Network formation in dilute amylose and amylopectin studied by TEM

The retrogradation, or reprecipitation, of dilute amylose and amylopectin aqueous solutions was investigated by transmission electron microscopy (TEM). Negative staining, shadowing, and cryo-TEM were combined to study the morphology of the molecular assemblies at different stages of precipitation. Amylose fractal-like networks formed within a few days. They are described as clusters of elementary semicrystalline 10-15 nm units, formed by associations of molecules into parallel double helices, linked by amorphous sections containing loosely organized chains. These networks subsequently condensed, yielding thick aggregates. Amylopectin was found to form similar networks whose branches also had a 10-15 nm lateral width. The elementary units are thought to be clusters of nanocrystals formed by association of the short side branches of the molecule into parallel double helices. As the amylopectin networks were stable in solution at this concentration during several months and did not undergo any further aggregation, the branched configuration of the molecule is believed to hinder the long-scale rearrangement of the crystallites.