The relationship between internal chain length of amylopectin and crystallinity in starch.

Molecular models of amylopectin were created and investigated by computer simulation. First, single and double helices of various lengths were constructed. The 1 --> 6 branching in double and single helices of amylopectin was studied. Subunits of single helices, double helices, and branch points were used as building blocks of larger systems. The possible makeup of amylopectin unit clusters was investigated via a series of models, including single-single, double-single, and double-double helix systems. The lengths of the single helix section that linked two branch points (internal chains) was systematically varied between values of 0-10 glucose residues. It was found that certain internal chain lengths lead to parallel double helices. Thus, it was postulated that the length of internal chains may determine the degree of local crystallinity. Furthermore, it was noted that some of the low-energy arrangement of double helices could be superimposed on either the two adjacent and nonadjacent double helices of crystalline A and B starch polymorphs. In other cases, the distance between the double helices is so large that it may in fact be a model for branching between two amylopectin crystals or unit clusters. Results obtained through this work were corroborated, where possible, with information available from crystallographic, branching, and enzymatic studies. Copyright 1999 John Wiley & Sons, Inc.