Branched Polysaccharides

Although AFM cannot easily detect short stubby branches it is possible to visualise larger branched polysaccharide structures. In order to detect branches the molecules have to be deposited onto substrates in an extended form. Height measurements allow branching to be distinguished from overlapping molecules.

The starch polysaccharide amylose is known to contain a small fraction of branched molecules. By forming soluble helical complexes it is possible to deposit amylose as extended molecules. The majority of the molecules are semi-flexible chains. When the molecule loops back and crosses itself the height doubles marking the junction point with a bright spot. The small fraction of branched molecules shows no change in height at the junction point. Different types of branched structures can be visualised and characterised.

For some cell-wall polysaccharides (arabinoxylans and pectins) it has been possible to observe previously unsuspected branching of the polysaccharides through the use of AFM.

In the case of sugar beet pectin about 17% of the pectin molecules were found to be branched. Similar studies on water-soluble wheat pentosans showed that about 15% of the arabinoxylan molecules were found to be branched structures.

Present evidence suggests that the images represent branched backbones rather than extended neutral sugar side-chains.

Further Reading:

Kirby AR, Alistair J. MacDougall AJ & Morris VJ
Sugar Beet Pectin – Protein Complexes. Food Biophysics 1 (1) 51-56 (2006)

Adams EL, Kroon PA, Williamson G & Morris VJ
AFM studies of water-soluble wheat arabinoxylans-effects of esterase treatment. Carbohydr. Res. 340 (2005) 1841-1845

Adams EL, Kroon PA, Williamson G, Gilbert HJ & Morris VJ.
Inactivated enzymes as probes of the structure of arabinoxylans as observed by atomic force microscopy. Carbohydr. Res. 339 (2004) 579-590.

Adams LL, Kroon P, Williamson G & Morris VJ.
Characterisation of heterogeneous arabinoxylans by direct imaging of individual molecules by atomic force microscopy. Carbohydrate Research 338 (2003) 771-780.

Morris VJ, Ring SG, McDougall AJ & Wilson RH.
Biophysical characterisation of plant cell walls. In ‘The Plant Cell Wall’, Ed. JKC Rose, Blackwell/CRC Press. Annual Plant Reviews 8, 2003, chapter 2, pp55-91.

Gunning AP, Giardina TP, Faulds CB, Juge N, Ring SG, Williamson G & Morris VJ. Surfactant mediated solubilisation of amylose and visualisation by atomic force microscopy. Carbohydrate Polymers 51 (2003) 177-182.

Round AN, Rigby NM, Ring SG & Morris VJ.
Investigating the nature of branching in pectins by atomic force microscopy and carbohydrate analysis. Carbohydr. Res. 331 (2001) 337-342.