Heterogeneity in branching of amylopectin

The angular dependence of scattered light from amylopectin and its β-limit dextrin, the mean square radius of gyration and the molecular weights M_w and M_n have been calculated on the basis of the cascade branching theory for the homogeneously branched model by Meyer &; Bernfeld (1940) (Model I) and for the two heterogeneously branched structures suggested by French (1972) (Model II) and by Robin et al. (1974, 1975) (Model III). The calculations take into account the particularities of topology in branched molecules and the experimentally determined ratio of the number of A- and B-chains, A/B = 1. Furthermore, an average branching density of 4% and an interconnecting chain length of ovbar|n_i2 = 22, found by gel permeation chromatography (GPC) after debranching, were used. The constraints lead to the conclusion that amylopectin is heterogeneously branched. Densely branched clusters containing 3·22 branching units are interconnected by longer chains of 22 units in length. Comparison of the calculated angular dependence of light scattering with measurements from a maize amylopectin β-limit dextrin in 1 n NaOH solution gives strong evidence for a modified Robin-Mercier model. The modification consists of the conclusion that the interconnecting chains are preferentially B-chains, such that these chains carry on the average 1·4 clusters, while Robin and Mercier assume exactly 2 clusters. Our result is in agreement with the distribution of chain length found after debranching the amylopectin β-limit dextrin.