On the interconnection of clusters and building blocks in barley amylopectin.

Amylopectin is a highly branched starch component built up of a large number of clustered α-D-glucose chains. A single C-chain possesses the reducing end and carries the rest of the macromolecule. The aim of this study was to investigate the interconnection of clusters and domains (groups of clusters) in barley amylopectin by isolation of the units with α-amylolysis and subsequent labelling of the C-chain in the φ,β-limit dextrins of these structural units with the fluorescent compound 2-aminopyridine. Because these C-chains were formed by α-amylolysis of B-chains in amylopectin, they were designated bc-chains to be distinguished from C-chains in amylopectin. Four barley samples were selected for the study, of which two had the amo1 genetic background. Longer bc-chains were found in domains suggesting their role in cluster interconnection. The average chain length of bc-chains was longer than the average chain length of B-chains and the size-distribution of the bc-chains was unimodal implying that the bc-chains comprise a unique category of chains. Extensive α-amylolysis of labelled amylopectin and clusters revealed the distribution of branched building blocks situated at the reducing end of these molecules. Any type of size group of building blocks can be situated at the reducing end, because the size-distribution of these blocks was similar to the distribution of all building blocks present in the sample. This suggested certain randomness in the distribution of the types of building blocks within the amylopectin macromolecule.