Arabinoxylan

Molecular structure

Arabinoxylans generally consist of between 1500 - 5000 residues. Fiber studies indicate the molecules take up a twisted ribbon conformation with 3-fold symmetry. However, the free molecules in solution may 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-molecular and inter-molecular hydrogen bonds associated with the 6-hydroxyl groups are necessarily absent. The presence of arabinose side chains reduces the interaction between the 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 [1387]. The loss of arabinose side chains also correlated with a decline in water binding capacity [1387].

 

 

 

The xylan backbone is shown right, with φ_H (H₁C₁OC₄), ψ_H (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 [920].

 

 

 

 

 

 

 

 

 

 

 

The β-Xylan loop structure (φ_H, ψ_H = 176°, 3°) above left shows the hydrophobic cavity produced when this link has the conformation of lowest potential energy. Such structuring behaves similarly to the cyclodextrins in binding materials such as aroma compounds.

 

There are substantial changes in the structure of the arabinoxylans during bread-making, with changes in its solubility, xylose substitution, and its fine structure [4249].

 

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Functionality

Wheat and rye arabinoxylans [4261] are essential functional ingredients in many baked products affecting water binding and holding, rheology, and starch retrogradation. They also protect the gas retention in dough due to their viscous influence on gluten-starch films. Arabinoxylans form the active ingredient in Psyllium husk that is used as a laxative [2509]. Unfortunately, psyllium has not proven useful as a fat substitute [4230]. 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), i t 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. The proportions of soluble and insoluble arabinoxylans have been proposed as a key feature in the function of arabinoxylan [2016].

 

Interactive structures, including the diferulic acid link, are available (Jmol).  [Back to Top ]