Two-Dimensional Size/Branch Length Distributions of a Branched Polymer

The first two-dimensional structural distributions are reported for a hyperbranched polymer, the two dimensions being macromolecular size and individual branch length. The 2D distributions for native starch, a polymer with both short- and long-chain branched components (amylopectin and amylose), were obtained by size fractionation using size-exclusion chromatography combined with enzymatic debranching. These distributions show distinct macromolecular architectures: separate mountains corresponding to amylopectin and amylose and two foothills assigned to hybrid populations. The distributions reveal new mechanistic information on the underlying polymer synthesis. The branch-length distributions for amylopectin are independent of macromolecular size, whereas a size variation is observed for amylose. Biological imperatives in amylopectin biosynthesis force the same branching structure for all macromolecular sizes because of evolutionary pressure to provide the crystallinity indispensable for the plant survival. Amylose biosynthesis does not have such restrictive biological constraints, offering a range of branched structures throughout macromolecular sizes and revealing a previously unsuspected size dependence.