The fine structure of cassava starch amylopectin. Part 1: Organization of clusters.

The enzyme alpha-amylase from Bacillussubtilis was applied to partly hydrolyze purified cassava amylopectin into groups of clusters, which were called domains. The domains were further size-fractionated by methanol precipitation and then subjected to a second stage of alpha-amylolysis until the rate of hydrolysis was slow in order to release the single clusters. All domain and cluster fractions were hydrolyzed with beta-amylase into beta-limit dextrins. The size distribution and chain composition of the beta-limit dextrins were analyzed by gel-permeation chromatography and high-performance anion-exchange chromatography with pulsed amperometric detection, respectively. The sizes of the clusters in the form of beta-limit dextrins were uniform with an average degree of polymerization of 67-78. The distribution profiles of B-chains were similar in all cluster fractions, which suggested that the internal structure of the cassava amylopectin clusters was homogenous. Long B-chains were involved in the interconnection of clusters in the domain fractions. These were cleaved and a new group of chains of intermediate length was produced by the alpha-amylase together with short chains. In the isolated clusters, however, some chains corresponding to long B-chains still remained, which is not predicted by the traditional cluster model of the amylopectin structure. Instead, the alternative two-directional backbone model could explain the mode of interconnection between the clusters.