Composition of building blocks in clusters from potato amylopectin

Abstract

The cluster structure of amylose-free potato starch was recently analysed with respect to the size and unit chain composition of the clusters [Bertoft, E. (2007). Composition of clusters and their arrangement in potato amylopectin. Carbohydrate Polymers, 68, 433–446]. The present work describes a further investigation of the cluster structure in terms of the internal organisation of the chains inside the clusters. This structural level represents the molecular organisation of the amorphous lamellae in starch granules. It was found that the chains were organised into very densely branched areas, characterised as building blocks, by which the clusters were constructed. The building blocks were isolated by extensive α-amylolysis of samples containing clusters. Doubly and triply branched blocks were identified in HPAEC-PAD, but larger dextrins with a degree of polymerisation (DP) ⩾ 20 were not resolved in the chromatograms. The majority of the branched blocks were, however, small, singly branched dextrins with DP 5–10. The overall composition of building blocks in different clusters was fairly similar, but based on the density of the blocks inside the clusters two major groups of clusters were distinguished. The average internal chain length between branches in the building blocks was only ∼2 glucosyl residues, whereas the inter-block chain length inside the clusters was estimated to 7–8 residues. A structural role of sub-groups of the short chains that form the clusters is suggested on the basis of the inter-block chain length. A minor part of the building blocks appeared to possess a strong tendency to form molecular aggregates that were detected by gel-permeation chromatography, but the structural background for this phenomenon remained unknown.