number of other conformations with similar energy whereas the chair conformation
of the pyranose residue is fixed.b [Back to Top ]
Arabinoxylans generally consist of between 1500 - 5000 residues.
Fiber studies indicate the molecules take up a twisted ribbon
conformation with 3-fold symmetry. The free molecules in solution
may, however, take up a wide variety of conformations with
only moderately extended structures. Although the backbone
xylan structure is similar to that occurring in cellulose
there is little driving force to produce crystalline type
structures as the intra- and inter-molecular hydrogen bonds
associated with the 6-hydroxyl groups are necessarily absent.
The presence of arabinose side chains reduce interaction between
chains due to their inherently more flexible water-hungry
furanose conformations. However, where there are sections
of disubstituted xylan the chain is relatively inflexible
and rod-like. Although relatively unsubstituted areas of the
backbone may be able to interact as ribbons with themselves
and β-glucans at higher temperatures such interactions are likely to be
relatively weak and these areas are more likely to form helical
entanglements. When the arabinose residues are stripped off the xylan backbone (using oxalic acid), aggregation appears at a Xyl/Ara ratio of about four and precipitation occurs when this is increased above ten . The loss of arabinose side chains also correlated with a loss in water binding capacity .
Xylan backbone with φH (H1C1OC4), ψH (C1OC4H4) torsion
angles of (a) 30°,-30°, (21 helix)
similar to cellulose but unstable; (b) 57°,-141°
(31 helix) forming a 3-fold right-handed
helix; (c) 60°, 27° (31 helix)
forming a 3-fold left-handed helix. A study using
the MM3 force field gives the structures (a) and (c) as the
most stable .
β-Xylan loop structure
(φH ,ψH = 176°,3°) showing the hydrophobic cavity that
may be produced as this link is the conformation of lowest potential energy.
Such structuring will behave similar to the cyclodextrins in binding materials such as aroma compounds. [Back to Top ]
Wheat and rye arabinoxylans are important functional
ingredients in baked products affecting water
binding and holding , rheology and starch retrogradation. They also protect the gas retention in dough
due to the viscous influence on gluten-starch films. Water
binding is dependent on the arabinose substitution. Where
this is absent the molecule binds less water and becomes less
soluble but water is also lost from sections that are 2-.
3-disubstituted where there are steric exclusion effects.
Although there are some reports that the degree of arabinose
substitution has little influence on the overall semi-flexible
conformation (and hence the viscosity),
it must influence areas within the structure and control over
the radius of gyration may be
lost due to the presence of under-substituted more-flexible
regions. Entanglement of the hydrated chains increases the water-holding capacity of the arabinoxylans. The presence of oxidative ferulic acid cross-links increases the strength and permanence of this water-holding capacity and
makes gels more elastic.
Interactive structures are available (Jmol). [Back to Top ]
a Unsubstituted xylans are very rare. Xylan biosynthesis has been reviewed . [Back]
using the AMBERS force field was used for the modeling presented on these