The fine structure of cassava starch amylopectin. Part 2: building block structure of clusters.

The aim of this work was to analyse the organization of unit chains inside clusters of cassava amylopectin. beta-Limit dextrins of the clusters and partly fragmented clusters (sub-clusters) were isolated previously [Laohaphatanaleart et al., Int. J. Biol. Macromol. (2010) doi:10.1016/j.ijbiomac.2010.01.0049] and were now hydrolysed extensively with the alpha-amylase (liquefying type) of Bacillus subtilis into small, branched building blocks. The blocks were size-fractionated and characterized structurally. The smallest blocks predominated in the clusters. They were single branched and possessed a degree of polymerization (DP) of 5-9. Blocks with DP 10-15 were double branched and constituted the second largest group. The clusters of cassava amylopectin, which were of rather uniform size, possessed typically 7-9 building blocks, and all clusters contained similar size-distributions of the blocks. The inter-block chain length was 7-8 residues. The possible mode of attack by the enzyme between the building blocks is discussed. A model of the building block organization in the clusters is presented, in which the structural roles of different sub-groups of clustered chains are suggested. A three-dimensional model suggests a possible organization of the building blocks inside the amorphous lamellae in the granular starch.