A-amylose single crystals: influence of amylose concentration, crystallization temperature and surface induction on the crystal morphology

Abstract

Narrow fractions of short-chain enzymatically-synthesized amylose have been crystallized from 0.05-5.0% w/v aqueous solutions by slow diffusion of acetone or ethanol vapors at 50-60 °C or addition of hot liquid acetone. The fractions were selected according to their ability to form a few micrometer-long A-type acicular single crystals with a well-defined facetted habit. The single crystals grew organized into rosettes or fan-like assemblies whose relative fraction depended on the initial amylose concentration. Rosettes became dominant with increasing concentration while the aspect ratio of the constituting crystals increased with crystallization temperature. In rosettes, the density of crystals increased as well but without forming solid spherulites, due to the topological constraint of packing isomorphous platelet crystals growing from a common nucleus inside a spherical volume. A higher density of crystals was achieved by nucleation on the surface defects of inductive glass slides. Due to the chemical polarity of A-type amylose single crystals, rosettes and crystal mats exposed faces concentrating reducing amylose chain ends, available for chemical labelling or grafting.